The Hubble tension—the persistent discrepancy between local and early-Universe measurements of the Hubble constant—remains one of the most intriguing puzzles in cosmology. The James Webb Space Telescope (JWST) now offers a fresh perspective on this issue by allowing an independent look at the same type of stars, Cepheids, used in the Hubble Space Telescope (HST) measurements that help define our best local estimate of cosmic expansion.  I’ll show how early JWST data, although still limited in size, serves as a powerful crosscheck of the HST-based distance ladder. When comparing results across multiple techniques and research groups, we find strong consistency with the HST measurements, lending confidence to their accuracy. These comparisons suggest that the observed tension is unlikely to stem from systematic errors in HST's Cepheid distances. Though JWST’s smaller sample size limits its precision for now, it already provides valuable validation of the HST approach. As more data accumulates, JWST will play an increasingly important role in testing and refining our understanding of the expanding Universe—and perhaps help us get to the bottom of the Hubble tension.

Bio: Adam Riess is a Bloomberg Distinguished Professor at Johns Hopkins University, a distinguished astronomer at the Space Telescope Science Institute and a member of the National Academy of Sciences.
He was awarded the 2011 Nobel Prize in Physics for his leadership in the High-z Supernova Search Team’s discovery that the expansion rate of the universe is accelerating. Currently, he leads the SH0ES Team in efforts to improve the measurement of the Hubble Constant and the HIgher-z Team to find and measure the most distant type Ia supernovae known to probe the origin of cosmic acceleration.

This Colloquium is part of the Invisibles25 Workshop (webpage)

Coffee and tea will be served at 16h

Organised by : Valerie Domcke, Miguel Escudero and Alexander Zhiboedov