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Strongly lensed quasars can be used to measure the Hubble constant, provided the time delays between the lensed images can be measured. The method depends on a model for the lensing galaxy but is fully independent of any other method to measure H0. It is therefore crucial to confirm (or not) the Hubble tension. The method comes however with its own systematics that are currently addressed by the TDCOSMO collaboration. I will explain the method and show its strength and weaknesses and I will show the latest results from the TDCOSMO collaboration.

BIO

I work mostly in observational cosmology with gravitational lensing in all regimes: strong, weak and micro, addressing the problems of Dark Matter, Dark Energy and primordial black holes as well as the study galaxy formation and evolution, quasar inner structure, quasar/galaxy co-evolution. A strong component of my research also resides in pure signal/image processing, with expertise in ill-posed inverse problems, often central to interpret astrophysical observations. I am a regular user of HST, JWST, and most world-class telescopes. In 2022, I was nominated Swiss representative on the ESO Scientific and Technical Committee.

My main achievement has been to transform the field of time delay cosmography from a niche topic to a competitive cosmological probe to address the Hubble tension, one of the most important challenges in modern cosmology. This was acknowledged by my COSMICLENS ERC AdG in 2018. This work also led to the RedH0T ERC Synergy project in 2025, of which I am one of the 4 PIs with Licia Verde, Julien Lesgourgues and Adam Riess.

Host: Dandan Xu