Exotic Gravitational Wave Signatures from Simultaneous Phase Transitions

TL;DR

This paper explores how overlapping multi-step phase transitions in the early universe can produce unique and complex gravitational wave signatures, differing from simple additive models, with implications for detecting new physics.

Contribution

It introduces the concept of overlapping phase transitions affecting gravitational wave spectra and demonstrates this effect explicitly in the singlet extension of the Standard Model.

Findings

Overlapping phase transitions can produce exotic gravitational wave spectra.

The temperature range for bubble nucleation can be as large as 20 GeV.

Standard models may need modification to account for non-trivial GW signatures.

Abstract

We demonstrate that the relic gravitational wave background from a multi-step phase transition may deviate from the simple sum of the single spectra, for phase transitions with similar nucleation temperatures $T_N$. We demonstrate that the temperature range $\Delta T$ between the volume fractions $f(T)=0.1$ and $f(T)=0.9$ occupied by the vacuum bubbles can span $\sim 20$ GeV. This allows for a situation in which phase transitions overlap, such that the later bubbles may nucleate both in high temperature and intermediate temperature phases. Such scenarios may lead to more exotic gravitational wave spectra, which cannot be fitted that of a consecutive PTs. We demonstrate this explicitly in the singlet extension of the Standard Model. Finally, we comment on potential additional effects due to the more exotic dynamics of overlapping phase transitions.