Large tensor non-Gaussianity from axion-gauge field dynamics

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 ${B}_{h}/{P}_{h}^{2}=\mathcal{O}(10)\ifmmode\times\else\texttimes\fi{}{\mathrm{\ensuremath{\Omega}}}_{A}^{\ensuremath{-}1}$, where ${\mathrm{\ensuremath{\Omega}}}_{A}$ 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.