Sacha Guerrini

Research Interests

The concordance model of cosmology, LCDM, predicts that the majority of the energy content of the Universe is in the dark sector. Unveiling the true nature of dark energy and dark matter is therefore a major stake of contemporary cosmology. The Euclid space mission is aimed at measuring the shape of galaxies on a large area on the sky to probe gravitational effects related to the latter forms of energy. I am currently undertaking a PhD called "Weak gravitational lensing statistics for the Euclid space mission" under Martin Kilbinger's supervision. The upcoming Stage-IV surveys such as Euclid will bring the tightest constraints on the growth of structure and evolving dark energy. I apply the tools I develop to compute those constraints within the UNIONS collaboration and the Euclid Consortium.

Part of my work focuses on caracterising the systematics coming from the Point Spread Function (PSF) in the measured signal. PSF systematics can induce additive and multiplicative bias on the shear measurements that can bias the estimate of the growth of structure. I also have interest in modern inference technique such as Implicit Likelihood Inference (also known as Simulation-Based Inference). I developed a forward model for the UNIONS survey and a toolbok for Implicit Likelihood Inference (JaxILI).

Papers

S. Guerrini, M. Kilbinger, H. Leterme, A. Guinot, et al. 2024. Galaxy-Point Spread Function correlations as a probe of weak lensing systematics with UNIONS data. accepted to A&A.

S. Guerrini & E. Mörtsell. 2023. Probing a scale dependent gravitational slip with galaxy strong lensing systems. Phys. Rev. D, 109, 023533.

N. Arendse, E. Mörtsell, L. Weisenbach, E. Hayes, et al. (incl. SG). 2025. Microlensing of lensed supernovae Zwicky & iPTF16geu: constraints on the lens galaxy mass slope and dark compact object fraction. submitted to OJAp.

Collaboration papers within UNIONS and Euclid available on the arxiv.

 

Education

Prior to my PhD, I obtained my Master's degree Ecole Polytechnique, completing my thesis "Strong Gravitational Lensing and Modified Gravity" under the supervision of Prof. Edvard Mörtsell at Stockholm University. During my course of study, I had the opportunity to work on two award-winning projects. The former in theoretical maths on the spectral rigidity of ellipses of small excentricties and the latter on the vertical dynamics of galactic disks during a research project under the supervision of Prof. James Binney at the university of Oxford.