Perturbations in Chromo-Natural Inflation
  Chromo-Natural Inflation is the first worked example of a model of inflation
in which slow-roll inflation is achieved by "magnetic drift" as opposed to
Hubble friction. In this work, we give an account of the perturbations at
linear order in this model. Our analysis uncovers two novel phenomena. First,
the amplitude of scalar curvature perturbations is not directly tied to the
shape of the inflationary potential. This allows the theory to violate na\"ive
formulations of the Lyth bound. Second, the tensor sector of the theory is
significantly altered from the usual case: the non-Abelian gauge field
perturbations have a tensor degree of freedom. One chirality of the this tensor
can be exponentially enhanced by a temporary instability near horizon crossing;
this chiral instability exists because of the classical gauge field background,
which violates parity. These tensor fluctuations of the gauge field also couple
to gravitational waves at linear order in perturbation theory and source a
chiral spectrum of gravitational waves. This spectrum can be exponentially
enhanced over the usual inflationary spectrum due to the instability in the
gauge sector. These new features cause the theory in its present form to be in
significant tension with current observational data. This is because the new
scalar physics leads to a significant reddening of the spectral tilt in the
same region of parameter space where the exponential enhancement of the
gravitational wave amplitude is small enough to satisfy current constraints on
the tensor-to-scalar index. Hence, the model either predicts a spectral tilt
that is too red, or it overproduces gravitational waves, or both.
