Constraining axion inflation with gravitational waves from preheating

TL;DR

This paper investigates how gauge preheating after inflation produces gravitational waves, affecting early universe parameters and offering observational tests through CMB experiments and constraints on inflaton-gauge field couplings.

Contribution

It provides a detailed analysis of gravitational wave production from gauge preheating across various inflationary models, linking it to observable parameters like $N_ ext{eff}$ and the tensor-to-scalar ratio.

Findings

Preheating into Abelian gauge fields generates a significant gravitational wave background.

The gravitational wave signal correlates with the tensor-to-scalar ratio, $r$.

Bounds on $N_ ext{eff}$ can constrain inflaton gauge couplings.

Abstract

We study gravitational wave production from gauge preheating in a variety of inflationary models, detailing its dependence on both the energy scale and the shape of the potential. We show that preheating into Abelian gauge fields generically leads to a large gravitational wave background that contributes significantly to the effective number of relativistic degrees of freedom in the early universe, $N_\mathrm{eff}$. We demonstrate that the efficiency of gravitational wave production is correlated with the tensor-to-scalar ratio, $r$. In particular, we show that efficient gauge preheating in models whose tensor-to-scalar ratio would be detected by next-generation cosmic microwave background experiments ($r \gtrsim 10^{-3}$) will be either detected through its contribution to $N_\mathrm{eff}$ or ruled out. Furthermore, we show that bounds on $N_\mathrm{eff}$ provide the most sensitive probe of the possible axial coupling of the inflaton to gauge fields regardless of the potential.