  We present a solution to the dark energy problem in terms of the Effective
Energy Momentum Tensor (EMT) of cosmological perturbations. The approach makes
use of the gravitational back-reaction of long wavelength (super-Hubble)
fluctuation modes on the background metric. Our results indicate that,
following preheating, the energy density associated with back-reaction is
sub-dominant and behaves as a tracker during the radiation era. At the onset of
matter domination, however, the effects of back-reaction begin to grow relative
to the matter density and the associated equation of state quickly approaches
that of a cosmological constant. Using standard values for the preheating
temperature and the amplitude of the inflaton following preheating, we show
that this mechanism leads to a very natural explanation of dark energy. We
comment on other recent attempts to explain the dark energy using back-reaction
and their relation to our work.
