Vacuum bubble collisions: from microphysics to gravitational waves
  We comprehensively study the effects of bubble wall thickness and speed on
the gravitational wave emission spectrum of collisions of two vacuum bubbles.
We numerically simulate a large dynamical range, making use of symmetry to
reduce the dimensionality. The high-frequency slope of the gravitational wave
spectrum is shown to depend on the thickness of the bubble wall, becoming
steeper for thick-wall bubbles, in agreement with recent fully 3+1 dimensional
lattice simulations of many-bubble collisions. This dependence is present, even
for highly relativistic bubble wall collisions. We use the reduced
dimensionality as an opportunity to investigate dynamical phenomena which may
underlie the observed differences in the gravitational wave spectra. These
phenomena include `trapping', which occurs most for thin-wall bubbles, and
oscillations behind the bubble wall, which occur for thick-wall bubbles.
