  Sonoluminescence is a process in which a strong sound field is used to
produce light in liquids. We explain sonoluminescence as a phase transition
from ordinary fluorescence to a superradiant phase. We consider a spin-boson
model composed of a single bosonic mode and an ensemble of $N$ identical
two-level atoms. We assume that the whole system is in thermal equilibrium with
a reservoir at temperature $\beta^{-1}$. We show that, in a
ultrastrong-coupling regime, between the two-level atoms and the
electromagnetic field it is possible to have a cooperative interaction of the
molecules of the gas in the interior of the bubble with the field, generating
sonoluminescence.
