Why is the Universe Accelerating?

Abstract

The universe appears to be accelerating, but the reason why is a complete mystery. The simplest explanation, a small vacuum energy (cosmological constant), raises three difficult issues: why the vacuum energy is so small, why it is not quite zero, and why it is comparable to the matter density today. I discuss these mysteries, some of their possible resolutions, and some issues confronting future observations.

Comments: 22 pages; Contribution to Measuring and Modeling the Universe, Carnegie Observatories Astrophysics Series Vol. 2, ed. W. L. Freedman; references improved