Full-Spectrum Analysis of Gravitational Wave Production from Inflation to Reheating

TL;DR

This paper provides a comprehensive analysis of gravitational wave production during inflation and reheating, identifying multiple channels and deriving the full spectrum, revealing significant high-frequency GW contributions from post-inflationary processes.

Contribution

It introduces a unified analytical framework for calculating the complete gravitational wave spectrum from both inflation and reheating, considering four distinct production channels.

Findings

Post-inflationary GWs can surpass inflationary GWs at high frequencies.

A simple analytical expression for GW spectra from various channels is derived.

The full GW spectrum during early universe epochs is now quantitatively characterized.

Abstract

In this work, we systematically study gravitational wave (GW) production during both the inflationary and post-inflationary epochs. While inflationary GWs can be readily derived from tensor perturbations during inflation, post-inflationary GWs arise from a variety of processes during reheating and require detailed treatment for quantitative analysis. We consider four distinct production channels: $(i)$ pure inflaton annihilation, $(ii)$ graviton bremsstrahlung from inflaton decay, $(iii)$ radiation-catalyzed inflaton-graviton conversion, and $(iv)$ scattering among fully thermalized radiation particles. For each channel, we solve the corresponding Boltzmann equation to obtain the GW spectrum and derive a simple yet accurate analytical expression for it. By employing a consistent treatment of all production channels, our analysis yields for the first time the full spectrum of GWs produced during the inflationary and post-inflationary epochs. We find that, while inflationary GWs dominate at low frequencies, post-inflationary processes generally produce high-frequency GWs with considerably high energy densities that may significantly exceed that of inflationary GWs.