Gravitational wave energy budget in strongly supercooled phase
  transitions
  We derive efficiency factors for the production of gravitational waves
through bubble collisions and plasma-related sources in strong phase
transitions, and find the conditions under which the bubble collisions can
contribute significantly to the signal. We use lattice simulations to clarify
the dependence of the colliding bubbles on their initial state. We illustrate
our findings in two examples, the Standard Model with an extra $|H|^6$
interaction and a classically scale-invariant $U(1)_{\rm B-L}$ extension of the
Standard Model. The contribution to the GW spectrum from bubble collisions is
found to be negligible in the $|H|^6$ model, whereas it can play an important
role in parts of the parameter space in the scale-invariant $U(1)_{\rm B-L}$
model. In both cases the sound-wave period is much shorter than a Hubble time,
suggesting a significant amplification of the turbulence-sourced signal. We
find, however, that the peak of the plasma-sourced spectrum is still produced
by sound waves with the slower-falling turbulence contribution becoming
important off-peak.
