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

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.

Automated summary

Research shows that stochastic gravitational wave backgrounds from first-order phase transitions can be significantly enhanced in hot dark sectors with a temperature higher than the standard model's thermal bath. However, the need to transfer entropy from the dark sector to the standard model can dilute the gravitational wave signal. By studying this dilution in detail, including the effect of number-changing processes in the dark sector (cannibalism), it is found that a net enhancement remains in large areas of the parameter space. Applying these findings to a specific example of a dark sector containing a dark Higgs boson and a dark photon, excellent detection prospects are identified for LISA and the Einstein telescope.