Back-Reaction: A Cosmological Panacea
  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. 
P. Martineau, R. Brandenberger