Description du sujet de thèse

Understanding the origin of the Universe's accelerated expansion and the nature of dark energy is one of the major challenges in modern cosmology. To tackle this, large-scale astronomical surveys such as LSST (Vera C. Rubin Observatory), Euclid (ESA), and Roman (NASA) will provide unprecedented and complementary data on billions of galaxies, ushering in a new era of observational cosmology.

Among the key tools leveraged by these surveys is weak gravitational lensing, which enables mapping the matter distribution in the Universe. However, this method is particularly sensitive to blending, the apparent overlap of astronomical sources (galaxies, stars, etc.), which distorts shape, brightness, and redshift measurements—thereby introducing significant systematic biases in cosmological analyses.

This PhD project aims to develop innovative methods to quantify, correct, and mitigate the effects of blending, by leveraging the unique complementarity of the three surveys. In particular, the high-resolution space-based images from Euclid and Roman can be used to train or inform models applied to LSST data, which are deeper but affected by atmospheric turbulence.

The research will unfold in three main phases:

Characterizing blending in early LSST data (commissioning images), using advanced probabilistic methods developed within the host team.

Assessing the impact of blending on key cosmological observables (gravitational shear, galaxy clustering), based on realistic simulations of upcoming surveys.

Developing correction algorithms using machine learning, notably probabilistic deep neural networks capable of integrating multi-band, multi-instrument data. These tools will then be used for a joint analysis of LSST, Euclid, and Roman data, aiming to maximize cosmological information while controlling systematics.

This thesis is embedded in a vibrant international research context. It will contribute to improving the reliability of cosmological measurements from next-generation surveys, with the goal of providing tighter constraints on dark energy properties. The PhD candidate will join the observational cosmology group at LPSC in Grenoble, collaborate closely with the University of Chicago, and be actively involved in the LSST DESC and Euclid international collaborations.

Contexte de travail

The Grenoble Laboratory of Subatomic Physics and Cosmology (LPSC)
(http://lpsc.in2p3.fr) is a joint research unit associating CNRS-IN2P3, Université Grenoble Alpes (UGA) and Grenoble INP school, for an average staff of around 230.
The PhD student will be assigned́ to the “Observational Cosmology” group made up of 10 LPSC staff and will be placed under the direct hierarchical authority of the group/department manager.
His/her thesis supervisor will be Cyrille DOUX.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

radioprotection

Informations complémentaires

PhD funded by a CNRS grant in collaboration with a team at the University of Chicago with regular meetings