Decay of baryon inhomogeneities in an expanding universe

TL;DR

This paper investigates how baryon inhomogeneities decay over time in the early universe due to diffusion, showing that most generated inhomogeneities diminish rapidly and have limited impact on later cosmological phase transitions.

Contribution

The study provides a detailed calculation of baryon inhomogeneity decay using temperature-dependent diffusion coefficients in an expanding universe, highlighting the minimal effect of typical inhomogeneities on subsequent phases.

Findings

Inhomogeneities decay faster due to universe expansion.

Most inhomogeneities generated at electroweak epoch are too small to affect later transitions.

Large inhomogeneities must have amplitudes >10^5 times background to survive until nucleosynthesis.

Abstract

Baryon inhomogeneities can be generated very early in the universe. These inhomogeneities then decay by particle diffusion in an expanding universe. We study the decay of these baryon inhomogeneities in the early universe using the diffusion equation in the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric. We have studied the decay starting from the electroweak phase transition. We calculate the interaction cross section of the quarks with the neutrinos, the electrons and the muons and obtain the diffusion coefficients. The diffusion coefficients are temperature dependent. We find that the expansion of the universe causes the inhomogeneities to decay at a faster rate. We find that the baryon inhomogeneities generated at the electroweak epoch have very low amplitudes at the time of the quark hadron phase transition. So unless inhomogeneities are generated with a very high amplitude (greater than $10^5$ times the background density), they will have no effect on the quark hadron phase transition. After the quark hadron phase transition, we include the interaction of the muons with the neutrons and the protons till 100 MeV. We also find that large density inhomogeneities generated during the quark hadron transition with sizes of the order of 1 km must have amplitudes greater than $10^5 $ times the background density to survive upto the nucleosynthesis epoch in an expanding universe.