"I'm seeing double! 4 quasars!" - An Overview of Gravitational Lensing and Time-delay Cosmography

by Cameron Lemon (Stockholm University)

Friday 3 Oct 2025, 16:00 → 17:30 Europe/Stockholm

Floor 6 - Fika Area (AlbaNova Main Building)

Gravitational lensing is the bending and distortion of light by massive objects like stars and galaxies. In rare cases—strong lensing—it produces multiple images of the same background source. Thanks to wide-field sky surveys and modern discovery methods, more than 1,000 strong lenses are now known. In this talk I'll present a history of gravitational lensing: early predictions, how Einstein got lucky, how Einstein was wrong, accidental discoveries, and modern telescope efforts. I will focus on one powerful application—time-delay cosmography—showing how differences in arrival times between images allow us to measure the Universe’s expansion rate, addressing the Hubble tension, today’s biggest debate in cosmology.

Note: This event is open to the junior audience only, including undergraduate, master’s, and PhD students, as well as postdocs.