The following article is a reformatted version of a preprint obtained from the LANL astro-ph preprint server ( The article was written on invitation of the refereed online journal Living Reviews in Relativity and the final article, an enhanced HTML version, is now at , where it will be regularly updated by its author.


Sean M. Carroll

Enrico Fermi Institute and Department of Physics
University of Chicago
5640 S. Ellis Ave.
Chicago, IL 60637 USA

Abstract. This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.

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