Constraining axion inflation with gravitational waves across 29 decades in frequency

TL;DR

This paper shows that gravitational waves from gauge preheating after axion inflation significantly impact cosmological parameters, providing new constraints on inflation models and axion-gauge couplings through current and future observations.

Contribution

It demonstrates that gravitational waves from gauge preheating constrain axion inflation models and gauge couplings more strongly than previous methods, especially with upcoming CMB data.

Findings

Gravitational waves from preheating affect the effective number of relativistic species.

Current Planck data already constrain axion-gauge couplings.

Future CMB experiments will test the entire efficient gauge preheating regime.

Abstract

We demonstrate that gravitational waves generated by efficient gauge preheating after axion inflation generically contribute significantly to the effective number of relativistic degrees of freedom $N_\mathrm{eff}$. We show that, with existing Planck limits, gravitational waves from preheating already place the strongest constraints on the inflaton's possible axial coupling to Abelian gauge fields. We demonstrate that gauge preheating can completely reheat the Universe regardless of the inflationary potential. Further, we quantify the variation of the efficiency of gravitational wave production from model to model and show that it is correlated with the tensor-to-scalar ratio. In particular, when combined with constraints on models whose tensor-to-scalar ratios would be detected by next-generation cosmic microwave background experiments, $r\gtrsim 10^{-3}$, constraints from $N_\mathrm{eff}$ will probe or rule out the entire coupling regime for which gauge preheating is efficient.