Sound shell model for acoustic gravitational wave production at a
  first-order phase transition in the early Universe
  A model for the acoustic production of gravitational waves at a first order
phase transition is presented. The source of gravitational radiation is the
sound waves generated by the explosive growth of bubbles of the stable phase.
The model assumes that the sound waves are linear and that their power spectrum
is determined by the characteristic form of the sound shell around the
expanding bubble. The predicted power spectrum has two length scales, the
average bubble separation and the sound shell width when the bubbles collide.
The peak of the power spectrum is at wavenumbers set by the sound shell width.
For higher wavenumber $k$, the power spectrum decreases as $k^{-3}$. At
wavenumbers below the inverse bubble separation, the power spectrum goes as
$k^5$. For bubble wall speeds near the speed of sound where these two length
scales are distinguished, there is an intermediate $k^{1}$ power law. The
detailed dependence of the power spectrum on the wall speed and the other
parameters of the phase transition raises the possibility of their constraint
or measurement at a future space-based gravitational wave observatory such as
eLISA.
