The Kinematics of Cosmic Reheating

TL;DR

This paper calculates the scalar field relaxation rate in a plasma to determine the cosmic reheating temperature, revealing complex temperature dependencies and effects of plasma interactions on early universe heating.

Contribution

It provides a detailed quantum field theory calculation of reheating rates, highlighting how plasma effects influence the efficiency and temperature of cosmic reheating.

Findings

Reheating rate depends nontrivially on temperature and plasma interactions.

Thermal masses can block or open dissipation channels, affecting reheating.

Fermion couplings can significantly contribute to reheating at high temperatures.

Abstract

We calculate the relaxation rate of a scalar field in a plasma of other scalars and fermions with gauge interactions using thermal quantum field theory. It yields the rate of cosmic reheating and thereby determines the temperature of the "hot big bang" in inflationary cosmology. The total rate originates from various processes, including decays and inverse decays as well as Landau damping by scatterings. It involves quantum statistical effects and off-shell transport. Its temperature dependence can be highly nontrivial, making it impossible to express the reheating temperature in terms of the model parameters in a simple way. We pay special attention to the temperature dependence of the phase space due to the modified dispersion relations in the plasma. We find that it can have a drastic effect on the efficiency of perturbative reheating, which depends on the way particles in the primordial plasma interact. For some interactions thermal masses can effectively close the phase space for the dominant dissipative processes and thereby impose an upper bound on the reheating temperature. In other cases they open up new channels of dissipation, hence increase the reheating temperature. At high temperatures we find that the universe can even be heated through couplings to fermions, which are often assumed to be negligible due to Pauli-blocking. These effects may also be relevant for baryogenesis, dark matter production, the fate of moduli and in scenarios of warm inflation.