What can the CMB tell about the microphysics of cosmic reheating?

TL;DR

This paper explores how cosmic microwave background (CMB) data can inform us about the microphysics of cosmic reheating, distinguishing between perturbative and non-perturbative processes and their observational implications.

Contribution

It analyzes the potential of CMB observations to constrain the microphysical parameters of reheating, highlighting differences between perturbative and preheating scenarios.

Findings

CMB data can constrain inflaton couplings during perturbative reheating.

Preheating scenarios involve complex microphysical parameter degeneracies.

CMB constraints are limited in non-perturbative reheating models.

Abstract

In inflationary cosmology, cosmic reheating after inflation sets the initial conditions for the hot big bang. We investigate how CMB data can be used to study the effective potential and couplings of the inflaton during reheating and constrain the underlying microphysics. If there is a phase of preheating that is driven by a parametric resonance or other instability, then the thermal history and expansion history during the reheating era depend on a large number of microphysical parameters in a complicated way. In this case the connection between CMB observables and microphysical parameters can only established with intense numerical studies. Such studies can help to improve CMB constraints on the effective inflaton potential in specific models, but parameter degeneracies usually make it impossible to extract meaningful best-fit values for individual microphysical parameters. If, on the other hand, reheating is driven by perturbative processes, then it can be possible to constrain the inflaton couplings and the reheating temperature from CMB data. This provides an indirect probe of fundamental microphysical parameters that most likely can never be measured directly in the laboratory, but have an immense impact on the evolution of the cosmos by setting the stage for the hot big bang.