CosmosClub: Celine Gouin (20/02/20)

Date: February 20th 2020, 10.00 am

Room: Kepler

Speaker: Céline Gouin (IAS, COSMIX)

Title: Probing the azimuthal environment of galaxies around clusters. From cluster core to cosmic filaments


Galaxy clusters are connected at their peripheries to the large scale structures by cosmic filaments that funnel accreting material.Therefore, the vicinity of galaxy clusters are ideal places to quantify the geometry and topology of the cosmic web.These filamentary structures are studied to investigate both environment-driven galaxy evolution and the growth of massive structures. In this presentation, I probe angular features in the distribution of galaxies around clusters by performing harmonic decompositions in large photometric galaxy catalogues around low-z clusters. In the clusters’ outskirts, filamentary patterns are detected in harmonic space: massive clusters seem to have a larger number of connected filaments than low-mass ones. Our results suggest also a gradient of galaxy activity in filaments around clusters.

CosmosClub: Irène Waldspurger (04/12/19)

Date: December 4rd 2019, 10.30am

Speaker: Irène Waldspurger (CEREMADE,  Université Paris-Dauphine)

Title: Convex and non-convex algorithms for phase retrieval

Room: Cassini 


Phase retrieval problems consist in recovering elements of a complex vector space from the modulus of their scalar product with a fixed family of measurement vectors. Traditional reconstruction algorithms rely on simple local optimization heuristics. Although they can in principle, because of the non-convexity of the problem, get stuck in local optima, they are observed to work well in many situations.

In this talk, we will see which theoretical correctness guarantees one can establish, in a particular setting, for the most well-known such algorithm. We will also present a different family of algorithms, based on so-called convexification techniques, describe its advantages and limitations.



CosmosClub: Miguel Zumalacarregui (06/11/19)

Testing Gravity and Dark Energy with Cosmology and Gravitational Waves

Date: November 06th 2019, 15h30

Speaker: Miguel Zumalacarregui (UC Berkeley & IPhT Saclay)

Title: Testing Gravity and Dark Energy with Cosmology and Gravitational Waves

Room: Cassini


Alternative theories of gravity may provide viable models of cosmic acceleration with the possibility of alleviating shortcomings of the standard paradigm such as discrepant measurement of the Hubble parameter. I will present recent progress in constructing viable, yet extremely predictive theories of gravity and dark energy, extracting their cosmological implications and testing them with data, current and forthcoming. I will also present how most of these theories affect the propagation of gravitational waves. In particular, the speed of gravitational waves provides the most stringent test for a large class of theories, which have been recently ruled out by the GW speed measurement following the neutron star merger GW170817. Other effects on gravitational wave propagation (damping, modified dispersion and oscillations) can be used to test the landscape of gravitational theories.

CosmosClub: Fangchen Feng (10/10/19)

Date: October 10th 2019, 15h00

Speaker: Fangchen Feng (Laboratoire Astroparticule & Cosmologie)

Title: Reconstruction and characterisation of polarisations of a gravitational-wave signal

Room: Kepler


Polarisation properties of gravitational waves carry crucial information about the physics of gravitational sources (binary compact systems of black holes or neutron stars, etc. ) such as precession effects. In practice, the reconstruction of the two polarizations h+(t) and h×(t) is made possible by the use of at least two non-aligned detectors. To this aim, we propose a complete analysis procedure of gravitational-wave signals. Starting from measurements, this procedure estimates the sky position of the source, reconstructs the two components h+(t) and h×(t) and estimates instantaneous Stokes parameters of the wave. This set of non-parametric observables encodes many fine properties of the astrophysical source without close bounds to a specific dynamical model, making them particularly suited to decipher precession effects.

CosmosClub: Catherine Heymans (22/10/19)

Date: October 22nd 2019, 11am

Speaker: Catherine Heymans (ROE, Edinburgh)

Title: low-z-vs-CMB tensions with KiDS and DES and  photo-z calibration

Room: Kepler


Catherine Heymans will give a 15-20min introduction on recent work on low-z-vs-CMB tensions with KiDS and DES ( and some more recent work on photo-z calibration. Then we will have a "bring a plot" session, in which each of us brings the plot we are most excited about and we comment and discuss on it together.

CosmosClub: Sebastian Rojas Gonzalez (12/07/19)

Date: July 12th 2019, 11am

Speaker: Sebastian Rojas Gonzales (KU Leuven)

Title: Gaussian processes for simulation optimization

Room: Kepler


The use of kriging metamodels (also known as gaussian processes) in simulation optimization has become increasingly popular during recent years. The majority of the algorithms so far uses the ordinary (deterministic) kriging approach for constructing the metamodel, assuming that observations have been sampled with infinite precision. This is a major issue when the simulation problem is stochastic: ignoring the noise in the outcomes may lead to inaccurate predictions. In this work, we propose a stochastic multiobjective simulation optimization algorithm that contains two crucial elements: the search phase implements a kriging method that is able to account for the inherent noise in the outputs when constructing the metamodel, and in the identification phase uses a Bayesian multiobjective ranking and selection procedure in view of maximizing the probability of selecting the true non-dominated points by optimally allocating the available computational budget. We evaluate the impact of these elements on the search and identification effectiveness on a set of artificial test problems with varying levels of heteroscedastic noise. Our results show that the characterization of the noise is crucial in improving the prediction efficiency; yet, the allocation procedure appears to lose effectiveness in settings with high noise. This emphasizes the need for further research on multiobjective ranking and selection methods.

CosmosClub: Doogesh Kodi Ramanah (20/06/2019)

Date: June 20th 2019, 11am

Speaker: Doogesh Kodi Ramanah (IAP)

Title: Fast complex dynamics emulators for cosmological inference

Room: Cassini


I will present an overview of our recent work in developing various aspects of Bayesian forward modelling machinery for an optimal exploitation of state-of-the-art galaxy redshift surveys. I will focus on the development of a generative model for mapping dark matter simulations to 3D halo fields using physically motivated neural networks. We employ the Wasserstein distance as a metric to train our halo painting emulator and demonstrate its efficacy in predicting 3D halo distributions using summary statistics such as the power spectrum and bispectrum. I will subsequently briefly review our novel cosmological parameter inference framework that extracts several orders of magnitude more information from the cosmic expansion relative to standard approaches, and a sophisticated likelihood that is robust to unknown foreground contaminations.

CosmosClub: Isabella Carucci (13/06/2019)

Date: June 13th 2019, 10:30am (/!\ unusual time)

Speaker: Isabella Carucci (CosmoStat)

Title: 21cm intensity mapping: using cosmic neutral hydrogen as tracer of large scale structure

Room: Kepler


This is an opportunity to introduce my past work to the group. I’ll start by briefly illustrating what we mean by intensity mapping (IM) and why to use the 21cm radiation coming from neutral hydrogen. I’ll then move to what its observational status is and how we can model this signal. In particular, I’ll show how IM will be a remarkable test both for dark energy and dark matter models, presenting forecasts for the bounds that the SKA telescope will be able to uniquely set. On the other hand, this signal is literally buried under foregrounds (about 4 orders of magnitude more intense). In this spirit, I’ll conclude by sketching what our plans are to use and optimise Cosmostat-developed tools for tackling the foreground and instrumental systematics problems of IM.

CosmosClub: Jean-Baptise Bayle (23/05/2019)

Date: May 23rd 2019, 11am

Speaker: Jean-Baptise Bayle (APC)

Title: Space-based Detection of Gravitational Waves with LISA: Opening the Low-Frequency Part of the Gravitational Spectrum [slides]

Room: Cassini


Einstein’s theory of General Relativity offers a powerful picture of the world, where the gravitational force is only the consequence of the curvature of spacetime. Gravitational waves are very tiny ripples in the fabric of spacetime, propagating at the speed of light. They do not interact much with the matter encountered on their way, making them very good messengers to probe astrophysical events at a distance... but also very difficult to detect. Scientists have looked for gravitational waves on Earth for more than 40 years and have developed incredibly precise instruments. In 2015, the first direct detection of such waves was announced by the LIGO and Virgo community: gravitational astronomy had opened a new observational window on the distant Universe. Unfortunately the limited sensitivities of such ground-based detectors does not enable the observation of the most massive object.

In two decades, the Laser Interferometer Space Antenna (LISA) mission will join the network of ground detectors. LISA aims to measure gravitational waves in the millihertz range, to help answer numerous astrophysical, cosmological and theoretical questions. Three spacecraft will orbit the Sun in a nearly equilateral triangular formation, with arm lengths of about 2.5 million kilometers. Each spacecraft will host two spacetime probes. Laser beams, exchanged between the spacecraft, will be reflected upon the probes and then made to interfere, in order to detect passing gravitational waves.

During this conversation, I will briefly recap how gravitational waves arise from Einstein equations, and present the astrophysical and cosmological sources expected throughout the gravitational spectrum. I will then describe the main detection principles, from ground-based interferometry to pulsar timing. I will then focus on the LISA mission, and discuss my PhD work on data pre-processing and instrumental modeling.