On metric preheating

We consider the generation of superhorizon metric fluctuations during an epoch of preheating in the presence of a scalar field chi quadratically coupled to the inflaton. We find that the requirement of efficient broad resonance is concomitant with a severe damping of superhorizon delta chi quantum fluctuations during inflation. We use perturbation theory and include the back reaction as spatial averages to second order in the scalar fields and in the metric, which for the potential considered gives the correct effective energy-momentum tensor on large scales even when the chi field fluctuations grow nonlinear. Within this framework we argue that the usual inflationary prediction for metric perturbations on scales relevant for structure formation is not strongly modified.