The curvature perturbations and induced gravitational waves induced by the first-order phase transition during reheating

TL;DR

This paper introduces a new mechanism where a first-order phase transition during reheating modulates decay rates, generating observable gravitational waves without significant vacuum energy release, detectable by future space-based interferometers.

Contribution

It proposes a novel process linking phase transitions to gravitational wave production during reheating, independent of vacuum energy release.

Findings

Gravitational wave spectrum can reach ^{-10} in energy density.

Detection prospects are promising for LISA, TianQin, and Taiji.

The mechanism allows probing early Universe phase transitions without large vacuum energy.

Abstract

We propose a novel mechanism where a first-order phase transition modulates the decay rate of a massive field. This modulation, even if the scalar field has negligible energy density, subsequently generates an observable stochastic gravitational-wave background. The stochastic nature of bubble nucleation leads to the asynchrony of phase transitions, generating superhorizon-scale density perturbations through spatial variations in the decay rate $\Gamma$. These perturbations subsequently source second-order gravitational waves with peak amplitudes governed by the phase transition parameter \(\beta/H_*\) and decay rate $\Gamma$. We apply this mechanism in the reheating scanario where the decay rate of inflaton are modulated by the scalar field that undergoes a first-order phase transition. Numerical calculations reveal that the gravitational wave energy spectrum typically reaches \(\Omega_{\text{GW}} \sim 10^{-10}\), demonstrating prospects for detection by space-based interferometers like LISA, TianQin and Taiji. This work establishes a new approach to probe phase transition processes in the early Universe without requiring significant vacuum energy release.