Model-independent features of gravitational waves from bubble collisions

TL;DR

This paper derives analytical expressions for the gravitational wave spectrum from bubble collisions in cosmological phase transitions, providing a model-independent framework that captures the spectrum's asymptotic behaviors and their dependence on source scales.

Contribution

It introduces a model-independent analytical method to characterize gravitational wave spectra from bubble collisions, covering both low and high frequency asymptotes.

Findings

Analytical asymptotic behaviors of the spectrum at low and high frequencies.

A simple interpolation accurately approximates the full spectrum.

Dependence of the spectrum on source time and size scales.

Abstract

We study the gravitational radiation produced by the collisions of bubble walls or thin fluid shells in cosmological phase transitions. Using the so-called envelope approximation, we obtain analytically the asymptotic behavior of the gravitational wave spectrum at low and high frequencies for any phase transition model. The complete spectrum can thus be approximated by a simple interpolation between these asymptotes. We verify this approximation with specific examples. We use these results to discuss the dependence of the spectrum on the time and size scales of the source.