Gravitational waves from bubble collisions in FLRW spacetime

TL;DR

This paper refines the analytical modeling of gravitational waves from bubble collisions during early universe phase transitions by incorporating universe expansion and finite transition durations, impacting detectability predictions.

Contribution

It extends previous Minkowski spacetime models to FLRW spacetime and accounts for finite phase transition durations, providing more accurate GW spectra estimates.

Findings

Maxima of GW energy spectra decrease by about an order of magnitude for certain parameters.

Even with reduced spectra, detection prospects remain promising for upcoming GW detectors.

The new model suggests more realistic estimates for GW signals from early universe phase transitions.

Abstract

Stochastic gravitational wave background (SGWB) is a promising tool to probe the very early universe where the standard model of particle physics and cosmology are connected closely. As a possible component of SGWB, gravitational waves (GW) from bubble collisions during the first order cosmological phase transitions deserve comprehensive analyses. In 2017, Ryusuke Jinno and Masahiro Takimoto proposed an elegant analysis approach to derive the analytical expressions of energy spectra of GW from bubble collisions in Minkowski spacetime avoiding large-scale numerical simulations for the first time[26]. However, they neglect the expansion of the universe and regard the duration of phase transitions as infinity in their derivation which could deviate their estimations from true values. For these two reasons, we give a new expression of GW spectra by adopting their method, switching spacetime background to FLRW spacetime, and considering a finite duration of phase transitions. By denoting $\sigma$ as the fraction of the speed of phase transitions to the expansion speed of the universe, we find when $\sigma$ is around $\mathcal{O}(10)$, the maxima of estimated GW energy spectra drop by around 1 order of magnitude than the results given by their previous work. Even when $\sigma=100$, the maximum of GW energy spectrum is only $65\%$ of their previous estimation. Such a significant decrease may bring about new challenges for the detectability of GW from bubble collisions. Luckily, by comparing new spectra with PLI (\textit{power-law integrated}) sensitivity curves of GW detectors, we find that the detection prospect for GW from bubble collisions is still promising for DECIGO, BBO, LISA, and TianQin in the foreseeable future.