Turn up the volume: Listening to phase transitions in hot dark sectors

TL;DR

This paper investigates how hot dark sectors with phase transitions can produce enhanced gravitational wave signals detectable by future observatories, considering entropy transfer effects and cannibalism processes.

Contribution

It demonstrates that hot dark sectors can significantly amplify gravitational wave signals from phase transitions, even after accounting for dilution effects.

Findings

Enhanced GW signals in hot dark sectors are detectable by LISA and Einstein telescope.

Entropy transfer can dilute GW signals, but net enhancement persists in many scenarios.

Cannibalism processes influence the strength of the GW background.

Abstract

Stochastic gravitational wave (GW) backgrounds from first-order phase transitions are an exciting target for future GW observatories and may enable us to study dark sectors with very weak couplings to the Standard Model. In this work we show that such signals may be significantly enhanced for hot dark sectors with a temperature larger than the one of the SM thermal bath. The need to transfer the entropy from the dark sector to the SM after the phase transition can however lead to a substantial dilution of the GW signal. We study this dilution in detail, including the effect of number-changing processes in the dark sector (so-called cannibalism), and show that in large regions of parameter space a net enhancement remains. We apply our findings to a specific example of a dark sector containing a dark Higgs boson and a dark photon and find excellent detection prospects for LISA and the Einstein telescope.