Quintessence, cosmic coincidence, and the cosmological constant

Recent observations suggest that a large fraction of the energy density of the Universe has negative pressure. One explanation is vacuum energy densit y; another is quintessence in the form of a scalar field slowly evolving do wn a potential. In either case, a key problem is to explain why the energy density nearly coincides with the matter density today. The densities decre ase at different rates as the Universe expands, so coincidence today appear s to require that their ratio be set to a specific, infinitesimal value in the early Universe. In this paper, we introduce the notion of a "tracker fi eld," a form of quintessence, and show how it may explain the coincidence, adding new motivation for the quintessence scenario.