CosmoSClub: 09-10-2018

Date: October 9th 2018

Speaker: Chieh-An Lin (IfA, University of Edinburgh)

Title: Predicting weak-lensing covariance with a fast simulator

Weak lensing has been shown as an outstanding tool to constrain cosmology. The state-of-the-art studies have used the power spectrum and peak counts as estimators, and the combination of the two can break down parameter degeneracies and maximize the information extraction.

To constrain cosmology with both estimators, understanding the joint covariance is crucial. However, calculating it analytically seems to be intractable for peaks, and the empirical approach with N-body simulations will be expensive as the size of lensing surveys increase.

I will present a fast solution to solve this problem. The proposed approach simulates lognormal fields and halo models to predict lensing signals. We compared the resulting joint covariance with the one from a large number of N-body simulations and found an excellent agreement. In addition, our approach is orders of magnitude faster than N-body runs.

CosmoSClub: 13-09-2018

Date: September 13th 2018

Speaker: Benjamin l'Huiller (Korea Astronomy and Space Science Institute)

Title: Cosmological structure formation in LCDM and beyond: Testing LCDM with N-body simulations and advanced statistical methods [slides]

The current concordance cosmological paradigm relies on a few assumptions: gravity is described by General Relativity, the Universe is Homogeneous and Isotropic on large scales, and a phase of inflation in the early Universe. Under these assumptions, the solution to the Einstein Equations is the Friedmann—Lemaître—Robertson—Walker (FLRW) metric, a general metric describing an expanding Universe. Observationally, the Universe seems flat, dominated by dark energy, thought to be responsible for the late-time acceleration of the Universe, and by a smooth dark matter component. Albeit reasonable, these are all assumptions. Therefore, it is important to test these assumptions in order to falsify the concordance model. 
In the first part of my talk, I will show how to probe extension to the LCDM paradigm via cosmological simulations (Modified Gravity and dark energy, primordial power spectrum): how do haloes form in modified gravity? can we use the large-scale structure to probe features in the primordial power spectrum?
I will then move on to the falsification of the concordance model via model-independent tests of the concordance model from the data at the background (FLRW metric, flatness, Lambda dark energy) and the perturbation (growth rate gamma), and obtain model-independent constraints on some key cosmological parameters. 

CosmoSClub: 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: 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: 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: 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: 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.