ON THE ENTROPY AND THE DENSITY-MATRIX OF COSMOLOGICAL PERTURBATIONS

We look at the transition to semiclassical behaviour and the decoheren ce process for the inhomogeneous perturbations in the inflationary uni verse. Two different decoherence mechanisms appear: one dynamical, acc ompanied with a negligible, if present at all, entropy gain, and the o ther, effectively irreversible dephasing, due to a rapid variation in time of the off-diagonal density matrix elements in the post-inflation ary epoch. We thus settle the discrepancies in the entropy content of perturbations evaluated by different authors.