Collisional equilibrium, particle production and the inflationary universe

TL;DR

This paper models particle production in the expanding universe, showing that equilibrium conditions and particle creation rates influence cosmological inflation, with implications for thermodynamics and universe evolution.

Contribution

It introduces a kinetic model linking particle creation rates to equilibrium conditions and demonstrates how this leads to power-law inflation.

Findings

Radiation and matter can be in equilibrium at the same temperature with specific particle creation rates.

Adiabatic creation of massive particles implies power-law inflation.

Exponential inflation conflicts with the second law of thermodynamics after a Hubble time.

Abstract

Particle production processes in the expanding universe are described within a simple kinetic model. The equilibrium conditions for a Maxwell-Boltzmann gas with variable particle number are investigated. We find that radiation and nonrelativistic matter may be in equilibrium at the same temperature provided the matter particles are created at a rate that is half the expansion rate. Using the fact that the creation of particles is dynamically equivalent to a nonvanishing bulk pressure we calculate the backreaction of this process on the cosmological dynamics. It turns out that the `adiabatic' creation of massive particles with an equilibrium distribution for the latter necessarily implies power-law inflation. Exponential inflation in this context is shown to become inconsistent with the second law of thermodynamics after a time interval of the order of the Hubble time.