The quintessential universe

Recent evidence suggests that 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 rate of the universe to accelerate. We review the evidence, including recent measurements of the cosmic microwave background by the DASI, BOOMerang and MAXIMA groups. The dark energy may consist of vacuum energy density (or, equivalently, a cosmological constant), or quintessence, a time-evolving, spatially inhomogeneous component with negative pressure. A key problem is to explain the initial conditions required to have the energy density nearly coincident with the matter density today. A possible solution is “k-essence,” a form of quintessence with an attractor-like solution which leads to cosmic acceleration today for a very wide range of initial conditions without fine-tuning and without invoking an anthropic argument.