Ruprecht Karls Universität Heidelberg

ITP Cosmology Seminars

Tuesday, 15h15, Phil 19, Seminar Room

Organizers: Guillem Domenech and Adrià Gómez-Valent

Seminar calendar

25 Feb 2020 15:15 - 16:15 Linda Blot (MPA Garching) - "TBA"
18 Feb 2020 15:15 - 16:15 Alexander Vikman (CEICO) - "TBA"
13 Feb 2020 15:15 - 16:15 Alessandra Silvestri () - "TBA"
04 Feb 2020 15:15 - 16:15 Anne-Christine Davis (Cambridge University) - "TBA"
28 Jan 2020 15:15 - 16:15 Florian Pacaud (Argelander-Institut fur Astronomie, University of Bonn) - "TBA"
23 Jan 2020 16:15 - 17:15 Salvatore Capozziello (Universita di Napoli Federico II) - "Cosmographic reconstruction to discriminate between modified gravity and dark energy"
Cosmography is a model independent approach, useful to discriminate among concurring cosmological scenarios. After reviewing the main features and shortcomings of LCDM model, we propose a cosmographic approach, based on some polynomial series like Pade, Chebyschev, etc. to investigate dark energy and modified gravity. The main result is that series convergence seems a powerful tool to extend the matching with the Hubble flow up to high redshift.
21 Jan 2020 15:15 - 16:15 Christian Fidler (RWTH, Aachen) - "TBA"
14 Jan 2020 15:15 - 16:15 Michele Moresco (Universita di Bologna) - "Beyond standard probes: exploring new paths to constrain the expansion history of the Universe with cosmic chronometers"
In the era of precision cosmology, exploring new and complementary approaches to measure how the Universe (and the structures therein) have evolved is of fundamental importance, to increase the accuracy in the measurements and keep under control systematic effects. In this talk, I will present a novel approach to obtain constraints on the expansion rate of the Universe based on the differential age evolution of "cosmic chronometers". The strength of this method is that it allows a direct measurement of the Hubble parameter H(z) without relying on any cosmological assumptions, providing an ideal framework to test cosmological models. I will review the latest results obtained, both in terms of H(z) measurements and of treatments of systematics, and I will discuss how this data can be used to constrain cosmological models, how these results compare with the ones obtained with more standard probes, and, finally, also how they can provide some helpful insight in the H0 controversy.
07 Jan 2020 15:15 - 16:15 Pavel Jirousek (CEICO, Prague) - "TBA"
19 Dec 2019 11:00 - 12:00 Khamphee Karwan (The Institute for Fundamental Study, Naresuan University) - "Generalized Conformal Transformation and Inflationary Attractors"
18 Dec 2019 11:00 - 12:00 Abdolali Banihashemi (Phil. 16, SR) (Shahid Beheshti University) - "TBA"
17 Dec 2019 15:15 - 16:15 Vedran Brdar & Alexander Helmboldt (Max-Planck-Institut fur Kernphysik) - "Testing seesaw models with gravitational waves"
10 Dec 2019 15:15 - 16:15 Yuko Urakawa (Bielefeld University) - "Axion like particle search through gravitational messenger"
Axion, which was proposed historically to solve the strong CP problem in QCD, is a compelling candidate of dark matter. Furthermore, exploring axion like particles (ALPs) provides a unique window to explore string theory. In this talk, I will explain our recent works which propose a new method to detect the imprint of axion or ALPs through gravitational messenger. I will show that the axions can be a prominent source of the GWs through the resonance process that takes place just after the onset of the oscillation. Because of the Chern-Simons coupling, that breaks the parity symmetry, the GWs emitted from the axion can be circularly polarized. If time permits, I will discuss another ALPs search through the birefringence of the polarized photons due to the coherently oscillating axion in a gravitational lensing system.
06 Dec 2019 11:00 - 12:00 Abdolali Banihashemi (Phil. 16, SR) (Shahid Beheshti University) - "TBA"
05 Dec 2019 13:15 - 14:15 Fazlollah Hajkarim (Phil. 12, SR 106) (Goethe University Frankfurt) - "Primordial Gravitational Waves and Dark Matter in Pre-BBN Cosmology"
Using the quantum chromodynamics (QCD) equation of state (EoS) from lattice calculations we investigated QCD effects on the first order primordial gravitational waves (PGWs) produced during the inflationary era. We also consider the cases for vanishing and nonvanishing lepton asymmetry where the latter one is constrained by cosmic microwave background experiments. Also, we investigate scenarios that inflation is succeeded by a phase where the energy density of the Universe was dominated by a scalar component with a general equation of state. Then we evaluate the spectrum of primordial gravitational waves induced in the post-inflationary Universe. We show that if the energy density of the Universe was dominated by some specific fluid phi before Big Bang Nucleosynthesis (BBN), its equation of state could be constrained by gravitational wave experiments. Moreover, we studied the effect of QCD and electroweak transitions on the induced (or second order) PGW from scalar perturbations which is different from the first order PGW spectrum. Additionally, we study the production of dark matter (DM) in an early matter era dominated by a heavy long lived scalar field, using the EoS of the standard model and assuming initially vanishing and nonvanishing radiation and DM densities. In that case new production mechanisms for DM beyond the thermally produced weakly interacting massive particles (WIMPs) are also possible. Finally, we focus on the production of neutralinos, as a popular DM candidate in supersymmetric models, in low reheating temperature scenarios and its status in current experimental searches.
03 Dec 2019 15:15 - 16:15 Sebastian Bramberger (Max Planck Institute for Gravitational Physics ) - "Bounces and Real Time Quantum Cosmology"
In this talk I will explore several possibilities of circumventing the singularity theorems by Hawking and Penrose. On the one hand I will take a closer look at the oscillatory BKL behaviour of the universe near the singularity. In particular we will see that remnants of this behaviour persist even when the singularity is resolved. On the other hand, when classical solutions do collapse to a singularity, quantum mechanics offers a way out as there is a non-zero probability for a contracting universe to tunnel to an expanding one. I will make this idea quantitative in the context of the Lorentzian path integral approach to quantum gravity.
26 Nov 2019 15:15 - 16:15 Elisabeta Lusso (Universita di Firenze & INAF) - "Active galactic nuclei as cosmological probes"
I will present the latest results on our analysis of the non-linear X-ray to ultraviolet luminosity relation in a sample of optically selected quasars from SDSS, cross-matched with the most recent XMM-Newton and Chandra catalogues. I will show that this correlation is very tight, implying that the observed relation is the manifestation of an ubiquitous (but still unknown) physical mechanism, that regulates the energy transfer from the accretion disc to the X-ray emitting corona in quasars. Moreover, I will show that this relation is valid over a very broad redshift range: from local AGN up to z~7 sources. I will discuss what the perspectives of quasars in the context of observational cosmology are and present new measurements of the expansion rate of the Universe in the redshift range z=0.5-5.5 based on a Hubble diagram of quasars. The distance-redshift relation of quasars at z<1.4 is in agreement with that of supernovae and with the concordance model. Nonetheless, a deviation from the LCDM model emerges at higher redshift, with a statistical significance of ~4 sigma. We found that, if an evolution of the dark energy equation of state is allowed, our data suggest a dark energy density increasing with time.
19 Nov 2019 15:15 - 16:15 Basundhara Ghosh (Geneva University) - "Lensing effects in large-scale structures and corrections to E_g statistics"
In this era of precision cosmology, galaxy surveys are aiming to probe high redshifts in the observable Universe, and it is important to include relativistic corrections in our theoretical predictions. I will briefly talk about the significance of lensing effects in density-shear correlations for present and future surveys. As an extension of this topic, I will discuss the E_g statistics as a tool for distinguishing modified gravity theories from general relativity. I shall show how this statistics is affected by lensing terms in density observations and how to correct for them.
12 Nov 2019 15:15 - 16:15 Christopher Sean Gallagher (Queen Mary University of London) - "Multi-Scale Perturbation Theory for Cosmologies with Nonlinear Structure"
Cosmological perturbation theory relies on the assumption that density contrasts are small. This assumption is valid for most of the history of the universe, but begins to break down at late times due to matter’s susceptibility to gravitational collapse. However, the post-Newtonian treatment of gravity is perfectly capable of dealing with highly nonlinear density contrasts, so long as the system under consideration is slow-moving and small in spatial extent - precisely the conditions present in large-scale structures such as superclusters! I will present a novel formalism for simultaneously applying a post-Newtonian expansion on short scales, while keeping a traditional approach to perturbation theory on large scales. This approach allows for the possibility of explicit coupling between terms from each sector. I will then use approximate solutions to calculate the matter bispectrum, using this statistic to highlight similarities and differences between traditional perturbation theory and our new multi-scale approach.
05 Nov 2019 15:15 - 16:15 Simon Friederich (gHS Phil 12) (University of Groningen) - "Multiverse theories -- are they testable? And what's fine-tuning got to do with it?"
Multiverse theories entail that there are distant regions, possibly separate universes, where the laws and constants of nature are different. Such theories are very difficult to test because we may not have empirical access to those distant regions or universes. Can we hope to test those theories nevertheless by assuming that, if there is a multiverse, we are “typical” inhabitants? Can we perhaps conclude from the fact that the laws of nature seem fine-tuned for life that there is a multiverse? Or is it blatantly fallacious to infer from the life-friendliness of this universe that there are other universes some of which are not life-friendly?
29 Oct 2019 14:45 - 15:45 Julien Lesgourgues (RWTH Aachen University) - "Could cosmology hint at new particle physics?"
Cosmological data offer an overall consistent picture up to a few anomalies, including two long-standing ones, the "small scale crisis" and "sigma8 anomaly", a more controversial Hubble tension, and an even more controversial Edges anomaly. A very interesting case for theorists is that if these tensions are not due to poorly understood systematics or astrophysical effects, they could be explained by particle physics ingredients such as decay and/or elastic scattering in the dark matter and/or neutrino sector, that may arise from the properties of a rich Dark Sector. I will review some of these models, trying to identify a few general trends, while providing a few examples based on recent works in my group.
22 Oct 2019 14:45 - 15:45 Bjoern Malte Schaefer (Zentrum fur Astronomie der Universitat Heidelberg) - "information entropies and inference in cosmology"
Information entropies are measures of the randomness of a distribution, which are based on an axiomatic approach and which have useful and practical analytical properties. In my talk I’m giving a short introduction about using information entropies in inference problems in cosmology and show analogies between entropies and conventional measures of experimental uncertainty. In Bayesian inference, experimental design and treatment of systematic errors information entropies can replace more traditional concepts like likelihood ratios and evidences.
17 Oct 2019 14:45 - 15:45 Yeinzon Rodriguez (Research Centre on Basic and Applied Sciences, Universidad Antonio Narino, Bogota) - "Einstein Yang-Mills Higgs dark energy revisited"
Inspired in the standard model of elementary particles, the Einstein Yang-Mills Higgs action with the Higgs field in the SU(2) representation was proposed in Rinaldi (2015 Class. Quantum Grav. 32 045002) as the element responsible for the dark energy phenomenon. We revisit this action emphasizing in a very important aspect not sufficiently explored in the original work and that substantially changes its conclusions. This aspect is the role that the Yang-Mills Higgs interaction plays at fixing the gauge for the Higgs field, in order to sustain a homogeneous and isotropic background, and at driving the late accelerated expansion of the Universe by moving the Higgs field away of the minimum of its potential and holding it towards an asymptotic finite value. We analyse the dynamical behaviour of this system and supplement this analysis with a numerical solution whose initial conditions are in agreement with the current observed values for the density parameters. This scenario represents a step towards a successful merging of cosmology and well-tested particle physics phenomenology.
15 Oct 2019 14:45 - 15:45 Subodh Patil (NBIA and Discovery Center, Niels Bohr Institute) - "Tensor bounds on the hidden universe (and a 1PI coda on loops in inflation)"
In this talk, we present the observation that primordial gravitational waves, if ever observed, can be used to bound the hidden field content of the universe. This is because a large number of hidden fields can resum to potentially observable logarithmic runnings for the graviton two-point function in the context of single field inflation, courtesy of a `large N' resummation. This allows one to translate ever more precise bounds on the tensor to scalar consistency relation into bounds on the hidden field content of the universe, with potential implications for phenomenological constructions that address naturalness with a large number of species. Along the way, we need to address certain subtleties regarding loop corrections on cosmological backgrounds, especially with regards to the correct implementation of dimensional regularization. We find a pleasing confirmation of our understanding by directly reproducing our results from the 1PI effective action (as one must) and present a simplified prescription to calculate loop corrections to certain correlation functions in inflation.
25 Sep 2019 11:00 - 12:00 Marcus Werner (Kyoto University) - "Optical Geometry and the Magnetoelectric Effect"
Optical geometry is a spatial formalism for light propagation in Lorentzian spacetimes. This provides a useful framework for gravitational lensing which is geometrically interesting and includes applications of non-Lorentzian geometries. In this talk, I will first consider Riemannian optical geometry, and review basic results as well as recent work using the Gauss-Bonnet method and curve-shortening flow. In particular, the first isoperimetric inequality in this context will be presented. Then I will discuss Finslerian optical geometry of stationary spacetimes, including the Kerr-Randers structure and extensions of the Gauss-Bonnet method. I will highlight the connection with Jacobi-Maupertuis metrics and the magnetoelectric effect. Finally, a framework for optics in non-metric spacetimes will be sketched briefly.
10 Sep 2019 16:00 - 17:00 Miguel Quartin (Universidade Federal do Rio de Janeiro) - "Probing the large scale structure with supernovae"
The supernova (SN) Hubble diagram residual contains valuable information on both the present matter power spectrum and its growth rate. SN lensing effects introduce non-Gaussianities, while the peculiar velocities of SN introduce correlations between different data points. I will show current constraints by combining both methods and forecasts for LSST. I will discuss how different survey properties affect the peculiar velocities forecasts, and show that although this information is peaked in the range z < 0.2, there is also plenty of information in 0.2 < z < 0.5. I will end by highlighting the challenge of a brute-force computational of the PV effect.
23 Jul 2019 11:00 - 12:00 Robert Brandenberger (Mc Gill University) - "Inflation and Alternatives in Light of Fundamental Physics"
Inflation is not the only early universe scenario which is consistent with the current cosmological data. I will introduce a couple of alternative scenarios, and will then ask which - if any - of these scenarios find a natural embedding in superstring theory.
17 Jul 2019 13:00 - 14:00 Chiara Moretti (note unusual time and date) (Universita degli Studi di Trieste ) - "Generating approximated halo catalogs with Modified Gravity"
Future generations of large scale structure surveys, such as the one provided by the Euclid satellite, will measure the clustering of matter with very high precision, yielding tight constraints on cosmological parameters and allowing to check for signatures of models that go beyond the standard LCDM one. In order to make predictions both for the standard model and for alternative ones based on modified theories of gravity, dedicated N-body simulations are needed. The latter are however very demanding in terms of computational time; an alternative approach is provided by approximate methods, that are able to generate distributions of dark matter halos for different models in a fast way, with minor compromises in accuracy. Such methods allow to explore the cosmological parameters' space and to compute covariance matrices. I will discuss the PINOCCHIO code, based on Lagrangian perturbation theory and ellipsoidal collapse, and describe the approach taken to extend the code to modified gravity theories. The new method developed is used to compute a reliable approximation to the second order LPT growth factor, which in turn allows to readily compute second order Lagrangian displacements. The accuracy of the results is tested against full N-body simulations run with f(R) gravity, by comparing the halo power spectrum computed with second order Lagrangian displacements. Our LPT-based method allows to recover the halo P(k) up to mildly non-linear scales, with the same accuracy obtained for the standard LCDM model.
16 Jul 2019 11:00 - 12:00 Daniele Bertacca (AIfA, Bonn University) - "GR corrections in Large Scale Structure"
On cosmological scales, relativistic effects that arise from observing on the past light-cone alter the observed galaxy number counts, therefore leaving their imprints on N-point correlators at all orders. In this talk, first of all, I will show that, working in spherical Bessel coordinates, it is possible to derive a compact expression for the power spectrum and bispectrum that encompasses all the physical effects at first and second order, including integrated (along the line of sight) terms. Then, I will quantify the effective local non-Gaussianity due to the relativistic light-cone projection effects in the squeezed limit, which could potentially contaminate the primordial non-Gaussianity signal if not properly accounted for. Finally, I will introduce a method to create mock galaxy catalogs in redshift space including all general relativistic effects to linear order in the cosmological perturbations. LIGER (i.e. LIght cones with General Relativity’) method takes a (N-body or hydrodynamic) Newtonian simulation as an input and outputs the distribution of galaxies in comoving redshift space. This result is achieved making use of a coordinate transformation and simultaneously accounting for lensing magnification. The calculation includes both local corrections and terms that have been integrated along the line of sight. Our fast implementation allows the production of many realisations that can be used to forecast the performance of forthcoming wide-angle surveys and to estimate the covariance matrix of the observables.
09 Jul 2019 11:00 - 12:00 Irina Dvorkin (cancelled) (Max Planck Institute for Gravitational Physics (Albert Einstein Institute)) - "TBA"
09 Jul 2019 11:00 - 12:00 Andreas Schachner (ITP Heidelberg) - "Introduction to the landscape-swampland program"
In this talk, I give a brief introduction to the landscape of string vacua. Specifically, I focus on the construction of viable string vacua possibly allowing for the realisation of inflation. Due to severe technical difficulties arising in this context, the so-called swampland has gained increasing attention in recent years. I therefore summarise some of the important conjectures about the swampland. To conclude, I talk about recent progress in challenging these conjectures based on work done in collaboration with A. Hebecker and D. Junghans published in JHEP 1903 (2019) 192.
02 Jul 2019 11:00 - 12:00 Giulia Cusin (Oxford University) - "Anisotropies of the astrophysical gravitational wave background in LIGO-Virgo and LISA frequency bands"
The astrophysical background of gravitational waves (AGWB) is composed by the incoherent superposition of gravitational wave signals emitted by a large number of resolved and unresolved astrophysical sources from the onset of stellar activity until today. I present a theoretical framework to fully characterize the AGWB in terms of energy density anisotropies and polarization and I show the first numerical predictions for the angular power spectra of anisotropies in different frequency bands and for cross-correlations with electromagnetic observables e.g. weak lensing and galaxy number counts. l then illustrate and discuss the astrophysical implications of this study.
27 Jun 2019 11:00 - 12:00 Ilia Musco (ICC University of Barcelona) - "Formation and Abundance of Primordial Black Holes"
Primordial black holes can form in the early Universe from the collapse of cosmological perturbations after the cosmological horizon crossing. They are possible candidates for the dark matter as well as for the seeds of supermassive black holes observed today in the centre of galaxies. If the perturbation is larger than a certain threshold, depending on the equation of state and on the specific shape of the perturbation, a black hole is formed. In this talk I will discuss the dependence of PBH formation from the initial shape of the curvature profile showing the relation between the threshold amplitude and the initial shape of the inflationary power spectrum of cosmological perturbations, taking into account also possible primordial non-Gaussianity. Although the abundance of PBHs could vary by several order of magnitudes depending on the model of inflation, we find that the threshold of PBH formation is rather solid against non linearities.
18 Jun 2019 11:00 - 12:00 Claus Kiefer (University of Cologne) - "Singularity avoidance in anisotropic quantum cosmology"
I discuss the fate of classical singularities in quantum cosmological models. After stating suitable criteria of singularity avoidance, I apply them to Friedmann-Lemaitre models as well as, in more detail, to the anisotropic case of a Bianchi I universe. One finds that the classical singularities are generally avoided in these cases.
17 Jun 2019 14:00 - 15:00 Tassia Ferreira (Note unusual time and date!) (Federal University of Espírito Santo / Carnegie Mellon University) - "A tale of two projects: covariance matrices and interaction in the dark sector"
Covariance matrices are important tools for obtaining accurate constraints on cosmological parameters and, as such, it is essential to be able to identify their most relevant elements. The first part of my talk will be focused on our efforts to single out the most important modes of the DESY1 covariance matrix for cosmic shear, and also how we can use them as a means of comparison between covariance matrices produced by different codes. In the second part of my talk, I will start with an overview of two cosmological models with interaction in the dark sector. I will then move on to explain our work to constrain the parameters of these models using background observables, such as Type IA Supernovae, Baryon Acoustic Oscillation, Cosmological Clocks and the acoustic angular scale of the CMB. I will conclude by showing our results and findings when we took the Bayesian evidence to compare these models to the standard one.
11 Jun 2019 11:00 - 12:00 Javier Rubio (Helsinki University) - "Higgs inflation and the nature of gravity"
Higgs inflation is an appealing inflationary model based on the inclusion of a non-minimal coupling between the Higgs field and gravity. Although the scenario was initially formulated as a metric theory in which the connection determining the Ricci scalar was identified with the Levi-Civita connection, this need not be the case. In particular, one could consider a Palatini formulation of gravity in which the metric and the connection are taken to be independent geometrodynamical variables. In this talk, I will discuss the pros and cons of these two alternative scenarios and the possibility of testing the fundamental nature of gravity with future cosmological observations.
04 Jun 2019 11:00 - 12:00 Vicente Atal (ICC University of Barcelona) - "Primordial Black Holes during Inflation and the Non-Gaussian Regime"
Primordial Black Holes (PBH) could be formed during inflation, possibly giving account for the dark matter in the Universe. The mass and abundances of the PBH depends on the shape of the collapsed regions as well as the statistics of the underlying random density field. These are usually determined assuming gaussian statistics for the inflaton perturbations. In this talk, I will show that this approximation fails for all single field models of inflation producing PBH known so far in the litterature. I will argue that a non-perturbative calculation of the non-gaussianities is necessary and show that a proper account on this implies the presence of a new regime for the creation of PBH
28 May 2019 11:00 - 12:00 Leong Khim Wong (Cambridge University) - "The impact of scalar fields on black hole binaries"
Scalar fields have become ubiquitous in our attempts to understand the nature of dark matter and dark energy. If these models are correct, they have interesting implications for astrophysical black holes. In this talk, I will present recent progress in our understanding of how black holes behave when interacting with a background scalar field. I will discuss how to construct an effective field theory (EFT) for such a system, and will show that the dynamics can be easily understood in terms of an induced scalar charge. We will then apply this EFT to learn how the inspiral of a black hole binary is affected when located inside a fuzzy dark matter halo.
21 May 2019 11:00 - 12:00 Arianna Renzini (Imperial College London) - "SKY MAPS of the GRAVITATIONAL WAVE BACKGROUND "
Since the direct detection of gravitational waves in 2016, we have focused on setting up a pipeline aimed at measuring anisotropies in the intensity of gravitational waves on the sky. We have been particularly interested in mapping the gravitational wave background (GWB), which is simply defined as the incoherent overlap of all signals which are unresolved by the detector (in this case, LIGO+Virgo). Different backgrounds may then be described and investigated depending on their nature. In this talk I will first describe and contextualise our map-making method, then present our results. Specifically, I will illustrate maps presented in our recent paper on stochastic GWB sky maps from Advanced LIGO O1 data and give a preview of our latest results with O2 data. Time permitting, I will also introduce our new work on detecting polarised GWBs.
14 May 2019 11:00 - 12:00 Sadra Jazayeri (Cambridge University) - "Solid Adiabatic Modes and Soft Theorems "
Cosmological observations, from the cosmic microwave background to large-scale structure, indicate that the initial condition for inhomogeneities in our universe has been adiabatic, namely, in the long wavelength limit, any perturbation in our universe locally mimics a coordinate change. I first review the concept of adiabatic modes; I demonstrate two empirical consequences of adiabaticity: conservation of the curvature perturbation and soft theorems for single field inflation. Adiabatic modes for perfect fluids and scalar fields have been extensively studied in the literature. In this talk, I investigate a solid coupled to gravity; I study its adiabatic modes and their corresponding soft theorems. Although almost every deformation in a solid—even in the long wavelength limit—is non-adiabatic, I will show that a solid exhibits infinitely many adiabatic modes.
07 May 2019 11:00 - 12:00 Moritz Platscher (MPI Heidelberg) - "Phenomenological implications of the Vainshtein screening mechanism -- a case study in bimetric gravity"
In this talk, I will discuss the astrophysical and cosmological implications of the Vainshtein screening mechanism, according to which the predictions of General Relativity are restored within a certain (spherical) region. Given that the radius of this Vainshtein sphere depends on the enclosed mass itself, the resulting effect is expected to appear on various physical length scales. It is shown how this can be used to address various anomalous observations in astrophysics and/or evade bounds on the parameter space at the same time. For definiteness I consider bigravity, the theory of two dynamical tensor fields which give rise to a massless and a massive spin-2 excitation.
30 Apr 2019 11:00 - 12:00 Neven Bilic (Rudjer Boskovic Institute) - "Tachyon inflation in the holographic braneworld"
The origin of the field that drives inflation is still unknown and is subject to speculations. Among many models of inflation a popular class comprise tachyon models. After a brief introduction to the AdS/CFT holography a tachyon inflation will be discussed in the framework of holographic cosmology. The model is based on a braneworld scenario with an effective tachyon field on a D3-brane located at the holographic boundary of an asymptotic ADS$_5$ bulk.
25 Apr 2019 11:00 - 12:00 Stefano Savastano (Note unusual date) (ITP Heidelberg) - "Primordial Dark Matter Halos from fifth-forces"
Dark matter is an essential ingredient of modern cosmology, its fundamental nature being one of the biggest open enigma in physics. I discuss the possibility to provide the entire dark matter content in form of screened dark compact objects which could be potentially generated during radiation dominated era. In the standard cosmological scenario this would be suppressed due the fast expansion rate of the Universe lead by radiative pressure. Nevertheless, the introduction of an additional long range attractive interaction stronger than gravity enhances the growth of fluctuations in the density field of a non-interacting matter fluid, which eventually collapse in Primordial Dark Matter Halos. Finally, I analyse in which conditions these structure are able to evade microlensing constraints and cosmic microwave background energy injection bounds.
16 Apr 2019 11:00 - 12:00 Lucas Lombriser (University of Geneva) - "Unravelling Cosmic Acceleration with Gravitational Waves and Large-Scale Structure"
Identifying the nature of the late-time accelerated expansion of our Universe remains a difficult puzzle to cosmology. Scalar-tensor modifications of gravity have long been considered as an alternative explanation to the cosmological constant. I will first discuss how the direct detection of gravitational waves and the measurement of electromagnetic counterparts, confirming a luminal speed of gravity, in combination with observations of the large-scale structure brought the anticipated challenge to the concept of cosmic self-acceleration from scalar-tensor gravity. I will then provide an outlook of how a more general model space will ultimately only be exhaustively probed by Standard Sirens. In a second part, I will show how a simple additional variation of the standard Einstein-Hilbert action with respect to the Planck mass may solve both the old and new cosmological constant problems. The additional variation yields a topological constraint that prevents vacuum energy from gravitating and when accounting for the nonlinear structure formation predicts a current energy density parameter of the cosmological constant of Omega_Lambda=0.704, in good agreement with observations.
09 Apr 2019 11:00 - 12:00 Pranjal Trivedi (CANCELLED) (ITP Hamburg) - "Axion-like Dark Matter Constraints from CMB Birefringence"
Axion-like particles (ALPs) are leading dark matter candidates originally motivated by the strong CP problem and also arise in theories of string compactifications. I will present a sensitive probe for ALPs or ultra-light dark matter - the birefringence in the cosmic microwave background (CMB). Birefringence arises from the oscillating ALPs' effective refractive index and is also relevant for laboratory axion searches. Constraints on the axion-photon coupling derived from birefringence of CMB polarization lead to orders of magnitude improvement over prior constraints, with further prospects for upcoming cosmological birefringence observations. These limits, in hitherto unconstrained regions of the coupling vs. ALP mass parameter-space, are independent of assumed magnetic fields and relatively robust to ALP dark matter fraction.
19 Mar 2019 11:00 - 12:00 Mahmood Roshan (Ferdowsi University Of Mashhad) - "Imprints of modified gravity on the long-term evolution of spiral galaxies"
In this talk I report the results of a series of galactic N-body simulations in the context of a specific subclass of modified gravity theories. Using high-resolution N-body simulations, we investigate the dynamical evolution of disk galaxies and compare the result with the standard dark matter viewpoint. Although the initial conditions of the baryonic matter are the same in both models, their long-term dynamics reveals some notable differences. For example, it turns out that the bar instability happens with different rate in modified gravity model compared with the standard case. On the other hand, at the end of the simulation, we find that bars are weaker and faster in modified gravity compared with the standard case. Finally we discuss the relevant observations that may help to discriminate between modified gravity effects and particle dark matter in the galactic scale.
12 Mar 2019 11:15 - : () - "TBA"
05 Mar 2019 11:00 - 12:00 Maximilian Duell (Heidelberg University) - "Constructive Gravity and Pre-metric Electrodynamics: Towards refined Friedmann equations"
The Constructive Gravity program provides new insights into the construction of (modified) gravity theories. With the mild physical assumption of common canonical evolution of matter and geometric background, one can show that all information about the gravity theory is already contained in a prescribed matter theory. The gravitational Lagrangian then arises as the solution of a set of partial differential equations, the gravitational closure equations. In the case of cosmology, a suitable symmetry reduction in the Constructive Gravity program allows to directly derive the cosmological field equations — refined Friedmann equations. That means, starting with Maxwell electrodynamics as matter input, one directly obtains the Friedmann equations in cosmology without having to know about the Einstein equations at all. Turning to a slightly more refined example, namely the most general theory of electrodynamics with linear field equations as a matter model, a similar approach will yield refined Friedmann equations.
26 Feb 2019 11:15 - : () - "TBA"
19 Feb 2019 11:15 - : () - "TBA"
12 Feb 2019 11:15 - : () - "TBA"
05 Feb 2019 11:15 - : Clement Stahl (Pontificia Universidad Catolica de Valparaiso) - "Relativistic cosmological large scale structures at one-loop"
In this talk, I will discuss some aspects of non-linear structure formation. The calculation of the bispectrum usually involves linear physics on large scales and non-linear (but Newtonian) physics on small scales. However, in the squeezed limit, the bispectrum (unlike the power spectrum) couples large scales with small scales. I will present the calculation of the non-linear relativistic corrections to the dynamics of dark matter perturbations. They are of the same order of magnitude, in the squeezed limit, than a primordial local non-Gaussianity signal. Such signal is expected to be within the accuracy of the next generation of LSS surveys.
29 Jan 2019 11:15 - : Julio Fabris (PPGCosmo/UFES, Brazil) - "Stiff Matter Solution in Brans-Dicke Theory and The General Relativity Limit"
Generally the Brans-Dicke theory reduces to General Relativity in the limit the BD parameter goes to infinity if the scalar field has an appropriate behaviour. However, it is also known that there are examples the Brans-Dicke does not tend to GR, specially when the trace of the energy-momentum tensor is zero (a gas of photons, for example). This is connected with an anomalous behaviour of the scalar field. We discuss another case: a homogeneous and isotropic universe filled with stiff matter. The power of time dependence of these solutions does not depend on the Brans-Dicke parameter, and there is no General Relativity limit even though the scalar field behaves appropriately in this limit. A perturbative and a dynamical system analysis of this exotic case are carried out.
22 Jan 2019 11:15 - : No seminar () - "TBA"
15 Jan 2019 11:15 - : No seminar () - "TBA"
08 Jan 2019 11:15 - 12:15 Laura Sagunski (York University) - "Testing Modified Gravity with Merging Neutron Stars"
The recent first ever direct detections of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the VIRGO detector have marked the dawn of a new era for probing the theory of general relativity (GR) as well as possible modifications of it. To test gravity at extreme conditions, current and future gravitational wave detectors are targeted to explore merging binary systems of black holes and/or neutron stars. Motivated by this, we study the final stages of evolution of a neutron star binary system and investigate how modifications of GR can effect the emitted gravitational wave signal. Already for the simplest $f(R)$ theory of modified gravity, $R^2$ gravity, we find characteristic features in the gravitational wave signal that make it clearly distinguishable from the one in GR and that should be detectable with LIGO.
18 Dec 2018 11:15 - : Satadru Bag () - " Discussing dark energy models from theoretical point of view"
The mysterious component called "dark energy", which comprises almost 70% of the current energy budget of the universe, is believed to be responsible for the observed cosmic acceleration at the present epoch. Interestingly, many of the recent observations predict that the properties of dark energy are consistent with the concordance cosmological model (LCDM), based on the Einstein's general theory of relativity and characterised by a constant energy density of the dark energy. But the LCDM model is plagued by quite a few theoretical and observational issues. Therefore, the importance of looking for the alternatives to the general relativity for explaining the cosmic acceleration, consistent with the observations, cannot be overstated. The talk will be focused on the following two compelling models of evolving dark energy. (i) New tracker models of dark energy: we introduced a new class of quintessence driven dark energy models which behave like cosmological trackers at early times and are consistent with the observed late-time accelerated expansion of the universe. These models are based on the α-attractor set of potentials, originally discussed in the context of inflation. (ii) Braneworld models of dark energy: the presence of a large extra dimension in the braneworld models can result in late-time accelerated expansion that is consistent with the observation. I will discuss some attractive features and the advantages of these models, and how well they match the current observations.
11 Dec 2018 11:15 - : Oliver Piattella (ITP - UFES) - "The effect of the cosmological constant on the bending of light"
Whether or not the cosmological constant Lambda affects the bending of light is a matter of debate. In principle, being Lambda a homogeneous component, one does not expect from it any contribution whatsoever. On the other hand, Ishak and Rindler have suggested in 2007 (arXiv:0709.2948) that a more appropriate definition of the bending angle in a Schwarzschild-de Sitter geometry makes Lambda appear in the formula for the bending angle. Since then many investigations appeared on the subject, some substantiating the result of Ishak and Rindler, others contradicting it. I will review part of this literature and present my own contribution, based on calculations on the McVittie metric (arXiv:1508.04763), which also allows to treat a more general cosmology, in which the Hubble factor is a function of time (arXiv:1609.00270)
06 Dec 2018 15:00 - : Michele Oliosi (Note unusual day and time) (Yukawa Institute) - "Two modified gravity theories with few degrees of freedom"
In this talk, I will present two specific modifications of gravity in which the number of degrees of freedom is minimized. I will focus on their construction, showing how different methods can be used to accomplish the same goal, i.e. keeping a low number of degrees of freedom. The first theory that I will describe is the minimal theory of quasidilaton massive gravity, a Lorentz-breaking theory of massive gravity + scalar field. As a second example, I will describe a class of modified gravity theories that propagate only the usual two polarizations of gravitational waves.
04 Dec 2018 11:15 - : Jorinde van de Vis (Nikhef) - "Preheating after Higgs inflation: self-resonance and gauge boson production"
In many inflationary models, inflation is driven by a scalar field. Since the Higgs is the only scalar particle detected so far, it is a natural candidate for the inflaton particle. When the Higgs field is nonminimally coupled to gravity, Higgs inflation can accurately predict cosmic microwave background parameters. Since the couplings between the Higgs and the other particles of the standard model are known, the dominant decay mechanisms of the Higgs-inflaton can be determined. We have studied resonant and perturbative particle production after Higgs inflation. We have coupled the Higgs-field to a U(1)-gauge field, in order to simulate the decay into standard model gauge bosons. We determine the dominant decay channels and reheating temperature after inflation for a range of values of the nonminimal coupling.
03 Dec 2018 15:00 - : Matteo Maturi (Special Seminar, Philosophenweg 12, room 060) (ITA, Heidelberg) - "Galaxy clusters as tools for cosmology"
Galaxy clusters are fundamental probes to investigate the nature of dark matter and the complex phenomena involving their baryonic content. Moreover, their number, mass and redshift distribution provide us with precious insights into the nature of our universe and of dark energy. To properly use them as a tool for cosmology, it is now of crucial importance to have large samples and a solid understanding of their observable and physical properties so as to obtain a reliable statistical sample and control over possible biases. Having this in mind, I will discuss ways (1) to detect galaxy clusters in weak lensing and optical photometric data sets; (2) to identify and characterize strong lensing features such as giant arcs in order to gain a deeper understanding of clusters; (3) to understand and exploit their weak lensing, Rees-Sciama and polarization imprints in the anisotropies of the Cosmic Microwave Background radiation. Moreover, also a method to model the images of galaxies, useful to produce realistic simulated astronomical images of galaxy clusters, is going to be discussed.
27 Nov 2018 11:00 - : No seminar today () - ""
20 Nov 2018 11:00 - : David Wands (ICG Portsmouth) - "Seeing stochastic inflation"
Cosmological inflation gives us a window onto the very early universe. In particular quantum fluctuations during inflation can perturb the local expansion and hence give rise to large-scale structure in our universe today. I will discuss the stochastic approach to inflation which incorporates the effect of quantum fluctuations into the dynamics of the coarse-grained fields above the characteristic Hubble scale during inflation. In some regimes this stochastic diffusion of the fields can become comparable to the classical drift leading to large density perturbations and even the formation of primordial black holes after inflation. I will discuss how we can use the stochastic formalism to calculate the non-perturbative density perturbation in such models and challenge recent claims that stochastic inflation fails beyond slow-roll inflation.
13 Nov 2018 11:00 - : Stefano Anselmi - CANCELLED (IAP, Paris) - "The Linear Point: a cleaner cosmological standard ruler in the galaxy correlation function"
06 Nov 2018 11:00 - : Guillem Domenech (ITP) - "Schwinger effect in (3+1) de Sitter: negative conductivity?"
Consider the possibility that there were strong electric (and magnetic) fields in the very early universe. How does the pair creation of charged particles out of vacuum (Schwinger effect) behave in a de Sitter (dS) spacetime? Can it shut off or enhance the production of electric fields? Formal exact calculations of the induced current show interesting peculiarities. For instance, one finds negative values of the conductivity for light charge carriers and weak fields — even in the limit of vanishing electric field — which could trigger an spontaneous electrification of dS. Furthermore, for heavy charge carriers one does not recover the exponential suppression of the current, expected from gravitational pair creation. What is the meaning of that? Is there anything wrong with the regularisation of the formal calculation? Perhaps we are simply misunderstanding the results? In this talk, I will clarify the above mentioned behaviors — some are related to the birefringence of the QED vacuum and some are spurious — and I will discuss the possible impacts in inflationary magnetogenesis.
30 Oct 2018 11:00 - : No seminar this day () - "TBA"
23 Oct 2018 11:00 - : Chunshan Lin (Warsaw University) - "Is GR unique?"
Probably not. I will introduce some different gravity theories which are as good as GR in the sense that all of constraints are first class and therefore a graviton has only 2 polarisations and the structure of the theory at low energies is thus expected to be stable against quantum corrections.
22 Oct 2018 14:00 - : Ryo Namba (Please note unusual day and time) (McGill University) - "TBA"
22 Oct 2018 14:00 - : Ryo Namba (Please note unusual date and time) (McGill University) - "Phenomenology of interacting gravitational waves during inflation"
Detection of tensor mode fluctuations at the largest cosmological scales is often expected to provide a robust evidence of inflation and to fix the inflationary energy scale. Such direct connection is however applicable only when gravitational waves (GWs), the source of tensor perturbations, are effectively decoupled from other energy contents. In this talk I discuss a case exceptional to this standard lore. Spin-1 particles can be produced efficiently during inflation due to interactions, and their energy is then transferred to GWs. When spin-1 fields have homogeneous part with a specific configuration, produced particles can source GWs to the amount considerably larger than the standard amplitude. Moreover, such GWs have non-trivial correlations with curvature perturbations, leading to interesting cross correlation functions, which would otherwise be absent. I demonstrate detectable GW signals even from low-energy inflation and discuss potentially observable correlations between tensor and scalar perturbations.
16 Oct 2018 11:00 - : Matteo Fasiello (ICG Portsmouth) - "Probes of late-time Cosmic Acceleration"
Operational and near-future astronomical surveys of the galaxy distribution will soon put to the test our best ideas as to the nature of the current cosmic acceleration. I will first briefly review some alternative probes of late-time acceleration and then focus on LSS ones. The perturbative treatment of LSS can well describe the quasi-linear regime of structure formation. This regime is a precious repository of key information on both the early and late-time Universe dynamics. I will elaborate on recent progress towards including a dark energy and a modified gravity component in this framework and detail on a phenomenological implementation of screening mechanisms.
24 Jul 2018 11:00 - : Seyed Mohammad Sadegh Movahed (IPM, Tehran) - "Multi-scale searching machine to detect the cosmic strings network"
The initial conditions for large-scale structures is a mysterious subject in cosmology. The inflationary paradigm is widely accepted scenario for seeding the structure in the Universe. However, there is room for alternative scenarios not only from observational points of view but also from theoretical approaches. Cosmic topological defects formed during phase transition in the very early universe are theoretically well-motivated. Cosmic strings network is parameterized by the energy scale of the phase transition represented by $G\mu$. The cosmic string can leave the imprint on the CMB stochastic field leading to emerging additional stochasticity behavior in the CMB map. In this talk, I will rely on the stochasticity nature of CMB superimposed by cosmic strings network due to Gott-Kaiser-Stebbins phenomenon. Some topological and geometrical measures accompanying multi-scale edge-detection algorithm to examine the minimum value of $G\mu$ for which, our pipeline is able to detect the cosmic strings network incorporating anticipated systematic noises for some surveys, will be introduced. On the noiseless sky maps with an angular resolution of $0.9'$, we show that our pipeline detects cosmic string with $G\mu$ as low as $G\mu\gtrsim 4.3\times 10^{-10}$. At the same resolution, but with a noise level typical to a CMB-S4 phase II experiment, the detection threshold would be to $G\mu\gtrsim 1.2 \times 10^{-7}$. We also explore the use of random forest and gradient boosting, two powerful tree-based machine learning algorithms to determine the feature importance of some topological and geometrical measures in detecting cosmic strings. Such approach opens new insight into utilizing prior information for detecting exotic features in a stochastic field.
17 Jul 2018 11:00 - : Catarina Cosme ( Universidade do Porto) - "Scalar field dark matter with spontaneous symmetry breaking and the 3.5 keV line"
We show that the present dark matter abundance can be accounted for by an oscillating scalar field that acquires both mass and a non-zero expectation value from interactions with the Higgs field. The dark matter scalar field can be sufficiently heavy during inflation, due to a non-minimal coupling to gravity, so as to avoid the generation of large isocurvature modes in the CMB anisotropies spectrum. The field begins oscillating after reheating, behaving as radiation until the electroweak phase transition and afterwards as non-relativistic matter. The scalar field becomes unstable, although sufficiently long-lived to account for dark matter, due to mass mixing with the Higgs boson, decaying mainly into photon pairs for masses below the MeV scale. In particular, for a mass of ∼7 keV, which is effectively the only free parameter, the model predicts a dark matter lifetime compatible with the recent galactic and extragalactic observations of a 3.5 keV X-ray line.
17 Jul 2018 17:05 - 18:00 Adria Gomez-Valent (Please note unusual date and place: gHS, Philosophenweg 12) () - "Hints of dynamical dark energy in the Universe "
In this talk I will start commenting on several historical aspects of the cosmological constant, and also on its potential link with the concept of vacuum energy. Then I will focus my attention to the class of the so-called Running Vacuum Models (RVMs), providing the most relevant expressions at the background and linear perturbations levels. I will discuss the fitting analysis carried out to constrain the various parameters entering the model, and finally I will present the results not only for the RVMs, but also for some other dark energy models and parametrizations. I will show that for some of them we find signals of dynamical dark energy, reaching the 3-4 sigma confidence level.
10 Jul 2018 11:00 - : Laura Bernard (CENTRA Lisbon) - "The challenge of testing gravity with gravitational waves"
The observations of gravitational waves from coalescing compact binary systems allow us to test gravity in its strong field regime. In the first part of this talk, I will focus on model-dependent tests and the problem of building accurate waveform templates in scalar-tensor theories. In a second part, I will discuss the prerequisite question of building consistent alternative theories of gravity. I will present some recent work on the construction of a theory of partially massless gravity on curved spacetimes.
10 Jul 2018 16:00 - : Barbara Sartoris (Note unusual time and place: Phil.12 Rm.106) (OATS) - "Cluster mass profiles as powerful tests of cosmological models and structure formation scenarios"
A key result of the hierarchical scenario of structure formation is that the halo density profiles have a universal shape across a wide range of mass scales. Such profiles have a central cusp proportional to r^-1 and an external slope proportional to r^-3, as come out from the N-body simulations. However, simulations have demonstrated that in the central part the slope of the profile could be shallower following a profile more similar to an Einasto. Moreover Diemer & Kravtsov 2014 demonstrated that in the outskirts of the clusters the slope of the density profile is steeper than expected. Such steepening is higher when the mass of the cluster or the accretion rate of the halo are higher. At that jump in the profile is located the splash back radius. Finally, the internal mass distribution of cluster can in principle be used to detect deviations from the General Relativity. Such deviations change the relations between the two scalar potentials in the perturbed Friedmann-Lemaitre-Robertson-Walker metric and the fluctuations of the matter density field. These relations enter in the determination of a cluster mass profile. By using a combination of data obtained as part of a VLT/VIMOS large program CLASH and from MUSE for the cluster RXJ2248 at redshift z=0.348 of mass ~2x10^15 M_sun,we test our knowledge of cosmological models and structure formation scenarios with a precise calculation of the cluster mass profile.
03 Jul 2018 11:00 - : Gonzalo Alonso (ITP) - "Non-minimally coupled scalar dark matter from inflationary fluctuations"
It is well known that light scalar fields present during inflation are coherently excited. We show that if the field couples to gravity in a non-minimal way, the fluctuations at large scales are suppressed with respect to the small scales ones. This fact allows for the field excitations to make a sizeable contribution to the energy density of the universe without generating too large isocurvature fluctuations at observable scales. We show that this mechanism could generate all the observable dark matter and study the main cosmological implications of this setup.
26 Jun 2018 11:00 - : Jose Maria Ezquiaga (IFT Madrid) - "Inflation and the factory of primordial black-holes"
Gravitational wave astronomy has opened a new window to test the population of black-holes in the universe. The origin of these BHs is still an open question. If the majority of BHs are produced in the early universe, they could constitute a fraction of the dark matter or act as seed for supermassive BHs. Taking this as a motivation, I will discuss how to produce primordial BHs during inflation. In particular, I will focus on single-field models where PBHs arise from a second plateau in the potential. I will show that in these models quantum diffusion plays an important role, impacting the estimation of the abundance of PBHs.
19 Jun 2018 11:00 - : Miguel Zumalacarregui (UC Berkeley & IPhT Saclay) - "Testing Dark Energy and Gravity with the propagation of Gravitational Waves"
Modern scalar-tensor theories of gravity provide self-accelerating cosmologies with the possibility of alleviating puzzles like the value of H0. I will present recent progress in constructing viable, yet extremely predictive self-accelerating models based on this paradigm, and present the conditions by which some of them modify the speed of gravitational waves (GW). This provides the most stringent test for a large class of theories, which have been recently ruled out by the GW speed measurement following GW170817. If time permits I will discuss a novel approach to test dark energy theories using the interaction between standard and additional polarizations.
12 Jun 2018 11:00 - : Ricardo Zambujal Ferreira (ICCUB, Universitat de Barcelona) - "When axions leave a footprint"
In this talk I'll discuss two examples where axions can leave interesting cosmological signatures. First I'll start with the most standard case, the QCD axion, and show how the axion can thermalize with the standard model at temperatures between the QCD and the EW phase transitions. This thermalization produces an axion relic abundance which contributes to the effective number of relativistic degrees of freedom and can be observable by near future CMB experiments. In the second part of the talk I'll discuss instead the case where the inflaton has axion-like couplings to gauge fields and fermions of the Standard Model. Due to the axial coupling there is an instability, and consequent particle production, in the equation of motion of the gauge fields. This particle production can be so large that the system thermalizes during inflation due to an equilibrium between the thermal masses and the instability. This scenario, dubbed Thai-flation, has very interesting predictions for inflation and, in particular, makes polynomial models of inflation in much better agreement with data.
05 Jun 2018 11:00 - : Valeri Vardanyan (Leiden University) - "alpha-attractor realization of quintessential inflation in the era of precision cosmology."
Over the last few years, a large family of cosmological \alpha-attractor models has been proposed, which can successfully match the latest inflation-related observational data. In this talk I will present possible quintessential extensions of $\alpha$-attractor models, where a single scalar field serves both as an inflaton field and as dynamical dark energy. We will see that in many of the scenarios studied the dark energy equation of state w=-1 is favored. However, there are some models with $w\neq -1$, which are compatible with the current observations. In the models with asymptotically vanishing potentials, one has the tensor to scalar ratio $r=\frac{12\alpha}{N^2}$ and the asymptotic equation of state $w=-1+\frac{2}{9\alpha}$. For example, in the seven disk M-theory related model with $\alpha = 7/3$ one finds $r \sim 10^{-2}$ and the asymptotic equation of state is w ~ -0.9. Future observations, including large-scale structure surveys as well as B-mode detectors will test these, as well as more general models presented in the talk. I will also discuss gravitational reheating in models of quintessential inflation and argue that its investigation may be interesting from the point of view of inflationary cosmology. Such models require a much greater number of e-folds, and therefore predict a spectral index $n_{s}$ that can exceed the value in more conventional models by about 0.006. This suggests a way to distinguish the conventional inflationary models from the models of quintessential inflation, even if they predict w = -1
29 May 2018 11:00 - : Ana Marta Pinho (ITP Heidelberg) - "Model-independent constraint of the anisotropic stress parameter eta"
We constrain the anisotropic stress parameter η in a model-independent way, without assumptions on the initial conditions or Dark Matter. The correspondence between this model-independent relation and the quasistatic limit for a Horndeski Lagragian makes these constraints a test for this class of Dark Energy models. The recent available data of Hubble expansion rate, Redshift Space Distortions and Lensing observables enable this analysis and we account for the problem of data reconstruction by comparing three different methods. I will present our results and important aspects of this work.
22 May 2018 11:00 - : Alaric Erschfeld (ITP Heidelberg) - "Cosmological perturbation theory beyond the single-stream approximation"
Exploring cosmic structure formation beyond the perfect pressureless fluid approximation is essential to understand the dynamics of vector and tensor perturbations of gravitational clustering. We consider a system of cold dark matter particles in a Vlasov-Poisson setting describing a fluid with velocity dispersion. By breaking the self-consistency of the single-stream approximation, vector and tensor perturbations are generated through non-linear interactions. In the linear limit we derive analytic expressions for the cosmic propagators and power spectra accounting for the dominant linear effects. The non-linear regime is studied within the effective field theory of large-scale structure formation, where we obtain one-particle irreducible corrections and develop a new prescription to derive a resummed non-linear propagator. We consider scalar initial conditions and show that vector and tensor power spectra are generated due to the coupling of modes. Finally the propagators and power spectra are numerically computed at one-loop, where we find that non-linear interactions with velocity dispersion perturbations are divergent for ultraviolet modes.
15 May 2018 11:00 - : Andrey Shkerin (EPF Lausanne) - "Gravity, Scale Invariance and the Hierarchy Problem"
Combining the quantum scale invariance with the absence of new degrees of freedom above the electroweak scale leads to stability of the latter against perturbative quantum corrections. Nevertheless, the hierarchy between the weak and the Planck scales remains unexplained. We argue that this hierarchy can be generated by a non-perturbative effect relating the low energy and the Planck-scale physics. The effect is manifested in the existence of an instanton configuration contributing to the vacuum expectation value of the Higgs field. We analyze such configurations in some theories encompassing the Standard Model and General Relativity. Dynamical gravity and a non-minimal coupling of it to the Higgs field play a crucial role in the mechanism.
08 May 2018 11:00 - : David Langlois (APC Paris) - "Dark energy and modified gravity in generalised scalar-tensor theories"
This talk will present the very general framework of Degenerate Higher-Order Scalar-Tensor (DHOST) theories, which includes and extends Horndeski and Beyond Horndeski theories. I will discuss the cosmological aspects of these theories and how they can lead to deviations from standard gravity in astrophysical bodies. I will also explain how these theories are constrained by the recent observation of the gravitational wave event GW170817 combined with its electromagnetic counterpart.
02 May 2018 17:00 - : Christian Byrnes, NOTE SPECIAL DATE AND TIME (University of Sussex) - "Black hole formation during the QCD phase transition"
Following the unexpected detection by LIGO of intermediate mass black hole mergers, there has been huge interest about whether these black holes are primordial and if they could constitute all of dark matter. In order to form primordial black holes, the primordial power spectrum needs to be boosted on small scales. In this talk I will show that the reduction in the equation of state during the QCD phase transition leads to a significant enhancement of primordial black hole formation with approximately one solar mass, an order of magnitude lighter than those detected by LIGO (and too light to have formed astrophysically).
01 May 2018 11:00 - : No seminar on this day () - ""
24 Apr 2018 11:00 - : Guillem Domenech (ITP, Heidelberg) - "Doppelganger dark energy: modified gravity with non-conformal couplings after GW170817"
We now know that Gravitational Waves propagate at the speed of light (or really really close to it) in the nearby universe. For dark energy cosmologists, this means that theories with higher derivatives terms - which modify the propagation speed of GWs - are at a first glance excluded. Only very particular cancellations among the functions (for a given background) in the Lagrangian are allowed. Nevertheless, the propagation speed of GWs could have been different than light at high redshifts - constraints there are still loose. In this talk, I will show a (surviving) model with higher derivative terms in which the current value of the propagation speed is a dynamical limit, i.e., the model is an attractor to c_{GW}=1. Interestingly, it predicts in general that the speed of GWs will be different than light at high redshifts. To do so, I will present a generalization of a class of models where dark energy and dark matter energy densities are proportional to each other - initially aimed to solve the "coincidence" problem. For these reason, we called them doppelganger dark energy.
28 Feb 2018 17:15 - : Pierre Fleury () - "Testing the equivalence principle on cosmological scales"
The equivalence principle is the main pillar of the general theory of relativity. Albeit exquisitely constrained on Earth and in the Solar System, its validity remains to be proved on cosmic scales, especially when the unknown dark matter is concerned. In this talk, I will show that relativistic effects in galaxy surveys offer the possibility to directly test the equivalence principle. Percent-level constraints can be achieved with future large surveys like the Square Kilometer Arrays.
21 Feb 2018 17:15 - : Daniela Saadeh () - "How isotropic is the Universe?"
A fundamental assumption of the standard model of cosmology is that the large-scale Universe is isotropic. Because of its centrality, it is essential to test this assumption. Breaking isotropy leads to Bianchi cosmologies, a set of solutions to the Einstein field equations of which only the subset describing rotating universes was previously tested against data. In this talk, I present a general test of isotropy considering, for the first time, all the degrees of freedom of anisotropic expansion. We analyse cosmic microwave background data from Planck, carrying out the first joint analysis of temperature and polarization data for this purpose. We also show that improved constraints on anisotropy may be obtained by extending the likelihood to high ell. For the vector mode (associated with rotating universes), we obtain a limit on the anisotropic expansion that is an order of magnitude tighter than previous Planck results using the CMB temperature only. We recover upper limits for all the other modes, with the weakest one arising from the regular tensor modes. We disfavour anisotropic expansion of the Universe with odds of 121,000:1 against.
20 Feb 2018 11:00 - : Alessandra Silvestri, PLEASE NOTE UNUSUAL TIME () - " Testing gravity with Large Scale Structure: the theoretical side of the challenge"
It is now an exceptional time for modern cosmology, when we can observe the universe with high precision and connect cosmological measurements with theory. The excitement about the advances of observational cosmology is accompanied by the awareness that we face some major challenges: we still lack compelling theoretical models for dark matter, (that accounts for the formation of the structure we see around us) , and dark energy, that drives cosmic acceleration, as well as a deeper understanding of the mechanism that set up primordial conditions, and these puzzles have deep roots in particle theory and gravity. I will focus on the challenge posed by cosmic acceleration, review approaches to it and discuss theoretical issues involved in finding an optimal framework to test gravity and the physics of dark energy from upcoming high precision measurements of large scale structure.
14 Feb 2018 17:15 - : Manuel Wittner (ITP Heidelberg) - "Is dRGT Massive Gravity Unique?"
Massive gravity is one of the most natural modifications of general relativity. The standard framework for describing a massive graviton is the so called dRGT theory, which has some problems fitting observations, however. In this talk I will review dRGT massive gravity and present a new theory for a massive graviton, dubbed beyond dRGT. The decoupling limit of this new theory lives inside the class of beyond-Horndeski theories, which proves the absence of ghosts in the decoupling limit and hints at their absence in the whole theory. This indicates that dRGT massive gravity is not the unique ghost-free theory to describe a massive graviton.
07 Feb 2018 17:15 - : Thejs Brinckmann () - "Interacting dark matter"
The LCDM paradigm has seen enormous success on large scales, but is faced with a growing number of anomalies. This may indicate a necessity to go beyond the LCDM model, in order to reconcile theory with observations. I will discuss a number of these anomalies and a possible extensions to the LCDM model that may alleviate them. In particular, I will focus on a model in which dark matter interacts with a species of dark radiation.
31 Jan 2018 17:15 - : Takeshi Kobayashi () - "Baryons, Dark Matter, and Light Scalars"
The appearance of scalar fields with small masses is ubiquitous in physics beyond the Standard Model. In this talk I will describe how such light scalars can play important roles in cosmology, especially in connection to the origin of the baryons and dark matter of our universe. With regard to the baryons, I will show that a scalar with an approximate shift symmetry, such as the QCD axion, can naturally induce baryogenesis. For dark matter, I will present cosmological constraints on scalar dark matter obtained from analyses of the Lyman-alpha forest and the cosmic microwave background; here I will particularly focus on the ultralight regime and examine the long-standing claim that ultralight scalar dark matter may solve the small-scale issues of the CDM paradigm.
24 Jan 2018 17:15 - : Sohyun Park (CEICO, Prague) - " Analysis of the Deser-Woodard nonlocal gravity model"
I will review a nonlocal gravity model proposed by Deser and Woodard to explain the current phase of cosmic acceleration without dark energy. I will present how the model reproduces the LambdaCDM expansion history without a cosmological constant and what is its prediction for cosmic structure formation, which turned out to be in better agreement with data than the one predicted by LambdaCDM. The studies of the structure formation were carried out by two groups, one in a localized formulation employed by Nersisyan, Fernandez Cid and Amendola and the other in the original nonlocal form by Dodelson and myself. By comparing these two formulations, I will show that they give the same solutions for perturbation variables provided the boundary conditions are set the same. I will also briefly discuss some conceptual issues in nonlocal gravity (including the Deser-Woodard model and other classes of nonlocal models introducing a new mass scale) such as causality, degrees of freedom and stability.
17 Jan 2018 14:00 - : Pierluigi Monaco, NOTE UNUSUAL TIME AND VENUE, Philosophenweg 12, SR 106 ( University of Trieste and INAF-Trieste observatory) - "Cosmological mock galaxy catalogs: approximate methods in the era of high precision cosmology"
The estimation of cosmological parameters and their confidence levels requires the construction of a large number of galaxy mock catalogs. The highest requirements are set by the computation of covariance matrices, that possibly requires thousands of realizations; here a brute-force, N-body approach is out of the question. I will review the techniques that are presently used for constructing galaxy mock catalogs, concentrating on those approximate methods, typically based on Lagrangian Perturbation Theory, that can be used to generate quick predictions for galaxy clustering and galaxy lensing. I will then discuss how these techniques are being applied beyond the LambdaCDM model, starting from the introduction of massive neutrinos and addressing the extension to modified gravity theories.
10 Jan 2018 17:15 - : No seminar on this day () - ""
20 Dec 2017 17:15 - : No seminar on this day () - "TBA"
13 Dec 2017 17:15 - : No seminar on this day () - ""
06 Dec 2017 17:15 - : Tommi Tenkanen (Queen Mary University of London) - "The Dawn of FIMP Dark Matter"
I will present an overview of scenarios where the observed Dark Matter (DM) abundance consists of Feebly Interacting Massive Particles (FIMPs), produced non-thermally by the so-called "freeze-in" mechanism. In contrast to the usual freeze-out scenario, frozen-in FIMP DM interacts very weakly with particles in the visible sector and never attained thermal equilibrium with them in the early Universe. This makes frozen-in DM very difficult but not impossible to test. In this talk I will present the freeze-in mechanism and its variations previously considered in the literature, compare them to the standard DM freeze-out scenario, discuss several aspects of model building, and pay particular attention to observational properties of such feebly interacting DM.
29 Nov 2017 17:15 - : Marco Baldi (Bologna University) - "Testing Cosmic Degeneracies with the SIMCODE project"
The upcoming era of precision cosmology, with several major observational enterprises expected to survey the Universe with unprecedented accuracy, will have the power to unveil even extremely feeble indications of possible new physics beyond the standard models of particle physics and cosmology. Many crucial aspects of our present understanding of fundamental physics, such as the laws of gravity, the nature of dark matter particles, the mass spectrum of neutrinos, or the driving mechanisms of inflation, will be tested and constrained with a level of precision that will allow us to rule out vast portions of our current landscape of alternative theories and hypotheses. Nonetheless, a serious potential source of systematics in such long-term observational program has been so far mostly overlooked, and may undermine the real discriminating power of future observations. This amounts to the possibility that different extensions of the standard model may co-exist and their observational footprints may partially interfere with each other, or even compensate or completely cancel. Such situation, known as a "Cosmic Degeneracy", could significantly bias — if not properly taken into account — the interpretation of cosmological observations, and will in any case significantly reduce the real constraining power of future experiments. In this talk I will provide an overview of the present activities of SIMCODE, a multi-year computational cosmology project aimed at testing the effects of various types of Cosmic Degeneracies on cosmological observables by means of detailed combined simulations of structure formation.
22 Nov 2017 17:15 - : Florian Kuhnel (KTH Royal Institute of Technology) - "Primordial Black Holes as Dark Matter"
Primordial black holes are black holes that may have formed in the early Universe. Their masses potentially span a range from as low as the Planck mass up to many orders of magnitude above the solar mass. This, in particular, includes black holes of the order of 10 solar masses, like those recently discovered by LIGO. These may be of primordial origin. In order to quantitatively asses this and related scenarios, a profound understanding of the holes' formation mechanism necessary. After a general introduction on primordial black holes, I will discuss the most consequential aspects of their formation, and elaborate on the observable imprints these may leave. I will give an overview about recent abundance limits, and comment on constraints of combined scenarios in which also particle dark matter is present.
15 Nov 2017 17:15 - : Guillem Domenech (ITP Heidelberg) - "Could there be observable effects from the matter frame during inflation?"
Pick your favourite inflationary model and redefine your metric. For example, go from an expanding metric to a contracting one and ... you get the same inflationary predictions! This is called changing "frame" and the "frame" invariance of observables. Two common choices are: the Einstein frame (i.e. the Einstein-Hilbert action) and the matter frame, where matter fields minimally couple to the metric. Does the frame invariance of observables imply that all inflationary models related by a redefinition of the metric are degenerate? Fortunately, the answer is no. In this talk, I will review inflation, the notion of frames and why are they relevant. In passing, I will briefly show the observable invariance for a generalised class of transformations. I will focus my talk on an explicit model where a light spectator scalar field yields an important contribution to the curvature power spectrum depending on which frame it minimally couples to, e.g. a super-inflating or bouncing frames. In the super-inflating case, it could lead to the formation of primordial black holes.
08 Nov 2017 17:15 - : Alessandro Codello () - "Covariant Effective Field Theory of Gravity and Cosmology"
Using covariant methods we construct the effective field theory of gravity as a loop expansion in inverse powers of the Planck mass, distinguishing classical, leading and next-to-leading quantum corrections. We then compute the leading corrections in a curvature expansion to quadratic order. We specialise the effective action to FRW spacetimes and we derive the related effective Einstein equations. We show that for particular values of the parameters we recover many different non-local modifications of general relativity recently proposed, that are able to describe both early and late time cosmology.
25 Oct 2017 17:15 - : Eleonora Villa () - "Lensing convergence in galaxy clustering in LCDM and beyond"
We study the impact of neglecting lensing magnification in galaxy clustering analyses for future galaxy surveys, considering the ΛCDM model and two extensions: massive neutrinos and modifications of General Relativity. Our study focuses on the biases on the constraints and on the estimation of the cosmological parameters. Our results show that the information present in the lensing contribution does improve the constraints on the modified gravity parameters whereas the lensing constraining power is negligible on the ΛCDM parameters. On the other hand the estimation is biased for all the parameters if lensing is not taken into account. This effect is particularly significant for the modified gravity parameters. Our findings show the importance of including lensing in galaxy clustering analyses for testing General Relativity and to constrain the parameters which describe its modifications.
24 Oct 2017 14:00 - : Disrael Camargo Neves da Cunha () - "Signature of Cosmic String Wakes in N Body Simulations "
On the first part of the talk I will present a review of cosmic strings and some of its observational consequences. One of the observable object produced by a cosmic string is the wake, which is a high density sheet-like structure in the matter distribution. The second part will be a description of the work arXiv 1508.02317 in which the stability of the wake is analysed. The important conclusion is that this object could be globally undisrupted until today. Finally, I will mention consequences of this work for wake detection in the large scale structure, using the results from n-body simulation of dark matter distribution with a wake.
18 Oct 2017 17:15 - : CANCELLED Thejs Brinckmann () - "Dark sector interactions"
The LCDM paradigm has seen enormous success on large scales, but is faced with a growing number of anomalies. This may indicate a necessity to go beyond the LCDM model, in order to reconcile theory with observations. I will discuss a number of these anomalies and possible extensions to the LCDM model that may alleviate them. In particular, we can allow for dark sector interactions in the form of Self-Interacting Dark Matter or dark matter-dark radiation interactions.
11 Oct 2017 17:15 - : No seminar on this day () - ""
04 Oct 2017 17:15 - : Jacopo Fumagalli (University of Amsterdam and Nikhef) - "Connecting low energy physics to inflation"
In the context of inflation, a model is said to be predictive if the predictions depend on a number of parameters smaller than the number of predictions themselves. In this talk we consider different types of single field inflationary models (such as Higgs inflation-type and Cosmological Attractors), discussing their unitarity and renormalizability properties. Once quantum corrections are considered, new physics is demanded. As a matter of fact, to consistently connect the low and high energy regimes of the effective field theories (described in terms of Standard model and inflationary parameters respectively), some threshold corrections are needed. This raises the question: how are the predictions sensitive to the required UV completion? We show under which circumstances the observables predicted by the model are insensitive to the threshold corrections. Finally, I will critically comment on the assumptions about the unknown UV completion on which the results rely.
26 Jul 2017 14:30 - : Andrea Maccio, NOTE UNUSUAL DATE, TIME AND VENUE , R105, Philosophenweg 12 (NYU Abu Dhabi ) - "CDM dies hard"
The current paradigm of structure formation is based on the existence of Cold Dark Matter (CDM). This component drives the formation of structures on all scales and it is extremely successful in reproducing all the large scale observables. On the other hand pure Nbody (gravity only) simulations have shown that such a model faces several problems when compared with observations on small (sub-galactic) scales. Are those problems a threat for the CDM paradigm? Do we need to revised our understanding on the nature of dark matter? In my talk will show that most of those alleged problems can be alleviated if not solved when the physics of the luminous sector of the universe (stars and gas) is taken into account in the simulations. Hydrodynamical simulations show that CDM faces no real problem on small scales when a proper comparison is carried out, making CDM a very successful model on all scales... besides the fact that we have still to detect it...
25 Jul 2017 11:30 - : Matteo Martinelli, NOTE UNUSUAL DATE AND TIME (Leiden Institute of Physics) - "Easing tensions between CMB and Weak Lensing; extensions of LCDM and the role of theoretical priors."
Recent observations of weak gravitational lensing, highlighted a tension in the measurements of the growth of cosmological structures with respect to CMB. Although it is not ruled out that this tension is due to some unforeseen systematic effect, it can also be seen as a sign of breakdown of the standard LCDM model. In this talk I will focus on some extensions or modifications of the standard cosmological model, highlighting how these can reconcile low and high redshift observations. Furthermore, I will show how imposing conditions of theoretical viability significantly impact the analysis of possible departures from LCDM when exploiting phenomenological parametrizations.
24 Jul 2017 11:30 - : Peter Dunsby, NOTE UNUSUAL DATE AND TIME (University of Cape Town) - "Travels in modified gravity"
In this talk I will present a comprehensive survey of work done over the past few years on f(R) theories of gravity and their astrophysical and cosmological consequences using both top-down and bottom-up constructions. Particular focus will be given to the perturbative and dynamical systems techniques used to unravel the fourth order nature of these theories and to shed light on the expansion history and evolution of large scale structure in the Universe, their imprint on the cosmic microwave background and the development of a scheme to investigate the production and features of gravitational waves generated by astrophysical sources. I will also consider bottom-up constructions of f(R) cosmologies, which are built by patching together sub-horizon-sized regions of perturbed Minkowski space. The comparison of these results with data coming from a range of cosmological surveys, together with future CMB and gravitational wave experiments will provide us with a much more detailed understanding of the nature of the gravitational interaction, as well as a possible explanation of the late time acceleration of the universe.
19 Jul 2017 17:00 - : Mafalda Dias (DESY) - "Complexity and universality in inflation"
An ultraviolet-complete description of inflation may not be simple. Indeed, our best paradigm for physics at high energies seems to point towards highly complex low-energy effective potentials with a large number of degrees of freedom interacting via non-trivial couplings. Building models of inflation in such scenarios is a challenging task. A promising approach is to incorporate stochastic tools in model building as a way of embracing the underlying complexity of the system, and take advantage of the universal behaviour which emerges when a system becomes sufficiently complex. This universality can lead to surprisingly simple phenomenological behaviours and sharp observable predictions. In this talk I will present some problems we have managed to address by taking this perspective and discuss future directions and open challenges.
12 Jul 2017 17:00 - : Thomas Schwetz-Mangold (Karlsruhe Institute of Technology) - "Axion dark matter miniclusters"
The QCD axion remains one of the most attractive dark matter candidates. In the case when the Peccei-Quinn phase transition happens after the end of inflation, we expect order-one density fluctuations for the cosmological axion. The typical size of the fluctuations is set by the horizon at the time of the QCD phase transition. Those fluctuations will lead to bound objects called axion miniclusters. If a significant fraction of axion dark matter is bound in miniclusters, we expect strong implications for axion dark matter searches. In the talk I will discuss how miniclusters are formed and give an estimate for the axion dark matter power spectrum as well as the minicluster mass distribution.
05 Jul 2017 17:00 - : Caroline Heneka (ITP Heidelberg) - "Line intensity mapping as a tool to probe Reionization and Cosmology"
Line intensity mapping opens up a new and exciting window for probing astrophysics as well as cosmology and fundamental physics, extending to redshifts previously untested by galaxy surveys. I present simulations of cosmological volumes during the Epoch of Reionization of 21-cm, as well as Lyman-alpha and H-alpha fluctuations, in order to derive corresponding power and cross-power spectra. For example, the cross-correlation of 21-cm, Ly-alpha and H-alpha fluctuations is shown to probe the structure of the IGM. Furthermore, 21-cm power spectra in modified gravity are derived, where additional parameters are the initial conditions of matter perturbations and the (in general) scale-dependent modified gravity parameter that measures deviations from GR in the Poisson equation, in order to put model-independent constraints. We show the ability of SKA jointly with other probes to measure the cross-correlation of 21-cm and Ly-alpha, as well as to constrain modifications of gravity with 21-cm power spectra. I will also shortly venture to mention a low redshift direction of my work, related to galaxy clusters as a probe of cold dark energy.
28 Jun 2017 17:00 - : Matteo Nori (University of Bologna) - "Simulating Ultra Light Axion Dark Matter through SPH "
I will present the new module I wrote for the parallel N-body code P-Gadget3 to perform cosmological hydrodynamical simulations with light bosonic non-thermal dark matter components. The dynamics of the so called Fuzzy Dark Matter (FDM), being Ultra light Axions or other dark matter candidates, features a highly non-linear quantum potential (QP) that suppress the growth of structures at small scales. Previous attempts of simulating FDM either completely neglected the QP or resorted to numerically challenging full-wave solver codes. AX-Gadget code follows the full quantum evolution without impairing the overall performances, computing acceleration contribution of QP through Smoothed Particle Hydrodynamical routines. Analytical consistency, cosmological test and performances are presented and discussed.
21 Jun 2017 17:00 - : Viviana Niro (ITP Heidelberg) - "Prospects for Detecting Galactic Sources of Cosmic Neutrinos with IceCube"
Air-Cherenkov telescopes have mapped the Galactic plane at TeV energies. Here we evaluate the prospects for detecting the neutrino emission from sources in the Galactic plane assuming that the highest energy photons originate from the decay of pions. Four promising sources are identified based on having a large flux and a flat spectrum. We subsequently evaluate the probability of their identification above the atmospheric neutrino background in IceCube data as a function of time. We show that observing them over the twenty-year lifetime of the instrumentation is likely, and that some should be observable at the 3$\sigma$ level with six years of data. In the absence of positive results, we derive constraints on the spectral index and cut-off energy of the sources, assuming a hadronic acceleration mechanism.
14 Jun 2017 17:00 - : Isabel Oldengott () - "Interacting neutrinos in cosmology: Exact description and constraints"
The cosmic microwave background (CMB) anisotropies have been providing useful insights into neutrino physics in the past decade. This naturally arises the questions whether we can also test non-standard neutrino interactions using cosmological observations. After a short introduction to the theory of cosmological perturbation theory, I present the Boltzmann hierarchy for neutrinos including self-interactions mediated by a massive scalar. Such interactions appear, for example, in majoron-like models of neutrino mass generation. I furthermore show the induced signal in the CMB angular power spectrum and compare our results to the approaches by others. Finally, I present constraints on the neutrino coupling, derived from a MCMC analysis of the Planck 2015 data.
07 Jun 2017 17:00 - : No seminar on this day () - ""
31 May 2017 17:00 - : Joan Sola (University of Barcelona) - " First compelling hints of dynamical vacuum energy in the expanding Universe"
In this talk, I will assess the possibility that a rigid cosmological constant, $\Lambda$, and hence the traditional concordance $\Lambda$CDM model of cosmology, might not be the best phenomenological description of the current cosmological data. In a series of recent works we have shown that a large class of dynamical vacuum models (DVMs), whose vacuum energy density consists of a nonvanishing constant term and a series of powers of the Hubble rate, provides a substantially better phenomenological account of the overall SNIa+BAO+H(z)+LSS+CMB cosmological observations. The main models within the class of DVMs appear significantly much more favored than the $\Lambda$CDM, at an unprecedented confidence level of roughly $4\sigma$. If the results presented here would be reconfirmed in the light of future investigations, the up to now hundred-years-old rigid status of the ``cosmological constant'' , in its traditionally accepted role for the optimal description of the cosmological data, would be seriously disputable.I will also discuss the implications of this framework on the current tension between the latest Planck determination of $H_0$ obtained from the CMB anisotropies and the local measurement from the Hubble Space Telescope, based on Cepheid variables.
24 May 2017 17:00 - : Martin Feix () - "Large-deviation statistics of the cosmic density field: count-in-cellsperspectives on tracer bias and dark energy"
I will review recent applications of large deviation theory to count-in-cells statistics of the cosmic density field which allow for analytic predictions in the mildly nonlinear regime of gravitational clustering down to scales of about 10 Mpc/h. Considering dark matter halos, I will present a simple approach that connects these results to biased mass tracers, and discuss the possibility of constraining bias and properties of the dark matter field from such tracers simultaneously. In view of future surveys like Euclid, this type of analysis might offer an interesting addition to other well-established cosmological probes.
17 May 2017 17:00 - : Alexandre Barreira (Max Planck Institute for Astrophysics, Garching bei Munchen) - " The matter covariance with power spectrum responses"
The power spectrum responses describe how the nonlinear matter power spectrum 'responds' to the presence of long-wavelength perturbations; and they can be measured accurately to any order in perturbation theory using (small-volume) N-body simulations. I will start by describing how these response functions can be embedded into cosmological perturbation theory calculations by defining them as interaction vertices with two hard and N soft modes. Crucially, unlike standard perturbation theory, the response-type terms are valid also in the nonlinear regime of structure formation. I will then apply the response formalism to evaluations of the matter covariance matrix. Without any free parameters, when including response-type terms up to 1-loop order, the calculation captures the bulk of the covariance as estimated from the cross-correlation of spectra from (many) N-body simulations. This analytical approach to the covariance has the enormous advantage of requiring far less computational resources compared to simulation-based methods, thereby permitting much needed systematic studies on covariance matrices, namely their dependence on cosmology and baryon effects.
11 May 2017 11:00 - : Alexey Golovnev, , NOTE UNUSUAL DATE AND TIME (Saint Petersburg State University) - "Teleparallel gravity, its extensions, and the issue of local Lorentz invariance "
In teleparallel theory, the gravitational interaction is described by the torsion tensor while the curvature is set to zero. It goes in terms of tetrads, and the spin connection is taken to vanish. The latter condition clearly violates the local Lorentz symmetry in the space of tetrads. However, for teleparallel equivalent of GR, this violation amounts to merely a surface term in its action. This is the classical formulation of teleparallel gravity. Nowadays, many experts prefer to use an arbitrary flat (purely inertial) spin connection. A local Lorentz rotation would change this spin connection, otherwise keeping the model invariant. Actually, it can be done even with more elegance: one can allow for variation with respect to the flat spin connection since it anyway affects only the surface term. The things differ for modified versions of teleparallel gravity, such as f(T) which is often used for cosmological model building. The local Lorentz violation goes beyond the surface terms, and leads to increased number of degrees of freedom. In this case the covariantisation by arbitrary flat spin connection works differently. The variation is not empty, however the field equations are redundant with those coming from tetrads. It seems that these issues are not well understood and appreciated even among experts, and there is some confusion in the literature. I will offer a pedagogical introduction to these topics, review some known results and discuss new ideas. The talk is based in part on arXiv:1701.06271 by me, Tomi Koivisto, and Marit Sandstad.
10 May 2017 17:00 - : Eva K. Grebel (ARI Heidelberg) - "Dwarf Galaxies - Fossils of Galaxy Evolution"
Dwarf galaxies are the most common type of galaxy in the Universe and include the most dark-matter-dominated objects known. They offer intriguing insights into evolutionary processes at low halo masses and low metallicities. Cosmological models of hierarchical structure formation describe the build-up of massive galaxies via the continuing accretion of smaller subhalos. If we consider dwarf galaxies to be the survivors of a once much more numerous population of such building blocks, their old stellar populations are key to understanding very early star and galaxy formation processes. The Local Group and in particular the Milky Way's dwarf galaxy entourage offer us the unique possibility to compare in detail dwarf and Galactic populations. This is an important step towards quantifying the magnitude and time scales of dwarf contributions to the build-up of massive galaxies the Milky Way and allows us to test predictions of cosmological scenarios.
03 May 2017 17:00 - : No seminar on this day () - ""
26 Apr 2017 17:00 - : Santiago Casas (ITP Heidelberg) - "Non-linearities in Modified Gravity: Forecasts and parameterizations"
In this talk I will show the impact that different prescriptions for non-linearities and different choices of parameterizations for Modified Gravity have on forecasts for Galaxy Clustering and Weak Lensing. Modified Gravity (MG) theories generally affect the Poisson equation and the gravitational slip in an observable way, that can be parameterized by two generic functions ($\eta$ and $\mu$) of time and space. Here, I will focus on forecasts for future surveys, like Euclid, SKA1, SKA2 and DESI. I will show that non-linearities and the inclusion of priors can reduce the strong correlations between parameters and that applying a decorrelation method can help identify the best constrained combinations of MG parameters measured with future experiments. Towards the end of the talk I will show the effect that different prescriptions for non-linearities have on the parameter constraints and show how this can be improved in the future.
30 Mar 2017 14:00 - : Georgios Karananas, NOTE UNUSUAL DATE AND TIME (EPF Lausanne) - "Gauge coupling unification without leptoquarks"
We propose an interpretation of the gauge coupling unification scale which is not related to any new particle threshold. We revisit Grand Unified Theories and show that it is possible to completely eliminate the scalar as well as vector leptoquarks from the particle physics spectrum. As a consequence, in our approach the gauge hierarchy problem is put on different grounds, and the proton may be absolutely stable. In order to achieve that, we employ a number of nonlinear gauge-invariant constraints which only affect the superheavy degrees of freedom. We illustrate our considerations in a model based on the SU(5) group, with the generalization to other groups being straightforward. We discuss how scale or conformal invariance may be added to our proposal.
22 Mar 2017 17:00 - : No seminar on this day () - ""
15 Mar 2017 17:00 - : Ilia Musco (LUTH - Observatoire de Paris) - "Primordial Black hole formation and trapping horizon dynamics"
In this talk I will give a review of my research on black hole formation. I will start introducing the concept of trapping horizons in spherical symmetry, characterising both black hole formation and the cosmological horizon. During gravitation collapse these horizons make their first appearance either within the collapsing matter or where it joins on to a vacuum exterior and the one moving outwards with respect to the matter have been proposed for replacing the global concept of an ``event horizon'' in the case of dynamical black holes. I will then explain how black holes can form in the early Universe from the collapse of cosmological perturbations when renter the cosmological horizon. In calculations of spherically symmetric collapse, starting with a a non linear curvature perturbation on the super horizon regime, the non linear evolution evolution is simulated using an hydrodynamical Lagrangian relativistic code. If the perturbation is larger than a threshold depending on the equation of state and on the specific shape of the perturbation, a black hole within an evacuated region is formed as an effect of the pressure gradients giving rise to a very strong relativistic wind, The mass of these objects spectrum is following the typical scaling law of critical collapse. Primordial lack holes formed in the early Universe are interesting candidates for the seeds of supermassive black holes that we observe today in the center of galaxies.
09 Mar 2017 14:00 - : Ippocratis Saltas, NOTE UNUSUAL DATE AND TIME (University of Lisbon) - "What can unimodular gravity teach us about the cosmological constant?"
Unimodular gravity became very popular over the last years as a theory that could shed light on the cosmological—constant problem. In this talk, I will explain the idea behind unimodular gravity, and discuss its (in)ability to bring a new perspective to the problem of the cosmological vacuum.
08 Mar 2017 14:00 - : Lavinia Heisenberg, NOTE UNUSUAL TIME (ETH Zurich) - " The role of vector fields in Cosmology"
Learned from lessons for constructing healthy and self consistent interactions for massive gravity and scalar-tensor theories, I will construct the Lagrangian of a vector field with derivative self-interactions, that propagates only the 3 desired polarisations associated with a Proca field. I will also discuss some cosmological implications of these generalized Proca theories and its extensions.
01 Mar 2017 17:00 - : Dario Bettoni (ITP Heidelberg) - ""Testing Lorentz invariance of dark matter with satellite galaxies""
"Lorentz symmetry is a fundamental pillar of General Relativity and Particle Physics. Nonetheless, certain theories of quantum gravity involve some degree of Lorenz violation which may have significant consequences on all scale. It is hence worthwhile to explore the observable signatures of Lorentz violations on the dynamics of the low energy spectrum of the theory. I will consider the case of a Lorentz violating vector field acting on the dark matter component of a satellite galaxy orbiting in a host halo and discuss the key observational signatures such as modifications to the line of sight velocity dispersion, mass profiles and shapes of satellites."
22 Feb 2017 17:00 - : Tessa Baker (University of Oxford) - "Multi-Messenger Time Delays with Gravitational Waves"
I’ll present an investigation into a new phenomenon precipitated by the birth of experimental gravitational wave astronomy. Gravitational waves follow null geodesics (in GR, at least), and hence can experience strong gravitational lensing by massive objects in exactly the same manner as photons do. Massive particles can also be gravitationally lensed, but the deflection they experience is subtly different from that of a gravitational wave or photon. In this talk I will consider some interesting effects of this distinction, focussing in particular on an analogue to familiar optical time delay measurements. This `massive time delay’ phenomenon turns out to be sensitive to the sum of the neutrino masses and cosmological parameters. It serves as an example of the kinds of novel GW probes of fundamental physics we can expect to uncover over the coming decades.
15 Feb 2017 17:00 - : Frank Konnig (ITP Heidelberg) - " Escaping the Riemannian World - From Non-Metricity to Ghost-Free Weyl Gravity"
The beauty of physics on manifolds comes along with the freedom to choose a connection. While the Levi-Civita connection is often assumed just for simplicity, the consideration of torsion or non-metric compatible connections opens the door to a completely new world of geometry. In this talk, we will introduce non-metricity and discuss recent developments towards a ghost-free Weyl gravity.
08 Feb 2017 17:00 - : No seminar on this day () - ""
01 Feb 2017 17:00 - : No seminar on this day () - ""
25 Jan 2017 17:00 - : Mario Herrero-Valea (EPF Lausanne) - "Horava gravity in a nutshell"
A model of renormalizable quantum gravity can be constructed if one assumes that Poincare invariance is an accidental symmetry that is only preserved by IR dynamics but that it is explicitly broken in the UV. In this proposal, named Horava gravity, usual scaling properties are substituted by an anisotropic scaling between space and time, which acts as a guide to construct a power-counting renormalizable theory of gravity. We will review the main fashions of HL gravity and we will prove its perturbative renormalizability in the projectable version of the theory, being the first known example of a fully renormalizable quantum field theory of gravitation.
24 Jan 2017 17:15 - : () - "TBA"
18 Jan 2017 17:00 - : Benjamin Joachimi (University College London) - "On the link between the bright and dark large-scale structure"
Distant galaxies are the beacons that trace the large-scale dark matter structures of the cosmos. Conversely, the dark environment of galaxies and clusters has decisive influence on the properties of observable structures and their evolution. A good understanding of the connection between the objects that we study and their dark environment is therefore key to progress in galaxy evolution and cosmological studies with large galaxy surveys. I will report on recent observational results in this area, especially with the KiDS and GAMA surveys, and highlight challenges for the forthcoming next generation of large galaxy surveys.
17 Jan 2017 17:15 - : () - "TBA"
11 Jan 2017 17:00 - : No seminar of this day () - ""
21 Dec 2016 17:00 - : No seminar on this day () - ""
14 Dec 2016 17:00 - : No seminar on this day () - ""
07 Dec 2016 17:00 - : No seminar on this day () - ""
02 Dec 2016 12:00 - : Nelson Lima (NOTE UNUSUAL DATE, TIME AND VENUE, PW 12, Room 58) (ITP Heidelberg) - "Phenomenology of modified theories of gravity. "
In the era of precision cosmology, it is more important than ever to fully understand the impact that modified theories of gravity can have on the cosmological observables. In this talk, I will focus on two particular scalar-tensor theories: the well known Brans-Dicke theory, and the hybrid metric-Palatini theory. For the former, I will introduce a designer approach that fixes the background dynamics to that of a "quasi-wCDM" cosmology and show the analytical solutions that describe the evolution of the scalar field. These, in turn, allow for a fully analytical description of the phenomenological parameters governing linear perturbations on sub-horizon scales. For the second theory, I will show the evolution of linear perturbations for a model that mimics LambdaCDM at the background level, and introduce an analytical correction to the sub-horizon parameters that allows us to constrain early modifications of gravity using the cosmic microwave background.
30 Nov 2016 17:00 - : Giulia Cusin (Geneva University) - "Generation of vorticity in the Universe: an analytical approach "
We present a formalism to compute the generation of vorticity in the dark matter fluid. For dark matter at zero temperature, Helmholtz's theorem dictates that no vorticity is generated and we therefore allow the dark matter fluid to have a non-vanishing velocity dispersion. This implies a modification to the usual hydrodynamical system (continuity and Euler equations): the Boltzmann hierarchy has to be considered up to the second moment. As a result, the Euler equation is modified with a source term that describes the effect of non-zero velocity dispersion. We derive an equation for the evolution of Eulerian vorticity in Lagrangian coordinates and show that it has a growing mode already at second order in perturbation theory.
23 Nov 2016 17:00 - : Hector Gil-Marin (Institut Lagrange de Paris - LPNHE) - "Constraining cosmological parameters using BOSS data: Power Spectrum and Bispectrum"
In this talk I will review the last results based on the analysis of the k-space products of the the final Data Release of SDSS III-BOSS survey. This survey has mapped the spatial distribution of luminous red galaxies (LRGs) and quasars from z=0.15 up to z=0.70, and is the largest spectroscopic galaxy survey with an effective volume of ~7.4Gpc3. I focus on the the analysis of the LRGs power spectrum isotropic and anisotropic signal of the LOWZ and CMASS samples. We have measured the Baryon Acoustic Oscillation signal from the post-reconstructed isotropic and anisotropic of the data. Using the Alcock-Paczynski scale relations we have determined cosmological constrains in the Hubble parameter and the angular diameter distance. Furthermore, we have performed an analysis of the redshift space distortions effect on both the monopole and quadrupole of the power spectrum. Consequently, we have been able to put constrains on the product of the growth of structure parameter times the amplitude of dark matter density fluctuations, fsigma8, at z=0.32 (LOWZ sample) and z=0.57 (CMASS sample). By adding higher order statistics, such as the bispectrum, we can improve the constrains on cosmological parameters. With the bispectrum signal we could, potentially, set competitive constrains on potential primordial non-Gaussianities that would have a characteristic signature in the large scale squeezed bispectrum. The constrains on fs8 along with the Hubble parameter and the angular diameter distance will be useful for setting constrains on neutrino masses, dark energy equation of state, gravity curvature as well as number of neutrino species.
16 Nov 2016 17:00 - : Wessel Walkenburg () - "Accurate initial conditions in mixed Dark Matter-Baryon simulations"
Even though hydrodynamic and N-body simulations have become a standard tool in modern cosmology, the interface between linear perturbation theory and fully nonlinear simulations has so far been oversimplified. After addressing relativistic gauge issues and discretising multi-species perturbation theory, I will show that contemporary state-of-the-art simulation suites that incorporate Dark Matter and Baryons, miss-estimate results up to ten percent. The root of this error lies in the initial conditions.
15 Nov 2016 11:00 - : Lucas Lombriser , NOTE UNUSUAL DATE AND TIME (University of Edinburgh) - "Challenges to Cosmic Self-Acceleration in Horndeski Gravity from Gravitational Waves and Large-Scale Structure"
Scalar-tensor modifications of gravity have long been considered as an alternative explanation for the late-time accelerated expansion of our Universe. I will first show that a rigorous discrimination between acceleration from modified gravity and from a cosmological constant or dark energy is not possible with observations of the large-scale structure alone. I will then demonstrate how gravitational-wave observations break this dark degeneracy and how the combination of the two challenges the concept of cosmic acceleration from a genuine scalar-tensor modification of gravity.
15 Nov 2016 11:00 - : Lucas Lombriser (NOTE UNUSUAL DATE & TIME) (University of Edinburgh) - "TBA"
11 Nov 2016 13:00 - : Sam Young (Sussex) - "Constraining the early Universe with primordial black holes"
Primordial black holes may have formed in the radiation dominated epoch in the early universe from the collapse of large overdensities. They are a viable dark matter candidate and may have been responsible for the observed gravitational waves from LIGO. In this talk I will discuss how they can be used to constrain the early Universe, and my research investigating the effect of non-Gaussianity on the abundance of and constraints from PBHs - with a particular focus on how modal-coupling can lead to the formation of dark matter isocurvature modes in the scenario that dark matter is made of PBHs. Such isocurvature modes would be observable in the CMB and are strongly constrained by results from the Planck satellite - which leads to extremely strong constraints on the non-Gaussianity parameters.
09 Nov 2016 17:00 - : Christian Arnold (Heidelberg ITS) - "Zoomed simulations of Milky-Way sized halos in f(R) gravity"
f(R) theories of gravity can explain the accelerated expansion of the universe without a cosmological constant. In our work, we explore the Hu & Sawicki model for f(R) gravity which is designed to reproduce the expansion history of a ΛCDM universe and features the chameleon screening mechanism to mimic GR in high density regions. Using the cosmological simulation code MG-Gadget, we for the first time perform zoomed cosmological simulations of Milky-Way sized dark matter halos in f(R) gravity. We find the velocity dispersions in f_R0 = -10^-6 to be increased by about 40% in unsceened regions with respect to LCDM. This difference is larger than reported in previous studies. Our simulations show that $f(R)$ gravity significantly affects the dark matter density profile of Milky-Way sized objects. Also, we calculate the fifth forces in ideal NFW-halos as well as in our cosmological simulations and compare them against analytic model predictions for the fifth force inside spherical objects. We find that these theoretical predictions match the forces in the ideal halos very well, whereas their applicability is somewhat limited for realistic cosmological halos. The Solar circle is fully screened in f_R0 = -10^-6 models for Milky Way sized halos, while this location is unscreened for slightly less massive objects.
02 Nov 2016 17:00 - : Dietrich von Harrach () - "Spacetime from growing singlets"
I will discuss a causally growing spin system as a model for an evolving universe. Requiring unitarity and zero total spin at each growth step leads to a finite, correlated quantum system exhibiting a fractal structure defined by the density and degeneracy of eigenvalues of different index exchange operators. The energy momentum operator leads to an energy density of 1/4. The eigenvalue structure of an velocity operator suggests the existence of exactly three spatial dimensions. In addition a three modal family stucture of states emerges.
26 Oct 2016 17:00 - : Mikhail Ivanov (CERN) - "Time-sliced perturbation theory for Large Scale Structures "
I will introduce a new framework to account for the non-linear clustering of large scale structures. This framework is based on the time - dependent probability distribution function considered at fixed time slices. I will first discuss the perturbative regime valid for mildly non-linear scales ranging from 10 to 100 Mpc. Then I will introduce techniques to account for the IR and UV modes and compare the results with N-body data. The second part of my talk will be devoted to some aspects of the high mass (non-perturbative) regime. Finally, I will discuss implications for new physics, e.g. modified gravity, neutrino masses and primordial non-gaussianity.
19 Oct 2016 17:00 - : Anna Tokareva (EPF Lausanne) - "Cosmology with a light Weyl ghost"
We study the creation and evolution of cosmological perturbations in renormalizable but non-unitary quadratic gravity with a Weyl term. The theory may still be predictive regardless the interpretation of non-unitary processes provided that their rate is negligible compared to the Universe expansion rate.This implies that the ghost is effectively stable. In such a setup, there are two scalar degrees of freedom excited during inflation. The first one is the usual curvature perturbation whose power spectrum appears to coincide with that of single-filed inflation. The second one is a scalar component of the ghost encoded in the shift vector of the metric in the uniform inflaton gauge. After inflation the ghost field starts to oscillate and its energy density shortly becomes dominant in the Universe. For all ghost masses allowed by laboratory constraints, ghosts should have ``overclosed" the Universe at temperatures higher than that of primordial nucleosynthesis.Thus, the model with a light Weyl ghost is ruled out.
12 Oct 2016 17:00 - : No seminar on this day () - ""
05 Oct 2016 17:00 - : Viraj A A Sanghai (Queen Mary) - "Using the post-Newtonian formalism to understand general relativity and its alternatives on cosmological scales "
I will split up my talk up into two parts. In the first part I will talk about how we can use the post-Newtonian formalism (within general relativity) to construct a bottom-up approach to cosmology to study the effect of small-scale inhomogeneities on the large-scale expansion. In this approach, we also extend the post-Newtonian formalism to include other forms of matter that are cosmologically relevant such as radiation and a cosmological constant. In second part of my talk I will generalise this bottom up approach to create a parametrization that can be used to test a large class of theories of gravity on cosmological scales in the weak field limit. We do not assume any set of field equations from the outset. The advantage of this parametrisation is that it requires only four functions of time. These four functions can be linked directly to the background cosmological expansion as well as the growth of structure. I will give a couple of specific examples where we have applied this formalism. We have applied it to scalar-tensor and vector-tensor theories of gravity.
28 Sep 2016 17:00 - : No seminar on this day () - ""
21 Sep 2016 17:00 - : Santiago Casas (ITP Heidelberg) - "Dynamics of neutrino lumps in growing neutrino quintessence"
We investigate the formation and dissipation of large scale neutrino structures in cosmologies where the time evolution of dynamical dark energy is stopped by a growing neutrino mass. In models where the coupling between neutrinos and dark energy grows with the value of the scalar cosmon field, the evolution of neutrino lumps depends on the neutrino mass. For small masses the lumps form and dissolve periodically, leaving only a small backreaction of the neutrino structures on the cosmic evolution. This process heats the neutrinos to temperatures much above the photon temperature such that neutrinos acquire again an almost relativistic equation of state. The present equation of state of the combined cosmon-neutrino fluid is very close to -1. In contrast, for larger neutrino masses the lumps become stable. The highly concentrated neutrino structures entail a large backreaction similar to the case of a constant neutrino-cosmon coupling. A present average neutrino mass of around 0.5 eV seems so far compatible with observation. For masses lower than this value, neutrino induced gravitational potentials remain small, making the lumps difficult to detect.
14 Sep 2016 17:00 - : Group meeting () - ""
03 Aug 2016 17:00 - : Group meeting () - ""
27 Jul 2016 17:00 - : Daniele Bertacca (University of Bonn) - "Light cone effects in the galaxy two-point correlation function"
Recently I worked on light cone effects come in to the measured 2-point correlation function in redshift space. This general relativistic wide-angle formalism recovers and generalizes previous results in the plane-parallel (flat-sky) and Newtonian wide-angle cases. These light cone effects become important on large scales and they lead to new terms in the wide-angle correlations. I will show that neglecting relativistic terms, especially lensing convergence, introduces an error in the forecasted precision in measuring cosmological parameters of the order of a few tens of percent, in particular when measuring the matter content of the Universe and primordial non-Gaussianity parameters. Finally I studied the parity-odd part (the doppler term) of the linear galaxy two-point correlation function that arises from wide-angle, velocity and doppler lensing. As it is important at low redshift and at large angular separations, the doppler term is usually neglected in the current generation of galaxy surveys. For future wide-angle galaxy surveys such as Euclid, SPHEREx and SKA, however, I will show that the Doppler term must be included.
21 Jul 2016 12:00 - : Nima Khosravi (SBU, Tehran) - "Ensemble Average Theory of Gravity"
(NOTE UNUSUAL DATE, TIME, AND VENUE: SR 061, Philosophenweg 12) We put forward the idea that all the theoretically consistent models of gravity have contributions to the observed gravity interaction. In this formulation, each model comes with its own Euclidean path-integral weight where general relativity (GR) has automatically the maximum weight in high-curvature regions. We employ this idea in the framework of Lovelock models and show that in four dimensions the result is a specific form of the f(R,G) model. This specific f(R,G) satisfies the stability conditions and possesses self-accelerating solutions. Our model is consistent with the local tests of gravity since its behavior is the same as in GR for the high-curvature regime. In the low-curvature regime the gravitational force is weaker than in GR, which can be interpreted as the existence of a repulsive fifth force for very large scales. Interestingly, there is an intermediate-curvature regime where the gravitational force is stronger in our model compared to GR. The different behavior of our model in comparison with GR in both low- and intermediate-curvature regimes makes it observationally distinguishable from LCDM.
19 Jul 2016 12:00 - : Chiara Caprini (IPhT, CEA Saclay) - "Cosmology with gravitational waves"
(NOTE UNUSUAL DATE AND TIME) Gravitational waves can constitute a unique probe of the universe. After a general introduction to the subject, this seminar presents two aspects of using GW to probe cosmology. First, for sources operating in the early universe, the characteristic frequency of the emitted GW is directly related to the energy scale at which the GW source acts. Consequently, different GW detectors can probe different energy scales in the evolution of the universe, going much beyond the recombination scale accessible through the CMB. I will discuss the special case of the space-based interferometer eLISA, which operates in the right frequency range to probe the electroweak scale. Second, compact binaries emitting GW can be used as standard sirens, to probe the energy content of the late universe and test dark energy. I will discuss the potential of eLISA in constraining the cosmological parameters and the models of early and interacting dark energy.
13 Jul 2016 17:00 - : No seminar on this day () - ""
06 Jul 2016 17:00 - : Alexey Koshelev (Vrije, Brussels) - "Non-local infinite derivative gravity"
In my talk I will introduce and review non-local infinite derivative gravities. In general such non-local theories originate from string field theory (SFT), the second quantized string theory, which is unitary and UV complete. This encourages the search for a unitary and UV complete gravity generalization in the framework of non-local theories. I will present exact cosmologically important solutions for bounce and inflation and explain what would be the observational signatures of such theories. Also, general arguments in favor of singularity-free space-times in such theories will be discussed.
29 Jun 2016 17:00 - : Florian Kuhnel (OKC, Stockholm) - "Primordial Black Holes"
Primordial black holes are black holes that may have formed in the early Universe. Their masses span potentially a range from as low as the Planck mass up to many orders of magnitude above the solar mass. Besides their conceptual importance regarding our understanding of quantum effects and gravity, they may provide the dark matter. In order to constrain this possibility, a proper understanding of their formation mechanism is crucial. In my talk, after a general introduction on primordial black holes, I will discuss recent investigations on this issue, including so-called critical collapse, non-sphericity and non-Gaussianity. Furthermore, I will discuss how to properly compare extended primordial black-hole mass spectra to observational constraints, such as those deriving from recent microlensing surveys.
23 Jun 2016 14:00 - : Yves Dirian (University of Geneva) - "Observational constraints and Bayesian model comparison in non-local gravity"
(NOTE UNUSUAL DATE, TIME, AND VENUE: ITP, Philosophenweg 12, SR 106) I present the cosmological predictions of two non-local modifications of General Relativity recently proposed by our group, the so-called RT and RR models. Both models have the same number of parameters as LCDM, with a mass parameter m replacing the cosmological constant. In implementing their cosmological background and perturbations equations into the CLASS Boltzmann code, we constrain the non-local models using the Planck 2015, isotropic and anisotropic BAO, JLA supernovae, H0 measurements and growth rate data. For both non-local models, Bayesian parameter estimations that include Planck data generically give a value of H0 higher than in LCDM, and in better agreement with the values obtained from local measurements. We also perform a Bayesian model comparison between the RT, RR and LCDM models, using the Savage-Dickey density ratio method. We find that, in the framework of the so-called Planck baseline, the RT model performs as well as LCDM whereas the RR model is disfavored.
22 Jun 2016 17:00 - : Bjoern Malte Schaefer (ITA Heidelberg) - "CMB-lensing by nonlinear (and non-Gaussian) structures"
Weak lensing of the cosmic microwave background is a source of cosmological information from the redshift range z=3-5 which is difficult to probe by other means. The statistical detection of the statistical homogeneity-breaking by Planck in excess of 40 sigma has allowed the determination of cosmological parameters on the percent-level. In the interpretation of the signal one usually assumes Gaussian statistics of the deflecting large-scale structure, which is only approximately true due to non-Gaussianity generation in nonlinear structure formation. I show a few results that we obtained in relaxing the assumption on Gaussianity, both using analytical and numerical methods, and conceptual results concerning the CMB-lensing formalism.
15 Jun 2016 17:00 - : Frank Koennig (ITP Heidelberg) - "A spectre is haunting the cosmos: Quantum stability of massive gravity with ghosts"
Many theories of modified gravity with higher order derivatives are usually ignored because of serious problems that appear due to an additional ghost degree of freedom. Most dangerously, it causes an immediate decay of the vacuum. However, breaking Lorentz invariance can cure such abominable behavior. By analyzing a model that describes a massive graviton together with a remaining Boulware-Deser ghost mode we show that even ghostly theories of modified gravity can yield models that are viable at both classical and quantum levels and, therefore, they should not generally be ruled out. Furthermore, we identify the most dangerous quantum scattering process that has the main impact on the decay time and find differences to simple theories that only describe an ordinary scalar field and a ghost. Additionally, constraints on the parameters of the theory including some upper bounds on the Lorentz-breaking cutoff scale are presented. In particular, for a simple theory of massive gravity we find that a Lorentz violation needs to occur below ~200 eV, which still agrees with observations. Finally, we discuss the relevance to other theories of modified gravity.
08 Jun 2016 17:00 - : No seminar on this day () - ""
07 Jun 2016 13:00 - 15:00 Frank Koennig (ITP, Heidelberg University) - ""
01 Jun 2016 17:00 - : No seminar on this day () - ""
25 May 2016 17:00 - : Adam R. Solomon (University of Pennsylvania) - "Quasilinear perturbation theory for massive (bi)gravity"
Massive gravity and bimetric gravity are promising frameworks for explaining the accelerating expansion of the Universe, but the models which lead to viable background evolution seem to be generically plagued by instabilities. There is reason, however, to believe that nonlinearities play an important role in the evolution of the perturbations due to a Vainshtein-like mechanism. I will introduce and discuss a new approach to cosmological perturbation theory in massive and bimetric gravity in which the nonlinearities in the helicity-0 mode of the massive graviton are fully retained. This setup is well-suited to perturbations in the subhorizon regime. The equations of motion can be applied to numerical studies of bimetric perturbations, including N-body codes.
18 May 2016 17:00 - : Michal Artymowski (Institute of physics, Jagiellonian University) - "Inflation from the almost most general f(R)/scalar theory"
The Starobinsky inflation, which is the simplest form of the f(R) theory has been proven to be perfectly consistent with the data. Nevertheless its ability to generate inflation can be spoiled due to the existence of the higher order terms. The same goes with scalar or scalar/tensor models of inflation. In my talk I will present theories, which consist of all possible higher powers of the Ricci scalar or scalar field, assumes inflation, and its power spectrum fits the Planck data. I will show that in both, f(R) and scalar approach the Starobinsky model is the very flattest potential to obtain. I will also investigate the vacuum stability of the theory.
11 May 2016 17:00 - : No seminar on this day () - ""
04 May 2016 17:00 - : No seminar on this day () - ""
27 Apr 2016 17:00 - 18:00 Janina Renk (ITP Heidelberg) - "Signatures of Horndeski's Gravity on ultra-large cosmic scales"
Upcoming galaxy surveys aim to map out almost the entire observable universe in the next decades. With these data we can test gravity on scales near the horizon by studying the signatures of modified gravity models on large scale structure observables. I will present the imprints on the galaxy number counts (GNC's) and on the cross-correlation of the GNC's with CMB temperature anisotropies from certain classes of scalar-tensor theories of gravity embedded in the Horndeski Lagrangian. Due to the remarkable modifications of relativistic effects which contribute to the signal, like the integrated Sachs-Wolfe effect, these observables allow us to constrain or even rule out some alternative models.
20 Apr 2016 17:00 - 18:00 Cornelius Rampf (ITP Heidelberg) - "Cosmological large-scale flows in the light of Newton theory and General Relativity"
In the first part of the talk, I show how to obtain time-analytic (i.e., convergent) perturbative solutions for the non-linear Newtonian fluid equations in Lambda CDM. These findings are fully constructive, allowing to develop novel algorithms which solve the fluid equations to unprecedented accuracy (even higher than conventional N-body simulations). Applications are for example the study of shell-crossing (blowup) or the cosmological reconstruction problem. In the second part of the talk, I show how general relativistic effects can be effectively incorporated within Newtonian (N-body) simulations, especially taking the full multi-fluid dynamics of CDM, baryons, neutrinos and photons into account. I show that at sufficiently late times (z<50), conventional Newtonian simulations for CDM include all first-order relativistic corrections, whereas at earlier times CDM particles receive 1 - 20% corrections on the momentum conservation (z= 50 - 1000), even well inside the horizon. These GR corrections arise from the gravitational coupling of matter to neutrinos and photons, and I show how these effects can be incorporated into conventional Newtonian simulations.
11 Mar 2016 15:00 - 16:00 Abhishek Basak () - "Cosmological perturbations of Unimodular Gravity and General Relativity are identical"
Unimodular Gravity (UG) is a restricted version of General Relativity (GR) in which the determinant of the metric is a fixed function and the field equations are given by the trace-free part of the full Einstein equations. The background equations in UG and GR are identical. It was recently claimed that, the first order contribution in the temperature fluctuation of the Cosmic Microwave Background (CMB) in UG is different from GR. In this work, we calculate the first order perturbation equations in UG and show that the Sachs-Wolfe effect in UG, in terms of gauge invariant variables, is identical to GR. We also show that the second order perturbation equation of Mukhnanov-Sasaki variable in UG, is identical to GR. The only difference comes from the gauge choices due the constraint on the metric determinant. Hence, UG and GR are identical and indistinguishable in CMB data on large scales.
09 Mar 2016 17:00 - 18:00 Jose Navarro (Department of Mathematics, Universidad de Extremadura, Badajoz, Spain) - "Foundations of Einstein's field equation"
In this talk, I will discuss the foundations of Einstein's field equation of General Relativity. On the one hand, I will focus on dimensional analysis, and, on the other, I will present the ideas behind Lovelock's theorem. The aim will be to clarify the different sets of hypothesis from which this equation can be derived.
24 Feb 2016 17:00 - 18:00 Miguel Quartin (UFRJ) - "The Cosmic Supernova Recycling Program"
Type Ia supernovae distance measurements revolutionized cosmology and are still one of the main dark energy probes. But I will NOT focus on that. Instead, I will show that there is much more information on what is often considered the noise in the supernova data. In the Hubble diagram, weak-lensing introduces non-Gaussianity to the dispersion, while peculiar velocities correlate the supernovae. Both effects can be well modeled, allowing us to recycle this noise into signal. I will then show that together they allow one to infer the amplitude of the matter power spectrum and its growth in the late universe and I'll quantify this with both present and future data.
10 Feb 2016 17:00 - 18:00 Phil Bull (JPL/Caltech) - "Reconstructing the KSZ effect"
Forthcoming high-resolution CMB experiments like Advanced ACTPol will turn secondary CMB anisotropies into precision probes of large-scale structure. One such secondary -- the kinetic Sunyaev-Zel'dovich (KSZ) effect -- can be used to measure the large-scale peculiar velocity field, but has the same frequency spectrum as the primary CMB, making them hard to disentangle. I describe a Bayesian method to extract the KSZ imprint of galaxy clusters, using samples from the joint posterior distribution for a combined primary CMB + cluster pressure/temperature profile + velocity field model. Marginal maps of both the reconstructed linear velocity field and primary CMB are a natural biproduct of the algorithm. This "double reconstruction" is valuable for reducing the variance of KSZ velocity estimators, and the marginal velocity map can be used as part of a multi-tracer analysis (e.g. to improve linear growth rate constraints).
27 Jan 2016 11:00 - 12:00 Urs Achim Wiedemann (Theoretical Physics Department, CERN) - "Learning from the Big Bang about little bangs – and vice versa"
While the physics underlying cosmology and heavy ion collisions is very different, of course, the theoretical challenges for understanding the dynamics of both systems show remarkable commonalities. In this colloquium, I explain on the one hand how the modern description of the expansion dynamics of heavy ion collisions parallels developments in cosmology. On the other hand, I discuss examples for how concepts widely used in heavy ion phenomenology may be employed to shed light on two central topics in cosmology, namely large-scale structure formation and the nature of dark matter.
20 Jan 2016 17:00 - 18:00 David Daverio (University of Geneva) - "gevolution, a relativistic n-body code ***Note unusual place: kHS Phil 12 ***"
The gevolution code is a particle-mesh code which solves General Relativity in the weak field limit. During this talk, I will discuss the approximation scheme used and I will overview the structure of the code gevolution. Finally, first results for LCDM will be shortly discussed.
13 Jan 2016 17:00 - 18:00 No seminar on this day () - ""
16 Dec 2015 17:00 - 18:00 José Pedro Pinto Vieira (University of Sussex) - "Cosmology with negative absolute temperatures"
Negative absolute temperatures (NAT) are an exotic thermodynamical consequence of quantum physics which has been known since the 1950's (having been achieved in the lab on a number of occasions). Recently, the work of Braun et al (Science, 2013) has not only rekindled interest in these counter-intuitive regimes but also sparked a debate which has forced a revision of the very foundations of statistical physics and hinted at a possibility of using NAT systems in the lab as dark energy analogues. In this talk we shall go one step further, looking into the cosmological consequences of the existence of an actual NAT component in the Universe. Interestingly, it is found that, thanks to an unavoidable connection between thermodynamical functions at positive and negative temperatures, all NAT expanding Universes and all NAT contracting Universes look qualitatively the same - NAT expanding Universes experience a borderline phantom expansion (w<-1) epoch with no Big Rip and their contracting counterparts are forced to bounce after the energy density becomes sufficiently large. Additionally, we shall discuss how thermally generated perturbations come about in this context, enabling a stringent test to NAT scenarios.
09 Dec 2015 17:00 - 18:00 No seminar on this day () - "TBA"
02 Dec 2015 17:00 - 18:00 Nima Khosravi (IPM, Tehran) - "Galileon or Weyleon? Italian or German?"
Weyl bi-connection model manifests a natural framework to automatically produce the Galileon structure. It is shown that this framework can explain scalar Galileon, vector Galileon as well as their interactions by generalizing the Weyl non-metricity. So it can be interpreted as a geometrical realization for Galileons. The non-metricity part enjoys a U(1) gauge invariance. The result is interestingly non-trivial since the Galileon structure appears spontaneously and not by demanding the absence of the Ostrogradsky ghost. This fact suggests a possible deeper conceptual relation between Weyl bi-connection model and the absence of Ostrogradsky ghost.
25 Nov 2015 17:00 - 18:00 Giuseppe Fanizza (University of Bari) - "High-precision cosmology and inhomogeneities: exact results in the geodesic light-cone gauge"
The remarkable properties of the recently proposed geodesic light-cone (GLC) gauge allow to get some new interesting results to face the problem of inhomogeneities and their backreaction. Indeed, GLC simply consists of gauge fixing the metric tensor on the past light-cone of the observer. Thanks to this choice, several interesting physical observables, related to photons, can be evaluated within this framework. In this talk, we will present an overview on these recent results: in particular, we will show how the geodesic deviation equation can be exactly solved, giving an exact expression for the so called Jacobi map. Its link with cosmological distances and weak gravitational lensing will be discussed. Finally, a recent extension of this formalism to ultra relativistic particles will be presented.
18 Nov 2015 17:00 - 18:00 Mikael von Strauss (IAP, Paris) - "General perturbations in bimetric theory"
In recent years there has been a vital resurgence in the interest of theories of interacting spin-2 fields, such as nonlinear massive gravity and its bimetric extension. Part of this interest comes from cosmology, where it is hoped that a broader understanding of the gravitational sector may shed light on the cosmological constant problem and the nature of dark energy. Another part comes from purely theoretical considerations, exploring the spectrum of classically consistent spin-2 theories with the hope of a deeper understanding of gravity. If these theories are realised in nature they hold potential to answer some of the mysteries concerning gravitational phenomena, while if they are not realised in nature it is important to understand why that is. A generic feature of these theories is the presence of a square-root matrix which allows for a rich analytical structure but also complicates many problems at the technical level. In this talk I will highlight some of the issues that arise in linear perturbation theory due to the presence of this square-root and in computing its variation. I will also discuss how the problem of the square-root can be circumvented altogether. As a simple application I will mention how this simplifies the problem of counting degree’s of freedom at the level of equations of motion.
11 Nov 2015 17:00 - 18:00 Benjamin Racine (University of Oslo) - "Bispectral non-Gaussianity in the Planck data"
In the current standard paradigm, quantum fluctuations during inflation are the seeds of all the perturbations in the Universe, including the Cosmic Microwave Background (CMB) anisotropies. In the simplest models, their statistics is Gaussian, and can be studied by statistics of order 2. But many other models predict deviations from Gaussianity, that we can study with higher order statistics. The ESA Planck Satellite, launched in 2009, observed the CMB temperature and polarization over the full sky with an unprecedented precision. In February 2015, the Planck collaboration published the best constraints to date on non-Gaussian models. In this talk, we will introduce the physics of non-Gaussianity, its estimation in the context of real data analysis, and present the latest results. We will focus on the Bispectrum, statistics of order 3, and the fNL parameter.
04 Nov 2015 17:00 - 18:00 Eleonora Villa (ICG, University of Portsmouth) - "Cosmology beyond the linear and Newtonian approximations"
Roughly speaking, cosmologists study large, linear scales and small, non-linear scales in two different ways: relativistic perturbation theory around a homogeneous and isotropic background describes scales where the growth of structures is at an early stage. At smaller scales, well inside the Hubble horizon, the relativistic effects are supposed to be completely negligible and General Relativity is replaced by Newtonian gravity. A post-Newtonian type approximation is a crucial improvement of this simple paradigm as it bridges the gap between relativistic perturbation theory and Newtonian structure formation. I focus on the relativistic corrections for both the Eulerian and the Lagrangian approaches to gravitational dynamics in standard perturbation theory and in the post-Newtonian approximation. I finally present the post-Newtonian extension of the Zel'dovich solution for the plane parallel dynamics and its application in the context of the cosmological back-reaction proposal.
22 Oct 2015 14:00 - 15:00 Tommi Tenkanen (NOTE UNUSUAL DATE & TIME) (University of Helsinki) - "Cosmological constraints on Dark Matter properties"
We show that dark matter abundance and inflationary energy scale could be intimately related and that cosmic inflation can be used to constrain the high-energy regime of extensions of the Standard Model. Standard Model extensions with Higgs mediated couplings to new physics typically contain extra scalars whose excursions at the end of inflation can be very large. If their coupling to the SM is weak, they will not thermalize and may easily constitute too much dark matter. As an example we consider SM extended by a real singlet scalar and a singlet fermion. We discuss the field dynamics during inflation together with the post-inflationary dark matter production mechanisms and derive stringent constraints on viable mass scales and coupling values.
21 Oct 2015 17:00 - 18:00 Johannes Noller (University of Oxford) - "Consistent graviton theories"
There has been a lot of progress recently in understanding what consistent graviton theories can be built in an effective field theory sense. This has led to the discovery of new graviton kinetic terms, potential interactions and ways of coupling matter to gravity. I will review and present this graviton theory space for local and Lorentz invariant theories. Along the way I will point out applications to cosmology, related discoveries of new dualities and symmetries, highlight which bits of theory space we already know to be unique and where there is still uncharted territory.
14 Oct 2015 17:00 - 18:00 Javier Rubio (ITP Heidelberg) - "Higgs inflation and vacuum stability"
Inflation is nowadays a well-established paradigm consistent with observations. The precise nature of the inflaton is however unknown and its role could be played by almost any candidate able to imitate a scalar condensate in the slow-roll regime. The discovery of a fundamental scalar in the LHC provides the less speculative candidate. 1) Could the Higgs field itself be responsible for inflation? 2) Which are the requirements for this scenario to be self-consistent? 3) Which is the relation between the Standard Model parameters and the inflationary observables (if any)? 4) What happens if our vacuum becomes unstable below the scale of inflation? Should we abandon the Higgs inflation idea? I will present an overview of Higgs inflation trying to provide some partial answers to the previous questions with special emphasis on the vacuum stability issue.
07 Oct 2015 17:00 - 18:00 No seminar on this day () - "TBA"
30 Sep 2015 17:00 - 18:00 Max Groenke (University of Oslo) - "Screened modified gravity theories on cluster scales"
Modified gravity theories are numerous. A common feature to all of them is the extra additional gravity-like force matter feels. This extra force violates solar system measurements which confirm predictions of general relativity (GR) with extremely high precision. Therefore, in order not to violate these constraints, a common technique is to employ the so-called `screening mechanisms' which weaken the deviations from GR in high-density environments. In my talk, I will first explain the concept of screening by means of the `Symmetron' and the `Chameleon’ mechanisms. As two examples of potential observables on cluster-scale, I discuss results from our N-body simulations showing deviations from LCDM in the velocity field & the depth of the potential well. In spite of the differences in the nature of the screening mechanisms, similar features are apparent. I will discuss these self-similarities in modified gravity theories further and propose a simple, three variable parametrization which might help us to constrain modified gravity models.
04 Aug 2015 13:00 - 14:00 Frank Koennig, Santiago Casas (ITP) - "Is $\Lambda$ a property of geometry?"
Believing in the Lovelock assumptions allows us to add a bare cosmological constant term to the LHS of the field equations. We explain by using the less-known theorem of Navarro-Sancho, that if we demand a global Weyl invariant theory, then we loose the possibility to add a CC. The question whether a bare CC is geometrically allowed is only a (philosophical) question of which properties of the space-time we are demanding.
22 Jul 2015 17:00 - 18:00 Phil Bull (ITA Oslo) - "Fundamental cosmology with the SKA"
Large-scale structure surveys with the Square Kilometre Array (SKA) will cover an unprecedented redshift range and area of the sky. This opens up the possibility of probing cosmology in important new regimes -- on ultra-large scales, and around the onset of cosmic acceleration. I will give an overview of the cosmological applications of "full-sky" radio surveys with the SKA, with a particular focus on the potential for testing fundamental physics -- General Relativity, dark energy, and inflation -- using galaxy and intensity mapping surveys of neutral hydrogen.
20 Jul 2015 17:00 - 18:00 Yashar Akrami (ITP Heidelberg) - "Anomalous universe in the eyes of Planck"
(UNUSUAL DAY AND LOCATION: SR 105, Phil. 12) The principles of isotropy and homogeneity of the Universe on large scales are two cornerstones of the cosmological concordance model. In addition, in the standard framework, the primordial distribution of matter fluctuations is assumed to be Gaussian. Despite the fact that these assumptions have so far been in good agreement with most cosmological observations of both the cosmic microwave background (CMB) anisotropies and the large scale structure (LSS) of the Universe, subtle hints of the contrary have been claimed by some studies of the CMB and LSS data, features that are dubbed "anomalies" in the cosmological literature. Any violation of these cosmological principles may have strong impacts on our current understanding of the Universe, and therefore, it is of crucial importance to verify whether such claims hold against the tide of various high-quality data or they are only the results of systematic errors or statistical flukes. In this talk, I will present the latest views on the nature and significance of the anomalous features based on the most recent CMB data provided by the Planck satellite. The emphasis will be on deviations from the statistical isotropy, and I will review various statistical analyses performed by the Planck collaboration as presented in the Planck 2015 paper on the isotropy and statistics of the CMB (arXiv:1506.07135).
15 Jul 2015 17:00 - 18:00 No seminar this day () - "TBA"
08 Jul 2015 17:00 - 18:00 Claire Zukowski (UC Berkeley and Perimeter Institute) - "Anthropic Origin of the Neutrino Mass from Cooling Failure"
Given a large landscape of vacua that statistically favors large values of the neutrino mass sum, m_nu, I will present the probability distribution over m_nu obtained by weighting this prior by the amount of galaxies that are produced. Using Boltzmann codes to compute the smoothed density contrast on Mpc scales, we find that large dark matter halos form abundantly for m_nu >~ 10 eV. However, in this regime structure forms late and is dominated by cluster scales, as in a top-down scenario. I will argue that this change of regime is catastrophic: baryonic gas will cool too slowly to form stars in an abundance comparable to our universe. Upon implementing this cooling boundary, the anthropic prediction for m_nu is consistent at better than 2 sigma with the entire range of values allowed by current experimental bounds, 58 meV <= m_nu <~ 0.23 eV. A degenerate hierarchy is mildly preferred. Without a catastrophic boundary at or below 10 eV, the theoretical expectation would conflict strongly with the observed mass range. Thus the asserted cooling failure can be regarded as a prediction of the anthropic solution to the neutrino mass problem.
01 Jul 2015 17:00 - 18:00 Tomi Koivisto (Nordita) - "On nonlocal gravity theories and their cosmology"
Ghosts can be avoided in renormalisable higher derivative gravity by nonlocal, i.e. infinite-order derivative operators. This will be shown and some cosmological implications of such ultraviolet-finite theories to the early universe will be considered. Theories with nonlocal infrared modifications of gravity will also be discussed, in particular considering their possible relevance to the cosmological constant and the dark energy problems.
24 Jun 2015 17:00 - 18:00 Thomas Konstandin (DESY) - "Analytic methods for the large scale structure of the Universe"
The large scale structure of the Universe can provide tremendous information about the expansion history of the Universe. In this talk, I start with an extensive introduction to the topic and the standard perturbative treatment. I discuss why the standard methods fail and some approaches to improve the situation.
17 Jun 2015 17:00 - 18:00 Alvaro de la Cruz Dombriz (Complutense U. Madrid and ACGC U. of Cape Town) - "Advantages and unexpected shortcomings of extended theories of gravity"
Modified gravity theories have attracted a lot of attention in order to find more natural explanations for open issues in Cosmology. Nevertheless, some key aspects have not received proper attention yet. For instance, how the formation of non-linear structures may affect the averaged cosmological expansion rate in scenarios beyond General Relativity has been widely overlooked. I shall present the backreaction formalism in cosmological frameworks including imperfect fluids as well as scalar-tensor theories of gravity. The application of averaging procedure to them shall determine the importance of the backreaction effects. On the other hand, in order to deal with the plethora of competing theories and to test the Cosmological Concordance model under different assumptions, model-independent methods in cosmology, such as the cosmographic approach, have become essential tools. I will briefly demonstrate how this method suffers from many shortcomings since it seems to be unable to rule out models or adequately reconstruct theories with higher-order derivatives in either the gravitational or matter sector. Consequently, in its present form, this method seems unable to provide reliable or useful results for cosmological applications.
10 Jun 2015 17:00 - 18:00 No seminar this day () - "TBA"
03 Jun 2015 17:00 - 18:00 No seminar this day () - "TBA"
02 Jun 2015 17:00 - 18:00 Bjoern Malte Schaefer (ITA Heidelberg) - "Weak lensing and intrinsic alignments of galaxies"
NOTE UNUSUAL DATE AND LOCATION (106 im Philosophenweg 12) Weak lensing is one of the primary probes to investigate cosmic structure formation and to measure cosmological parameters. It operates under the assumption of intrinsically uncorrelated galaxy shapes, which is flawed due to interactions of galaxies with the surrounding large-scale tidal fields in a number of different processes. After a review of the most important alignment processes I discuss the interference between lensing and intrinsic shape correlations, and forecasts on the estimation of cosmological parameters.
27 May 2015 17:00 - 18:00 No seminar this day () - "TBA"
20 May 2015 17:00 - 18:00 Jeremy Sakstein (Portsmouth) - "Recent progress in testing alternate theories of gravity"
Alternate theories are an active area of modern cosmological research. In this talk I will present some recent progress in looking for new and novel astrophysical probes of some popular theories. In particular, I will discuss the new phenomena exhibited by stars and galaxies when the theory includes the Vainshtein screening mechanism and will present new constraints on disformal theories using solar system probes.
19 May 2015 17:00 - 18:00 Fabio Finelli (Bologna U.) - "Planck 2015 results on inflation"
NOTE UNUSUAL DAY AND LOCATION (16:30-17:30, SR 106, Philosophenweg 12) We present the implications of the Planck full mission measurement of CMB anisotropies in temperature and polarization for inflation and for the main cosmological parameters. We present the constraints on single field inflationary models, on the primordial power spectrum of curvature perturbations, and on isocurvature perturbation.
06 May 2015 17:00 - 18:00 Miguel Zumalacarregui (ITP Heidelberg) - "Non-linear evolution of the BAO scale in alternative theories of gravity"
The scale of Baryon Acoustic Oscillations (BAO) imprinted in the matter distribution is known to suffer sub-percent deviations from being a perfect standard ruler due to non-linear corrections. However, such corrections have never been computed in alternative theories of gravity. I will give an overview of historical and recent developments in alternative theories of gravity and present the first computation of the BAO shift based on perturbation theory (à la Bellini-Sawicki) and a peak-background split (à la Sherwin-Zaldarriaga),valid for any Horndeski theory. The BAO shift is modified by the enhanced growth of linear perturbations, as well as by novel, non-linear gravitational effects. The modified gravity result can range from a negligible to order-unity correction at z=0 depending on the model, and in some cases presents a characteristic time evolution. Although small, this effect might be within the reach of future galaxy surveys, and might have implications for BAO reconstruction in theories other than Einstein's.
29 Apr 2015 17:00 - 18:00 Dragan Huterer (University of Michigan) - "How to Falsify a Dark Energy Paradigm"
The physical mechanism behind the acceleration of the universe remains one of the great mysteries of modern cosmology. Given the proliferation of dark energy models on the market, an obvious question is: How can we rule out whole classes of dark energy models? And what quantities, at what redshift, and with what accuracy, should be measured in order to rule out these classes of models? I present answers to these questions by presenting results from two interrelated analyses whose motivation is to compare the geometrical quantities with the growth of structure using current or future cosmological observations.
23 Apr 2015 17:45 - 18:45 Marco Baldi (U. Bologna) - "A tale of three characters: dark couplings, dark scattering, and the biased voids"
In this talk I will discuss some recent develpments in the field of structure formation for cosmological models characterized by various types of non-standard physics, in particular by multiple couplings or elastic scattering in the dark sector. I will also present some preliminary results on how galaxy bias might strongly affect the possibility to investigate dark sector physics through the properties of cosmic voids. (The seminar will take place in room 106 of Phil 12 right after the Teilchentee - NOTE UNUSUAL DAY, TIME AND LOCATION)
22 Apr 2015 17:00 - 18:00 No seminar this day () - "TBA"
20 Apr 2015 17:00 - 18:00 Claudia Antolini (SISSA) - "CMB lensing signal analysis: prospects for measurements and characterisation for future surveys "
Within the last few years, Cosmic Microwave Background gravitational lensing has become a new tool for cosmology and astrophysics. As a new independent kind of measurement in this field, it can help to break the degeneracy between the cosmological parameters and determining their values with a better accuracy. This field of research is becoming increasingly prominent and fertile in the latest years, both as a way to investigate the evolution of the Universe at late times, and in relation to the EUCLID mission preparatory science work, aiming at mapping galaxies on about 30% of the sky reaching a redshift of about 2. In this talk, I will characterise the lensing signal in the CMB and to study the efficiency of lensing extraction at small angular scales from simulated CMB maps lensed by N-body simulations, and how this translates in constraining Dark Energy and its relevant parameters for the expansion. After investigating the balance between the tensor modes and lensing in the B−mode power spectrum, the feasibility of lensing extraction on CMB lensed N-body simulations is demonstrated: the faithfulness of the implemented pipeline is assessed and verified for upcoming exper- imental setups and validated ranging from the degree to the arcminute scale. Future applications to neutrino simulations and forecasting will be discussed. NOTE UNUSUAL DATE!
15 Apr 2015 17:00 - 18:00 Adam Solomon (Heidelberg ITP) - "Bigravity: Dead or Alive?"
After many years of wandering in the desert, we are finally in sight of the Promised Land: massive and interacting gravitons. However, we have not yet reached this holiest of terrains: in massive gravity, stable cosmological solutions have long been known not to exist, while bigravity once looked more promising but has since been shown to possess its own potentially-deadly problems. I will summarize some of these issues, and then discuss possible ways out. I will conclude with my own outlook on this theory and where, if anywhere, we should go from here.
18 Mar 2015 17:00 - 18:00 Santiago Casas (ITP Heidelberg) - "FisherTools, CosmoTools and Cosmomathica"
I will present a set of Mathematica packages that can be used to perform very easily Fisher Forecasts in Galaxy Clustering and Weak Lensing. In order to make the code flexible with respect to different cosmological models, nonlinear corrections and observational effects, the code relies on two packages CosmologyTools and Cosmomathica. The first one calculates all needed cosmological functions in various models while the second one calculates the linear and nonlinear power spectrum using CAMB, Eisenstein&Hu, CosmicEmulator, Halofit, Copter (and others to come). Apart from the usual Fisher Matrix computation, FisherTools integrates functions for post-analysis, file exports and plotting. I will also use the opportunity to explain a little bit how to use Mathematica as an efficient programming language by reviewing some tricks and properties of its versatile computational capabilities.
11 Feb 2015 17:00 - 18:00 Matthias Bartelmann (ITA Heidelberg) - "A field-theoretical approach to classical many-particle systems"
Classical kinetic theory can be formulated in a way which is structurally equivalent to statistical quantum field theory: Ensembles of many classical particles in and out of equilibrium are then described by a generating functional from which collective and statistical information can be extracted by suitable functional derivatives. This generating functional is based on the complete microscopic initial phase-space distribution of the ensemble. Specialising this approach to cosmological structure formation by introducing suitably correlated initial ensembles, the statistical properties of collective fields such as the density can be studied in a way which avoids important limitations of the conventional approach to cosmic structure formation. In particular, the evolution of the cosmic matter power spectrum can be followed well into the non-linear regime. The theory itself is applicable to classical many-particle systems with arbitrary interactions.
04 Feb 2015 17:00 - 18:00 Miguel Quartin (UFRJ) - "On the significance of power asymmetries in CMB data at all scales"
Power anomalies in the CMB data have been reported and investigated for over ten years. We will discuss in-depth two of these: the "hemispherical asymmetry" and the "dipolar power modulation". We will show how correcting for 3 spurious effects implies no significant deviation from cosmological isotropy all the way to ℓ =~ 2000, except for a small discrepancy on large scales. The three effects are: (i) mask-induced biases; (ii) the Doppler and aberration effects due to our peculiar velocity; (iii) the anisotropy of the noise. Interestingly, without proper removal of Doppler and aberration effects there are significant spurious discrepancies. We will also discuss the problem of "a posteriori" statistics and how Bayesian model selection techniques affect the final results.
28 Jan 2015 17:00 - 18:00 There will be no seminar this day () - ""
22 Jan 2015 17:00 - 18:00 () - "TBA"
21 Jan 2015 17:00 - 18:00 Matteo Martinelli (ITP Heidelberg) - "Including birefringence into time evolution of CMB: current and future constraints"
We introduce birefringence effects within the propagation history of CMB, considering the two cases of a constant effect and of an effect that increases linearly in time, as the rotation of polarization induced by birefringence accumulates during photon propagation. Both cases result into a mixing of E and B modes before lensing effects take place, thus leading to the fact that lensing is acting on spectra that are already mixed because of birefringence. Moreover, if the polarization rotation angle increases during propagation, birefringence affects more the large scales that the small scales. We put constraints on the two cases using data from WMAP 9yr and BICEP 2013 and compare these results with the constraints obtained when the usual procedure of rotating the final power spectra is adopted, finding that this dataset combination is unable to distinguish between effects, but it nevertheless hints for a non vanishing value of the polarization rotation angle. We also forecast the sensitivity that will be obtained using data from Planck and PolarBear, highlighting how this combination is capable to rule out a vanishing birefringence angle, but still unable to distinguish the different scenarios. Nevertheless, we find that the combination of Planck and PolarBear is sensitive enough to highlight the existence of degeneracies between birefringence rotation and gravitational lensing of CMB photons, possibly leading to false detection of non standard lensing effects if birefringence is neglected.
17 Dec 2014 17:00 - 18:00 There will be no seminar this day () - ""
10 Dec 2014 17:00 - 18:00 Caroline Heneka (DARK Cosmology Center) - "Cluster cosmology, the halo mass function and clustering dark energy"
Galaxy clusters represent an important probe in modern cosmology. I will start by reviewing current cosmological constraints coming from cluster X-ray data, as well as halo mass functions (HMF) in use for deriving contraints. Using the example of clustering dark energy, it will be shown how we proceed to include non-linear effects into HMF-estimates; as we aim at an accurate description of the mass function for parameter estimates, dealing with theoretical uncertainties is crucial.
03 Dec 2014 17:00 - 18:00 There will be no seminar this day () - ""
26 Nov 2014 17:00 - 18:00 Jerome Gleyzes (IPhT Paris) - "Healthy theories beyond Horndeski"
In search for a candidate that could explain the current acceleration of the Universe, a lot of attention has been given recently to Galileon theories, or in their generalized form, Horndeski theories. They are interesting as they represent the most general scalar tensor theories that do not lead to equations of motion containing more than two derivatives. This restriction is generally thought to be of great importance, as generically, higher order derivatives lead to ghost instabilities. I will present a new class of scalar tensor theories that are broader than Horndeski and, as such, do bring higher order derivatives. However, when studying carefully the theories, it was shown that they do not propagate any additional ghostly degree of freedom. I will give details on how and why this is possible, and I'll further talk about the uncommon phenomenology associated. Indeed, these theories exhibit a new type of coupling to matter, even when the latter is minimally coupled.
19 Nov 2014 17:00 - 18:00 Pierre Fleury (Institut d’Astrophysique de Paris) - "Interpreting cosmological observations in a clumpy universe"
In the standard cosmological framework, observations are interpreted as if light propagated through a smooth, fluid-filled universe. However, the typical light beams involved in astronomical observations—such as supernovae—are extremely narrow, and thus probe the Universe at scales where the fluid description should no longer be valid. In this talk, I will explain how the clumpiness of the distribution of matter can affect light propagation through the cosmos, and thus the interpretation of our observations.
12 Nov 2014 17:00 - 18:00 Mark Sullivan (Southampton) - "The Transient Universe: Cosmic Explosions and Dark Energy"
Type Ia Supernovae (SNe Ia) provide our most mature probe of dark energy. I will present the latest dark energy constraints from the Supernova Legacy Survey, including new astrophysical effects in the SN Ia population that are refining their use as cosmological probes. I will then show recent results from the next generation of local transient surveys, including details of new constraints on SN Ia progenitors.
05 Nov 2014 17:00 - 18:00 There will be no seminar this day () - ""
29 Oct 2014 17:00 - 18:00 Linda Blot (LUTH, Observatoire de Paris, Meudon) - "The challenges of precision cosmology: estimating covariance matrices"
The upcoming generation of galaxy surveys will probe the distribution of matter in the universe with unprecedented accuracy. Measurements of the matter power spectrum at different scales and redshifts will provide stringent constraints on the cosmological parameters. However, on non-linear scales this will require an accurate evaluation of the covariance matrix. Using the large set of simulations of the DEUS PUR project we computed the covariance matrix of the 3D matter density power spectrum for the concordance ΛCDM cosmology. In this talk I discuss the numerical systematics involved in the estimation of the covariance from simulations and the effects of the non-linear evolution of the density field on the probability distribution of the matter power spectrum and its covariance.
22 Oct 2014 17:00 - 18:00 There will be no seminar this day () - ""
15 Oct 2014 17:00 - 18:00 Gerasimos Rigopoulos (ITP Heidelberg) - "TBA"
08 Oct 2014 17:00 - 18:00 Graduate days - There will be no seminar () - ""
26 Sep 2014 12:00 - 13:00 Eiichiro Komatsu () - "Polarisation of the Cosmic Microwave Background: Toward an Observational Proof of Cosmic Inflation"
In the seminar room of Philosophenweg 16. Note unusual day and time.
11 Sep 2014 17:00 - 18:00 Antony Lewis () - "New observables for future CMB anisotropy observations"
30 Jul 2014 17:00 - 18:00 Johannes Lange (Heidelberg University, Chinese University of Hong Kong) - "Probing Dark Matter with Gamma-Ray Anisotropies"
The weakly interacting massive particle (WIMP) is one of the most promising candidates for cosmological dark matter. If such particles are thermally produced in the early universe, they could self-annihilate in the cores of dark matter halos today. This would give rise to a gamma-ray signal that could be detected with present-day instruments such as the Fermi satellite. One of the prime targets for such an indirect dark matter search is the diffuse gamma-ray background (DGRB). In particular, dark matter annihilation could induce a peculiar anisotropy signal in the DGRB. I discuss the prospects of detecting such an anisotropy pattern and compare it to recent measurements of the gamma-ray anisotropy with Fermi.
23 Jul 2014 17:00 - 18:00 Riccardo Catena (Goettingen) - "Prospects for direct detection of galactic dark matter"
Dark matter constitutes about five sixth of the total matter in the observable Universe, and it forms large spheroidal halos hosting the majority of the astrophysical structures, including our Milky Way. Yet, the particles forming the Milky Way dark matter halo have so far escaped detection, and the dark matter mass, as well as its interactions, remain unknown. The detection of dark matter particles in the solar neighborhood through their scattering off the nuclei of a terrestrial detector would provide us with a direct measurement of the dark matter particle mass, and of the strength of the dark matter-nucleon interaction. To accomplish this goal is the aim of the dark matter direct detection technique. In this talk, I review the status and illustrate the prospects of this rapidly evolving research area. The focus of my talk will be on a new theoretical approach that combines the notion of effective field theory with advanced statistical methods.
16 Jul 2014 17:00 - 18:00 There will be no seminar this day () - ""
09 Jul 2014 16:00 - 17:00 Jaiyul Yoo (University of Zurich) - "Relativistic Effect in Galaxy Clustering"
Recent developments in CMB and large-scale galaxy surveys have led to the standard cosmological model, but the physical understanding of its ingredients remains elusive so far. In response to the gravity of these issues, numerous large-scale galaxy surveys are ongoing or planned to be operational in a near future. However, precision measurements in future galaxy surveys bring in new challenges, demanding substantial advances in theoretical modeling and observational methods. I will discuss the recent theoretical development in modeling galaxy clustering in a relativistic context and the observational issues associated with this recent development. The relativistic effect in galaxy clustering or the deviation from the standard Newtonian description becomes substantial on large scales, in which dark energy models or alternative theories of modified gravity deviate from general relativity, and in which the fingerprint of the inflationary epoch remains in its pristine form. I will discuss how the subtle relativistic effect in galaxy clustering can be used to test general relativity on large scales and probe signatures of the early Universe. Note unusual location and time: SR 204 im Phil 12 at 4pm
02 Jul 2014 17:00 - 18:00 Antonio Enea Romano (Medellin) - "Non perturbative effects of primordial curvature perturbations on the apparent value of a cosmological constant"
We study effects on the luminosity distance of a local inhomogeneity seeded by primordial curvature perturbations of the type predicted by the inflationary scenario and constrained by the cosmic microwave background radiation. We find that a local underdensity originated from a one, two or three standard deviations peaks of the primordial curvature perturbations field can induce corrections to the value of a cosmological constant of the order of 0.6 respectively. These effects cannot be neglected in the precision cosmology era in which we are entering. Our results can be considered an upper bound for the effect of the monopole component of the local non linear structure which can arise from primordial curvature perturbations and requires a fully non perturbative relativistic treatment.
25 Jun 2014 17:00 - 18:00 Francesco Montanari (Geneva) - "Parameter estimation with galaxy number counts "
We propose to determine the truly observed two-point statistics from observations, either in terms of the angular power spectra, or in terms of the angle and z-dependent correlations functions. This information can be used, e.g., to constrain cosmological parameters via an Alcock-Paczinski test, or to test GR. Future surveys like Euclid will be sufficiently large to profit maximally from this cleaner cut between observations and modeling.
18 Jun 2014 17:00 - 18:00 Andreu Font-Ribera (LBNL Berkeley) - "Studying the Expansion of the Universe with BOSS quasars"
After five years of observations, the Baryon Oscillation Spectroscopic Survey (BOSS) was finished at the end of March. During these years, it has used the SDSS telescope to obtain spectra of 1.5 million galaxies to get very accurate measurements of the Baryon Acoustic Oscillations (BAO) scale at redshift z ~0.5. Roughly 20% of the fibers of the spectrograph, however, are pointing to high redshift quasars with the goal of detecting the BAO feature in the clustering of the intergalactic medium, using a technique known as the Lyman alpha forest. In this talk I will overview several recent results from the Lyman alpha forest working group in BOSS, including the measurement of BAO at z=2.4 both from the auto-correlation of the Lyman alpha absorption, and from its cross-correlation from quasars. I will also present preliminary results on the cosmological parameters constrains obtained when combining all BOSS BAO analyses with Cosmic Microwave Background and supernovae probes, including the most precise measurement of the current expansion of the Universe.
11 Jun 2014 17:00 - 18:00 Alexis Helou (APC Lab, Paris) - "The Apparent Universe"
The usual depiction of black hole physics is based on the static Schwarzschild metric and teleological event horizon. There is an analogy with cosmological models, i.e the static de Sitter metric and its event horizon. However, since our real Universe is highly dynamical, the analogy remains veiled when one works with the static black hole. Using a dynamical description of black hole horizon, we find a striking parallel with our cosmological horizon. Such a dynamical formalism has been proposed by Hayward for black holes: it singles out the trapping horizon, or apparent horizon, as the relevant object to work with. We will first explain the difference between apparent and event horizons, and then go through Haywards formalism for black holes. The context being set, we will apply this machinery to our dynamical Universe, and therefore take the apparent horizon as our preferred boundary. We will be able to recover the Friedmann Equations from thermodynamics, and explain what we have in common with Jacobson’s work on thermodynamical Gravity. Moreover, an interesting quantity appears in the equation, which is reminiscent of the beta function used in the renormalization-group flow technics in the AdS-CFT correspondence. Finally, we will derive a Hawking temperature for the apparent horizon, in two different ways which turn out to give the same expression for the surface gravity. We will conclude with possible interpretations in terms of cosmology, and the role of the apparent horizon as the relevant notion for the evolution of our Universe.
04 Jun 2014 17:00 - 18:00 There will be no seminar this day () - "TBA"
28 May 2014 17:00 - 18:00 Adam Solomon (DAMPT Cambridge) - "Are Two Metrics Better than One?: The Cosmology of Massive Bigravity"
The ghost-free theory of massive gravity with two dynamical metrics can produce viable cosmological expansion, where the late-time acceleration of the Universe is due to the finite range of the gravitational interaction rather than a nonzero cosmological constant. Furthermore, this theory is of intrinsic theoretical interest, as it is only in the last four years that a healthy non-linear theory of a massive graviton has been discovered and studied. This has opened up an exciting new avenue in modifying general relativity. I will motivate these theories and discuss their cosmological behavior, focusing on recent work investigating the evolution of cosmic structure in massive bigravity. Models which are indistinguishable from LCDM at the level of the background are shown to have quite different predictions for structure formation in the linear, subhorizon regime. These deviations should be detectable by near-future experiments such as the ESA Euclid satellite. If time permits, I will discuss some more fundamental issues arising from having two metrics, particularly in the case where matter couples to both metrics. This brings up ambiguities in the physical role of the metric, which raises both conceptual questions as well as very practical difficulties to do with comparing the theory to observations. These challenges may be a call to go back to the basics, and rediscover the justifications for what we have taken for granted over the better part of the last century.
21 May 2014 17:00 - 18:00 Luisa Jaime (ITP Heidelberg) - "Revisiting f(R)"
f(R) gravity is, perhaps, the most popular modification to general relativity, the treatment in general have been performed by using the mapping to scalar tensor theories (STT) and it is very natural because we have a demostration about the equivalence (under some conditions) between this two theories, and also because it is simpler to work in STT than in the 4th order field equations in f(R). In this talk I will show a different approach that was developed in order to avoid the mapping to STT. This approach was used in order to solve the "controversy" about the existence of compact objects in f(R) and also in order to explore the cosmological evolution for different models. Finally I would like to introduce a discussion about the definition of the equation of state in this kind of theories.
16 May 2014 13:10 - 14:10 Emilio Bellini (ITP - Heidelberg & ICC-UB) - "Maximal freedom in general scalar-tensor theories"
NOTE UNUSUAL DAY, TIME AND LOCATION (ITP Philosophenweg 12 - Room 106 / Tunch room) In this talk I shall discuss the properties of linear perturbations of scalar-tensor theories belonging to the Horndeski class of actions whilst evolving on a cosmological background. To do so, I will discuss the physical interpretation of five independent functions of time, which are sufficient in order to describe the exact linear dynamics of these models. Finally, I will explain how to possibly constrain these functions by observations and how to reduce the full theory to well-known models. NOTE the unusual time and place, the talk will be during lunch, in the tunch room
07 May 2014 17:00 - 18:00 There will be no seminar this day () - ""
30 Apr 2014 17:00 - 18:00 Hans-Juergen Pirner (Heidelberg) - "The physical world and possible worlds"
One method to understand our actual world better is to regard it as one of possible worlds. I will discuss the current concept of possible worlds in philosophy and then cover various parts in physics where possible worlds occur: The principle of least action and Leibniz's concept of our world as the best of all worlds, possible microstates in an ensemble, possibility in quantum mechanics, the multiverse and a possible mass ranking of elementary particles.
23 Apr 2014 17:00 - 18:00 Valeri Vardanyan (ITP, Heidelberg U) - "Seeking the Epoch of Maximum Luminosity for Dusty Quasars"
Infrared luminosities vlv(7.8 um) arising from dust reradiation are determined for optically discovered Sloan Digital Sky Survey (SDSS) quasars having 1.4 < z < 5 using detections at 22 um with the Wide-Field Infrared Survey Explorer. Infrared luminosity does not show a luminosity maximum at any redshift z < 5, reaching a plateau for z >~ 3 with maximum luminosity vLv(7.8 um) >~ 10^{47} erg per s. Using vLv(0.25 um) to define ultraviolet luminosity, the most ultraviolet luminous quasars have the largest values of the ratio vLv(0.25 um)/vLv(7.8 um) with a maximum ratio at z = 2.9. These results indicate that the most ultraviolet luminous quasars have the smallest dust content and appear luminous primarily because of lessened extinction. Based on the observed ultraviolet/infrared luminosity ratios, obscured quasars are defined as having more than five magnitudes of ultraviolet extinction compared to the SDSS quasars. To compare luminosity functions of obscured and unobscured quasars, infrared vLv(7.8 um) luminosity functions at z ~ 2.1 are determined for the SDSS quasars and the optically discovered quasars in the AGN and Galaxy Evolution Survey. These are compared with the luminosity function of optically obscured quasars determined using a new summary of obscured quasars discovered with the Spitzer Infrared Spectrograph. The comparison indicates comparable numbers of obscured and unobscured quasars at z ~ 2.1 with a possible excess of obscured quasars at fainter luminosities.
16 Apr 2014 17:00 - 18:00 There will be no seminar this day () - ""
09 Apr 2014 17:00 - 18:00 Matteo Maturi (ITA Heidelberg) - "Some tools for cosmology"
Many different approaches are available to investigate galaxy clusters, supernovae and constrain cosmology. I'm going to discuss: the 'Eigen-cosmologies', a convenient approach to constrain the expansion rate of the universe via supernovae (and an alternative to MCMC?); EasyLens, a new pipeline under development here in Heidelberg which aims at studying galaxy clusters joining all data from X-rays to micro-waves plus gravitational lensing; the detection of filamentary structures in the surroundings of galaxy clusters; constrain cosmology with weak lensing peak counts and gravitational giant arcs.
02 Apr 2014 17:00 - 18:00 Frank Koennig (ITP, Heidelberg U) - "Viable cosmological solutions in massive bimetric gravity"
In this talk I will present general conditions for viable cosmological solutions in massive bimetric gravity theories and discuss especially simple single-parameter models. Such minimal bimetric models have nice properties, analytic solutions of the background evolution and are a valid and testable alternative to LCDM. Firsts results in linear perturbation theory for those models indicate significant differences in the growth of structure compared to standard cosmology, though they can fit observed growth data very well.
26 Mar 2014 17:00 - 18:00 Elena Sellentin (ITP, Heidelberg U) - "DALI: A new method for posterior forecasting"
We present the new method DALI (Derivative Approximation for LIkelihoods) for reconstructing and forecasting posteriors. DALI extends the Fisher Matrix formalism but allows for a much wider range of posterior shapes. While the Fisher Matrix formalism is limited to yield ellipsoidal confidence contours, our method can reproduce the often observed flexed, deformed or curved shapes of known posteriors. This gain in shape fidelity is obtained by expanding the posterior to higher order in derivatives with respect to parameters, such that non-Gaussianity in the parameter space is taken into account. The resulting expansion is positive definite and normalizable at every order. Here, we present the new technique, highlight its advantages and limitations, and show a representative application to a posterior of dark energy parameters from supernovae measurements.
19 Mar 2014 17:00 - 18:00 Ixandra Achitouv (University Observatory Munich) - "Abundance of Halos and Voids in the Excursion Set Approach"
Our picture of the present universe is mainly composed of voids and dark matter halos, the building blocks of cosmic structure. The halo mass function quantifies their number density as function of mass, and it is a key quantity to predicting several large-scale-structure observables. In this talk, I will review the latest theoretical advancements on the derivation of a halo mass function which encodes physical aspects of the halo formation process and is capable of reproducing numerical N-body simulation results with unprecedented accuracy. Finally I will apply the same formalism to compute void abundances and I will discuss current limitations of the void hierarchy model.
12 Mar 2014 17:00 - 18:00 Valerio Marra (ITP, Heidelberg U) - "so long, and thanks for all the Physik"
I will present the research I carried out during my 2.5 years at the ITP. I will focus mainly on i) weak lensing of standard candles, ii) Bayesian search for systematics-driven supernovae and iii) inhomogeneous models, in particular the effect of cosmic variance on dark-energy parameters and tests of the Copernican principle.
10 Mar 2014 17:00 - 18:00 Camilla Penzo (MPIA, Heidelberg) - "Galaxy formation in dynamical and coupled dark energy cosmologies."
Firstly, I will review the basis of dynamical dark energy and coupled dark energy models, to then introduce zoom-in simulations, an extremely useful tool in order to investigate collapsed structures with a much higher resolution and, at the same time, maintain the cosmological context. I will start with the Dark MaGICC project, which aims to investigate the effect of dynamical Dark Energy on galaxy formation via hydrodynamical cosmological simulations. We have selected four dynamical dark energy scenarios with different time varying equations of state, and performed high resolution SPH simulations for three different galaxies. The dark energy parameterization has a non negligible impact on galaxy evolution and on structural properties of galaxies at z=0, in striking contrast with predictions from pure Nbody simulations. In fact, different background evolutions can, depending of the behavior of the DE equation of state, either enhance or quench star formation with respect to a LCDM model with profound effects on the final galaxy rotation curves. Finally, I will show preliminary results regarding zoom-in dark matter only simulations on galactic scales in coupled dark energy cosmologies and their future developments.
05 Mar 2014 17:00 - 18:00 There will be no seminar this day. () - "Please choose another day."
19 Feb 2014 17:00 - 18:00 There will be no seminar this day. () - "Please choose another day."
12 Feb 2014 17:00 - 18:00 There will be no seminar this day. () - "Please choose another day."
29 Jan 2014 17:00 - 18:00 There will be no seminar this day. () - "Please choose another day."
22 Jan 2014 17:00 - 18:00 Godfrey Leung (University of Nottingham) - "Can We Constrain or Even Rule Out Multifield Inflation Models?"
Recent Planck satellite results suggest negligible (local) non-gaussianity, fNL less than O(5), which are consistent with simple single field inflation. Though multifield models could potentially give large fNL, they generically gives Gaussian statistics as well. It is thus important to test multifield models as well, since they are considered to be more natural from particle physics models point of view. It has been known that predictions in multifield models are sensitive to initial conditions, though recently it has been shown they are more predictive as one may expect. However, this doesn't take (p)reheating into account. In this talk, I will discuss about the fate of primordial inflationary observables such as non-gaussianity in multifield inflation models during the reheating stage. In various 2-field models studied, we found that the reheating period is important for correctly evaluating the statistics of primordial curvature perturbations, even in the simplest perturbative reheating scenario. This suggests that one need to be cautious when comparing model predictions in multifield models with observations if the details of reheating are unknown. Consistency relations however seem to be more robust and may thus provide hope to test and constrain different models without specifying the physics of reheating.
15 Jan 2014 17:00 - 18:00 Alexander Wiegand (MPG, Potsdam) - "Minkowski functionals as a tracer of cosmic inhomogeneity"
With the discovery of larger and larger coherent structures, it is worthwhile to test the consistency of the observations with the standard models of structure formation. For this purpose, Minkowski functionals are very useful, as they contain more information than the usual two point statistics. I will discuss what precisely this supplementary information contains and give an overview over the use of the Minkowski functionals for the characterization of structure. In particular, I will introduce the boolean grain model for which the Minkowski functionals may be analytically related to a series of the (higher order) correlation functions of the studied point distribution. This allows us to quantify the influence of higher order clustering beyond the simple two point correlations. In the second part, I will present the results of a Minkowski functional analysis of the structure of the galaxy distribution in the luminous red galaxy (LRG) sample of the Sloan Digital Sky Survey. Comparing the Minkowski functionals of the observed galaxies to those of galaxies obtained from a set of N-body simulations, we find that the two are only marginally consistent. Expanding the functionals into a series of integrals over n-point correlation functions and comparing the coefficients with the theoretical prediction, gives also a consistent picture and allows to extract the values of cosmological parameters. Finally, we use the series expansion to show the importance of the higher order correlations for the shape of the functionals.
13 Jan 2014 17:00 - 18:00 Shinji Tsujikawa (Tokyo University of Science) - "Planck constraints on single-field inflation"
18 Dec 2013 17:00 - 18:00 Nima Khosravi (AIMS, Cape Town) - "Geometric Massive Gravity in Multi-Connection Framework"
What is the right way to interpret a massive graviton? We generalize the kinematical framework of general relativity to multiple connections. The average of the connections is itself a connection and plays the role of the canonical connection in standard General Relativity. At the level of dynamics, the simplest choice of the Einstein-Hilbert action is indistinguishable from the single-connection case. However, inspired by Weyl geometry, we show how one can construct massive gravity to all orders in perturbation theory compatible with the de Rham-Gabadadze-Tolley ghost-free model. We conclude that the mass of the graviton can be interpreted as a geometrical property of spacetime arising from two connections. Furthermore in the multi-connection framework there is no ambiguity in the definition of physical metric and consequently coupling to matter.
18 Dec 2013 12:30 - 13:30 Guillermo Ballesteros (ITP, Heidelberg U) - "Cosmological phonons in multi-component fluids: towards a theory ofDM-DE interactions (Phil12, SR 106)"
The effective field theory (EFT) of fluids is a powerful framework based on symmetry principles that describes sound waves in continuous media using a derivative expansion. I will apply it to study interacting cosmological species and answer questions such as: 1) how to distinguish between species and 2) what are the possible intreactions between them. I will also show that one of the operators of the EFT behaves as a cosmological constant when it dominates the dynamics.
09 Dec 2013 16:15 - 17:15 Timothy Clifton (Queen Mary, University of London) - "An exact treatment of back-reaction in cosmology - (Teilchentee seminar)"
The precise effect of inhomogeneity on the large-scale expansion of the Universe is still an open question. In this talk I will outline how one can investigate this problem by considering cosmological models in which the matter content is taken to be confined to regularly distributed discrete sources, rather than a continuous fluid. Using exact, fully relativistic, non-perturbative methods one can then determine a number of properties of such models. We find that not all behaviour approaches that of the Friedmann solutions, and in particular that accelerating expansion is possible without any exotic matter content.
04 Dec 2013 17:00 - 18:00 There will be no seminar this day. () - ""
27 Nov 2013 17:00 - 18:00 There will be no seminar this day. () - "TBA"
20 Nov 2013 17:00 - 18:00 Enea Di Dio (Geneva University) - "Cosmological Parameter Estimation with Large Scale Structure Observations"
I will present a recent Fisher matrix forecast on cosmological parameters based on 2D angular power spectrum for Euclid-like surveys. I will show that the analysis with redshift dependent angular power spectra leads to comparable results as the usual analysis based on the 3D matter power spectrum. Contrarily to the 3D analysis, the 2D-tomography analysis is model independent and directly observable. All the results have been obtained using CLASSgal, a code made now publicly available, which computes, accurately and efficiently, the galaxy number fluctuations in linear perturbation theory, including all relativistic corrections.
13 Nov 2013 17:00 - 18:00 Julian Adamek (Geneva U.) - "Weak field general relativistic N-body simulations for cosmic large scale structure"
I present a numerical framework which, for the first time, allows one to abandon the Newtonian concept of absolute space and time and do a simulation of cosmic structure formation in a truly dynamical spacetime. In order to avoid the complexity of full general relativity, the framework employs a weak field approximation which is suitable for cosmology, even in the deeply non-linear regime. Since the description is fully relativistic to begin with, any observables which depend on relativistic effects (like lensing or redshift) can be constructed in a transparent way. Moreover, the scheme naturally allows for completely general sources of stress-energy, and can therefore be useful for the investigation of possible alternatives to the standard dark components of the Universe.
30 Oct 2013 17:00 - 18:00 Michele Maggiore (Geneva U.) - "Dark energy from non -local massive gravity"
We propose an infrared modification of General Relativity, based on the addition of non-local terms to the Einstein equations. This construction provides a theory of massive gravity which is fully covariant, without the need of introducing an external reference metric. We discuss the consistency of the theory and its cosmological consequences. We find that the model generates a dynamical dark energy that can account for the presently observed value, without introducing a cosmological constant. Tuning a free mass parameter to a value $m= 0.67 H_0$ we reproduce the observed value $\Omega_{DE}\simeq 0.68$. This leaves us with no free parameter and we then get a pure prediction for the EOS parameter of dark energy. Writing $w_{\rm DE}(a)=w_0+(1-a) w_a$, we find $w_0= -1.040$ and $w_a= -0.020$, consistent with the Planck data, and on the phantom side. We also show that the model displays a sort of degravitation mechanism where the cosmological constant is reabsorbed into the boundary conditions of a scalar field
28 Oct 2013 17:00 - 18:00 Christos Tsagas (University of Thessaloniki, Greece) - "On the question of cosmic magnetism"
Although magnetic fields seem to be everywhere in the universe, we know little about their origin, their evolution and their potential implications. The idea of primordial magnetism is attractive, because it could in principle explain all the large-scale fields seen in the universe today. Nevertheless, primordial magnetogenesis is still not problem free. We outline how cosmological magnetic fields could have affected the scenarios of structure formation, discuss the basic problems faced by the proposed mechanisms of magnetic generation and indicate some possible ways out.
23 Oct 2013 17:00 - 18:00 There will be no seminar this day. () - ""
16 Oct 2013 17:00 - 18:00 There will be no seminar this day. () - ""
09 Oct 2013 17:00 - 18:00 Takahiro Tanaka (Kyoto University ) - "Graviton Oscillation in a viable bigravity model"
We discuss graviton oscillations based on the ghost free bi-gravity theory. We point out that this theory possesses a natural cosmological background solution which is very close to the case of general relativity. Furthermore, interesting parameter range of the graviton mass, which can be explored by the observations of gravitational waves, is not at all excluded by the constraint from the solar system tests. Therefore the graviton oscillation with possible inverse chirp signal would be an interesting scientific target of KAGRA, adv LIGO, adv Virgo and GEO.
07 Oct 2013 16:00 - 17:00 Claudia Antolini (SISSA, Trieste) - "Prospects for CMB lensing: constraints on tensor modes and signal extraction from N-body lensed CMB maps"
After multiple high precision detections (ACT, SPT, Planck) gravitational lensing has become a new source of relevant cosmological information: combining it with other probes (e.g. the large scale structure) can give significant insight on the evolution of the Dark Energy component. In my talk I will analyse how future polarization experiments can constrain the standard LCDM Universe along with an evolving Dark Energy component and how including the lensing phenomenology in the analysis of primordial B modes can significantly influence the constraints on the amplitude on tensor modes. Furthermore, I will present the first results of a flat sky lensing extraction algorithm, aiming at the characterization of Dark Energy imprints in N-body lensing CMB realizations for temperature maps.
25 Sep 2013 17:00 - 18:00 There will be no seminar this day. () - ""
18 Sep 2013 11:00 - 12:00 Lingfei Wang (Lancaster University) - "A new mechanism for curvature perturbations"
I will explain the spectator scenario and show its advantages compared to the curvaton scenario, under the latest Planck constraints. The spectator scenario generates a smaller non-Gaussianity, negligible isocurvature perturbations, and less tuning. It agrees well with the current Planck data in the visible sector inflation setup, as an example. It also explains the CMB dipole asymmetry with a brief fast roll phase. I will close the talk by proposing the CMB power multipoles, as an alternative approach independent of the CMB dipole asymmetry model.
12 Sep 2013 17:00 - 18:00 Japan-Germany collaboration () - "Short talks and discussion"
05 Sep 2013 17:15 - 18:15 Jianhua He (Astronomical Observatory of Brera, Italy) - "Testing f(R) gravity with the large-scale structure"
In this talk, I will focus on f(R) theory, addressing the very important role of this kind of theory played in a wider family of modified gravity theories. I will focus on several fundamental questions as to why we need to study f(R) theory and what we can learn from it. As a working example, I will focus on a specific model, discussing the linear perturbation as well as the non-linear perturbation for the Large-scale structure formation using N-body simulations. I will address the role of Chameleon mechanism in the non-linear behaviors for f(R) gravity. I will also discuss several questions with regard to the Chameleon mechanism. How does it work? Is it natural or really a cure for f(R) models to evade the local tests?
24 Jul 2013 17:00 - 18:00 Alicia Bueno Belloso (ITP, Heidelberg) - "Using isolated galaxy pairs as cosmological tracers"
The Alcock-Paczynski effect uses the fact that, when analyzed with the correct geometry, we should observe structure that is statistically isotropic in the Universe. For structure undergoing cosmological expansion with the background, this constrains the product of the Hubble parameter and the angular diameter distance. However, the expansion of the Universe is inhomogeneous and local curvature depends on density. We argue that this distorts the Alcock-Paczynski effect on small scales. After analyzing the dynamics of galaxy pairs in the Millennium Simulation , we find an interplay between peculiar velocities, galaxy properties and local density that affects how pairs trace cosmological expansion. We find that only low-mass, isolated galaxy pairs trace the average expansion with a minimum "correction" for peculiar velocities. Other pairs require larger, more cosmology- and redshift-dependent peculiar velocity corrections and, in the small separation limit of being bound in a collapsed system, do not carry cosmological information.
17 Jul 2013 17:00 - 18:00 There will be no seminar () - ""
15 Jul 2013 16:00 - 17:00 Ippocratis Saltas (The University of Nottingham) - "Successes and challenges of asymptotically safe cosmology"
Asymptotic Safety is a promising scenario for a non-perturbative UV completion of gravity, if a non--trivial fixed point under the Renormalisation Group (RG) exists . In this talk, I will first introduce the main idea behind Asymptotic Safety, and present evidence for the existence of a non--trivial fixed point in the UV. I will then proceed by focusing on some successes and challenges of the emerging cosmology. In particular, I will show how an RG improvement of the Einstein--Hilbert action in this context can naturally yield a primordial and late time acceleration period with a large scale hierarchy, on the same time also allowing for a radiation and matter domination era, as well as recovery of classical GR in solar system. I will then explain how the production of primordial fluctuations can pose a challenge for such a scenario, discussing also ongoing work towards this direction.
10 Jul 2013 17:00 - 18:00 Alessio Notari (Universitat de Barcelona) - "CMB Aberration and Doppler Effects as a Source of Hemispherical Asymmetries"
03 Jul 2013 17:00 - 18:00 Miguel Quartin (Universidade Federal do Rio de Janeiro) - "Measuring sigma_8 with Weak Lensing of Supernovae"
Soon the number of type Ia supernovae (SNe) measurements should surpass 10^5. Understanding weak lensing effects in these objects will then be more important than ever. Although SNe lensing is usually seen as a source of systematic noise in this talk I will show how this noise can be in fact turned into signal. To accomplish this I will first describe how we were able to accurately model the lensing effects and provide simple analytical fits to describe it. I will then use these fits to show that the non-gaussianity introduced by lensing in the SNe Hubble diagram dispersion is basically modulated by Omega_m0 and sigma_8. Finally, I will argue that the modelling of such non-gaussianity allows for an independent measurement also of sigma_8 with supernova data.
27 Jun 2013 17:30 - 18:30 Silvia Galli (IAP, Paris) - "Shedding light on fundamental physics with Planck"
The universe as pictured by the recent data release of the Planck satellite is remarkably “simple”, well described by a LCDM model (apart from few hints of anomalies). However, the nature of some of the basic components of this model, i.e. of dark matter and dark energy, is still unknown and represents one of the major puzzles of modern cosmology and modern physics; it might even be the hint that currently known physics is not enough to describe our universe. In this talk, I will propose two ways to investigate these problems with the Planck data. First, I will show how the CMB is an extremely powerful tool to constrain fundamental characteristics of dark matter particles, as it strongly constrains dark matter annihilation. Annihilation is a standard prediction of many popular current dark matter models, and it is one of the candidates for the interpretation of the anomalies detected in cosmic and gamma ray data by several experiments such as Pamela, AMS02 and Fermi. I will review the general ideas behind searches of dark matter annihilation with the CMB and present the most recent developments in the field. Second (time permitting), I will show how the detection of hundreds of new galaxy clusters through the Sunyaev-Zel'dovich effect by Planck and other ongoing experiments is a potentially powerful new probe of fundamental physics. In particular, the x-ray and SZ observations of these objects can be used to test the value of fundamental constants, i.e. to test the validity of currently known physics, at redshifts z< 1. I will show that current data can constrain the value of the fine structure constant at the level of 0.8%, comparable to CMB constraints.
19 Jun 2013 17:00 - 18:00 Mustafa Amin (KICC, Cambridge) - "Lumps and bumps in the early (and late) universe"
Our understanding of the universe between the end of inflation and production of light elements is incomplete. How did inflation end? What did the universe look like at the end of inflation? In this talk, I will discuss different end of inflation scenarios. I will concentrate on a particular case: the fragmentation of the inflaton into localized, long-lived excitations of the inflaton field (oscillons), which can end up dominating the energy density of the universe. I will argue that oscillons can be produced in a large class of theoretically well-motivated and observationally consistent inflationary models. I will provide conditions for their existence, emergence and stability. Based on some ongoing work, I will also introduce a novel, analytic approach to understanding the interactions of nonlinear soliton-like objects. Finally, I will discuss some theoretical and observational consequences of different end of inflation scenarios.
12 Jun 2013 17:00 - 18:00 Martin Crocce (Institute for Space Science (IEEC/CSIC), Barcelona) - "Nonlinear clustering from theory and simulations"
Present and upcoming galaxy surveys require an unprecedented ability to describe nonlinear physics happening on large-scales and limiting key observational probes for Cosmic acceleration, such as the baryon acoustic oscillations or redshift space distortions. I will discuss advances in the field of perturbation theory happened over the past years that can help match this new standard. And the prospects for turning them into practical tools for interpreting observational data. In addition, a unprecedented effort to develop large numerical simulations is needed and undergoing. I will also discuss advances in this field as well as numerical requirements imposed by the so called "precision cosmology" era.
10 Jun 2013 16:00 - 17:00 Erminia Calabrese (University of Oxford) - "Small scale CMB from the Atacama Cosmology Telescope"
The Atacama Cosmology Telescope has measured the CMB angular power spectra to arcminute scales at frequencies of 148 and 218 GHz, from 3 seasons of data. At small scales the fluctuations of the primordial CMB become increasingly obscured by extragalactic and secondary signals. We build a multi-frequency likelihood to describe the data including four late-time astrophysical components: thermal and kinetic SZ, emission from CIB galaxies, and emission from radio galaxies. Modeling these foregrounds emission allow us to probe the primordial CMB down to an angular resolution of 4 arcminutes. The accurate ACT foreground characterization is moreover extremely useful in cleaning the primary high resolution CMB measurements from the Planck satellite.
05 Jun 2013 17:00 - 18:00 There will be no seminar () - ""
04 Jun 2013 13:00 - 14:00 Arpine Piloyan (Yerevan State University, Armenia) - "Testing multi-dark-energy models with Supernovae Ia"
[Unusual location: Room 204, Phil12] The recently proposed model of multi-coupled Dark Energy (Baldi 2012) brings new effects in cosmology. We present a complete analysis of critical points and the first comparison of this model with supernovae Type Ia. Our analysis shows a very good agreement with SN data for a wide range of couplings: even a coupling two orders of magnitude larger than gravity is not excluded, a much larger range than in typical coupled dark energy models.
29 May 2013 17:00 - 18:00 Thorsten Moos (FEST, Heidelberg) - "Symbol theory and religion in the context of modern physics"
"We can only speak in metaphors and analogies" (Werner Heisenberg). Many modern physicists have been dealing with the issue of religion. How can religion be understood from a physicist's perspective? Atheists and theists among them have published their opinions about God, life, the universe and everything in essays, talks, and books. Some try to short-circuit physical results to religious confessions, such as Frank Tipler. He tried to show that Christian dogmatics, especially the issues concerning resurrection, could be proven by means of physical theories and computer technology. Usually then, laughter and bashing come from both sides, and rightly so. In a more promising approach, others such as Werner Heisenberg deal with the specific epistemological status of physical and religious statements. Heisenberg's considerations will be scrutinized in the talk by Thorsten Moos, physicist and theologian, from the Protestant Institute for Interdisciplinary Research (FEST), Heidelberg.
27 May 2013 16:00 - 17:00 Emilio Bellini (ITP Heidelberg and Padua University) - "The Dark Matter Bispectrum in Galileon cosmologies"
Galileon cosmologies offer a good description of the late-time cosmic acceleration. However, we expect that the new degree of freedom affects the large-scale structure of the universe. Thus, the Dark Matter Bispectrum generated by gravitational instability is an important tool in order to understand how the growth of structures is modified w.r.t. the LCDM model. In this talk I will summarize the results obtained by means of a semi-analytic approach to second-order perturbations in the context of the cubic Galileon theory, assuming Gaussian initial conditions. In particular, I will show that, even in the presence of large deviations of the linear growth-rate w.r.t. the LCDM one, at the bispectrum level such deviations are reduced to a few percent. Then, I will generalize the results to the coupled Galileon theory.
22 May 2013 17:00 - 18:00 Christian Byrnes (University of Sussex) - "Non-Gaussianity after Planck"
Testing the Gaussianity of the primordial perturbations provides a window on the early universe. Despite not detecting any new non-zero parameters, the Planck results have provided the most accurate measurements ever about the physics of the early universe. I will reflect on these precision results and their implications for inflation, as well as the prospects for future measurements. In particular, I will focus on the implications of the results on non-Gaussianity, and what they might imply for the many other signatures that have not yet been searched for. Finally I will discuss how one may probe the small scale perturbations. Even in the "precision era" of cosmology, measurements of the cosmic microwave background and large scale structure only cover around three orders of magnitude in length scales. Another 30 orders of magnitude in length scales must also have exited the horizon during inflation, but these smaller scales remain unobserved. Primordial Black Holes (PBHs) can form in the early universe from the collapse of large density fluctuations. Tight observational limits on their abundance provide the best upper limit on the size of the primordial fluctuations on small scales, with PBHs only forming from extremely large and rare density fluctuations. The number of PBHs formed is therefore highly sensitive to small changes in the tail of the fluctuations probability density function, which probes non-Gaussianity.
15 May 2013 17:00 - 18:00 There will be no seminar () - ""
08 May 2013 17:00 - 18:00 Maria Archidiacono (Department of Physics and Astronomy, Aarhus) - "Neutrino and Dark Radiation properties in light of recent CMB observations"
Neutrinos deeply affect the cosmological observables, such as the Cosmic Microwave Background and the power spectrum of matter fluctuations. Thanks to these fingerprints cosmology can constrain the absolute neutrino mass scale and the cosmic neutrino background. In the past years cosmology has also provided some hints for a non standard value of the effective number of relativistic degrees of freedom, pointing towards the existence of an extra dark component of the radiation content of the Universe. In my talk I will review recent cosmological constraints on neutrino and dark radiation properties. Indeed recent CMB measurements at high multipoles from the South Pole Telescope and from the Atacama Cosmology Telescope seem to disagree in their conclusions for the neutrino and dark radiation properties. I will discuss how this tension can be alleviated by adding external data sets or extending the cosmological model. Finally I will interpret the Planck results concerning neutrino and dark radiation properties in light of these considerations.
24 Apr 2013 17:00 - 18:00 There will be no seminar () - ""
17 Apr 2013 17:00 - 18:00 Matteo Martinelli (SISSA, Trieste) - "Observational probes for Dark Energy"
In this talk I will present my current work topic, i.e. methods to constrain dynamic Dark Energy and possible deviation from the cosmological constant equation of state. In particular I will focus on expected results using redshift-drift (also known asSandage-Loeb test) and on how these observations, performed in the future by CODEX experiment, will be able to break degeneracies between Dark Energy parameters and other cosmological quantities. Moreover I will introduce results obtained with current data using a model indipendent method, i.e. binning the equation of state parameter w in redshift intervals, showing how this results give some hints for a possible tension between data and possible alternatives to the cosmological constant.
10 Apr 2013 17:00 - 18:00 Rhiannon Gwyn (MPGP, Potsdam) - "Degeneracies between canonical and non-canonical inflation"
Non-canonical inflationary models are characterized by higher powers of the standard kinetic term X in the effective Lagrangian p(X,φ) and arise for instance in the context of the DBI action in string theory. We look for potential observational degeneracies between canonical and non-canonical models of inflation of a single field. An on-shell transformation is introduced that transforms non-canonical inflationary theories to theories with a canonical kinetic term. The 2-point function observables of the original non-canonical theory and its canonical transform are found to match in the case of DBI inflation. Separately, at the 3-pt function level we show that multiple sources of resonant NG, which arise in axion inflation for instance, can result in equilateral-type nongaussianity even for a single scalar field with canonical kinetic term. (arxiv: 1211.0070 and 1212.4135)
26 Mar 2013 17:00 - 18:00 Thomas Kitching (University of Edinburgh) - "Weak Lensing : past, present and future"
I will present the state-of-the-art in weak lensing data analysis, showing the results from the ground-based CFHT Legacy Survey covering 154 square degrees to redshifts of 3. Looking towards the future I will discuss the space-based Euclid survey that covers 100 times the area of CFHTLenS whose aims are to determine the nature of dark energy, dark matter and nature of gravity on cosmic scales.
25 Mar 2013 16:00 - 17:00 Julien Lesgourgues (CERN and Lausanne) - "Planck results and implications for cosmology"
I will present a summary of the new CMB results released by the Planck collaboration, focusing mainly on their implications for theoretical cosmology.
13 Mar 2013 17:00 - 18:00 There will be no seminar () - ""
06 Mar 2013 17:00 - 18:00 There will be no seminar () - ""
27 Feb 2013 17:00 - 18:00 There will be no seminar () - ""
20 Feb 2013 17:00 - 18:00 There will be no seminar () - ""
13 Feb 2013 17:00 - 18:00 There will be no seminar () - ""
06 Feb 2013 17:00 - 18:00 There will be no seminar () - ""
30 Jan 2013 17:00 - 18:00 There will be no seminar () - ""
29 Jan 2013 15:00 - 16:00 Eloisa Menegoni (ITP, Heidelberg) - "Cosmological constraints on variations of fundamental constants from CMB data"
There is ample experimental evidence showing that fundamental couplings run with energy, and many particle physics and cosmology models suggest that they should also roll with time. In this talk I will show how a time varying fine structure constant can leave an imprint on CMB anisotropies by changing the time of recombination and the size of the acoustic horizon at photon-electron decoupling. The CMB datasets have been extensively used to constrain alpha by parametrizing a variation in the fine structure constant as Delta alpha = (alpha - alpha_0)/alpha_0, where alpha_0 =1/137.03599907 is the standard, local, value and alpha is the value during the recombination process. I performed a Monte Carlo Markov of Chain analysis using WMAP-5 years data and I will show the results. The interesting point is that the CMB is an observable potentially sensitive to variations also in both alpha and G. It is therefore interesting to perform a combined analysis of CMB data considering simultaneous variations in alpha and the gravitational constant in order to investigate the possible correlations and deviations from the standard values. In order to take tighter constraint I used CMB data and I included also the Big Bang Nucleosynthesis data, and I will show the results of my analysis. I consider also the possibility of an early dark energy (EDE) component and its implications for fundamental couplings. The presence of a scalar field at recombination could induce variations in the fine structure constant. Searching for relations in the variations of the fine structure constant and a non-negligible scalar field at recombination, it is possible to describe the scalar field with a EDE model, where the dark energy density parameter and equation of state are parametrized in the way to be coupled. I modified the CAMB code for early dark energy including the variations of the fine structure constant using WMAP-7 years data and HST data. Using Planck and CMBPol experimental specifications I found better and strong constraints on the parameter. Moreover, forthcoming or future missions, such as Planck Survey or and CMBPol, can allow to see new scenarios on constraints of fundamental physics. Finally I will show the ability of future weak lensing surveys, as those expected from the Euclid satellite experiment, to constrain variation in fundamental constants. This procedure involves a simple Fisher Matrix analysis.
23 Jan 2013 17:00 - 18:00 Subodh Patil (CERN) - "Heavy fields, Decoupling and the Effective theory of Inflation"
Heavy fields can play a surprisingly large role in the dynamics of the adiabatic mode during inflation, consistent with the validity of an effective single field description. In this talk, we will review recent developments in the effective theory (EFT) of inflation, offer a brief tour of the two main approaches, and along the way point out various subtleties of how decoupling is operative on time dependent backgrounds. In the context of a two field system (with one field much more massive than the other) we will derive the EFT for the light degrees of freedom and show how heavy fields can be excited whenever the background trajectory `bends' sufficiently in field space. This occurs even when the mass of the heavy field is several orders of magnitude larger than the Hubble scale during inflation, entirely consistent with slow roll and the decoupling of the true high energy modes of the system. At the level of the EFT this is due to certain operators briefly transmuting into relevance, the result of which imprints features in the power spectrum and can generate a scale dependent non-gaussianity that peaks for equilateral shapes. We comment on the prospects seeing any such imprints of higher dimensional operators in future CMB and LSS observations.
16 Jan 2013 17:00 - 18:00 Riccardo Catena (Goettingen University) - "Cosmological parameter estimation: impact of CMB aberration and Doppler"
The peculiar motion of an observer with respect to the CMB rest frame induces an apparent deflection of the observed CMB photons, i.e. aberration, and a shift in their frequency, i.e. Doppler effect. Both effects distort the temperature multipoles via a mixing matrix at any multipole l. The common lore when performing a CMB based cosmological parameter estimation is to consider that Doppler affects only the l = 1 multipole, and neglect any other corrections. In this talk I will reconsider the validity of this assumption, showing that it is actually not robust when sky cuts are included to model CMB foreground contaminations. Assuming a simple fiducial cosmological model with five parameters, we simulated CMB temperature maps of the sky in a WMAP-like and in a Planck-like experiment and added aberration and Doppler effects to the maps. We then analyzed with CosmoMC these maps (with and without including aberration and Doppler effects). We find that, depending on the specific realization of the simulated data, the parameters can be biased up to one standard deviation for WMAP and almost two standard deviations for Planck when the data are analyzed with the wrong theoretical assumption of zero peculiar velocity. Therefore we conclude that in general it is not a solid assumption to neglect aberration and Doppler effects in a CMB based cosmological parameter estimation.
09 Jan 2013 17:00 - 18:00 Wessel Valkenburg (Heidelberg, ITP) - "A gradient expansion"
I will discuss a gradient expansion as an ansatz for solving the Einstein equations in a perturbed cosmology. After relating it to Lagrangian perturbation theory, I will show how this approach could be used to solve for perturbation theory in many inhomogeneous models.
12 Dec 2012 17:00 - 18:00 There will be no seminar () - ""
05 Dec 2012 17:00 - 18:00 Andrea Maccio' (MPIA, Heidelberg) - "Some like it Warm"
Observational data on matter distribution on galactic and subgalactic scales seem to challenge the currently favored Cold Dark Matter model and possibly indicate the need for a WARM dark matter component. I will present high resolution numerical simulations for Warm Dark Matter (WDM) cosmologies to critically address the issue of WDM can and cannot do to solve/ameliorate problems on small scales. I will also show that on such scales the effects of ordinary matter (baryons) cannot be neglected and must be taken into account when extracting cosmological constraints on the nature of dark matter. I will conclude summarizing current and forthcoming astrophysical constraints on the "temperature" of the DM.
28 Nov 2012 17:00 - 18:00 There will be no seminar () - ""
21 Nov 2012 17:00 - 18:00 Fabio Fontanot (HITS, Heidelberg) - "AGN/Galaxy co-evolution: using observations to constraint our theoretical perspective"
In recent years, a number of observational constraints on the AGN/galaxy co-evolution has become available, both locally (e.g. the distribution of AGN classes as a function of host stellar mass and parent dark matter halo mass) and at higher redshifts (e.g. the differential redshift evolution of the AGN luminosity function, or "AGN downsizing"): in my talk I will show that it is possible to compare those results with the predictions of theoretical models, in order to get strong insight in our understanding of these phenomena. I will first review the different theoretical approaches to the problem of co-evolution between Active galactic nuclei (AGN) and galaxies, with particular emphasis on the different paradigms describing gas accretion onto super massive black holes and their impact on the properties of their host galaxies. I will thus show that AGN feedback is nowadays a crucial ingredient for models of galaxy and AGN formation and evolution, not only to explain the redshift evolution of the AGN population but also for setting up a variety of observed host galaxy properties, such as the evolution of their stellar masses and their star formation levels. I will then extensively discuss the different "modes" of AGN feedback, their triggering mechanisms and relate them to the physical condition of the host galaxies. Finally, I will discuss the main successes and failures of the current paradigm for AGN/galaxy co-evolution and introduce possible improvements.
19 Nov 2012 17:30 - 18:30 Mariano Cadoni (Cagliari University, Italy) - "Holographic duals of scalar black branes"
Black brane solutions with scalar hair and with AdS and non-AdS asymptotics are very interesting for holographic applications. I will describe their most important features and their application for reproducing, holographically condensed matter-like behaviour such as phase transitions, superconductivity, exotic metals and hyperscaling violation.
14 Nov 2012 17:15 - 18:15 Camille Bonvin (Cambridge U.) - "Testing General Relativity with 21cm intensity mapping"
I will discuss the potential of a new observational technique, called 21cm intensity mapping, to constrain modified theories of gravity. I will first derive the relation between the observable fluctuations in the 21cm brightness temperature and the underlying distribution of matter. I will show that in linear perturbation theory the temperature fluctuations are affected by the matter density fluctuations, redshift-space distortion as well as various relativistic effects. I will then discuss the prospects for testing deviations from General Relativity at large scale with this new signal.
07 Nov 2012 17:00 - 18:00 Bjoern Malte Schaefer (Centre for Astronomy, Heidelberg) - "Squeezing information out of weak lensing surveys"
Future weak lensing surveys are regarded as a source of information about cosmological parameters with a similar precision as the cosmic microwave background anisotropies. in particular tomographic surveys harness the full power of weak lensing data by splitting the galaxy sample in redshift slices. I will explain the sensitivity of tomographic weak lensing surveys in parameter estimation, and introduce a data weighting method based on orthogonal polynomials which are designed to access statistically independent data and to optimise parameter estimation. I compare this method to standard tomography and to 3-dimensional weak lensing.
24 Oct 2012 17:00 - 18:00 There will be no seminar () - ""
10 Oct 2012 17:30 - 18:30 Phil Bull (Oxford U) - "Measures of acceleration in cosmology"
A wide range of cosmological observations imply that the expansion of the Universe is accelerating -- if they are interpreted within a homogeneous and isotropic "FLRW" model. On considering inhomogeneous models, however, we find that "acceleration" becomes a rather ambiguous concept. In this talk, I describe how the question of whether spacetime is accelerating depends on what measure of acceleration is being used, and explain how this affects attempts to solve the dark energy problem.
02 Oct 2012 10:00 - 11:00 Juliane Behrend (ITP, Utrecht) - "Einstein-Cartan Theory as an Averaged Theory of Gravity"
In this talk I will show that the construction of a macroscopic theory of gravity for a corpuscular medium along the lines of classical electromagnetism naturally leads to an Einstein-Cartan theory. This theory is assumed to describe the universe on large scales.
01 Oct 2012 10:00 - 11:00 Alessio Notari (Universitat de Barcelona) - "The Higgs mass range and Inflationary models"
For a narrow band of values of the top quark and Higgs boson masses, the Standard Model Higgs potential can develop a false minimum at energies of about 10^15-10^16 GeV, where primordial Inflation could have happened. I will present two mechanisms to achieve a successful transition from Inflation to a radiation dominated era: by adding a Brans-Dicke scalar to gravity or by introducing a hybrid model with a new scalar very weakly coupled to the Higgs. In both cases this can happen only for a narrow window of values for the Higgs mass, of about (126 +- 3) GeV, which has already coincided with the hint of the Higgs boson detection, as reported very recently by the LHC. We discuss the features of the models and point out that they can be further checked with: more precise measurements of the top quark mass, improvement of theoretical uncertainties on the Standard Model RGE equations, and by cosmological observables (especially the tensor-to-scalar ratio) by forthcoming experiments, such as Planck.
26 Sep 2012 17:00 - 18:00 Iain Brown (University of Oslo) - "Ambiguities in Cosmological Averaging"
The issue of averaging in cosmology has received significant interest in the last decade in the hope that corrections to the Hubble rate from inhomogeneities in the recent universe could behave as a dark energy. However, an averaging procedure is sensibly defined only in a volume-preserving coordinate system. An evaluation of the corrections also naturally requires a definition of the Hubble rate. We construct two (comoving) volume-preserving coordinate systems and analyse the corrections to two common definitions of the Hubble rate.
19 Sep 2012 17:00 - 18:00 Shinji Tsujikawa (Tokyo University of Science) - "Discrimination between theoretical models of dark energy from recent observations"
We discriminate between theoretical models of dark energy from recent observations. We first place constraints on dark energy models from the background cosmology by using the data of supernovae Ia, cosmic microwave background, and baryon acoustic oscillations. We further show that the recent measurement of the peculiar velocity of galaxies is powerful to discriminate between models based on modified gravitational theories.
18 Jul 2012 17:00 - 18:00 Gennady Y. Chitov (Laurentian University) - "Quintessence and Majorana Neutrinos: Proposal for Unification of the Dark Sector of the Universe"
The origin of the neutrino mass, dark matter (DM), and dark energy (DE) are among the most challenging problems of fundamental physics. We address these questions from analyses of the models where the neutrino mass is generated via Yukawa coupling to the quintessence field which represents the DE. It has been shown in a recent work on the model with a single Dirac fermion coupled to the quintessence that by choosing parameters of the DE potential to match the present DE density, the model allows to lock the neutrino mass at $m \sim 0.01$ eV and yields consistent estimates for other parameters of the Universe. To include the DM component into this framework we propose to add the right-handed Majorana term(s) into Lagrangian with the quintessence-generated mass. As a results, the DE field is responsible for generation of the masses of the light active Majorana neutrinos as well as of the heavy sterile neutrinos. The latter are natural DM candidates.
11 Jul 2012 17:00 - 18:00 Gaveshna Gupta (Center For Theoretical Physics, New Delhi) - "Dark side of the Universe and its observational signature"
We look at observational constraints on the thawing class of scalar field models proposed to explain the late time acceleration of the universe. Using the recently introduced ‘Statefinder Hierarchy’, we compare these thawing class of models with other widely studied dark energy (and modified gravity) models to check the underlying parameter degeneracies. We put constraints on the deviations of these thawing models from the canonical ΛCDM model using a large class of observational data, e.g, the Supernova Type Ia data, the BAO data, the CMB data and data from the measurements of the Hubble parameter using red-envelope galaxies. We also forecast constraints using a simulated dataset for the future JDEM SNe survey. Our study shows that, although with current data it is difficult to distinguish different thawing models from ΛCDM, a future JDEM like mission would be able tell apart thawing models from ΛCDM for currently acceptable values of Ωm0 We also study the linear growth function f for large scale structures in a cosmological scenario where Generalised Chaplygin Gas (GCG) serves as dark energy candidate. We parametrize the growth index parameter as a function of redshift and do a comparative study between the theoretical growth rate and the proposed parametrization. Moreover, we demonstrate that growth rates for a wide range of dark energy models can be modeled accurately by our proposed parametrization. By fitting our proposed parametrization for f to growth data, we show that growth history of large scale structures of the universe although allows a transient acceleration, one cannot distinguish it at present with an eternally accelerating universe.
04 Jul 2012 17:00 - 18:00 Amna Ali (Saha Institute of Nuclear Physics, India) - "The Expanding Universe"
An important challenge of cosmology is to understand the late time acceleration of universe. High precision cosmological observations in last decade suggest that about 73% of our universe’s energy density is in so called “Dark Energy.There are various models to describe dark energy. One of them is through Scalar fields. In my talk I will discuss about the Tachyon Dark Energy. Tha Observational constraint on the model. The another way to explain the current epoch of accelerated is by modifying gravity at large scales. In the past few years, several schemes of large scale modifications have been actively investigated.I will also discuss about the F(R) gravity model and Galileon gravity.
22 Jun 2012 11:30 - 12:30 Barbara Sartoris (University of Trieste) - "Constraining cosmological models with surveys of galaxy clusters"
20 Jun 2012 17:00 - 18:00 Savvas Nesseris (Universidad Autónoma de Madrid) - "A new perspective on Dark Energy modeling via Genetic Algorithms"
We use Genetic Algorithms to extract information from several cosmological probes, such as the type Ia supernovae (SnIa), the Baryon Acoustic Oscillations (BAO) and the growth rate of matter perturbations. This information consists of a model independent and bias-free reconstruction of the various scales and distances that characterize the data, like the luminosity $d_L(z)$ and the angular diameter distance $d_A(z)$ in the SnIa and BAO data, respectively, or the dependence with redshift of the matter density $\om_m(a)$ in the growth rate data, $f\sigma_8(z)$. This information can then be used to reconstruct the expansion history of the Universe, and the resulting Dark Energy (DE) equation of state $w(z)$ in the context of FRW models, or the mass radial function $\om_M(r)$ in LTB models. In this way, the reconstruction is completely independent of our prior bias. Furthermore, we use this method to test the Etherington relation, ie the well-known relation between the luminosity and the angular diameter distance, $\eta \equiv \frac{d_L(z)}{(1+z)^2 d_A(z)}$, which is equal to 1 in metric theories of gravity. We find that the present data seem to suggest a 3-$\sigma$ deviation from one at redshifts $z\sim 0.5$.
13 Jun 2012 17:00 - 18:00 Ignacy Sawicki (ITP, Heidelberg) - "How I learned to stop worrying and love scalar fields"
In the end, in modelling dark energy and modified gravity, cosmologists usually reach for the same trusty tool in the toolbox: the scalar field. Both classes of models can replicate background expansion histories for the universe that are arbitrarily close to LCDM, but can give different predictions for the evolution of perturbations. However, the approach to the study of these cosmological perturbations is completely different. Dark energy is usually modelled through hydrodynamics. Modified-gravity predictions are usually based on modifying the Poisson equation for gravity. Why the difference if a scalar equation of motion underlies both? I will show that hydrodynamics is the correct description only in some limits while modifications of gravity are a somewhat counterintuitively the result of the clustering of the scalar field. I will discuss a unified framework for treating this whole spectrum of models valid for any theory of the accelerating sector comprising a single scalar.
30 May 2012 17:00 - 18:00 Wessel Valkenburg (ITP Heidelberg) - "Taking into account that the homogenous universe is not homogenous: the Dark Energy equation of state for an ignorant observer"
The Lambda-CDM universe it not what it seems. I will discuss how one can take into account that an average observer in a standard inflationary universe does not live in a homogeneous fluid, but in structures in stead. Using the LTB metric, I show that structures that are common in standard cosmology, do affect our perception of global parameters such as the equation of state of Dark Energy and the age of the universe.
16 May 2012 17:00 - 18:00 Shaun Hotchkiss (Helsinki Institute of Physics) - "Is the integrated Sachs-Wolfe effect from super-structures still a problem for LambdaCDM?"
In any region of the universe where the gravitational potential decays over time large-scale structure will produce a redshift in the cosmic microwave background. This is known as the integrated Sachs-Wolfe (ISW) effect. A cosmological constant causes such a decay and as such the ISW effect is an important consistency test of the standard cosmological model. The ISW imprint of very large (super) structures has reportedly been detected with >4\sigma significance. I will discuss this observation and the LambdaCDM prediction for it. It is seen that the observed signal is >3-sigma larger than expectations. No solution to this 'ISW mystery' is yet known; however I will discuss some interesting directions worth further exploration.
09 May 2012 17:00 - 18:00 Stefan Gottloeber (Leibniz-Institut für Astrophysik Potsdam) - "Near field cosmology with CLUES (Constrained Local UniversE Simulations)"
During the last decade our understanding of the formation of structure in the universe grew substantially. Due to the non-linear nature of the gravitational dynamics and the complicated gas-astrophysical processes numerical simulations have been the driving force behind much of this theoretical progress. Cosmological simulations must cover a large dynamical and mass range. A representative volume of the universe should be large, but this comes at the expense of the resolution. To overcome this problem we have developed a new approach which consists of using observations of the nearby universe as constraints imposed on the initial conditions of the simulations. The resulting constrained simulations successfully reproduce the observed structure within a few tens of megaparsecs around the Milky Way. I will discuss the formation of the Local Group and the Local Volume based on a series of simulations performed within the CLUES project - Constrained Local UniversE Simulations (
25 Apr 2012 11:00 - 12:00 Marco Baldi (Excellence Cluster Universe) - "Multiple Dark Matter and dark sector interactions"
Present cosmological constraints and the absence of a direct detection and identification of any dark matter particle candidate leave room to the possibility that the dark sector of the Universe be actually more complex than it is usually assumed. In particular, more than one new fundamental particle could be responsible for the observed dark matter density in the Universe, and possible new interactions between dark energy and dark matter might characterize the dark sector. I will discuss the possibility that two dark matter particles exist in nature, with identical physical properties except for the sign of their coupling constant to dark energy. I will describe the main features of such Multiple Dark Matter scenario on the background and linear perturbations evolution of the Universe. Finally, I will move to the non-linear regime of structure formation by presenting the results of the first N-body simulations ever performed for this kind of cosmological models.
28 Mar 2012 17:00 - 18:00 Giovanni Marozzi (College de France) - "The light-cone averaging and the luminosity-redshift relation"
I will show a general gauge invariant formalism for defining cosmological averages that are relevant for observations based on light-like signals. Such averages involve either null hypersurfaces corresponding to a family of past light-cones or compact surfaces given by their intersection with timelike hypersurfaces. Afterwards, using such formalism, together with adapted "geodesic light-cone" coordinates, I will show as induced backreaction effect emerges from correlated fluctuations in the luminosity distance and covariant integration measure. Considering a realistic stochastic spectrum of inhomogeneities of primordial (inflationary) origin, we find that the induced backreaction on the luminosity-redshift relation is larger than naively expected but by itself cannot account for the observed effects of dark energy at large-redshifts. A full second-order calculation, or even better a reliable estimate of contributions from the non-linear regime, appears to be necessary before firm conclusions on the correct interpretation of the data can be drawn.
21 Mar 2012 17:00 - 18:00 Gerasimos Rigopoulos (RWTH Aachen) - "A non-linear approximation for cosmological inhomogeneities"
Perturbation theory has been a powerful and extremely valuable tool for confronting cosmological models with the real Universe. However, it is naturally limited by the regime of non-linear structure formation. I will describe an alternative approximation technique, based on an expansion in spatial gradients that can probe the evolution of perturbations deeper into the non-linear regime. Two applications will be given: The evolution of perturbations in LCDM and the calculation of backreaction of cosmological inhomogeneities. I will also discuss the possible role of this technique for the study of dark energy and its potential relevance for precision cosmology.
14 Mar 2012 17:00 - 18:00 Maik Weber (ITP Heidelberg) - "Simulating Growing Neutrino Quintessence"
Growing Neutrino Quintessence is a model of dynamical dark energy coupled to neutrinos which provides a solution to the "why now" problem of dark energy. Its dynamics include a strong attractive force between neutrinos leading to a rapid growth of neutrino perturbations in recent cosmological times. As a consequence, linear perturbation theory breaks down. A full understanding of the non-linear evolution, while essential for testing the validity of the model, is still lacking. In this talk, I will present a specific simulation method adjusted to the features of the model. At its current stage, the method is successful until redshift z=1 and already reveals a rich phenomenology. I will show some illustrative results including the formation of large-scale neutrino structures as well as their impact on the evolution of matter perturbations and the overall cosmology.
07 Mar 2012 17:00 - 18:00 Dieter Gromes (ITP Heidelberg) - "Optimal covariant fitting to a Robertson-Walker metric and smallness of backreaction"
We define a class of optimal coordinate systems by requiring that the deviation from an exact Robertson-Walker metric is as small as possible within a given four dimensional volume. The optimization is performed by minimizing several volume integrals which would vanish for an exact Robertson-Walker metric. Covariance is automatic. Foliation of space-time is part of the optimization procedure. Only the metric is involved in the procedure no assumptions about the origin of the energy-momentum tensor are needed. A scale factor does not show up during the optimization process the optimal scale factor is determined at the end. The general formulation is non perturbative. An explicit perturbative treatment is possible. The shifts which lead to the optimal coordinates obey Euler-Lagrange equations which are formulated and solved in first order of the perturbation. The extension to second order is sketched but turns out to be unnecessary. The only freedom in the choice of coordinates which finally remains are the rigid transformations which keep the form of the Robertson-Walker metric intact i.e. translations in space and time spatial rotations and spatial scaling. Spatial averaging becomes trivial. In first order of the perturbation there is no backreaction. A simplified second order treatment results in a very small effect excluding the possibility to mimic dark energy from backreaction. This confirms (as well as contradicts) statements in the literature.
29 Feb 2012 17:00 - 18:00 Jean-Sebastien Gagnon (ITP Heidelberg) - "Dark goo: Bulk viscosity as an alternative to dark energy"
We present a simple (microscopic) model in which bulk viscosity plays a role in explaining the present acceleration of the universe. The effect of bulk viscosity on the Friedmann equations is to turn the pressure into an ``effective'' pressure containing the bulk viscosity. For a sufficiently large bulk viscosity, the effective pressure becomes negative and could mimic a dark energy equation of state. Our microscopic model includes self- interacting spin-zero particles (for which the bulk viscosity is known) that are added to the usual energy content of the universe. In the first part of this talk, we present results coming from the study of both background equations and linear perturbations in this model. We show that a dark energy behavior is obtained for reasonable values of the two parameters of the model (i.e. the mass and coupling of the spin-zero particles) and that linear perturbations are well-behaved. In the second part, we discuss in some details the assumptions underlying this model, in particular the conditions under which hydrodynamics holds.
22 Feb 2012 17:00 - 18:00 Tommaso Giannantonio (Excellence Cluster Universe Munich) - "Constraining dark energy and primordial non-Gaussianity with the CMB and the large-scale structure"
The integrated Sachs-Wolfe (ISW) effect is a component of the CMB anisotropies produced at late times in the presence of dark energy or curvature. It can be measured by correlating the CMB with the large-scale structure (LSS) of the Universe. Additionally, the observed auto- and cross-correlations of the LSS depend on the galactic bias, which is altered in the presence of primordial non-Gaussianity: this is a key signature of the physics of the early universe. I will present an updated compilation of the ISW effect obtained by measuring the 2-point functions between the CMB and galaxy catalogues. The galaxy data have been extended to the latest data release of the SDSS, and the CMB data to WMAP7. I will discuss an extended analysis of systematics, commenting on related recent works. I shall then show cosmological results. In particular, by combining the ISW data with the complete set of galaxy auto- and cross-correlations, it is also possible to measure the galaxy bias of these catalogues, and through this we constrain the amount of primordial non-Gaussianity.
15 Feb 2012 17:00 - 18:00 Diego Blas (CERN) - "Technically natural dark energy from Lorentz breaking"
I will present a model of dark energy with a technically natural small contribution to cosmic acceleration. The proposed acceleration mechanism appears generically in the low-energy limit of gravitational theories with violation of Lorentz invariance that contain a derivatively coupled scalar field. Furthermore, the model is a valid effective field theory up to a high cutoff. Even if the expansion history of the Universe is essentially indistinguishable from that of ΛCDM, cosmological perturbations allow to discriminate between both models. I'll show how the matter power spectrum is enhanced at subhorizon scales for realistic parameters, which may be tested with current cosmological data.
08 Feb 2012 17:00 - 18:00 Tomi Koivisto (Oslo U) - "C-gravity"
New approaches to gravitational theories are presented: bimetric variational principle and C-theories. Assuming the spacetime connection to be generated by an independent metric extends general relativity by adding nonmetric degrees of freedom and propagating torsion. The C-theories, where the connection is conformally metric, unify Einstein and Palatini gravity and contain also new viable theories. Stability, PPN limit and some cosmological implications are discussed.
01 Feb 2012 17:00 - 18:00 Miguel Quartin (UFRJ Rio de Janeiro) - "Measuring our Peculiar Velocity by Pre-deboosting the CMB"
It was recently shown that our peculiar velocity beta with respect to the CMB induces mixing among multipoles and off-diagonal correlations at all scales which can be used as a measurement of beta which is independent of the standard measurement using the CMB temperature dipole. The proposed techniques rely however on a perturbative expansion which breaks down for l > ~ 800. Here we propose a technique which consists of deboosting the CMB temperature in the time-ordered data and show that it extends the validity of the perturbation analysis multipoles up to l ~ 10000. We also obtain accurate fitting functions for the mixing between multipoles valid in a full non-linear treatment. Finally we forecast the achievable precision with which these correlations can be measured in a number of current and future CMB missions. We show that ACTPol could measure in 4 years the velocity with a precision of around 70 km/s Planck around 60 km/s while proposed future experiments could shrink this error bar by over a factor of 2.
25 Jan 2012 17:00 - 18:00 Ion Stamatescu (ITP Heidelberg) - "The Philosophy of Physicists - Is Physics Philosophy-able?"
Physicists have often felt compelled to philosophise about physics, and their contribution to the discussion is typically unorthodox but relevant at the same time: they are not philosophically accurate but they know what the physical problems are. A modern history of this involvement will be sketched, followed by an exemplification of the problems one is confronted with in raising epistemological questions. A brief overview of the philosophy of science perspective will then be given together with a hint on the present day discussion.
18 Jan 2012 17:00 - 18:00 Jan Hamann (Physics and Astronomy Dept. U. Aarhus) - "Dark radiation"
In the cosmological standard model the relativistic sector of the Universe's energy budget is made up from photons and three species of neutrinos.Interestingly recent data seem to be hinting at the possible existence of an additional component dubbed dark radiation which would profoundly impact the physics of decoupling and primordial nucleosynthesis.In my talk I will review the physics of the relativistic sector discuss the observational signatures of dark radiation present constraints from recent measurements give an outlook on future measurements and finally indulge in some speculation on the identity of the particles making up the dark radiation.
11 Jan 2012 17:00 - 18:00 Robert Schmidt (ARI Heidelberg) - "Cosmology with dynamically relaxed galaxy clusters"
The X-ray surface brightness profiles of galaxy clusters can be broadly classified into (1) shallow -~^~- and (2) steep /\ profiles. I shall show how this simple distinction can be specified formally and how this relates to the dynamical state of galaxy clusters. I also show how gravitational lensing and dynamical data are used in addition and how dynamically relaxed clusters can be used for dark matter and dark energy studies.
21 Dec 2011 17:00 - 18:00 Stefano Anselmi (Physics Dept. U. Padua) - "Mildly non-linear regime of structure formation: evolution equation approaches"
One of the most interesting challenges of present cosmology concerns the understanding of structure formation and evolution. Relevant physical information is encoded in scales smaller than O(100 Mpc) where the structures are more clustered and the standard perturbation theory breaks down. In the last years different semi-analytical approaches to the problem have appeared as alternative and/or complementary tools to the N-body simulations.In this context a really powerful technique is the implementation of non-linear evolution equations which greatly simplify the computation and allow to scan over non-standard cosmologies. I will discuss the latest results on these approaches and we will give an example on how to apply these methods to the Dark Energy clustering case.
14 Dec 2011 17:00 - 18:00 Guillermo Ballesteros (Physics Dept. U. Padua) - "Non-linear dark energy clustering"
If the acceleration of the universe is not driven by a cosmologicalconstant but by some form of dark energy its small inhomogeneitieswould affect the CMB and the formation of structures at large scales. Iwill first discuss dark energy perturbations at the linear level usinga phenomenological fluid approach and commenting on the future prospectsfor detection. Then I will move on to their effects at non-linearscales using the time renormalization group to compute the densitypower spectra.
30 Nov 2011 17:00 - 18:00 Krzysztof Bolejko (Astrophysics Dept. U. Oxford) - "Light propagation in the inhomogeneous universe"
Light propagation plays an important role in cosmological research as it is mainly via astronomical observations that we gain our knowledge about the Universe.During my talk I will discuss how inhomogeneities affect the light propagation and the distance-redshift relation. I will also discuss what implications these effects havefor the cosmological research and our analysis of cosmological observations.
23 Nov 2011 17:00 - 18:00 Alejandro Guarnizo Trilleras (ITP Heidelberg) - "Geodesic Deviation Equation in f(R) Gravity"
In the context of metric f(R) gravity we study the Geodesic Deviation Equation (GDE) in a FLRW background. First we give general expressions for the GDE in f(R) for any space-time and energy-momentum tensor. We focus our attention in two particular cases of the GDE in FLRW background: for fundamental observers and for null vector fields past directed. The last one give us interesting properties and a general differential equation for the diametral angular distance in f(R) gravity which could be solved and compared with the solutions in standard cosmology (in progress). Finally we obtain a Dyer-Roeder like equation in f(R) gravity opening the possibility to study cosmological distances in inhomegeneous universes and a general treatment of gravitational lensing in these theories.
16 Nov 2011 17:00 - 18:00 Parvin Moyassari (LMU Munich) - "On Stability of a Consistent Deformation of Einstein's Gravity"
Relevant deformations of gravity present an exciting window of opportunity to probe the rigidity of gravity on cosmological scales. For a single-graviton theory the leading relevant deformation constitutes a graviton mass term. We construct deformations of general relativity that are consistent and phenomenologically viable since they respect in particular cosmological backgrounds. These deformations have unique symmetries in accordance with their Minkowski cousins (Fierz-Pauli theory for massive gravitons) and incorporate a background curvature induced self-stabilizing mechanism. Self-stabilization is essential in order to guarantee hyperbolic evolution in and unitarity of the covariantized theory as well as the deformations uniqueness. We show that the deformations parameter space contains islands of absolute stability that are persistent through the entire cosmic evolution.
26 Oct 2011 17:00 - 18:00 Julian Merten (ITA Heidelberg) - "Towards an understanding of galaxy clusters"
Clusters of galaxies are a powerful tool to study the formation of structure in a cosmological context and to dissect the interaction between the different matter components. Particularly interesting are merging clusters of galaxies where matter can be observed under extreme conditions. But also an understanding of a relaxed population at the end of its assembly sheds light on e.g. a universal density profile of dark matter structures.In my talk I will present several observational programmes we got recently involved in including the spectacular merger Abell 2744 dubbed Pandora's Cluster and the large HST multicycle treasury programme CLASH. Powerful numerical techniques including parallel GPU implementation to harvest those programmes shall be discussed. In this context a comprehensive analysis of numerical simulations of individual clusters but also of large cosmological boxes is inevitable to compare observations to the theory of structure formation. Strategies to perform this analysis in a sensible way will close my presentation.
19 Oct 2011 17:00 - 18:00 Marco Bruni (ICG Portsmouth) - "From small scales to the horizon: relativistic effects in structure formation"
The LCDM is the successful standard model of cosmology. Alternatives tothe cosmological constants include models of dark energy modified theoriesof gravity as well as general relativistic (GR) models that either weakenthe symmetry assumptions of the cosmological principle or try to constructan average universe to explain acceleration as a back-reaction effect. At atime when we are going to have observational data allowing measurements withunprecedented precision it is however also worth reconsidering the finedetails of how we model structure formation in LCDM cosmology and how thismay affect how we interpret observations. By and large we model very largescales with relativistic perturbation theory small scales - wherenon-linearity is important - with Newtonian N-body simulations and weinterpret many observations (e.g. supernovae) as if light was propagating ina homogeneous-isotropic background. In the first part of this talk I willillustrate two examples of relativistic effects: on the power spectrumon large scales and on how redshift and distances are affected if wepropagate light in a inhomogeneous universe. In the second part of the talkI will present a new non-linear post-Friedmannian scheme which is a sort ofgeneralisation to cosmology of the post-Newtonian approximation. Using a 1/cexpansion of Einstein equations a set of non-linear approximate equationsare obtained in the Poisson gauge which include the full non-linearity ofthe Newtonian regime at small scales and when linearised give standardscalar and vector linear relativistic perturbations (TT modes arenon-dynamical at the order of approximation considered). I will end with anoutline of possible applications and extensions of this new formalism.

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