CosmosClub: Santiago Casas (09/07/2018)

Date: July 9th 2018

Speaker: Santiago Casas (CEA Saclay)

Title: Dark Energy with Euclid

Euclid is an ESA medium-class mission expected to launch in 2020 that will map the geometry of the Universe by imaging 10^9 galaxies and measuring 10^7 galaxy redshifts in 15000 square degrees of the sky. This will provide us detailed information about the accelerated expansion, the evolution of large-scale structure and the matter-energy content of the Universe up to a redshift of about z≈2.
In this talk, I will review how the main probes of Euclid, namely galaxy clustering and weak lensing, will be able to constrain theories beyond the standard cosmological ΛCDM model and how we will be able to pin down the equation of state of dark energy with about 1% precision. Galaxy clustering measures mainly the movement of tracers along geodesics, while weak lensing is an almost direct mapping of the gravitational potentials at large scales. Using both of these observables, we can obtain valuable information about the growth of perturbations and the geometrical quantities of the Universe and therefore constrain the properties of General Relativity. Since the measurements of Euclid will also give insights on the properties of dark matter and neutrinos at cosmological scales, I will also show how we can measure non-standard couplings between matter species and dark energy and how we can give tight constraints on many alternative theories of gravity.

CosmosClub: Viviana Niro (25/06/2018)

Date: June 25th 2018

Speaker: Viviana Niro (University of Heidelberg)

Title: Galaxy rotation curves in modified gravity models [slides]

I'll present the possibility that galaxy rotation curves can be explained in the framework of modified gravity models. A Yukawa term is considered in the gravitational potential and dark matter is included in the fit.
Using a set of 40 galaxy rotation curves data from the SPARC catalogue, I'll present constraints on the strength, β, and the range, λ, of the Yukawa fifth force. The global best-fit is found to be β=0.34±0.04 and λ=5.61±0.91 kpc and the dark matter content is on average 20% smaller than in the standard gravity parametrization. The Bayesian evidence for a NFW profile and a Yukawa term is higher than 8σ with respect to the case without the Yukawa term.

CosmosClub: Alessio Spurio Mancini (03/05/2018)

Date: May 3rd 2018

Speaker: Alessio Spurio Mancini

Title: Weak gravitational lensing as a (3D) probe of gravity


Cosmic shear, the weak gravitational lensing effect caused by the
large-scale structure, is one of the primary probes to test gravity on
cosmological scales with current and future surveys. In particular,
cosmic shear is sensitive to both standard GR cosmological parameters
and those that describe modified theories of gravity, such as those
belonging to the Horndeski class. These models include the majority of
universally coupled extensions to ΛCDM with one scalar degree of freedom
in addition to the metric, which are still in agreement with current
In this talk I will discuss some aspects concerning the constraining
power of current and future cosmic shear datasets on this broad class of
theories. I will compare in particular two main techniques to analyse a
cosmic shear survey: a tomographic method, where correlations between
the lensing signal in different redshift bins are used to recover
redshift information, and a 3D approach, where the full redshift
information is carried through the entire analysis. Due to its increased
amount of redshift information, a future 3D analysis can constrain both
standard gravity and Horndeski theories better than a tomographic one,
in particular with a decrease in the errors on the Horndeski parameters
of the order of 20%.

CosmosClub: Benjamin Joachimi (13/04/2018)

Date: April 13th 2018

Speaker: Benjamin Joachimi (University College London)

Title: Cosmic shear cosmology - where we stand


I will review the recent weak lensing cosmology results obtained by the ESO Kilo-Degree Survey, which display an intriguing, marginal discrepancy with the primary Planck CMB constraints on structure growth. Key analysis choices and challenges will be highlighted, and new approaches to validating the measurements presented. I will also briefly discuss the relation to the Dark Energy Survey Year 1 results and some lessons learnt for the forthcoming generation of cosmological galaxy surveys.

CosmosClub: Elena Sellentin (05/04/2018)

Date: April 5th 2018

Speaker: Elena Sellentin (University of Geneva)

Title: The skewed weak lensing likelihood: why biases arise, despite data and theory being sound


We derive the essentials of the skewed weak lensing likelihood via a simple Hierarchical Model. Our likelihood passes four objective and cosmology-independent tests which a standard Gaussian likelihood fails. We demonstrate that sound weak lensing analyses are naturally biased low, and this does not indicate any new physics such as deviations from ΛCDM. Mathematically, the biases arise because noisy two-point functions follow skewed distributions. This form of bias is already known from CMB analyses, where the low multipoles have asymmetric error bars. Weak lensing is more strongly affected by this asymmetry as galaxies form a discrete set of shear tracer particles, in contrast to a smooth shear field. We demonstrate that the biases can be up to 30 percent of the standard deviation per data point, dependent on the properties of the weak lensing survey. Our likelihood provides a versatile framework with which to address this bias in future weak lensing analyses.

CosmosClub: Anais Möller (28/03/2018)

Date: March 28th 2018

Speaker: Anais Moller (Australian National University)

Title: The next-generation of type Ia supernovae Dark Energy studies


Currently Dark Energy studies with type Ia supernovae rely on a spectroscopically classified sample. The Dark Energy Survey (DES) is entering its last year of observations and a first cosmological analysis with the spectroscopically confirmed supernova sample is on the way. However, present and future surveys as DES and LSST will do cosmological analysis with a photometrically classified type Ia supernova sample. For this, a reliable photometric classification is necessary, which can process large number of candidates and obtain a high-purity sample. 
In this talk, I will first present preliminary cosmological parameter constraints from the first 3-years of the DES supernova survey. The sample is composed by  251 spectroscopically confirmed Type Ia Supernovae (0.02 < z < 0.9) discovered during the first 3 years of the Dark Energy Survey Supernova Program. I will also discuss about the future analysis with the DES 5-year photometric supernova sample. In particular, I will discuss a photometric classification method based on recurrent neural networks that can classify quickly large number of supernovae with high accuracy using only photometric measurements and time as input. This method includes a bayesian interpretation of classification probabilities which will be fundamental for a cosmology analysis. In addition, this method also classifies partial light-curves with high accuracy and speed which will allow to distribute resources towards promising candidates and can be applied to other transients.

CosmosClub: Nicolas Martinet (07/03/2018)

Date: March 7th 2018

Speaker: Nicolas Martinet (Universität Bonn)

Title: Cosmological constraints from shear peaks in the Kilo Degree Survey (KiDS)


The peak statistic in weak lensing reconstructed mass maps is a new powerful tool to probe cosmology. Peaks trace the Universe large scale structure, but on the contrary to the two-point correlation function of the shear, they are also sensitive to the non-Gaussian part of the matter distribution. In that sense they are similar to galaxy clusters. In this talk, I will introduce weak lensing peak statistics and review its application to the Kilo Degree Survey (Martinet, Schneider, Hildebrandt et al. 2018, MNRAS 474, 712). I will in particular discuss how combining shear peaks with the two-point correlation function of the shear can improve cosmological constraints.


CosmosClub: Sandrine Codis (01/02/2018)

Date: February 1st 2018

Speaker: Sandrine Codis (IAP)

Title: Intrinsic alignments : theoretical and numerical insights


In this talk, I will show how both dark matter and hydrodynamical simulations predict that the morphology of galaxies is correlated with the cosmic web. This large-scale coherence of galaxy shapes could possibly induce some non-negligible level of contamination for future cosmic shear experiments. Because this effect is very sensitive to the small scale baryonic physics, it is difficult to use dark matter-only simulations as the sole resource to predict and control intrinsic alignments. I will show how state-of-the-art hydrodynamical simulations like Horizon-AGN can be used to shed light on the level of intrinsic alignment we should expect for future weak lensing measurements.  On the theoretical side, I will describe an analytical Lagrangian model that reproduces qualitatively the correlations between the intrinsic angular momentum of galaxies and the cosmic filaments. The key ingredient is to take into account the anisotropy of the cosmic web in the standard theory of spin acquisition by tidal torquing.