Vacuum bubble collisions: from microphysics to gravitational waves

TL;DR

This paper investigates how bubble wall thickness and velocity influence the gravitational wave signals from vacuum bubble collisions, revealing that wall properties significantly affect the spectral shape.

Contribution

It provides a comprehensive numerical analysis of bubble collision dynamics, highlighting the impact of wall thickness and velocity on gravitational wave spectra, including phenomena like trapping and oscillations.

Findings

High-frequency slope depends on bubble wall thickness

Thick-wall bubbles produce steeper spectra

Dynamical phenomena like trapping and oscillations are identified

Abstract

We comprehensively study the effects of bubble wall thickness and speed on the gravitational wave emission spectrum of collisions of two vacuum bubbles. We numerically simulate a large dynamical range, making use of symmetry to reduce the dimensionality. The high-frequency slope of the gravitational wave spectrum is shown to depend on the thickness of the bubble wall, becoming steeper for thick-wall bubbles, in agreement with recent fully 3+1 dimensional lattice simulations of many-bubble collisions. This dependence is present, even for highly relativistic bubble wall collisions. We use the reduced dimensionality as an opportunity to investigate dynamical phenomena which may underlie the observed differences in the gravitational wave spectra. These phenomena include `trapping', which occurs most for thin-wall bubbles, and oscillations behind the bubble wall, which occur for thick-wall bubbles.