Quantum production of gravitational waves after inflation

TL;DR

This paper proposes a new mechanism for generating high-frequency gravitational waves from vacuum fluctuations after inflation, driven by scalar metric perturbations breaking conformal flatness.

Contribution

It introduces a novel source of gravitational waves from quantum graviton production induced by inhomogeneities post-inflation, with detailed spectrum calculations.

Findings

GW signal peaks around GHz frequencies

The mechanism depends on the primordial scalar power spectrum

Distinct from other astrophysical and cosmological backgrounds

Abstract

A variety of mechanisms in the early Universe lead to the generation of gravitational waves (GWs). We introduce here a novel source of GWs generated by vacuum fluctuations after inflation. Given that gravitons are minimally coupled particles, their quantum creation takes place during inflation, but is absent in an unperturbed Universe during the radiation-dominated epoch, since they behave as conformally coupled particles. However, the presence of inhomogeneities breaks the conformal flatness of the metric, allowing scalar metric perturbations to induce the quantum production of gravitons. We compute the resulting GW spectrum from this mechanism for different models of the primordial scalar power spectrum. We find that this GW signal peaks around the GHz frequency range, distinguishing it from other astrophysical and cosmological backgrounds and underscoring the need for detectors sensitive to these high frequencies.