5 March 2025: Ian Harrison

Speaker: Ian Harrison (Cardiff)

Date: Wednesday 5 March 2025

Time: 15:00

Location: Physics 3.21

Lensing. Lensing everywhere... and what it tells us about structure formation

I will give an overview of weak gravitational lensing across optical, radio and microwave observations and what it can tell us about cosmology. Any map of distant light emission is distorted by the lensing effect of all of the gravitating matter along the line of sight to the source. These distortions are most often used to measure a combination of the total amount of matter in the Universe and how it clusters together - the 'S8' parameter. Recent measurements of S8 from lensing have been causing consternation, as they (somewhat) disagree with predictions for low-redshift structures made by combining primary CMB measurements with the standard cosmological model. I will describe the various physics — systematics, astrophysics and exotic new physics — which could be causing this disagreement. In the near future we will be able to measure lensing at high signal to noise across all of optical galaxies, radio galaxies and the Cosmic Microwave Background using new facilities like the Vera C. Rubin Observatory, SKAO and the Simons Observatory. I will show how cross-correlations of these lensing maps will both improve robustness to systematics and help us tease out the redshift and scale dependence of measurements of S8 and use this to understand the physics driving structure formation.