Cosmological perturbation with two scalar fields in reheating after inflation

TL;DR

The paper analyzes cosmological perturbations in a two-scalar-field model during reheating after inflation, showing that coherent oscillations of the inflaton induce parametric resonance in the massless field and amplify iso-curvature metric perturbations in the long-wavelength limit. Using a renormalization-group approach, the authors obtain a detailed background evolution and demonstrate that the iso-curvature component of the Bardeen parameter grows during the resonance epoch, while the adiabatic mode remains conserved. They derive bounds on the growth of iso-curvature perturbations and confirm the results with numerical simulations, highlighting potential impacts on the small-scale power spectrum. The findings imply that non-linear multi-field dynamics during reheating can leave observable imprints on cosmological perturbations, motivating further study of the resulting power spectrum evolution.

Abstract

We investigate the cosmological perturbation of two-scalar field model during the reheating phase after inflation. Using the exact solution of the perturbation in long-wavelength limit, which is expressed in terms of the background quantities, we analyze the behavior of the metric perturbation. The oscillating inflaton field gives rise to the parametric resonance of the massless scalar field and this leads to the amplification of the iso-curvature mode of the metric perturbations.