On the Back Reaction Problem for Gravitational Perturbations
  We derive the effective energy-momentum tensor for cosmological perturbations and prove its gauge-invariance. The result is applied to study the influence of perturbations on the behaviour of the Friedmann background in inflationary Universe scenarios. We found that the back reaction of cosmological perturbations on the background can become important already at energies below the self-reproduction scale. 
V. Mukhanov (ETH Zuerich), L.R.W. Abramo (Brown Univ.) and R.   Brandenberger (Brown Univ.)