Gravity, parametric resonance, and chaotic inflation

We investigate the possibility that nonlinear gravitational effects influence the preheating era after inflation. Our work is based on numerical solutions of the inhomogeneous Einstein field equations, and is free of perturbative approximations. The one restriction we impose is to limit the inhomogeneity to a single spatial direction. We compare our results to perturbative calculations and to solutions of the nonlinear field equations in a rigid (unperturbed) spacetime, in order to isolate gravitational phenomena. We consider two types of initial conditions: where only one mode of the field perturbation has a non-zero initial amplitude, and where all the modes begin with a non-zero amplitude. Here we focus on preheating following inflation driven by a scalar field with a quartic potential. We confirm the broad picture of preheating obtained from the nonlinear field equations in a rigid background, but gravitational effects have a measurable impact on the dynamics for both sets of initial data. The rigid spacetime results predict that the amplitude of a single initially excited mode drops rapidly after resonance ends, whereas in the relativistic case the amplitude is roughly constant. With all modes initially excited, the longest modes in the simulation grow much more rapidly in the relativistic calculation than with a rigid background. However, we see no evidence for the sort of gravitational collapse associated with the formation of primordial black holes. The numerical codes described here are easily extended to more complicated resonant models, which we will examine in the future.