Gravitational wave background from vacuum and thermal fluctuations during axion-like inflation

TL;DR

This paper explores how axion-like inflation with a coupling to topological charge density can generate a gravitational wave background through vacuum and thermal fluctuations, with potential observational signatures in the LISA frequency range.

Contribution

It introduces a framework for warm axion-like inflation including thermal effects and derives a formula for tensor perturbations combining vacuum and thermal contributions.

Findings

Thermal effects produce a characteristic frequency shape in gravitational waves.

The amplitude relates to the shear viscosity of the thermal epoch.

Detectability depends on the model parameters and viscosity magnitude.

Abstract

We revisit the framework of axion-like inflation in view of the possibility that the coupling of the inflaton to a non-Abelian topological charge density could lead to the generation of a rapidly thermalizing heat bath. Both dispersive (mass) and absorptive (friction) effects are included. For phenomenologically viable parameters, the system remains in a weak regime of warm inflation (thermal friction $\ll$ Hubble rate). For tensor perturbations we derive an interpolating formula that incorporates both vacuum and thermal production. The latter yields a model-independent frequency shape $\sim f_0^3$ in the LISA window, whose coefficient allows to measure the maximal shear viscosity of the thermal epoch. It is a challenge, however, to find models where the coefficient is large enough to be observable.