Effect of density fluctuations on gravitational wave production in first-order phase transitions

TL;DR

This paper investigates how density fluctuations in the early Universe influence the size of nucleated bubbles during first-order phase transitions, leading to an enhancement in gravitational wave signals detectable today.

Contribution

It introduces a mechanism showing that large-scale density fluctuations can significantly amplify gravitational wave signals from early Universe phase transitions.

Findings

Large-scale density fluctuations increase effective bubble size at collision.

Enhanced gravitational wave amplitude due to density fluctuations with moderate amplitudes.

Significant GW signal enhancement for transitions with large eta/H* ratio.

Abstract

We study the effect of density perturbations on the process of first-order phase transitions and gravitational wave production in the early Universe. We are mainly interested in how the distribution of nucleated bubbles is affected by fluctuations in the local temperature. We find that large-scale density fluctuations ($H_* < k_* < \beta$) result in a larger effective bubble size at the time of collision, enhancing the produced amplitude of gravitational waves. The amplitude of the density fluctuations necessary for this enhancement is ${\cal P}_\zeta (k_*) \gtrsim (\beta / H_*)^{-2}$, and therefore the gravitational wave signal from first-order phase transitions with relatively large $\beta / H_*$ can be significantly enhanced by this mechanism even for fluctuations with moderate amplitudes.