Gravitational waves from bubble walls
  We present a general method for computing the gravitational radiation arising
from the motion of bubble walls or thin fluid shells in cosmological phase
transitions. We discuss the application of this method to different wall
kinematics. In particular, we derive general expressions for the bubble
collision mechanism in the envelope approximation and the so-called bulk flow
model, and we also consider deformations from the spherical bubble shape. We
calculate the gravitational wave spectrum for a specific model of deformations
on a definite size scale, which gives a peak away from that of the bubble
collision mechanism.
