# ITP Cosmology Seminars

Tuesdays, 3:15 pm, Webinar Room:

#### Organizers: Luca Amendola and Adrià Gómez-Valent

### Seminar calendar

** 2nd March 2021 **

**Speaker:** Chiara Caprini

**Institution:** Laboratoire Astroparticule et Cosmologie, Université de Paris

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** 23th February 2021 **

**Speaker:** Caroline Heneka

**Institution:** University of Hamburg

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** 16th February 2021 **

**Speaker:** Luca Amendola

**Institution:** Institut für Theoretische Physik (ITP) Heidelberg

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** 9th February 2021 **

**Speaker:** Balakrishna S. Haridasu

**Institution:** SISSA-International School for Advanced Studies

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** 2nd February 2021 **

**Speaker:** Raquel Emy Fazolo

**Institution:** Universidade Federal do Espírito Santo (UFES)

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** 26th January 2021 **

**Speaker:** Adrià Gómez-Valent

**Institution:** Institut für Theoretische Physik (ITP) Heidelberg

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** 19th January 2021 **

**Speaker:** David Camarena

**Institution:** Universidade Federal do Espírito Santo (UFES)

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** 12th January 2021 **

**Speaker:** Andreas Doll

**Institution:**

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** 15th December 2020 **

**Speaker:** Diego Blas

**Institution:** King's College London

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** 8th December 2020 **

**Speaker:** Miguel Quartin

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

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** 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 https://arxiv.org/pdf/1912.08806.pdf, 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.