Large Tensor Non-Gaussianity from Axion-Gauge Fields Dynamics

TL;DR

This paper demonstrates that a spectator axion coupled to an SU(2) gauge field during inflation can generate a large non-Gaussian signal in primordial gravitational waves, potentially detectable in CMB observations.

Contribution

It introduces a model where axion-gauge field interactions produce significant tensor non-Gaussianity on CMB scales, a novel mechanism in inflationary cosmology.

Findings

The bispectrum amplitude is proportional to the inverse of the gauge field energy fraction.

The model predicts a large equilateral non-Gaussianity in gravitational waves.

Potential observability of the non-Gaussian signal in future CMB experiments.

Abstract

We show that an inflation model in which a spectator axion field is coupled to an SU(2) gauge field produces a large three-point function (bispectrum) of primordial gravitational waves, $B_{h}$, on the scales relevant to the cosmic microwave background experiments. The amplitude of the bispectrum at the equilateral configuration is characterized by $B_{h}/P_h^2=\mathcal{O}(10)\times \Omega_A^{-1}$, where $\Omega_A$ is a fraction of the energy density in the gauge field and $P_h$ is the power spectrum of gravitational waves produced by the gauge field.