Ultra-high Frequency Gravitational Waves from Scattering, Bremsstrahlung and Decay during Reheating

TL;DR

This paper analyzes various mechanisms of ultra-high frequency gravitational wave production during inflationary reheating, comparing their spectra and identifying dominant sources depending on reheating temperature and frequency.

Contribution

It systematically compares four different GW production processes during reheating, highlighting conditions under which each process dominates.

Findings

$2 \to 2$ inflaton scattering can dominate GW production at high reheating temperatures.

Bremsstrahlung is generally a significant GW source, especially at lower frequencies.

One-loop inflaton decay contributes less to GW spectrum compared to other mechanisms.

Abstract

We investigate ultra-high frequency gravitational waves (GWs) from gravitons generated during inflationary reheating. Specifically, we study inflaton scattering with its decay product, where the couplings involved in this $2 \to 2$ scattering are the same as those in the $1 \to 3$ graviton Bremsstrahlung process. We compute the graviton production rate via such $2 \to 2$ scattering. Additionally, we compare the resulting GW spectrum with that from Bremsstrahlung as well as that from pure $2 \to 2$ inflaton scatterings. For completeness, the GW spectrum from graviton pair production through one-loop induced $1 \to 2$ inflaton decay is also analyzed. With a systematic comparison among the four sources of GWs, we find that $2 \to 2$ inflaton scattering with its decay product can dominate over Bremsstrahlung if the reheating temperature is larger than the inflaton mass. Pure inflaton $2 \to 2$ scattering is typically subdominant compared to Bremsstrahlung except in the high-frequency tail. The contribution from one-loop induced $1 \to 2$ inflaton decay is shown to be suppressed compared to Bremsstrahlung and pure inflaton $2 \to 2$ scattering.