Progress in Measuring the Hubble Constant

Abstract

The Hubble Space Telescope is breaking a long-standing impasse in physical cosmology. The distances of galaxies sufficiently remote for their random velocities to be negligible can now be measured in two steps, the first using Cepheid variable stars as standard candles to approximately 20 Mpc, the second using a variety of secondary distance indicators to distances 10 times larger. The present key project on the Hubble Constant aims to measure Ho to 10%. Current results with approximately 20% uncertainty suggest that cosmologists will be offered a dilemma: an open Universe or a vacuum energy dominated Universe.