Barnacles -- A Novel Channel for Vacuum Decay
  I show that cosmological bubble walls in the thin wall approximation are
unstable to the creation of "barnacles" -- loci of different wall tension
adjacent to regions filled with a third vacuum. Barnacle formation leads to the
same observational consequences as the extensively studied bubble collision
scenario, but occurs exponentially more often. The process is described by a
saddle point of the thin wall action with two negative modes.
