Quintessence and the Missing Energy Problem

Recent evidence suggests that the total matter density of the universe is significantly less than the critical density. The shortfall may be explained by curvature (an open universe), vacuum energy density (a cosmological constant), or quintessence (a time-evolving, spatially inhomogeneous component with negative pressure). In all three cases, 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 "tracker fields," a form of quintessence with an attractor-like solution which leads to the same conditions today for a very wide range of initial conditions. Tracker field make quintessence a more viable candidate for the missing energy component and produce more distinctive predictions.