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. 1 a