title: A quintessential introduction to dark energy.
abstract: Most of the energy in the Universe consists of some form of dark energy that is gravitationally self-repulsive and that is causing the expansion of the Universe to accelerate. The possible candidates are a vacuum energy density (or, equivalently, a cosmological constant) and quintessence, a time-evolving, spatially inhomogeneous component with negative pressure. In this review, we focus on quintessence and ideas on how it might solve the cosmic coincidence problem, how it might be distinguished observationally from a cosmological constant, and how it may affect the overall cosmic history of the Universe.
authors.affiliation: Department of Physics, Princeton University, Princeton, NJ 08540, USA.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Steinhardt