The following article is a reformatted version of a preprint obtained from the LANL astro-ph preprint server (http://xxx.lanl.gov/abs/astro-ph/0004075). 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 http://www.livingreviews.org/Articles/index.html , where it will be regularly updated by its author.
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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