Dark, Cold, and Noisy: Constraining Secluded Hidden Sectors with
  Gravitational Waves
  We explore gravitational wave signals arising from first-order phase
transitions occurring in a secluded hidden sector, allowing for the possibility
that the hidden sector may have a different temperature than the Standard Model
sector. We present the sensitivity to such scenarios for both current and
future gravitational wave detectors in a model-independent fashion. Since
secluded hidden sectors are of particular interest for dark matter models at
the MeV scale or below, we pay special attention to the reach of pulsar timing
arrays. Cosmological constraints on light degrees of freedom restrict the
number of sub-MeV particles in a hidden sector, as well as the hidden sector
temperature. Nevertheless, we find that observable first-order phase
transitions can occur. To illustrate our results, we consider two minimal
benchmark models: a model with two gauge singlet scalars and a model with a
spontaneously broken $U(1)$ gauge symmetry in the hidden sector.
