Nonequilibrium inflaton dynamics and reheating: Back reaction of parametric particle creation and curved spacetime effects

Abstract

We present a detailed and systematic analysis of the nonperturbative, nonequilibrium dynamics of a quantum field in the reheating phase of inflatonary cosmology, including full back reactions of the quantum field on the curved spacetime, as well as the fluctuations on the mean field. We use the O(N) field theory with unbroken symmetry in a spatially flat FRW universe to study the dynamics of the inflaton in the post-inflaton, preheating stage. Oscillations of the inflaton's zero mode induce parametric amplification of quantum fluctuations, resulting in a rapid transfer of energy to the inhomogeneous modes of the inflaton field. We adopt the coupled nonperturbative equations for the mean field and variance derived in a preceding paper [gr-qc/9706001] by means of a two-particle-irreducible (2PI), closed-time-path (CTP) effective action for curved spacetime while specialized to leading order in the 1/N expansion. Adiabatic regularization is employed. The renormalized dynamical equations are evolved numerically from initial data which are generic to the end state of slow roll in many inflatonary cosmological scenarios. The initial conditions consist of a large-amplitude, quasiclassical, oscillating mean field, and a variance given by the de Sitter-invariant vacuum. We find that for sufficiently large initial mean-field amplitudes in this model, the parametric resonance effect alone (in a collisionless approximation) is not an efficient means to "preheat" the quantum field. For small initial mean-field amplitude, damping of the mean field via parametric amplification of quantum fluctuations is seen to occur. Our results indicate that the self-consistent dynamics of spacetime plays an important role in determining the physics of the post-inflatonary Universe.