Measuring the Hubble constant

Abstract

Measuring the Hubble constant (H0), or the rate of expansion of the universe, has proved to be a much more difficult enterprise than originally anticipated. Recent, rapid progress in measuring accurate distances to galaxies, coupled with the prospects for measuring the Hubble constant using other physical techniques offers the promise for measuring the expansion rate to 10% accuracy, including systematic errors. The notorious factor-of-two uncertainty has already been eliminated.