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