Collapse of Small-Scale Density Perturbations during Preheating in Single Field Inflation

TL;DR

This paper investigates how small-scale density perturbations grow and become non-linear during the preheating phase after inflation, potentially affecting gravitational wave production and primordial black hole formation.

Contribution

It provides a detailed analysis of metric preheating instability in single-field inflation models with quadratic and quartic potentials, highlighting the early non-linear growth of perturbations.

Findings

Sub-Hubble scale perturbations grow rapidly during preheating.

Density perturbations can become non-linear before radiation domination.

Potential implications for gravitational waves and primordial black holes.

Abstract

After cosmic inflation and before the transition to radiation domination, the cosmic energy density may have been dominated during an extended period by an oscillating massive scalar condensate. We show that during this period, sub-Hubble scale perturbations are subject to a metric preheating instability in the narrow resonance regime. We analyze in detail both, quadratic and quartic potentials. The instability leads to the growth of density perturbations which in many cases become non-linear already before the beginning of a radiation dominated Universe. This is particularly the case when requiring a phenomenologically preferred low reheat temperature. These early structures may lead to the emission of gravitational waves and the production of primordial black holes. Furthermore, it is not clear if they could modify the prediction of linear curvature perturbations on very large scales.