Published in "Measuring and Modeling the Universe", Carnegie Observatories Astrophysics Series Vol. 2, ed. W. L. Freedman (2004).
astro-ph/0310342

For a PDF version of the article, click here.

WHY IS THE UNIVERSE ACCELERATING?

Sean M. Carroll

Enrico Fermi Institute, Department of Physics, and Center for Cosmological Physics, University of Chicago
Kavli Institute for Theoretical Physics, University of California, Santa Barbara
carroll@theory.uchicago.edu

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.

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