Resonant backreaction in axion inflation
  Axion inflation entails a coupling of the inflaton field to gauge fields
through the Chern-Simons term. This results in a strong gauge field production
during inflation, which backreacts on the inflaton equation of motion. Here we
show that this strongly non-linear system generically experiences a resonant
enhancement of the gauge field production, resulting in oscillatory features in
the inflaton velocity as well as in the gauge field spectrum. The gauge fields
source a strongly enhanced scalar power spectrum at small scales, exceeding
previous estimates. For appropriate parameter choices, the collapse of these
over-dense regions can lead to a large population of (light) primordial black
holes with remarkable phenomenological consequences.
