Preheating and the Einstein Field Equations
  We inaugurate a framework for studying preheating and parametric resonance after inflation without resorting to any approximations, either in gravitational perturbation theory or in the evolution of the field(s). We do this by numerically solving the Einstein field equations in the post-inflationary universe. In this letter we show how to compare our results to those of gauge invariant perturbation theory. We then verify Finelli and Brandenberger's analysis (hep-ph/9809490) of super-horizon modes in $m^2\phi^2$ inflation, showing that they are not amplified by resonant effects. Lastly, we make a preliminary survey of the nonlinear couplings between modes, which will be important in models where the primordial metric perturbations undergo parametric amplification. 
Matthew Parry and Richard Easther (Brown U.)