  We investigate first order phase transitions arising from hidden sectors
which are in thermal equilibrium with the Standard Model bath in the Early
Universe. Focusing on two simplified scenarios, an higgsed U(1) and a two
scalar singlet model, we show the impact of friction effects acting on the
bubble walls on the gravitational wave spectra and on the consequences for
present and future interferometer experiments. We further comment on the
possibility of disentangling the properties of the underlying theory featuring
the first order phase transition should a stochastic gravitational wave signal
be discovered.
