The devil is in the distance

摘要

The age of the Universe is inversely proportional to the Hubble constant-the rate at which the Universe is currently expanding. The Hubble constant, H_0, is defined by the relation V = H_0d, where V is the velocity of a galaxy at a distance d from us. So establishing a value for H_0 requires working out accurate velocities (which is relatively easy) and exact distances (much more problematic) of extragalactic objects. The difficulty in measuring such distances is highlighted by Maoz et aV on page 351 of this issue.