Parametric Amplification of Metric Fluctuations During Reheating in Two   Field Models
  We study the parametric amplification of super-Hubble-scale scalar metric fluctuations at the end of inflation in some specific two-field models of inflation, a class of which is motivated by hybrid inflation. We demonstrate that there can indeed be a large growth of fluctuations due to parametric resonance and that this effect is not taken into account by the conventional theory of isocurvature perturbations. Scalar field interactions play a crucial role in this analysis. We discuss the conditions under which there can be nontrivial parametric resonance effects on large scales. 
F. Finelli (Purdue U. & Bologna U. & INFN, Bologna), R. Brandenberger   (Brown U.)