Gravity, parametric resonance, and chaotic inflation - art. no. 103503

We investigate the possibility that nonlinear gravitational effects influen ce the preheating era after inflation. Our work is based on numerical solut ions of the inhomogeneous Einstein field equations, and is free of perturba tive approximations. The one restriction we impose is to limit the inhomoge neity 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 co nsider two types of initial conditions: where only one mode of the field pe rturbation 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 pic ture of preheating obtained from the nonlinear field equations in a rigid b ackground, but gravitational effects have a measurable impact on the dynami cs for both sets of initial data. The rigid spacetime results predict that the amplitude of a single initially excited mode drops rapidly after resona nce ends, whereas in the relativistic case the amplitude is roughly constan t. With all modes initially excited, the longest modes in the simulation gr ow much more rapidly in the relativistic calculation than with a rigid back ground. However, we see no evidence fur the sort of gravitational collapse associated with the formation of primordial black holes. The numerical code s described here are easily extended to more complicated resonant models, w hich we will examine in the future.