Warming up cold inflation

Abstract

The axion is a well-motivated candidate for the inflaton, as the radiative corrections that spoil many single-field models are avoided by virtue of its shift symmetry. However, axions generically couple to gauge sectors. As the axion slow-rolls during inflation, this coupling can cause the production of a non-diluting thermal bath, a situation known as "warm inflation." This thermal bath can dramatically alter inflationary dynamics and observable predictions. In this paper, we demonstrate that a thermal bath can form for a wide variety of initial conditions, including starting from zero initial temperature in the universe. Furthermore, we find that axion inflation becomes warm over a large range of couplings, and explicitly map the parameter space for two axion inflation potentials. We show that in large regions of parameter space, axion inflation models once assumed to be safely "cold" are in fact warm, and must be reevaluated in this context.

Axion dark matter from frictional misalignment

“…Axion-gauge couplings may lead to tachyonic, non thermal enhancement of gauge modes which results in rich phenomenology in many aspects of cosmology such as inflation [108][109][110][111][112], dark matter [113][114][115], gravitational waves [116][117][118], cosmological relaxation [119][120][121][122] or dark energy [123]. On the other hand, the cosmological implications of thermal backgrounds of gauge fields have only been recently explored in a limited setting and mainly in the context of inflation and not axion dark matter (see also [121,[124][125][126] for the transition from the non-thermal to thermal regime). Our work fills this gap in the literature and provides a natural extension to the standard misalignment mechanism that is based on a set of reasonable assumptions which however have a strong impact on the size of the viable parameter space for axion dark matter.…”

On sphaleron heating in the presence of fermions

“…In order to guarantee validity of effective field theory and ensure a definite hierarchy of energy scales, we will impose f < M P and C < M P . Throughout, we shall assume that the system is already thermalized (see [8,10,38,74,75,143] for partially controversial discussions about how a thermal state can be reached).…”

Some open questions in axion theory

“…The instability shuts off if the background modes thermalize. One mechanism can precede the other, setting the initial conditions for thermal friction to take over [240]. The nonlinear friction arising from tachyonic instability has a different set of evolution equations, that requires tracking each individual mode which can be computationally demanding [241].…”