  We derive a formula for the nonequilibrium entropy of a classical stochastic
field in terms of correlation functions of this field. The formalism is then
applied to define the entropy of gravitational perturbations (both
gravitational waves and density fluctuations). We calculate this entropy in a
specific cosmological model (the inflationary Universe) and find that on scales
of interest in cosmology the entropy in both density perturbations and
gravitational waves exceeds the entropy of statistical fluctuations of the
microwave background. The nonequilibrium entropy discussed here is a measure of
loss of information about the system. We discuss the origin of the entropy in
our cosmological models and compare the definition of entropy in terms of
correlation functions with the microcanonical definition in quantum statistical
mechanics.
