Quintessence and the missing energy problem

Recent evidence suggests that the total matter density of the universe is s ignificantly less than the critical density. The shortfall may be explained by curvature (an open universe), vacuum energy density (a cosmological con stant), or quintessence (a time-evolving, spatially inhomogeneous component with negative pressure). In all three cases, a key problem is to explain t he initial conditions required to have the energy density nearly coincident with the matter density today, ii possible solution is "tracker fields," a form of quintessence with an attractor-like solution which leads to the sa me conditions today for a very wide range of initial conditions. Tracker fi eld make quintessence a more viable candidate for the missing energy compon ent and produce more distinctive predictions.