The magnitude of the non-adiabatic pressure in the cosmic fluid

Abstract

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.

5 pages, 4 figures. Replaced with version accepted by MNRAS. One figure removed, added some discussion

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• Degeneracy between nonadiabatic dark energy models and $\Lambda$CDM: ISW effect and the cross correlation of CMB with galaxy clustering data
• Isocurvature initial conditions for second order Boltzmann solvers
• Cosmological Perturbations: Vorticity, Isocurvature and Magnetic Fields