The Hubble constant, H_o, provides a measure of the current expansion rate of the universe. In recent decades, there has been a huge increase in the accuracy with which extragalactic distances, and hence, H_o can be measured. While the historical factor-of-two uncertainty in H_o has been resolved, a new discrepancy has arisen between the values of H_o measured in the local universe, and that estimated from cosmic microwave background measurements, assuming a Lambda cold dark matter model. I will review the advances that have led to the increase in accuracy in measurements of H_o, as well as describe exciting future prospects with the James Webb Space Telescope (JWST) and Gaia, which will make it feasible to measure extragalactic distances at percent level accuracy in the next decade.

Argonne Physics Division Colloquium Schedule