Abstract: One of the most basic questions in cosmology is: How fast is the Universe expanding today? Two powerful approaches give answers that should agree but do not. Measurements based on the cosmological model calibrated by the cosmic microwave background predict one value for the Hubble constant H0, while direct measurements using the cosmic distance ladder give a higher value. The difference, now exceeding 5σ, is known as the Hubble tension and has persisted for more than a decade and challenges LambdaCDM. The James Webb Space Telescope (JWST) provides a new way to test the local distance ladder that underlies the direct measurement of H0. By observing Cepheid variable stars and other distance indicators in galaxies that host Type Ia supernovae, and that were previously studied with the Hubble Space Telescope (HST), JWST allows a direct and independent check of those earlier measurements at higher resolution. These observations are also part of a broader effort to build a more robust local distance network, combining multiple independently calibrated distance indicators across the Milky Way and nearby galaxies. This network approach allows different rungs of the distance ladder to cross-check one another and helps expose potential systematic errors. I will review several tests of possible systematics. The JWST observations closely match the HST results across multiple methods and wavelengths, ruling out several suspected sources of systematic error and strengthening the case that the Hubble tension is real. In addition, the distance network results show that the tension does not depend on any single source, team, or sample. Rather, it may signal that something important is missing from our current picture of the Universe.
About the Speaker
Adam Riess is a Bloomberg Distinguished Professor, the Thomas J. Barber Professor in Space Studies at the Krieger School of Arts and Sciences, a distinguished astronomer at the Space Telescope Science Institute and a member of the National Academy of Sciences. He received his bachelor’s degree in physics from the Massachusetts Institute of Technology in 1992 and his PhD from Harvard University in 1996. His research involves measurements of the cosmological framework with supernovae (exploding stars) and Cepheids (pulsating stars). Currently, he leads the SHOES 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. In 2011, he was named a co-winner of the Nobel Prize in Physics and was awarded the Albert Einstein Medal for his leadership in the High-z Supernova Search Team’s discovery that the expansion rate of the universe is accelerating, a phenomenon widely attributed to a mysterious, unexplained “dark energy” filling the universe. The discovery was named by Science magazine in 1998 as “the Breakthrough Discovery of the Year.” His accomplishments have been recognized with a number of other awards, including a MacArthur Fellowship in 2008, the Gruber Foundation Cosmology Prize in 2007 (shared), and the Shaw Prize in Astronomy in 2006.