Gravitational waves from spectator Gauge-flation

Abstract

We investigate the viability of inflation with a spectator sector comprised of non-Abelian gauge fields coupled through a higher order operator. We dub this model "spectator Gauge-flation". We study the predictions for the amplitude and tensor tilt of chiral gravitational waves and conclude that a slightly red-tilted tensor power spectrum is preferred $n_T=-{\cal O}(0.01)$. As with related models, the enhancement of chiral gravitational waves with respect to the single-field vacuum gravitational wave background is controlled by the parameter $\gamma=g^2 Q^2/H^2$, where $g$ is the gauge coupling, $H$ is the Hubble scale and $Q$ is the VEV of the $SU(2)$ sector. The requirement that the $SU(2)$ is a spectator sector leads to a maximum allowed value for $\gamma$, thereby constraining the possible amplification. In order to provide concrete predictions, we use an $\alpha$-attractor T-model potential for the inflaton sector. Potential observation of chiral gravitational waves with significantly tilted tensor spectra would then indicate the presence of additional couplings of the gauge fields to axions, like in the spectator axion-SU(2) model, or additional gauge field operators.