Impact of cosmic expansion on gravitational wave spectra from strongly supercooled first-order phase transitions

TL;DR

This paper analyzes how cosmic expansion affects gravitational wave signals from supercooled phase transitions, revealing that the spectral shape is mostly unchanged but peak features depend less on transition rate than previously thought.

Contribution

It introduces a detailed computation of gravitational wave spectra considering background metric evolution during supercooled phase transitions, highlighting new dependencies of peak features on transition rate.

Findings

Spectral shape remains largely unchanged with a super-horizon tail.

Peak amplitude and frequency depend weakly on the transition rate for slow transitions.

Standard scaling laws with transition rate are modified in this context.

Abstract

We compute the gravitational wave spectra from strongly supercooled first-order phase transitions, explicitly incorporating the evolution of the background metric across the transition from thermal inflation to radiation domination. We find that the spectral shape remains largely unchanged apart from a causality-induced super-horizon tail. However, in contrast to standard expectations, for slow transitions we show that the peak amplitude and frequency exhibit a weaker dependence on the transition rate $\beta$ than the usual scaling of $\propto \beta^{-2}$ and $\propto\beta$, respectively.