  Gravity can play a role in critical phenomena. Topological singularities
induce ground state degeneracy and break the continuum symmetry of the vacuum.
They also generate momenta oscillations about an average momentum and a
positive gravitational susceptibility. Gravitational analogues of the laws of
Curie and Bloch have been found for a one-dimensional model. The critical
temperature for a change in phase from unbound to isolated vortices has also
been calculated in a $XY$-model.
