General Properties of the Gravitational Wave Spectrum from Phase
  Transitions
  In this paper we discuss some general aspects of the gravitational wave
background arising from post-inflationary short-lasting cosmological events
such as phase transitions. We concentrate on the physics which determines the
shape and the peak frequency of the gravitational wave spectrum. We then apply
our general findings to the case of bubble collisions during a first order
phase transition and compare different results in the recent literature.
