  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.
