Großes Physikalisches Kolloquium

The present-day expansion rate of the Universe, or Hubble constant (H0), is a fundamental parameter of cosmology. H0 sets the age of the Universe, its critical density, and the absolute scale for a wide array of cosmological observables that probe the laws of physics on extremely large scales and across time. However, the last decade has brought about an increasing disagreement between the values of H0 measured directly in today's Universe and the values of H0 determined indirectly from observations of the very early Universe. This 5-6 sigma disagreement has come to be known by the term "Hubble constant tension" and is one of the most pressing issues in cosmology. Yet, the Tension has thus far escaped a satisfactory resolution. Following a brief summary of the status quo, I will describe the observational setup that achieves the most accurate direct H0 measurement using an extragalactic distance ladder in which pulsating stars anchor type-Ia supernovae to geometrically measured distances. In turn, I will present key developments that have slashed the uncertainty on H0 by nearly a factor of 10 since the 2001 Hubble key project as well as the ruthless efforts ongoing to identify and mitigate even the smallest systematics. Third, I will discuss the usefulness of the James Webb Space Telescope for cross-checking the state of the art as well as its limitations for doing better. In closing, I will offer some thoughts on the possibly far-reaching consequences of the Hubble constant tension and useful ways forward.