Found 88 talks archived in Cosmology

Tuesday September 13, 2022
Dr. Martín López Corredoira


Cosmological observations (redshifts, cosmic microwave background radiation, abundance of light elements, formation and evolution of galaxies, large-scale structure) find explanations within the standard Lambda-CDM model, although many times after a number of ad hoc corrections. Nevertheless, the expression ‘crisis in cosmology’ stubbornly reverberates in the scientific literature: the higher the precision with which the standard cosmological model tries to fit the data, the greater the number of tensions that arise. Moreover, there are alternative explanations for most of the observations. Therefore, cosmological hypotheses should be very cautiously proposed and even more cautiously received.

There are also sociological and philosophical arguments to support this scepticism. Only the standard model is considered by most professional cosmologists, while the challenges of the most fundamental ideas of modern cosmology are usually neglected. Funding, research positions, prestige, telescope time, publication in top journals, citations, conferences, and other resources are dedicated almost exclusively to standard cosmology. Moreover, religious, philosophical, economic, and political ideologies in a world dominated by anglophone culture also influence the contents of cosmological ideas.

Tuesday May 10, 2022
Dr. Alessia Ritacco


The search for the primordial B-modes polarization in the cosmic microwave background (CMB) radiation,
carrying the signature of the primordial gravitational waves from the inflation epoch, motivated a significant
technological progress enabling the next generation of CMB instruments (e.g. CMB-S4, LiteBIRD)
to reach an unprecedented sensitivity. However, such a challenging detection demands a very high control
of the instrumental systematics and CMB foreground emissions.
Among those, the galactic dust polarized emission spectral dependence, not yet fully
characterized, could leave a high level of uncertainty in the cosmological polarization data
producing an ambiguous detection of the CMB B-modes.
Characterizing the dust spectral energy distribution (SED) spatial variations became one of
the most critical issues in the quest for primordial B-modes.
In the work that I will present we have used the release of the Planck satellite HFI data
obtained with the software Sroll2 (Delouis+2019, A&A 629, A38), in order to characterize
and compare the SEDs for polarization and total intensity.
The mean SEDs for dust polarization and total intensity from 353 to 100 GHz are confirmed
to be remarkably close. However, the data show evidence for spatial variations of the
polarization SED. These variations are correlated with variations of dust temperature
measured on total intensity data but the correlation is tight only in the Galactic plane.
At higher latitudes, by considering 90% of useful sky fraction and less, the amplitude of the dust
emission residuals in polarization suggests that an additional contribution, coming from
variations of the polarization angle, becomes dominant. Current models, which extrapolate
the SED spatial variations from total intensity to polarization, would be therefore grossly
simplifying and underestimating the foreground signal to CMB polarization.

Thursday February 10, 2022
Dr. Marcos Pellejero


A key problem that we are facing in cosmology nowadays is that we cannot make accurate predictions with our current theoretical models. We have all of the pieces of the standard model but it doesn't have an analytical solution. The only way to have accurate predictions is to run a cosmological simulation. Then, why not use these simulations as the theory model? Well, for one main reason, if we want to explore the full parameter space comprised in the standard model, we need thousands of such simulations, and they are terribly computationally expensive. We wouldn't be able to do it in years! In this talk, I will tell you how in the last few years we have come up with a way to circumvent this problem.

Thursday November 4, 2021
Dr. Sergio Contreras



On the LCDM cosmology, dark matter collapses into virialised objects called haloes. The abundance and distribution of these haloes are a direct consequence of the cosmology of the Universe. By constraining the dark matter halo clustering, we could also constraint the cosmology from our Universe. Since dark matter haloes can not be observed, we need to use galaxies to trace them.

In this talk, I will present a new method that we develop capable of constraining cosmological information from the redshift space galaxy clustering.  We use the scaling of cosmological simulations and the SubHalo Abundance Matching extended (SHAMe) empirical model to produce realistic galaxy clustering measurements over a wide range of cosmologies. We generate more than 500,000 clustering measurements at different cosmological and SHAMe parameters to build an emulator capable of reproducing the projected correlation function, monopole and quadrupole of the galaxies. We run an MCMC using this emulator to constrain the cosmology of the TNG300 hydrodynamic simulation. We correctly predicted the cosmology of the TNG300 simulation constraining sigma8 between [0.75,0.83] and Omega matter h^2 between [0.127,0.162]. The best constraints are obtained when including scales below 2 Mpc/h and when combining all different clustering statistics. We conclude that our approach can be used to constrain cosmological and galaxy formation parameters from the galaxy clustering of galaxy surveys.

Tuesday October 19, 2021
Dr. Alberto Dominguez


The light emitted by all galaxies across the history of the Universe is encoded in the intensity of the extragalactic background light (EBL), the diffuse cosmic radiation field at ultraviolet, optical, and infrared wavelengths. The EBL is a source of opacity for very high energy gamma rays via pair production, leaving a characteristic attenuation imprint in the spectra of distant gamma-ray sources. In this seminar, I will report on new measurements of the EBL using gamma-ray data from both the Large Area Telescope on board the Fermi Gamma-ray Space Telescope and ground-based Imaging Atmospheric Cherenkov Telescopes. These unprecedented measurements have allowed us to derive the cosmic star-formation history, the number density of faint galaxies during the re-ionization epoch, and also the expansion rate of the Universe and its matter content. These results demonstrate that gamma-ray astrophysics has matured to the point of providing competitive measurements of cosmic properties previously restricted to techniques used by more traditional astronomy.

Thursday September 16, 2021
Dr. Siddharth Mishra-Sharme


The next decade will see a deluge of new cosmological data that will enable us to accurately map out the distribution of matter in the local Universe, image billions of stars and galaxies to unprecedented precision, and create high-resolution maps of the Milky Way. Signatures of new physics as well as astrophysical processes of interest may be hiding in these observations, offering significant discovery potential. At the same time, the complexity of astrophysical data provides significant challenges to carrying out these searches using conventional methods. I will describe how overcoming these issues will require a qualitative shift in how we approach modeling and inference in cosmology, bringing together several recent advances in machine learning and simulation-based (or likelihood-free) inference. I will ground the talk through examples of proposed analyses that use machine learning-enabled simulation-based inference with an aim to uncover the identity of dark matter, while at the same time emphasizing the generality of these techniques to a broad range of problems in astrophysics, cosmology, and beyond.

Meeting ID: 831 9395 9785
Passcode: 343950O


Thursday September 9, 2021
Dr. Carlos Hernández-Monteagudo


In cosmology, it is customary to convert observed redshifts into distances in order to study the large scale distribution of matter probes like galaxies and quasars, and to obtain cosmological constraints thereof. In this talk, I describe a new approach which bypasses such conversion and studies the "field of redshifts" as a new cosmological observable, dubbed thereafter as angular redshift fluctuations (ARF). By comparing linear theory predictions to the output of N-body cosmological simulations, I will show how the ARF are actually sensitive to both the underlying density and radial peculiar velocity fields in the universe, and how one can obtain cosmological and astrophysical constraints from them. And since "the prove of the pudding is in the eating", I will demonstrate how ARF provide, under a very simple setup, competitive constraints on the nature of peculiar velocities and gravity from BOSS DR13 data. Furthermore, I will also show that by combining ARF with maps of the cosmic microwave background (CMB), we can unveil the signature of the missing (and moving) baryons, doubling the amount of detected baryons in disparate cosmic epochs ranging from z=0 up to z=5, and providing today's most precise description of the spatial distribution of baryons in the universe.


Thursday June 3, 2021
Dr. Javier Redondo
Universidad de Zaragoza


We introduce the strong CP problem and the existence of the Axion as a possible solution. 

We discuss the possibility that axions are the dark matter of the Universe and the possible ways to

detect it or disprove it using: direct laboratory experiments as well as astrophysical and cosmological


Tuesday May 25, 2021
Drs. Olga Mena


In this talk, we shall review the impact of the neutrino properties on the different cosmological observables. We shall also present the latest cosmological constraints on the neutrino masses and on the effective number of relativistic species. Special attention would be devoted to the role of neutrinos in solving the present cosmological tensions.

Thursday May 13, 2021
Prof. Diego Blas
Imperial College


Bosonic ultra-light dark matter (ULDM) in the mass range m ~ $10^{-22} - 10^{-21} \rm eV$ has been invoked as a motivated candidate with new input for the small-scale `puzzles' of cold dark matter. Numerical simulations show that these models form cored density distributions at the center of galaxies ('solitons'). These works also found an empirical scaling relation between the mass of the large-scale host halo and the mass of the central soliton. We show that this relation predicts that the peak circular velocity of the outskirts of the galaxy should approximately repeat itself in the central region. Contrasting this prediction to the measured rotation curves of well-resolved near-by galaxies, we show that ULDM in the mass range m ~ $10^{-22} - 10^{-21} \rm eV$ is in tension with the data.