On the validity of the perturbative description of axions during
  inflation
  Axions play a central role in many realizations of large field models of
inflation and in recent alternative mechanisms for generating primordial tensor
modes in small field models. If these axions couple to gauge fields, the
coupling produces a tachyonic instability that leads to an exponential
enhancement of the gauge fields, which in turn can decay into observable scalar
or tensor curvature perturbations. Thus, a fully self-consistent treatment of
axions during inflation is important, and in this work we discuss the
perturbative constraints on axions coupled to gauge fields. We show how the
recent proposal of generating tensor modes through these alternative mechanisms
is in tension with perturbation theory in the in-in formalism. Interestingly,
we point out that the constraints are parametrically weaker than one would
estimate based on naive power counting of propagators of the gauge field. In
the case of non-Abelian gauge fields, we derive new constraints on the size of
the gauge coupling, which apply also in certain models of natural large field
inflation, such as alignment mechanisms.
