Vacuum decay in the presence of a cosmic string
  We study the false vacuum decay and bubble nucleation in the presence of a
cosmic string in dS spacetime. A cosmic string induces a deficit angle in
spacetime around itself so the nucleated bubble has the shape of a rugby ball.
Working in thin wall approximation and using the junction conditions we study
the dynamics of the bubble wall and calculate the Euclidean action. An
interesting feature in our analysis is that the tension of the string is
screened by the bubble such that an observer inside the bubble measures a
different value of the tension than an outside observer. We show that the
string can act as a catalyzer in which the nucleation rate is enhanced compared
to the Coleman-de Luccia instanton. However, in general, the nucleation rate is
not a monotonic function of the difference between the two tensions so in some
regions of the parameters space the nucleation rate may be smaller than the
Coleman-de Luccia bubble.
