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 Wilkinson Microwave Anisotropy Probe 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≈ 0.2 h Mpc−1, while at late times (z= 500) it peaks at k≈ 0.02 h Mpc−1.