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

TL;DR

This paper investigates the evolution of non-adiabatic pressure perturbations in the early universe, revealing their growth and peak behavior around matter-radiation equality, with implications for cosmological observations.

Contribution

It provides a detailed numerical analysis of non-adiabatic pressure evolution across cosmic history, highlighting its non-zero nature even with adiabatic initial conditions.

Findings

Non-adiabatic pressure perturbation is non-zero with adiabatic initial conditions.

The perturbation peaks at matter/radiation equality epoch.

Power spectrum peaks shift from small to large scales over time.

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.