Gravitational Wave Signature of Aspherical Bubbles Driven by Thermal Fluctuation

TL;DR

This paper investigates how thermal fluctuations during cosmological phase transitions distort bubble shapes and alter gravitational wave signals, revealing effects that could be detectable in future observations.

Contribution

It introduces lattice simulations with thermal initial conditions to quantify the impact of thermal fluctuations on bubble profiles and GW spectra, extending beyond idealized spherical models.

Findings

Thermal fluctuations break spherical symmetry early, enabling isolated bubbles to emit GWs.

Thermal effects reshape the GW spectrum, suppressing low-frequency and enhancing high-frequency components.

Analytical estimates match lattice results, indicating thermal fluctuations dominate the ultraviolet GW spectrum.

Abstract

Cosmological first-order phase transitions are a well-motivated source of stochastic gravitational waves (GWs), but most predictions are made based on the highly idealized model of perfectly spherical vacuum bubbles, neglecting thermal fluctuations. In this work we use $(3+1)$-dimensional lattice simulations of a scalar model with thermal initial conditions to quantify how thermal fluctuations distort bubble profiles and modify the resulting GW spectrum. We find that thermal fluctuations can strongly break spherical symmetry at early times, allowing even an isolated bubble to emit GWs. In multi-bubble simulations, thermal fluctuations systematically reshape the spectrum, suppressing the infrared part while enhancing and broadening the high-$k$ tail. We further provide an analytical estimate for the ultraviolet regime of the GW spectrum, which is in good agreement with our lattice results and suggests that this regime is dominated by thermal fluctuations. These effects could leave observable imprints in future GW searches.