Model-independent features of gravitational waves from bubble collisions
  We study the gravitational radiation produced by the collisions of bubble
walls or thin fluid shells in cosmological phase transitions. Using the
so-called envelope approximation, we obtain analytically the asymptotic
behavior of the gravitational wave spectrum at low and high frequencies for any
phase transition model. The complete spectrum can thus be approximated by a
simple interpolation between these asymptotes. We verify this approximation
with specific examples. We use these results to discuss the dependence of the
spectrum on the time and size scales of the source.
