Special Colloquium

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.

Abstract: The proximity of our Galaxy's center presents a unique opportunity to study a galactic nucleus with orders of magnitude higher spatial resolution than can be brought to bear on any other galaxy. After more than a decade of diffraction-limited imaging on large ground-based telescopes, the case for a supermassive black hole at the Galactic center has gone from a possibility to a certainty, thanks to measurements of individual stellar orbits.