Hearing without seeing: gravitational waves from hot and cold hidden
  sectors
  We study the spectrum of gravitational waves produced by a first order phase
transition in a hidden sector that is colder than the visible sector. In this
scenario, bubbles of the hidden sector vacuum can be nucleated through either
thermal fluctuations or quantum tunnelling. If a cold hidden sector undergoes a
thermally induced transition, the amplitude of the gravitational wave signal
produced will be suppressed and its peak frequency shifted compared to if the
hidden and visible sector temperatures were equal. This could lead to signals
in a frequency range that would otherwise be ruled out by constraints from big
bang nucleosynthesis. Alternatively, a sufficiently cold hidden sector could
fail to undergo a thermal transition and subsequently transition through the
nucleation of bubbles by quantum tunnelling. In this case the bubble walls
might accelerate with completely negligible friction. The resulting
gravitational wave spectrum has a characteristic frequency dependence, which
may allow such cold hidden sectors to be distinguished from models in which the
hidden and visible sector temperatures are similar. We compare our results to
the sensitivity of the future gravitational wave experimental programme.
