Gauge-flation confronted with Planck
  Gauge-flation is a recently proposed model in which inflation is driven
solely by a non-Abelian gauge field thanks to a specific higher order
derivative operator. The nature of the operator is such that it does not
introduce ghosts. We compute the cosmological scalar and tensor perturbations
for this model, improving over an existing computation. We then confront these
results with the Planck data. The model is characterized by the quantity \gamma
= (g^2 Q^2)/H^2 (where g is the gauge coupling constant, Q the vector vev, and
H the Hubble rate). For \gamma < 2, the scalar perturbations show a strong
tachyonic instability. In the stable region, the scalar power spectrum n_s is
too low at small \gamma, while the tensor-to-scalar ratio r is too high at
large \gamma. No value of \gamma leads to acceptable values for n_s and r, and
so the model is ruled out by the CMB data. The same behavior with \gamma was
obtained in Chromo-natural inflation, a model in which inflation is driven by a
pseudo-scalar coupled to a non-Abelian gauge field. When the pseudo-scalar can
be integrated out, one recovers the model of Gauge-flation plus corrections. It
was shown that this identification is very accurate at the background level,
but differences emerged in the literature concerning the perturbations of the
two models. On the contrary, our results show that the analogy between the two
models continues to be accurate also at the perturbative level.
