Sean M. Carroll The Cosmological Constant 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. 2001-01-29 2001-02-07 vacuum energy cosmology cosmological constant lrr-2001-1 Max Planck Institute for Gravitational Physics