Thermodynamics and kinetics of elementary particles in cosmology

TL;DR

This paper reviews the thermodynamics and kinetics of elementary particles in the early universe, analyzing equilibrium conditions, deviations, and implications for cosmological phenomena like baryogenesis.

Contribution

It provides an accessible overview of kinetic equations in cosmology, including effects of T-invariance and CPT violation on particle distributions and baryogenesis.

Findings

Neutrinos deviate from equilibrium despite being massless.

Standard equilibrium distributions remain unchanged under T-invariance breaking.

Modified kinetics may influence baryogenesis mechanisms.

Abstract

Several lectures for non-experts on equilibrium and non-equilibrium kinetics in expanding universe are presented. An establishment of thermal equilibrium in the ealry universe as well as mechanisms leading to deviations from the equilibrium are discussed. In the first lecture an elementary introduction to cosmology is presented. Next, kinetic equation in Friedman-Robertson-Walker mertric is considered. Properties of its solutions for different types of particles, in particular, for photons of cosmic microwave background radiation, neutrinos, and massive particles, which could be cold dark matter ones, are analyzed. It is shown, in particular, that despite being massless, neutrinos noticeably deviate from equilibrium. Next the modification of the detailed balance condition in the case of broken T-invariance is discussed and it is shown that the standard equilibrium distribution functions remain unchanged. Possible modification of the standard kinetics in the case of broken CPT and applications to baryogenesis are considered.