Ruprecht Karls Universität Heidelberg

ITP Cosmology Seminars

Tuesdays, 3:15 pm, Webinar Room:


Meeting-ID: 937 8535 5577

Password: 703531

Organizers: Luca Amendola, Adrià Gómez-Valent and Jenny Wagner

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Seminar calendar

13th July 2021

Speaker: Francesco Pace

Institution: Università di Bologna; Jodrell Bank Centre for Astrophysics, University of Manchester

Title: Phenomenology of the dark sector

Abstract: Despite being the standard cosmological model (general relativity plus cosmological costant) a rather good explanation for the current observations, it is nevertheless affected by some problems, both from a theoretical and an observational point of view. Therefore, and also to better understand gravity, models beyond the LCDM are actively investigated. In this talk, I will concentrate on the dark sector which generically indicates the component, be it a fluid or a modification to gravity, invoked to accelerate the expansion of the universe. I will first briefly present the Equation of State approach, a tool developed to study linear perturbations of dark energy/modified gravity models considering them as a fluid with non-trivial properties and then show its applications, compared also to other techniques, for three selected cases of interest; f(R) models, gravitational waves and a validation of the quasi-static approximation for Horndeski models

6th July 2021

Speaker: Kazuya Koyama

Institution: Institute of Cosmology and Gravitation, University of Portsmouth

Title: General relativistic weak-field limit and Newtonian N-body simulations

Abstract: Future galaxy surveys such as Euclid, LSST and SKA will cover larger and larger scales where general relativistic effects become important. On the other hand, our study of large scale structure still relies on Newtonian N-body simulations. I explain how standard Newtonian N-body simulations can be interpreted in terms of the weak-field limit of general relativity. Our framework allows the inclusion of radiation perturbations and the non-linear evolution of matter. I show you how to construct the weak-field metric by combining Newtonian simulations with results from Einstein-Boltzmann codes and discuss observational effects on weak lensing and ray tracing, identifying important relativistic corrections. Finally, I demonstrate that this framework can be extended to gravitational theories beyond general relativity.

29th June 2021

Speaker: Max Tegmark

Institution: Massachusetts Institute of Technology

Title: AI for physics & physics for AI

Abstract: A central goal of physics is to discover mathematical patterns in data. For example, after four years of analyzing data tables on planetary orbits, Johannes Kepler started a scientific revolution in 1605 by discovering that Mars' orbit was an ellipse. I describe how we can automate such tasks with machine learning and not only discover symbolic formulas accurately matching datasets (so-called symbolic regression), equations of motion and conserved quantities, but also auto-discover which degrees of freedom are most useful for predicting time evolution (for example, optimal generalized coordinates extracted from video data). The methods I present exploit numerous ideas from physics to recursively simplify neural networks, ranging from symmetries to differentiable manifolds, curvature and topological defects, and also take advantage of mathematical insights from knot theory and graph modularity.

23rd June 2021 (3:00 pm) [NOTE UNUSAL DATE AND TIME]

Speaker: Laura Wolz

Institution: Jodrell Bank Centre for Astrophysics, University of Manchester

Title: Cold gas constraints via HI Intensity Mapping

Abstract: Intensity mapping surveys of neutral hydrogen (HI) are a new way to measure the large-scale matter distribution of our Universe over a wide range of redshifts, and thus constrain cosmological parameters describing the Universal expansion. The next generation of radio telescopes and interferometers are being designed and built to optimise the detection of the HI line at low spatial resolution allowing efficient mapping of large volumes. The impact of instrumental systematics of radio observations on cosmological measurements can be significantly reduced by cross-correlating the HI signal with galaxy surveys. The cross-correlation also offers new ways to measure correlations between HI and properties of the optically-selected galaxy samples. I will give an introduction into the HI intensity mapping technique and prospects of the on-going and future experiments, such as the Square Kilometre Array (SKA). I will showcase studies on the potential of future intensity mapping experiments in constraining HI properties, such as the global HI density as well as HI scaling relations in galaxies. I will also present the latest HI intensity mapping detection of Green Bank Telescope data in cross-correlation with the SDSS eBOSS galaxy survey samples.

22nd June 2021

Speaker: Roy Maartens

Institution: University of the Western Cape

Title: Cosmological surveys with the SKA

Abstract: I will describe the exciting prospects for advances in cosmology with the advent of the Square Kilometre Array (SKA), which will open a new window on the cosmos and cover larger cosmic volumes than ever before. The combination of optical surveys like Euclid and the Rubin Observatory (LSST) with the SKA promises to provide excellent precision in measuring Dark Energy and Dark Matter. But in addition, SKA surveys will deliver a new capacity to make tests of the foundations of our model of the Universe. In particular, I will discuss probing the primordial Universe via fossil signals in the galaxy distribution, and testing the Cosmological Principle via radio galaxies.

15th June 2021

Speaker: Suhail Dhawan

Institution: University of Cambridge

Title: Cosmology with Type Ia supernovae: A view from the Zwicky Transient Facility

Abstract: Type Ia supernovae are excellent stellar explosions that have been developed for precision cosmology. Calibrating their absolute luminosity with the local distance ladder indicates a value of the Hubble constant in tension with the inference from the early universe. In my talk, I will present recent work on the impact of non-standard dark energy models on the inference local $H_0$. I will showcase recent results and ongoing work with the Year 1 sample of SNe Ia from the Zwicky Transient Facility (ZTF). ZTF is an untargeted search and follow-up of hundreds of SNe Ia, with the aim of uniquely controlling systematic uncertainties in cosmology.

Complementary to the SNe Ia Hubble diagram, I will present work to compute time-delays, lensing magnification for the first strongly lensed SN Ia, iPTF16geu. Lensed SNe are an excellent, independent probe of $H_0$ as well as the matter distribution and dust in high-$z$ galaxies. I will preview some ongoing work to find a sample of lensed SNe with the ZTF survey.

8th June 2021

Speaker: Daniela Doneva

Institution: Theoretical Astrophysics, IAAT, University of Tübingen, and INRNE—Bulgarian Academy of Sciences

Title: Nonlinear development of black hole scalar hair

Abstract: Black holes are the ideal testbed for probing the strong field regime of gravity. Particularly interesting are classes of extended scalar-tensor theories that are perturbatively equivalent to general relativity in the weak field regime while leading to nonlinear development of a scalar field for strong gravity, that is the so-called scalarization. In my talk I will discuss such scalarized black hole solutions and their dynamical formation. I will focus on their astrophysical implications and the viability of the corresponding extended scalar-tensor theory within cosmological context.

1st June 2021

Speaker: Joan Solà Peracaula

Institution: Departament de Física Quàntica i Astrofísica, and Institute of Cosmos Sciences, Universitat de Barcelona

Title: Running vacuum: theoretical aspects and application to the $H_0$ and $\sigma_8$ tensions

Abstract: The vacuum energy in cosmology is a most subtle concept which has challenged theoretical physicists and cosmologists for many decades. The problem stems from the interpretation of the cosmological constant term, $\Lambda$, in Einstein's equations as a term being connected with the notion of vacuum energy density in quantum field theory (QFT). Its renormalization is usually plagued with theoretical pitfalls. Apart from these formal issues, a variety of practical problems or “tensions” besiege the concordance or $\Lambda$CDM model, in particular the $H_0$ and $\sigma_8$ tensions. I will consider the class of running vacuum models (RVMs), which can describe inflation followed by essentially the standard model evolution. The vacuum energy density takes the form of a constant plus a series of (even) powers of the Hubble rate, which can be motivated theoretically on several accounts. In addition, the RVM's can help to alleviate the mentioned tensions and predict that the dark energy is mildly dynamical and appears effectively as quintessence.

25th May 2021

Speaker: Andrew Robertson

Institution: Institute for Computational Cosmology, Durham University

Title: Probing the nature of dark matter with galaxy clusters

Abstract: I will begin with a brief discussion of dwarf galaxy rotation curves, outlining a number of proposed mechanisms for producing their ‘puzzling diversity’ (the fact that some appear to have constant density dark matter ‘cores', while others have centrally-concentrated ‘cusps'). One of these possibilities is that dark matter is more complicated than CDM, with self-interacting dark matter (SIDM) able to modify the distribution of dark matter in the centres of galaxies. SIDM would not only affect dwarf galaxies, but more massive systems as well. I will present the first large-volume, cosmological simulations including both SIDM and baryonic physics. SIDM has little effect on the distribution of gas and stars within massive galaxies and galaxy clusters. However, gravitational lensing observables can discriminate between dark matter models, and I will show how the distribution of Einstein radii can be used to constrain the SIDM cross-section. I will discuss using these simulations to test analytic methods for modelling SIDM density profiles in the presence of baryons, before presenting future tests of SIDM, including using strong lensing by galaxy-cluster substructures.

18th May 2021

Speaker: Ue-Li Pen

Institution: Canadian Institute for Theoretical Astrophysics

Title: Wave Optics Lensing: cosmology and fundamental physics from gravitational and plasma lensing of FRBs and other coherent sources

Abstract: I describe recent progress in wave optics lensing theory and observation. Picard-Lefschetz theory enables the wave optics inverse problem of reconstructing lens properties from observed interference patterns, including ’tunnelling' from imaginary images. This opens up probing dark matter properties, tests of gravitational theories, measurement of cosmic geometry, and more.

11th May 2021

Speaker: Hendrik Hildebrandt

Institution: Ruhr-Universität Bochum

Title: Precision measurements of large-scale structure challenge the standard model of cosmology

Abstract: The gravitational lensing effect of all the massive structures in the Universe distorts our view of the sky. These distortions can be used to map the structures irrespective of their physical composition, essentially giving us a tool to make dark matter visible. The statistical properties of the matter field contain lots of information on the underlying cosmological model. Recent precision measurements of this cosmic shear effect show a potentially interesting discrepancy to measurements of the cosmic microwave background with the Planck satellite, similar to the by now well-known tension in the Hubble parameter. If this discrepancy holds after further scrutiny it might pose a serious challenge to the standard model of cosmology. In this talk I will review the basics of cosmic shear, present the most recent results from such measurements, and discuss the challenges in their interpretation. Furthermore, I will give an outlook on the immediate future of this field of research as well as the next decade that will see unprecedented results from so-called stage-IV surveys.

4th May 2021

Speaker: Matteo Martinelli

Institution: Instituto de Física Teórica, Universidad Autónoma de Madrid (UAM-CSIC)

Title: Testing the assumptions of the standard cosmological model: the case of the Distance Duality Relation

Abstract: Despite the availability of several cosmological models alternative to the standard LCDM, and the presence of observational tensions in the context of this model, still no alternative is preferred over standard cosmology. This might indicate that in order to solve the current observational issues, one might need to relax some of the fundamental assumptions that are at the foundation of the LCDM model. A direct consequence of some of these assumptions is the Distance Duality Relation (DDR), relating the different cosmological distances with each other, which might breakdown in extended theories, e.g. when photons are coupled to beyond standard model particles. In this talk I will discuss how upcoming cosmological surveys will be able to improve current constraints on the DDR, showing both the impact of observational improvements and how new observational windows (standard sirens and strong lensing) will provide new opportunities and new challenges.

27th April 2021

Speaker: Prasenjit Saha

Institution: Institute for Computational Science and Physics Institute, University of Zurich

Title: Quasars canons and the Hubble constant

Abstract: Follow the link for the first video abstract of this seminar:

20th April 2021

Speaker: Jordi Salvadó

Institution: Departament de Física Quàntica i Astrofísica and Institut de Ciències del Cosmos, Universitat de Barcelona

Title: Long Range Interactions in Cosmology: Implications for Neutrinos

Abstract: Cosmology is well suited to study the effects of long range interactions due to the large densities in the early Universe. In this talk, we will explore how the energy density and equation of state of a fermion system diverge from the commonly assumed ideal gas form under the presence of scalar long range interactions with a range much smaller than cosmological scales. In this scenario, “small”-scale physics can impact our largest-scale observations. We will apply the formalism to self-interacting neutrinos, performing an analysis to present and future cosmological data. The results will show that the current cosmological neutrino mass bound is fully avoided in the presence of a long range interaction, opening the possibility for a laboratory neutrino mass detection in the near future. We will also see an interesting complementarity between neutrino laboratory experiments and the future EUCLID survey.

13th April 2021

Speaker: Jenny Wagner

Institution: Institut für Theoretische Physik (ITP) Heidelberg

Title: Lensing of '69 – free strong gravitational lensing from its heuristic models

Abstract: In this talk I will give an introduction into observation-based strong gravitational lensing to infer local light-distorting properties of the deflecting mass density without any a priori assumptions about the deflecting mass density profile. Upcoming galaxy cluster surveys e.g. based on JWST will be able to exploit this approach well. Since data remains sparse until then, I will also show how the most common mass density profiles, in particular the Navarro-Frenk-White profile, can be derived from fundamental physical principles. Most of them are currently based on heuristical fitting functions inferred from simulations.

2nd March 2021

Speaker: Raquel Emy Fazolo

Institution: Universidade Federal do Espírito Santo (UFES)

Title: Skewness as a test for dark energy perturbations

Abstract: In previous work we noticed a significant signature of dark energy perturbations in the skewness of the matter distribution when dealing with a single fluid universe. Now in a more recent study we take a more general approach dividing the universe in two fluids (one of matter and other of dark energy) and reach in a more general set of equations until the second order of perturbation levels to study this same effect. The results show a different scenario from previous work, indicating a closer result (compared to the other work) to $\Lambda$CDM than before, showing that this more general approach affects in a significant way the behavior of dark energy perturbations. We are also interested in providing an approximation to calculate this effect fitting our results in terms of $\Omega_m$ and the equation of state for dark energy $w_{de}$ and study dark energy models with these equations. We are also applying modified gravity to test its effects in this set of equations and making the fits with these new variables.

23th February 2021

Speaker: Caroline Heneka

Institution: University of Hamburg

Title: Deep learning for 'cosmological imaging'

Abstract: Deep learning is increasingly applied as well to cosmological studies, due to its ability of modelling e.g. complex and 'big data' imaging problems in a data-driven fashion. After reviewing recent deep learning studies, I will highlight two applications in particular, the use of deep neural networks to estimate the photometry and shape of galaxies in monochrome space images, similar to the ones that will be delivered by the Euclid space telescope, as well as the direct parameter inference from intensity maps and even tomopraphic imaging (such as 21cm measurements expected for the Square Kilometre Array), that naturally includes information beyond the power spectrum.

16th February 2021

Speaker: Luca Amendola

Institution: Institut für Theoretische Physik (ITP) Heidelberg

Title: Measuring H(z) independently of cosmology

Abstract: I discuss a recent proposal to measure H(z) independently of any assumption concerning the underlying cosmological model, based on standard candles and their spatial and velocity correlations (see arXiv:1912.10255).

9th February 2021

Speaker: Balakrishna S. Haridasu

Institution: SISSA-International School for Advanced Studies

Title: Late- and early-time modifications to resolve $H_0$ tension

Abstract: In this talk, I shall discuss the implications of a few late-time and early-time modifications suggested to alleviate the $H_0$ tension, which is now arriving to a significance of $\sim 5 \sigma$. Contrasting the physics against data these proposals have been put forth as possible resolutions. However, a deeper look at the physics and relevant data from the epochs at which the physics is modified, might not support the claims. We find that the late-time decaying dark matter resolution is indeed unable to resolve the tension. On the other hand, the early time physics within the Cosmic Microwave Background data doesn't show any evidence for the early-time dark energy scenario. We speculate that the possible resolutions for this tension should be looked within the late-time physics and low-$l$ modeling within the CMB analysis.

5th February 2021 (11:00 am) [NOTE UNUSAL DATE AND TIME]

Speaker: Eoin Ó. Colgáin

Institution: Physics Dept., Sogang University, Seoul

Title: Some comments on $H_0$

Abstract: Hubble tension brings model building back into fashion. I will review arguments for and against early Universe resolutions to Hubble tension. I will comment on how one attempts to make model independent statements. Finally, I will spell out the implications for your friendly neighbourhood string theorist (or swamplander).

26th January 2021

Speaker: David Camarena

Institution: Universidade Federal do Espírito Santo (UFES)

Title: Can a $\Lambda$LTB model explain the Hubble tension?

Abstract: Despite current standard cosmology assumes we live in a homogeneous and isotropic universe, inhomogeneous models have been largely considered throughout cosmological history, mainly to explain the accelerated expansion of the universe. Although these models have not survived as an alternative to dark energy, they have been lately presented as a possible solution to the Hubble tension, due they allow for a transition to higher values of the Hubble constant at low redshift. In this talk, I will discuss if the current data supports a inhomogenous $\Lambda$LTB model and if this model is a plausible solution to the Hubble constant problem.

19th January 2021

Speaker: Adrià Gómez-Valent

Institution: Institut für Theoretische Physik (ITP) Heidelberg

Title: Boosting Monte Carlo sampling with a non-Gaussian fit

Abstract: Monte Carlo analyses are a key ingredient in many branches of natural and social sciences. Also in cosmology. They are typically used to sample posterior distributions (built from data) in high-dimensional parameter spaces and infer the confidence regions of the parameters that enter the model under study. When the evaluation of the likelihood is computationally expensive, Monte Carlo analyses can demand prohibitive computational times, even with the use of powerful clusters. In this talk I will describe a new method, called Monte Carlo Posterior Fit, which allows to reduce in some cases an order of magnitude the time spent in the Monte Carlo sampling process. The idea is to approximate the posterior function by an analytical multidimensional non-Gaussian fit. The many free parameters of this fit can be obtained by a smaller sampling than is needed to derive the full numerical posterior, and the evaluation of the resulting analytical distribution can be quite faster than the original one. I will show some examples of the performance of this method in cosmology, based on supernovae and cosmic microwave background data. The method was recently introduced by Prof. Amendola and me in arXiv:2007.02615 [Mon.Not.Roy.Astron.Soc. 498 (2020) 1, 181-193]. Finally, I will present some preliminary results obtained also in collaboration with Dr. Marco Baldi by applying our method to cases in which N-body simulations are required to evaluate the likelihood.

12th January 2021

Speaker: Andreas Doll

Institution: Department of Physics of Heidelberg University

Title: A theory of type-II minimally modified gravity

Abstract: We propose a modified gravity theory that propagates only two local gravitational degrees of freedom and that does not have an Einstein frame. According to the classification by Aoki et al. (JCAP 01 (2019) 017 [arXiv:1810.01047 [gr-qc]]), this is a type-II minimally modified gravity theory. The theory is characterized by the gravitational constant $G_{\rm N}$ and a function $V(\phi)$ of a non-dynamical auxiliary field $\phi$ that plays the role of dark energy. Once one fixes a homogeneous and isotropic cosmological background, the form of $V(\phi)$ is determined and the theory no longer possesses a free parameter or a free function, besides $G_{\rm N}$. For $V'(\phi) = 0$ the theory reduces to general relativity (GR) with $G_N$ being the Newton's constant and $V=const.$ being the cosmological constant. For $V'(\phi) \ne 0$, it is shown that gravity behaves differently from GR but that GR with $G_{\rm N}$ being the Newton's constant is recovered for weak gravity at distance and time scales sufficiently shorter than the scale associated with $V(\phi)$. Therefore this theory provides the simplest framework of cosmology in which deviations from GR can be tested by observational data.

15th December 2020

Speaker: Diego Blas

Institution: King's College London

Title: New ideas on quenching and detecting BH rotational superradiances

Abstract: In this talk I'll discuss two recent results on BH superradiance: first, I will describe how BH photon superradiance is typically quenched by interactions of the photon cloud with the ambient electrons. Second, I will explain how an axionic cloud may impact the CMB if it decays into low energy photons which quickly heat and ionise the surrounding medium to Mpc scales.

8th December 2020

Speaker: Miguel Quartin

Institution: Instituto de Física e Observatório do Valongo, Universidade Federal do Rio de Janeiro

Title: First constraints of the intrinsic CMB dipole

Abstract: The CMB dipole is usually assumed to be completely due to the relative velocity between the solar system and the CMB restframe. We test this hypothesis by measuring independently the Doppler and aberration effects on the CMB using Planck 2018 data on TT and EE. We make improvements upon the measurements by the Planck team and arrive at measurements which are independent from the CMB dipole itself. Combining these new measurements with the dipole one we get the first constraints on the intrinsic CMB dipole. Neglecting a dipolar lensing contribution we can put an upper limit on its amplitude: 3.5 mK (95% CI). We also obtain the estimate of the peculiar velocity of the solar system which does not assume a negligible intrinsic dipole contribution. We also discuss how this approach compares with tests of the dipole in radio galaxies and with local peculiar velocity surveys.

1st December 2020

Speaker: Valerio Marra

Institution: Universidade Federal do Espírito Santo (UFES)

Title: The Hubble-constant crisis

Abstract: The determination of the Hubble constant $H_0$ - the present-day expansion rate of the universe - is one of the key goals of cosmology, for it impacts all measurements of cosmological properties. Determinations of $H_0$ from Cosmic Microwave Background observations disagree with local determinations from calibrated supernovas Ia: this tension is now approaching the 5 sigma level. After reviewing the present status of cosmology, I will present recent work on model-independent determinations of H0 and their implications for cosmologies beyond $\Lambda$CDM.

27th November 2020 (11:00 am) [NOTE UNUSAL DAY AND TIME]

Speaker: Antonio De Felice

Institution: Yukawa Institute for Theoretical Physics (YITP), Kyoto University

Title: Weakening gravity for dark matter in a type-II minimally modified gravity

Abstract: I propose a new cosmological framework in which the strength of the gravitational force acted on dark matter at late time can be weaker than that on the standard matter fields without introducing extra gravitational degrees of freedom. The framework integrates dark matter into a type-II minimally modified gravity that was recently proposed as a dark energy mimicker. The idea that makes such a framework possible consists of coupling a dark matter Lagrangian and a cosmological constant to the metric in a canonically transformed frame of general relativity (GR). On imposing a gauge fixing constraint, which explicitly breaks the temporal diffeomorphism invariance, we keep the number of gravitational degrees of freedom to be two, as in GR. We then make the inverse canonical transformation to bring the theory back to the original frame, where one can add the standard matter fields. This framework contains two free functions of time which specify the generating functional of the above mentioned canonical transformation and which are then used in order to realize desired time evolutions of both the Hubble expansion rate $H(z)$ and the effective gravitational constant for dark matter $G_{\rm eff}(z)$. The aim of this paper is therefore to provide a new framework to address the two puzzles present in today's cosmology, i.e. the $H_0$ tension and the $S_8$ tension, simultaneously. When the dark matter is cold in this framework, we dub the corresponding cosmological model the V Canonical Cold Dark Matter (VCCDM), as the cosmological constant Λ in the standard ΛCDM is replaced by a function V(ϕ) of an auxiliary field ϕ and the CDM is minimally coupled to the metric in a canonically transformed frame.

17th November 2020

Speaker: Javier de Cruz Pérez

Institution: Institute of Cosmos Sciences (ICC), Universitat de Barcelona

Title: BD-$\Lambda$CDM and Running Vacuum Models: Theoretical background and current observational status

Abstract: We study Brans-Dicke gravity with a Cosmological Constant and cold dark matter (BD-$\Lambda$CDM hereafter). This theory is the first historical attempt to extend Einstein's General Relativity by promoting the Newtonian coupling constant into a dynamical entity. We present the background and the perturbation equations, which allow us to test the theoretical predictions with a complete and updated data string, formed by: CMB+BAO+LSS+$H(z)$+SNIa. Additionally, we contemplate the possibility of including alternative data in order to cover a wide variety of different scenarios. The BD-$\Lambda$CDM turns out to be observationally favored as compared to the concordance model (GR-$\Lambda$CDM). We pay special attention to the ability of the BD-$\Lambda$CDM model to smooth out not only the $H_0$-tension but also the $\sigma_8$ one. An exhaustive study can be found in arXiv:2006.04273. Due to the possible connection with the Running Vacuum Models (RVM) (see arXiv:1703.08218 and references therein), where a time-evolving vacuum energy density in the context of QFT is considered, we deem it is worthwhile to also present the background and the perturbation equations, as well as, the performance, of this kind of models, when they are put in the light of the observational data.

10th November 2020

Speaker: Emmanuel Saridakis

Institution: National Observatory of Athens

Title: Cosmology and gravity in the new era of multi-messenger astronomy

Abstract: We present the recent possibility of using multi-messenger astronomy, namely data from gravitational waves observations alongside their electromagnetic counterparts, in order to constrain various theoretical models and scenarios and test general relativity. Additionally, we analyze the possibility of using multi-messenger data as a smoking gun for modified gravitational theories.

3rd November 2020

Speaker: Marcos Pellejero-Ibañez

Institution: Donostia International Physics Center (DIPC)

Title: How to estimate cosmological constraints using the least amount of theoretical evaluations through Gaussian processes

Abstract: From a very naive point of view, we could describe our work as theoretical cosmologists in two main stages: first, the development of models encoding the main physical processes (such as structure formation) and that depend on the physical quantities of interest (such as the amount of Dark Energy), and second, the comparison of these models with actual observations. This last stage can be approached in several ways depending on how the model development went. With simple models, it is usually the case that the evaluation of the underlying theory is computationally cheap and the parameter estimation is done through simple Markov Chain Monte Carlo (MCMC) techniques. However, with the increasing amount of data provided by the cosmological surveys, the need for accurate modeling has lead to the creation of computationally heavy models. Typical MCMC's require hundreds of thousands of model evaluations, which becomes an issue when using heavy computations. In this talk, I'll try to explain how we tackled this problem in the case of the power spectrum in redshift space by using Gaussian processes directly in parameter space. Although we used this power spectrum example in, the method can be easily generalized to different observables.

27th October 2020

Speaker: Seshadri Nadathur

Institution: Institute of Cosmology and Gravitation, University of Portsmouth

Title: Beyond BAO: cosmology with voids in BOSS and eBOSS

Abstract: Large galaxy surveys allow the measurement of baryon acoustic oscillations (BAO) and the full shape of the galaxy power spectrum, which have been key to constraining models of cosmology, especially dark energy. But can we extract yet more information from the data? I will describe a new type of analysis, based on the anisotropic distribution of galaxies around low-density cosmic void regions, which allows simultaneous constraints on the geometry and growth rate of structure in the Universe. I will present results of this measurement applied to the BOSS galaxy data as well as to the latest eBOSS DR16 release. For these surveys the use of voids far outperforms BAO in measurement of the Alcock-Paczynski parameter, and adds information equivalent to quadrupling the survey data volume compared to using galaxy clustering alone. I will then discuss the cosmological implications, including the tightest constraints on curvature and the dark energy equation of state.

This talk will be based on arXiv:1904.01030, arXiv:2001.11044, arXiv:2008.06060 and the eBOSS DR16 results, arXiv:2007.08991.

20th October 2020

Speaker: Hermano Velten

Institution: Universidade Federal de Ouro Preto (UFOP)

Title: Emergence of dissipative effects through cosmological radiation-matter transition

Abstract: We study the evolution of an expanding universe consisting of two perfect fluids which are allowed to interact and establish thermal equilibrium within a certain time scale $\tau$. While both components evolve adiabatically according to their internal perfect fluid structure within the time interval $\tau$, as expected in the standard cosmology, one realizes that the global system experiences the emergence of an effective bulk viscous pressure at background level. We quantify the magnitude of such effect for the early universe around the matter-radiation equality epoch (i.e., $z_{eq}$). As a consequence of this mechanism and depending on the $\tau$-value there are two main consequences, namely: i) since a small negative bulk viscous pressure is added to the effective total pressure this leads to a new contribution to the expansion rate around $z_{eq}$ yielding to a larger $H_0$ value and ii) large scale structure formation is impacted by suppressing the amplitude of matter overdensities growth via a new viscous friction term contribution to the Mészáros effect. Also, similar imprints on the gravitational potential of super-horizon scales arise. Both results have a direct impact on current tensions of the standard cosmological model.

13th October 2020

Speaker: Rubén Arjona Fernández

Institution: Instituto de Física Teórica UAM-CSIC, Universidad Autónoma de Madrid

Title: Exploring the nature of dark energy with Machine Learning

Abstract: One of the most pressing mysteries in physics is the accelerating expansion of the Universe, usually attributed to a dark energy component. The standard model of cosmology, which contains only six free parameters describing the matter and dark energy content of the Universe is so far the best phenomenological fit to the data to percent level precision. Observations of future experiments will provide a vast amount of data for a broad span of redshifts with hundreds of thousands of supernovas type Ia along with millions of galaxies and quasars. Machine Learning (ML) techniques will play a big role in testing accurately the standard model of cosmology, but will also help in the search for new physics and tensions in the data by placing tighter constraints on cosmological parameters. I will present a unified ML analysis of all the currently available cosmological data in order to reconstruct several key background and perturbations variables in a model-independent manner in order to explore the nature of dark energy.

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