On the Phase-Space Volume of Primordial Cosmological Perturbations
  We show how to determine the typical phase space volume $\Gamma$ for primordial gravitational waves produced during an inflationary stage, which is invariant under squeezing. An expression for $\Gamma$ is found in the long wavelength regime. The quasi-classical entropy of a pure vacuum initial state defined as the logarithm of $\Gamma$ modulo a constant remains zero in spite of the generation of fluctuations (creation of real gravitons). 
Julien Lesgourgues, David Polarski and Alexei A. Starobinsky