Gravitational Waves from Particles Produced from Bubble Collisions in First-Order Phase Transitions

TL;DR

This paper introduces a novel gravitational wave source from particle production during bubble collisions in first-order phase transitions, which can significantly alter the expected GW spectrum and is relevant for future observations.

Contribution

It presents a semi-analytical model for GWs from particle distributions produced in bubble collisions, highlighting a new contribution to the GW signal.

Findings

New GW spectral feature at low frequencies due to particle-induced anisotropy

Significant impact on GW signals when the background field has strong self-couplings

Potential observability of the modified GW spectrum by future experiments

Abstract

We discuss a new source of gravitational waves (GWs) from first-order phase transitions. The collisions of bubbles of the new phase can efficiently produce particles that couple to the background field undergoing the transition, thereby transferring a significant fraction of the released vacuum energy into a distribution of inhomogeneous and dynamic particle populations that persist long after the bubbles have disappeared. We study the GWs produced by such particle distributions, showing that GWs arise from the quadrupolar anisotropy in the radiation emitted from the bubble collisions, and present a semi-analytical calculation of the two-point correlation function for the associated energy distributions. We find that this new contribution can qualitatively modify the overall GW signal from such phase transitions, creating a distinct shift in the spectral slope at low frequencies that could be observed by future GW experiments. It is therefore important to take this new contribution into account for any transition where the background field has significant self-coupling or couplings to other fields that could lead to efficient particle production at bubble collision.