  A criterion is presented and discussed to detect when a divergence-free
perfect fluid energy tensor in the space-time describes an evolution in local
thermal equilibrium.
  This criterion is applied to the class II Szafron-Szekeres perfect fluid
space-times solutions, giving a very simple characterization of those that
describe such thermal evolutions. For all of them, the significant
thermodynamic variables are explicitly obtained.
  Also, the specific condition is given under which the divergence-free perfect
fluid energy tensors may be interpreted as an ideal gas.

General Relativity and Quantum Cosmology