CosmosClub: Florent Mertens (10/03/20)

Date: March 18th 2020, 10.30am

Speaker: Florent Mertens (LERMA / Kapteyn Astronomical Institute)

Title: The challenges of observing the Epoch of Reionization and Cosmic Dawn

Room: Cassini 

Abstract

Low-frequency observations of the redshifted 21cm line promise to open a new window onto the first billion years of cosmic history, allowing us to directly study the astrophysical processes occurring during the Epoch of Reionization (EoR) and the Cosmic Dawn (CD). This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under astrophysical foregrounds orders of magnitude brighter and contaminated by numerous instrumental systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are currently underway aiming at statistically detecting the 21-cm brightness temperature fluctuations from the EoR and CD. While no detection is yet in sight, considerable progress has been made recently. In this talk, I will review the many challenges faced by these difficult experiments and I will share the latest development of the LOFAR Epoch of Reionization and NenuFAR Cosmic Dawn key science projects.

 

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 

Abstract

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.

[Slides]

 

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


Abstract

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


Abstract

Catherine Heymans will give a 15-20min introduction on recent work on low-z-vs-CMB tensions with KiDS and DES (https://arxiv.org/abs/1906.09262) 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.