Ruprecht Karls Universität Heidelberg

ITP Cosmology Tunch

* This page refers to Tunch meetings up to the Winter Semester 2016/17. See the page Joint Tunch/JC for an updated list of Journal Clubs and Tunches *

* For subsequent semesters, see Joint JC/Tunch *

Tuesday, PW 12, Room 056

13:00 - 15:00

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Go to the calendar of Wednesday Cosmology seminars (Normally on Wednesday, 5pm, Phil 16)

Seminar calendar

25 Sep 2017 13:00 - : Tunch available () - "TBA"
11 Sep 2017 13:00 - : Tunch available () - "TBA"
08 Aug 2017 13:00 - : Tunch available () - "TBA"
25 Jul 2017 13:00 - : Tunch available () - "TBA"
11 Jul 2017 13:00 - : Tunch available () - "TBA"
27 Jun 2017 13:00 - : Cornelius Rampf (ITP, Heidelberg University ) - "Shell-crossing in quasi one-dimensional flow "
So far exact analytic solutions to the cosmological fluid equations existed for initial data that only depend on one space variable. Exact solutions (until shell-crossing) play an important role in cosmology, not only because they are simple but because the breakdown of smooth 3D solution through the development of infinite density caustics begins generically as an almost 1D phenomenon with the formation of pancakes. I present recent work on quasi-one-dimensional (Q1D) flow that depends on all three coordinates but differs only slightly from a strictly 1D flow, thereby allowing a perturbative treatment of shell-crossing using the Euler-Poisson equations written in Lagrangian coordinates. The signature of shell-crossing is then just the vanishing of the Jacobian of the Lagrangian map, a regular perturbation problem. In essence, the problem of the first shell-crossing, which is highly singular in Eulerian coordinates, has been desingularized by switching to Lagrangian coordinates, and can then be handled by perturbation theory. All-order recursion relations are obtained for the time-Taylor coefficients of the displacement field, and it is shown that the Taylor series has an infinite radius of convergence. This allows the determination of the time and location of the first shell-crossing, which is generically shown to be taking place earlier than for the unperturbed 1D flow.
13 Jun 2017 13:00 - : Santiago Casas (ITP, Heidelberg University) - "Non-linear structure formation"
30 May 2017 13:00 - : Javier Rubio (ITP, Heidelberg University) - "Scale invariance: connecting inflation and dark energy"
Inflation and dark energy share many essential properties. I will show that these two eras can be accommodated into a common framework based on scale invariance. I will discuss the phenomenological consequences of two scenarios based on i) exact scale symmetry and ii) broken scale invariance with symmetry resurgence at UV and IR fixed points.
16 May 2017 13:00 - : Alefe Almeida (ITP, Heidelberg University) - " A method for evaluating models that use galaxy rotation curves to derive matter density profiles"
There are some approaches, either based on GR or modified gravity that use galaxy rotation to derive the matter density of the corresponding galaxy. In this work, we proposed a test for evaluating them.
25 Apr 2017 13:00 - : Yves Dirian (Geneva University ) - "''A numerical relativity scheme for cosmological simulations''"
I present a new 3+1 integration scheme which allows one to pass an adaptation of the robustness test to the cosmological context, at least in the case of General Relativity with a pressureless perfect fluid field. As an interesting by-product of this construction, a novel constraint-damping method is obtained.
14 Mar 2017 13:00 - : Frank Koennig (ITP, Heidelberg University) - "Cosmological viability of massive spin-2 fields "
02 Mar 2017 13:00 - : Margherita Lembo (ITP, Heidelberg University) - "Rotation curve of the Milky Way "
We are exploring the idea that the rotation curve of the Milky Way can be explained in a framework of modified gravity theories without excluding the existence of dark matter.
14 Feb 2017 13:00 - : No TUNCH today () - ""
31 Jan 2017 13:00 - : Collective Tunch (ITP, Heidelberg University ) - ""
17 Jan 2017 13:00 - : Group Discussion () - ""
20 Dec 2016 13:00 - : Luca Amendola (ITP, Heidelberg University) - "Model-independent constraints on modified gravity from current data and from the Euclid and SKA future surveys"
06 Dec 2016 13:00 - : Collective Tunch (ITP, Heidelberg University) - ""
22 Nov 2016 13:00 - : Ronaldo Batista (Federal University of Rio Grande do NorteBrazil) - "Spherical Collapse of non-top-hat profiles in the presence of clustering dark energy"
We study the spherical collapse of non-top-hat profiles fluctuations in dark matter and dark energy with arbitrary sound speed. We show how clustering of dark energy impacts the matter profiles and discuss some issues regarding dark energy models with w<-1 that appear in the non-linear evolution. We also show the impact of dark energy sound speed in the critical density for collapse.
15 Nov 2016 13:00 - : Salvador Enrique Ramirez Ciarreta (Complutense University of Madrid) - "Non-local theories and Scale Holography"
We present a new correspondence between a d-dimensional dynamical system and a whole family of (d+1)-dimensional systems. This new scale-holographic relation is built by the explicit introduction of a dimensionful constant which determines the size of the additional dimension. Scale holography is particularly useful for studying non-local theories, since the equivalent dual system on the higher dimensional manifold can be made to be local, as we illustrate with the specific example of the p-adic string theory.
25 Oct 2016 13:00 - : Sara Jamali (ITP, Heidelberg University) - "The phase space view of MOG"
We investigate the cosmological consequences of a scalar-vector-tensor theory of gravity known as modified gravity (MOG). In MOG, in addition to metric tensor, there are two scalar fields, and one vector field .Using the phase space analysis, we explore the cosmological consequences of MOG. We find that this theory possesses a true sequence of cosmological epochs. However, the matter-dominated phases are different from the standard matter-dominated epoch. Considering these results we discuss the cosmological viability of MOG.
11 Oct 2016 13:00 - : Florian Fuehrer (ITP, Heidelberg University) - ""
27 Sep 2016 13:00 - : Group Discussion () - ""
13 Sep 2016 13:00 - : Group discussion () - ""
02 Aug 2016 13:00 - : Group discussion () - ""
19 Jul 2016 13:00 - 15:00 Alberto Bailoni (ITP, Heidelberg University) - "Improving the galaxy clustering Fisher matrix: window function and correlated bins"
The Fisher matrix is a widely used tool to forecast the performance of future experiments and approximate the likelihood of large data sets. Most of the forecasts for cosmological parameters in galaxy clustering studies rely on the Fisher matrix approach for large-scale experiments like DESI, Euclid, or SKA. In this presentation, I will focus on an approach for improving the standard method by taking into account two effects: the finite window function and the correlation between redshift bins. Whereas for an infinite survey these effect can be neglected, for a finite survey they might play a significant role. I will show how to take into account both effects and consider their impact on forecasting an Euclid-type experiment. The main result of my study is the conclusion that the windowing and the bin cross-correlation induce a sizeable change in the forecasted errors, of the order of 10-30% for most cosmological parameters.
05 Jul 2016 13:00 - 15:00 Luca Amendola (ITP, Heidelberg University) - "Modified gravity from galaxy clusters"
21 Jun 2016 13:00 - 15:00 Ana Marta Pinho (ITP, Heidelberg University) - "EPIC Heidelberg project"
EPIC: Equivalence Principle Tests in Cosmology is my Master in Astronomy dissertation, a project that included a 6 months internship at ITP, Heidelberg. On this talk, I will show you my motivations and my work so far on using fine-structure constant measurements, to set constrains and search for a time and a spatial variation by Bayesian statistics and Monte Carlo Markov Chains simulations.
07 Jun 2016 13:00 - 15:00 Frank Koennig (ITP, Heidelberg University) - ""
24 May 2016 13:00 - 15:00 Adrian Fernandez (ITP, Heidelberg) - "Cosmological perturbations in non-local gravity"
10 May 2016 13:00 - 15:00 Group discussion () - ""
19 Apr 2016 13:00 - 15:00 Group discussion () - ""
05 Apr 2016 13:00 - 15:00 Group discussion () - ""
08 Mar 2016 13:00 - 14:00 (Phil. 12) - "Journal Club"
01 Mar 2016 13:00 - 14:00 Marvin Lueben (ITP) - "Background cosmology with three interacting spin-2 fields"
Cosmological solutions in ghost-free bimetric gravity are either indistuingishable from standard cosmology or plagued by ghost or gradient instabilities. That makes it interesting to study multi-metric theories. The simplest multi-metric theory beyond bimetric gravity involves three interacting metrics. In this talk, I will present those trimetric gravity theories, that are not plagued by the Boulwere-Deser ghost: star and path trigravity. For some select models I will discuss the background cosmology.
23 Feb 2016 13:00 - 14:00 (Phii.12) - "Journal Club"
02 Feb 2016 13:00 - 14:00 No Tunch : Fake Interview () - ""
26 Jan 2016 13:00 - 14:00 (Phil 12) - "Journal Club"
19 Jan 2016 13:00 - 14:00 Laura Taddei (ITP) - "Cosmological constraints on modified gravity from currend and future data"
In this talk, I will review the cosmological constraints on the anisotropic stress and on the effective gravitational constant for modified gravity in a model-independent way, that have been found in the past. Then, I will show the preliminary results on the constraints on the effective gravitational constant by combining RDS measurements with Supernovae. We use all currently available values of f\sigma_8 and, in order to constrain the background, the Supernovae from the JLA catalog. We also forecast the precision of a future estimation on G_eff in a Euclid-like redshift survey, and in a SKA-like redshift survey.
08 Dec 2015 13:00 - 14:00 9th TRR33 Winter School - no Tunch this day () - ""
01 Dec 2015 13:00 - 14:00 (Phil.12) - "Journal Club"
24 Nov 2015 13:00 - 14:00 Sergey Pavluchenko () - "Exact cosmological solutions in Lovelock gravity"
The talk is dedicated to exact flat anisotropic solutions in Lovelock gravity. Two main ansatz -- power-law and exponential -- are considered. Special attention is paid to how the number of dimensions and order of correction affect the structure and abundance of the solutions. Additionally, the influence of non-flatness is analyzed.
24 Nov 2015 : - : Luisa Jaime (unusual time) - "About one cosmological test and the existence of hair black holes inf(R)"
During this talk I will present the results of my latest two papers: One cosmological test and black holes in f(R). 1) We perform the two-point diagnostic for the Om(z) function proposed by Sahni et al (2014) for two f(R) models. We show that the observed values of this model independent test can be explained in f(R) models, while in LCDM the value of the function is constant. 2) We consider several f(R) models for spherically symmetric black holes and discuss about the existence of non-trivial solutions and the applicability of the No-hair theorems. If such theorems can not be applied we perform a numerical analysis and show that for a asymptotically flat solutions there is no hair.
17 Nov 2015 13:00 - 14:00 (Phil.12) - "Journal Club"
10 Nov 2015 13:00 - 14:00 Marco Raveri (SISSA) - "The Effective Field Theory approach to gravitation on cosmological scales"
One of the major challenges in modern cosmology is to explain the phenomenon of cosmic acceleration. Thanks to the enormous amount of precise data that have been gathered and will be provided by cosmological surveys it will soon be possible to discriminate between competing models. I will discuss the relevance of the Effective Field Theory (EFT) approach in testing modified gravity and dark energy models that aim at solving the problem of cosmic acceleration. In particular I will briefly review the EFT construction and its implementation into the Einstein-Boltzmann solver EFTCAMB that is publicly available and can be used to test a wide range of models against cosmological observations. I will then comment on the theories that have been recently studied with this tool and the corresponding observational constraints.
03 Nov 2015 13:00 - 14:00 Behnam Javanmardi (Argelander-Institut for Astronomie, Bonn) - "Probing Cosmic Isotropy with Type Ia Supernovae"
Based on the cosmological principle, the acceleration of the cosmic expansion is expected to be isotropic. We present a method to test the isotropy of the magnitude-redshift relation of Type Ia Supernovae (SNe Ia) and single out the most discrepant direction (in terms of the signal-to-noise ratio) with respect to the all-sky data. Our technique accounts for possible directional variations of the corrections for SNe Ia and yields all-sky maps of the best-fit cosmological parameters with arbitrary angular resolution. To show its potential, we apply our method to the recent Union2.1 compilation, building maps with three different angular resolutions. We use a Monte Carlo method to estimate the statistical significance with which we could reject the null hypothesis that the magnitude-redshift relation is isotropic based on the properties of the observed most discrepant directions. We find that, based on pure signal-to-noise arguments, the null hypothesis cannot be rejected at any meaningful confidence level. However, if we also consider that the strongest deviations in the Union2.1 sample closely align with the dipole temperature anisotropy of the cosmic microwave background, we find that the null hypothesis should be rejected at the 95 - 99 per cent confidence level, slightly depending on the angular resolution of the study. If this result is not due to a statistical fluke, it might either indicate that the SN data have not been cleaned from all possible systematics or even point towards new physics. We finally discuss future perspectives in the field for achieving larger and more uniform data sets that will vastly improve the quality of the results and optimally exploit our method.
27 Oct 2015 09:00 - 18:30 Heidelberg - Geneva Meeting () - "Gravity at the Largest Scales"
20 Oct 2015 13:00 - 14:00 Alessio Spurio Mancini (ITP) - "A quadratic estimator for the matter power spectrum from weak gravitational lensing"
Weak gravitational lensing is one of the most powerful tools available to cosmologists to gain an insight into our Universe. In particular, measurements of this relativistic effect can provide information about the matter power spectrum. We present a new quadratic estimator for the matter power spectrum, based on weak lensing convergence. According to the Cramér-Rao inequality, the estimator is guaranteed to have the minimum variance and is therefore the best unbiased estimator of the matter power spectrum. The properties of this estimator are described, in particular its window functions which, being as narrow as possible in k-space, would permit to isolate the effects of physical processes that act on different scales.
13 Oct 2015 13:00 - 14:00 No Tunch () - ""
04 Aug 2015 13:00 - 14:00 Frank Koennig, Santiago Casas (ITP) - "Is Λ 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.
21 Jul 2015 13:00 - 14:00 Cosimo Bambi (Fudan University) - "Testing astrophysical black hole candidates"
Astrophysical black hole candidates are thought to be the Kerr black hole of general relativity, but a direct observational confirmation is still lacking. In this talk, I will discuss how the study of the properties of the electromagnetic radiation emitted by the gas in the accretion disk can test the Kerr nature of black hole candidates.
14 Jul 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
07 Jul 2015 13:00 - 14:00 Tunch available () - ""
30 Jun 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
23 Jun 2015 13:00 - 14:00 Tunch available () - ""
16 Jun 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
15 Jun 2015 13:00 - 14:00 Sumit Kumar (Centre for Theoretical Physics, Jamia Millia Islamia, India) - "Models of inflation and interacting Dark energy-dark matter in the light of observational data"
To explain some theoretical problems in standard model of cosmology as well as to explain observational facts, we need two phases of accelerated expansion in the cosmological history of universe. One is inflationary epoch which happened very early in the Universe and other is more recent phenomenon explained by cosmological constant or "dark energy". We shall discuss a specific two brane warped geometry model in which brane separation stabilizes and at the same time it gives inflation on visible brane which is consistent with the present data. We shall also discuss an interacting dark energy model in which the scalar field is coupled with dark matter sector. Finally, I shall also introduce a python package "ScalPy" for studying late time scalar field cosmology. This code solves the dynamical system for scalar field models (minimally coupled scalar field, tachyon field and Galileon field) and gives cosmological observable quantities. This code can be easily hacked for an arbitrary scalar field model with a given potential. This code is integrated with publicly available MCMC sampler `emcee' to constrain model parameters.
09 Jun 2015 13:00 - 14:00 Tunch available () - ""
02 Jun 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
26 May 2015 13:00 - 14:00 There will be no Tunch this day () - ""
19 May 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
12 May 2015 11:00 - 18:00 Internal Bi-Gravity Meeting () - ""
05 May 2015 13:00 - 14:00 There will be no Tunch this day () - ""
17 Mar 2015 13:00 - 14:00 Henrik Nersisyan, Valeri Vardanyan (ITP Heidelberg) - "Cultural Tunch"
10 Mar 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
03 Mar 2015 13:00 - 14:00 Io Odderskov (University of Aarhus) - "Local Variations in the Hubble Constant"
The motion of galaxies is affected by the distribution of matter in the universe. In areas that are much more dense than the mean, this will give rise to an expansion rate that is somewhat smaller than the mean expansion rate, and in very underdense areas, the expansion rate will be larger than the mean. The significance of these effects can be investigated using, for example, N-body simulations. In this talk, I will present the results of such an investigation, and answer the question of whether the effect is big enough to explain the difference between the Hubble constant observed in our local universe and the one estimated from the CMB. Furthermore, using the same method I show that measuring the Hubble constant from observations of Cepheids in the Coma Cluster, as has been proposed, can not be expected to give a reliable result.
24 Feb 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
17 Feb 2015 13:00 - 14:00 Tunch available () - ""
10 Feb 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
03 Feb 2015 13:00 - 14:00 Santiago Casas, Alejandro Guarnizo (ITP Heidelberg) - "Cultural Tunch"
27 Jan 2015 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
20 Jan 2015 13:00 - 14:00 Santiago Casas, Frank Koennig (ITP) - ""
We will present the CURLY website, which lists the codes, scripts, snippets and packages of all cosmici users on a single page automatically for you. It has also support for external repositories. We will also explain how to use the internal GitLab platform to mantain a version control system.
16 Dec 2014 13:00 - 14:00 Tunch available () - ""
09 Dec 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
02 Dec 2014 13:00 - 14:00 Luisa Jaime, Suttiluk Othatawong (ITP Heidelberg) - "Cultural Tunch"
25 Nov 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
18 Nov 2014 13:00 - 14:00 Miguel Zumalacárregui (ITP Heidelberg) - "Shaken, not stirred: kinetic mixing in general scalar-tensor gravities"
The propagating degrees of freedom in scalar-tensor theories (ST) of gravity interact with each other via their derivatives. This leads to the mixing of the higher derivatives in the equations of motion, a phenomenon known as kinetic gravity braiding (KGB). I will introduce the physics of KGB, starting from the simplest theories and building up towards increasingly complex Horndeski theories (the most general ST governed by second order equations) and recently proposed theories beyond Horndeski (that do not introduce additional degrees of freedom). I will also present a procedure to covariantly "unbraid" quartic Horndeski theories, point out its shortcomings and discuss its usefulness to understand phenomenological applications.
11 Nov 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
04 Nov 2014 13:00 - 14:00 There will be no Tunch this day () - ""
28 Oct 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
21 Oct 2014 13:00 - 14:00 Guillermo Ballesteros (ITP Heidelberg) - "Effective fluids at NLO and dark energy"
I will present the effective theory of fluids at next-to-leading order in derivatives. In a perturbed FLRW geometry the theory presents a set of features that make it very rich for modelling the acceleration of the Universe. These include anisotropic stress, a non-adiabatic speed of sound and modifications to the standard equations of vector and tensor modes. These effects are determined by an energy scale which controls the size of the high derivative terms and ensures that no instabilities appear.
05 Aug 2014 13:00 - 14:00 Frank Koennig (ITP Heidelberg) - "Bigravity meets Cambridge"
After massive gravity (and later its parents, bigravity) has recovered from several studies claiming non-physicality (e.g. the vDVZ discontinuity or the Bouldware-Deser ghost), instable first-order perturbations are challenging the theory again. This tunch will demonstrate the immortality of bigravity by presenting an infinite branch model (IBB, or infinite-branch bigravity) that fits well the background and growth data and has stable first-order perturbations at all times. Along this way, I will present hints why IBB's presumptive birthplace is Cambridge and show the results of our own empirical ghost-free proof.
22 Jul 2014 13:00 - 14:00 Massimo Pietroni (INFN Padova) - "A coarse grained perturbation theory for the Large Scale Structure"
Standard cosmological perturbation theory (SPT) for the Large Scale Structure (LSS) of the Universe fails at small scales (UV) due to strong nonlinearities and to multistreaming effects. A new framework is proposed in which the large scales (IR) are treated perturbatively while the information on the UV, mainly small scale velocity dispersion, is obtained by nonlinear methods like N-body simulations. In this framework it is possible to reproduce the fully nonlinear power spectrum (PS) by combining a simple (and fast) 1-loop computation for the IR scales and the measurement of a single, dominant, correlator from N-body simulations for the UV ones. The agreement between this method and N-body simulations is at percent level for wave numbers up to k?0.4h Mpc^1 down to z=0. We then discuss how, once this correlator has been measured in a given cosmology, there is no need to run a new simulation for a different cosmology. Indeed, by rescaling this correlator by a proper function computable in SPT, the reconstruction procedure works also for the other cosmologies and for all redshifts, with comparable accuracy. Finally, we clarify the relation of this approach to the Effective Field Theory methods recently proposed in the LSS context.
10 Jun 2014 13:00 - 14:00 Iason Baldes (University of Melbourne) - "Particle-antiparticle asymmetries from annihilations"
Common mechanisms invoked to explain particle antiparticle asymmetries involve the out-of-equilibrium and CP violating decay of a heavy particle. In this talk I discuss how asymmetries can arise purely from 2 <-> 2 annihilations instead of the usual 1 <-> 2 decays and inverse decays. I will present a simple toy model to point out the salient features of such a scenario and to illustrate how an asymmetry can arise while respecting CPT and S-matrix unitarity. I will also discuss a baryogenesis scenario in which annihilations and decays compete in creating the baryon asymmetry. Annihilations may actually dominate over decays in determining the final asymmetry in certain areas of parameter space. Such ideas could also have applications in asymmetric dark matter models.
27 May 2014 13:00 - 14:00 Yashar Akrami (ITA - Oslo University) - "Searches for violations of statistical isotropy: power asymmetry in WMAP and Planck sky maps as measured by a local variance estimator"
The principles of isotropy and homogeneity of the Universe on large scales are two cornerstones of the cosmological concordance model. 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. 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 a powerful but conceptually and practically simple numerical tool for the statistical analysis of various cosmological data sets in searches for signatures of violations of statistical isotropy. I will show how this principle can be tested in a model-independent way by measuring the local variances over the sky and on disks of various sizes. For the 2013 Planck temperature sky map we find that the data, in comparison with simulated isotropic maps, suggests the presence of an anisotropic signature formally significant at least at the 3.3$\sigma$ level. Similar results are obtained from analyzing the WMAP 9-year sky map. The preferred directions for the asymmetry are in good agreement with previous reports of the same hemispherical power asymmetry. I will finally discuss the implications of this finding for theoretical cosmology and the prospects for applying the same technique to the forthcoming Planck CMB polarization data and measurements of the LSS by future generations of galaxy surveys such as the ESA Euclid mission.
13 May 2014 13:00 - 14:00 Alejandro Guarnizo (ITP Heidelberg) - "A cosmological exclusion plot for modified gravity"
In this talk I will discuss the current and future observational constraints on the effective anisotropic stress or gravitational slip $\eta=-\Phi/\Psi$, using a model-independent approach which is independent of the perturbation initial conditions, of the cosmic evolution outside the observed range and of the bias, beside the assumption of linearity. We use data for the growth rate $f\sigma_{8}$ coming from Redshift Space Distortion (RSD) measurements, observations of the Hubble expansion $H(z)$, and a constraint for the quantity $P_{2}$ defined as the expectation value of the ratio between galaxy-galaxy and galaxy-velocity cross correlations. We find with current data a value at $z=0.32$ of $\eta=0.981\pm0.667$, in agreement with the predicted value for the $\Lambda$CDM model. Modeling a future large-scale survey along the lines of the ESA Eucid mission we produce a cosmological exclusion plot for modified gravity in analogy with the exclusion plots produced in laboratory experiments. Adopting the specific form of $\eta$ predicted by Horndeski Lagrangian, we find how well future clustering and weak lensing surveys can constrain its strength and range.
29 Apr 2014 13:00 - 14:00 Miguel Zumalacárregui (ITP Heidelberg) - "May the force not be with you"
I will present the different effects by which modified theories of gravity may have sizeable cosmological implications while remaining indetectable in the Solar System. Particularly, I will focus on those relying on derivative interactions of the scalar field, the Vainshtein and the disformal screening mechanisms, and present them as clearly as possible. If time permits, I will talk about how they can be realized in general scalar-tensor theories.
22 Apr 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
08 Apr 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
01 Apr 2014 13:00 - 14:00 Emilio Bellini (ITP Heidelberg) - "An introduction to xAct: a package to manipulate tensors"
In this talk I will introduce through examples the logic and the capabiliies of xAct. "//xAct/ is a suite of free packages for tensor computer algebra in /Mathematica/ . /xAct/ implements state-of-the-art algorithms for fast manipulations of indices and has been modelled on the current geometric approach to General Relativity. It is highly programmable and configurable. Since its first public release in March 2004, /xAct/ has been intensively tested and has solved a number of hard problems in GR./" (source
25 Mar 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
24 Mar 2014 13:00 - 14:00 Martina Schwind (ITP Heidelberg) - "Software engeneering (for a tour operator)"
I want to give You a brief introduction into software engineering with the example of an international company with a world wide network and a complex system landscape. Starting with the business life cicle I give You a rough overview of the system landscape behind it and how this is managed with different small teams being responsible for the single systems. Afterwards I give You an introduction into the software developing process.beginning with a project idea or an idea for optimization up to the rollout of the software.
18 Mar 2014 13:00 - 14:00 There will be no Tunch this day () - "TBA"
11 Mar 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
04 Mar 2014 13:00 - 14:00 There will be no Tunch this day () - "TBA"
25 Feb 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
18 Feb 2014 13:00 - 14:00 There will be no Tunch this day () - "TBA"
11 Feb 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
04 Feb 2014 13:00 - 14:00 Adrian Vollmer (ITP Heidelberg) - "Efficiently computing non-linearities"
As part of my PhD project, I developed a Fortran code that implements the Time Renormalization Group formalism to compute non-linear corrections. Here, I give a short overview of the current state of my work.
28 Jan 2014 13:00 - 14:00 (Phil.12 R105) - "Journal Club"
21 Jan 2014 13:00 - 14:00 Suttiluk Othatawong (ITP Heidelberg) - "Effect of inhomogeneous dark energy on distribution of clusters of galaxies"
We investigate theoretical phenomena on the roles of dark energy in spherical collapse, among homogeneous, intermediate and inhomogeneous forms, on the cosmic structure. Within the same dark energy model, we include these forms in structure formation and then consider dark energy degeneracies with the parameter, $\beta$, which describes a degree of local dark energy inhomogeneity in the continuity equation. We study the chosen dark energy models which are, $w=-0.8$, $w=-1.2$ including the $\Lambda\textrm{CDM}$ model and CPL models to differ significantly the effect of dark energy perturbation from the generally homogeneous assumption of dark energy via the study of the growth rate and the cluster number count.
14 Jan 2014 13:00 - 14:00 ( Phil.12 R105) - "Journal Club"
17 Dec 2013 13:00 - 14:00 Collective Tunch () - "TBA"
10 Dec 2013 13:00 - 14:00 Luca Amendola (ITP Heidelberg) - "Why you should use Lyx"
26 Nov 2013 12:45 - 14:15 There will be no Tunch this day () - "TBA"
19 Nov 2013 12:45 - 14:15 Laura Taddei (ITP Heidelberg) - "Spherical collapse and halo-mass function in the symmetron model"
"We study the gravitational clustering of spherically symmetric overdensities and the statistics of the resulting dark matter halos in the 'symmetron model', in which a new long range force is mediated by a Z2 symmetric scalar field. Depending on the initial radius of the overdensity, we identify two distinct regimes: for small initial radii the symmetron mediated force affects the spherical collapse at all redshifts; for initial radii larger than some critical size this force vanishes before collapse because of the symmetron screening mechanism. In both cases overdensities collapse earlier than in the LCDM and statistically tend to form more massive dark matter halos. Regarding the halo-mass function of these objects, we observe order one departures from standard LCDM predictions. The formalism developed here can be easily applied to other models where fifth-forces participate to the dynamics of the gravitational collapse."
12 Nov 2013 13:00 - 14:15 Miguel Zumalacarregui (ITP Heidelberg) - "Transforming Gravity: Islands of Stability beyond the Land of Horndeski"
Horndeski's theory of gravity has recently attracted considerable attention as the most general, physically reasonable scalar-tensor theory. I will challenge this assumption by considering derivative couplings to matter of both the conformal and disformal type, which have second order field equations and hence represent a genuine extension of the Horndeski Lagrangian. The study of the Jacobian for the metric transformation between the Einstein and Jordan frames allows one to determine certain viability conditions on the form of the metric, related with the existence of such transformation. Although the Jordan-frame version of the theory does not belong to the Horndeski Lagrangian, projecting the metric equations along the eigenvector of the Jacobian allows one to write down dynamical equations with only second or lower derivatives. This feature signals a loophole in Horndeski's Theorem and enlarges the set of (potentially) viable scalar-tensor theories.
05 Nov 2013 12:45 - 14:15 There will be no Tunch this day () - "TBA"
30 Oct 2013 13:00 - 14:15 Cornelius Rampf (ITP Aachen) - "The fundamental role of the observer in relativistic cosmological structure formation"
According to the standard model of cosmology the large-scale structure (i.e., clusters of galaxies) is largely the result of gravitational instability. By studying the gravitational evolution we can verify the theory of gravitation, but we are also able to shed light into the energy composition of our Universe. In this talk I will concentrate on the fundamental role of the observer in relativistic structure formation. I will show how relativistic effects affect the interpretation of observations and Newtonian numerical simulations. This becomes increasingly important when we interpret precision data from current and forthcoming large-scale surveys (such as SDSS or Euclid). The key analytic technique for such considerations is the Lagrangian perturbation theory [1-3] which I will also review in this talk. References: [1] C. Rampf & G. Rigopoulos, Mon. Not. Roy. Astron. Proc. 430 (2013) L54-L58, arXiv:1210.5446 [2] C. Rampf & G. Rigopoulos, Phys. Rev. D87 (2013) 123525, arXiv:1305.0010 [3] C. Rampf, (2013), arXiv:1307.1725
29 Oct 2013 13:05 - 14:15 Santiago Casas (ITP Heidelberg) - "Fitting the non-linear power spectrum of coupled dark energy cosmological simulations."
First we review the coupled quintessence model and its effects on structure formation due to linear and nonlinear perturbations. In the highly nonlinear regime, all present analytical and numerical approaches fail to describe properly the power spectrum. In order to test these models with future observations, we need to evolve them using N-body simulations. In this talk I will explain how to obtain smooth power spectra out of the simulations and find fitting formulas that describe the evolution, scale and coupling dependence of the power spectrum at the few percent level.
22 Oct 2013 13:05 - 14:15 Eloisa Menegoni (ITP Heidelberg) - "Constraints on a k-dependent bias from galaxy clustering"
Many experiments in the near future will test dark energy through its efects on the linear growth of matter perturbations, bias functions, and on the cosmological parameters. Simultaneous information on geometry and growth rate, bias function can be obtained by measuring the galaxy power spectrum or the 2-point correlation function and their anisotropies observed in redshift space. We forecast the constraints that future observations can put on the bias function by using the galaxy power spectrum. In particular we use representative assumptions for the parameters of the Euclid survey to provide a baseline for future experiments and we focus on the following issues. We assess how well one can constrain the bias function from the analysis of the power spectrum itself in two models: 1) we taken in account a model in which the bias is written as a power law, 2) we taken in account a model based on the "Cole et al. arXiv:astro-ph/0501174 [astro-ph].
17 Sep 2013 13:00 - 14:15 Elena Sellentin (ITP Heidelberg) - "Marginalizations and Fisher-Analysis"
Wolz et al (2012) claim the Fisher-matrix analysis of cosmological data does not produce reliable confidence contours for dark energy parameters. However, tracing their steps seems to show that their claim might be a homemade problem due to their chosen priors
16 Jul 2013 13:00 - 14:15 Ignacy Sawicki (ITP Heidelberg) - "Seeding super-massive black holes with a non-vortical dark-matter subcomponent"
We propose a new dark-matter subcomponent, fluid dark matter, which behaves effectively like cold dark matter at background and linear level but, as a consequence of its hydrodynamical nature, cannot have vortical flow and therefore forms a cosmic skellington around which cold dark matter virializes, while at its joints black hole seeds form. This model can predict black hole seeds of mass of 10^4--10^-6 solar masses, thus providing an efficient way to account for the super massive black holes observed at redshift greater than six.
02 Jul 2013 13:00 - 14:15 Cristiano Germani (Fakultät für Physik /LMU München) - "The many saddle saddle points of quantum Black Holes"
In this talk I will show that a two dimensional Black Hole is semiclassically described by two quantum states. In this respect I will back up the idea that usual Black Hole paradoxes, are mainly related to the assumption that the classical gravitational (Schwartzschild) saddle point is the dominant contribution to the gravity-matter path integral.
28 Jun 2013 13:00 - 14:15 Alexandre Posada (ITP Heidelberg) - "Search of systematic driven subsets in SNe catalogues with a genetic algorithm."
Since 1998 and the discovery of the accelerated expansion of the universe, the golden age of the modern cosmology has begun. A great theoretical effort has been made to explain this phenomenon and now a greater work is needed to get more and more precise data to discriminate the new theories. With the increases of data from supernovae the statistical errors in cosmology decreased to systematics uncertainty level. This presentation deals with the search and identification of systematic contaminated datasets with a statistical tool, the internal robustness, and a genetic algorithm in the Union2.1 supernovae catalogue. It shows a fast way to purge datasets from systematics and open opportunity to transpose this method to other cosmological probes or field of physics.
18 Jun 2013 13:00 - 14:15 Valerio Marra (ITP Heidelberg) - "Cosmic variance and the measurement of the local Hubble parameter"
There is an approximately 9% discrepancy, corresponding to 2.4sigma, between two independent constraints on the expansion rate of the universe: one indirectly arising from the cosmic microwave background and baryon acoustic oscillations, and one more directly obtained from local measurements of the relation between redshifts and distances to sources. We argue that by taking into account the local gravitational potential at the position of the observer this tension - strengthened by the recent Planck results - is partially relieved and the concordance of the standard model of cosmology increased. We estimate that measurements of the local Hubble constant are subject to a cosmic variance of about 2.4% (limiting the local sample to redshifts z>0.010) or 1.3% (limiting it to z>0.023), a more significant correction than that taken into account already. Nonetheless, we show that one would need a very rare fluctuation to fully explain the offset in the Hubble rates. If this tension is further strengthened, a cosmology beyond the standard model may prove necessary.
21 May 2013 12:45 - 14:15 Caroline Heneka (ITP Heidelberg) - "Robustness of SN Ia data - systematics or new signals?"
Advanced statistical analysis tools are crucial for cosmological parameter estimates and the search for new signals. I will thus introduce you to a new Bayesian method - the internal robustness formalism - that is able to detect subsets better described by underlying model parameters which differ from the ones preferred by the overall dataset. Our analysis of the SN Ia Union compilation by means of the formalism introduced will be reviewed and some of the results discussed.
05 Feb 2013 12:45 - 14:15 TBA () - "TBA"
29 Jan 2013 12:45 - 14:15 TBA (TBA) - "TBA"
22 Jan 2013 12:45 - 14:15 Emer Brady (ITP Heidelberg) - "General models of Chameleon Coupling"
I´ll give an overview of the Chameleon method for screening coupled dark energy, the motivation for such models and their cosmological shortcomings.
15 Jan 2013 12:45 - 14:15 TBA () - "TBA"
08 Jan 2013 12:45 - 14:15 TBA () - "TBA"
18 Dec 2012 12:45 - 14:15 TBA () - "TBA"
11 Dec 2012 12:45 - 14:15 Date not available due to winter school () - "No talk"
No talk
04 Dec 2012 12:45 - 14:15 Youness Ayaita (ITP Heidelberg) - "Neutrino lump fluid in growing neutrino quintessence"
Growing neutrino quintessence addresses the "why now" problem of dark energy by assuming that the neutrinos are coupled to the dark energy scalar field. The coupling mediates an attractive force between the neutrinos leading to the formation of large neutrino lumps. This work proposes an effective, simplified description of the subsequent cosmological dynamics. We treat neutrino lumps as effective particles and investigate their properties and mutual interactions. The neutrino lump fluid behaves as cold dark matter coupled to dark energy. The methods developed here may find wider applications for fluids of composite objects.
27 Nov 2012 12:45 - 14:15 Santiago Casas (ITP Heidelberg) - "The non-linear power spectrum in coupled dark energy cosmologies and the halo model of structure formation."
I will review the importance of N-body simulations to study the evolution of non-linear structure formation. In the case of cosmologies with coupled dark energy (cDE), little is know about the behaviour of the non-linear power spectrum for different models. I will also explain the basics of the halo model and the current fitting formulae and try to motivate the search for a new fit that works for these cDE cosmologies.
20 Nov 2012 13:00 - 14:15 Marc Rohloff (ITP Heidelberg) - "The Universe as a Lie group"
Lie groups form a class of Lorentz manifolds that intrinsically link the signature of the metric with compactness of the spatial universe. This talk outlines the basic constructions of Lie groups as Lorentz manifolds and properties of the solution, such as every semisimple Lie group having a unique vacuum Einstein metric.
13 Nov 2012 12:45 - 14:15 TBA () - "TBA"
06 Nov 2012 12:45 - 14:15 TBA () - "TBA"
30 Oct 2012 12:45 - 14:15 Ignacy Sawicki (ITP Heidelberg) - "Ostrograski's theorem: why higher-order equations of motion are just bad"
23 Oct 2012 12:45 - 14:15 Mariele Motta (ITP Heidelberg) - "The Horndeski Lagrangian and some observables in Dark Energy Cosmology"
The aim of this letter is to answer the following two questions: (1) Supposing we had infinitely precise cosmological observations of the expansion history and linear perturbations in a range of redshifts and scales, which properties of the dark energy could actually be reconstructed without imposing any parametrization? (2) Are these observables sufficient to rule out not just a particular dark energy model, but the entire general class of viable models comprising a single scalar field? This paper bears both good and bad news. On one hand, we find that the goal of reconstructing dark energy models is fundamentally limited by the unobservability of the present values of the matter density $\Omega_{m0}$, the perturbation normalization $\sigma_{8}$ as well as the present matter power spectrum. On the other, we find that, under certain conditions, cosmological observations can nonetheless rule out the entire class of the most general single scalar-field models, i.e. those based on the Horndeski Lagrangian.
16 Oct 2012 12:45 - 14:15 Arpine Piloyan (Yerevan State University, Armenia. / ITP Heidelberg) - "An aloud thought of possible relationships between theories of extra dimensions and Averaged Gravity theories"
02 Oct 2012 12:45 - 14:15 Miguel Quartin (Instituto de Física - UFRJ) - "Fisher Matrix in Cosmology"
Fisher Matrix, Markov Chain Monte Carlo and other methods to estimate likelihood of cosmological parameter.

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