Barnacles and Gravity
  Theories with more than one vacuum allow quantum transitions between them,
which may proceed via bubble nucleation; theories with more than two vacua
posses additional decay modes in which the wall of a bubble may further decay.
The instantons which mediate such a process have $O(3)$ symmetry (in four
dimensions, rather than the usual $O(4)$ symmetry of homogeneous vacuum decay),
and have been called `barnacles'; previously they have been studied in flat
space, in the thin wall limit, and this paper extends the analysis to include
gravity. It is found that there are regions of parameter space in which, given
an initial bubble, barnacles are the favoured subsequent decay process, and
that the inclusion of gravity can enlarge this region. The relation to other
heterogeneous vacuum decay scenarios, as well as some of the phenomenological
implications of barnacles are briefly discussed.
