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.



Radio Astronomical Images Restoration with Shape Constraint


Journal: Proceedings of SPIE
Year: 2019
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Weak gravitational lensing is a very promising probe for cosmology that relies on highly precise shape measurements. Several new instruments are being deployed and will allow for weak lensing studies on unprecedented scales, and at new frequencies. In particular, some of these new instruments should allow for the blooming of radio-weak lensing, specially the SKA with many Petabits per second of raw data. Hence, great challenges will be waiting at the turn. In addition, processing methods should be able to extract the highest precision possible and ideally, be applicable to radio-astronomy. For the moment, the two methods that already exist do not satisfy both conditions. In this paper, we present a new plug-and-play solution where we add a shape constraint to deconvolution algorithms and results show measurements improvement of at least 20%.

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.