The magnitude of the non-adiabatic pressure in the cosmic fluid
  Understanding the non-adiabatic pressure, or relative entropy, perturbation
is crucial for studies of early-universe vorticity and Cosmic Microwave
Background observations. We calculate the evolution of the linear non-adiabatic
pressure perturbation from radiation domination to late times, numerically
solving the linear governing equations for a wide range of wavenumbers. Using
adiabatic initial conditions consistent with WMAP seven year data, we find
nevertheless that the non-adiabatic pressure perturbation is non-zero and grows
at early times, peaking around the epoch of matter/radiation equality and
decaying in matter domination. At early times or large redshifts (z=10,000) its
power spectrum peaks at a comoving wavenumber k~0.2h/Mpc, while at late times
(z=500) it peaks at k~0.02 h/Mpc.
