Primordial Gravitational Waves from Axion-Gauge Fields Dynamics
  Inspired by the chromo-natural inflation model of Adshead&Wyman, we reshape
its scalar content to relax the tension with current observational bounds.
Besides an inflaton, the setup includes a spectator sector in which an axion
and SU(2) gauge fields are coupled via a Chern-Simons-type term. The result is
a viable theory endowed with an alternative production mechanism for
gravitational waves during inflation. The gravitational wave signal sourced by
the spectator fields can be much larger than the contribution from standard
vacuum fluctuations, it is distinguishable from the latter on the basis of its
chirality and, depending on the theory parameters values, also its tilt. This
production process breaks the well-known relation between the tensor-to-scalar
ratio and the energy scale of inflation. As a result, even if the Hubble rate
is itself too small for the vacuum to generate a tensor amplitude detectable by
upcoming experiments, this model still supports observable gravitational waves.
