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.
NB: The author will not update this review anymore, however, some of its topics are subject of other reviews. In May 2008, the article was republished in the revised Living Reviews layout, therefore the pagination has changed. The publication number lrr-2001-1 has not been altered.
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