Cosmic Bubble Collisions
  I briefly review the physics of cosmic bubble collisions in false-vacuum
eternal inflation. My purpose is to provide an introduction to the subject for
readers unfamiliar with it, focussing on recent work related to the prospects
for observing the effects of bubble collisions in cosmology. I will attempt to
explain the essential physical points as simply and concisely as possible,
leaving most technical details to the references. I make no attempt to be
comprehensive or complete. I also present a new solution to Einstein's
equations that represents a bubble universe after a collision, containing
vacuum energy and ingoing null radiation with an arbitrary density profile.
