Preheating and Entropy Perturbations in Axion Monodromy Inflation
  We study the preheating of gauge fields in a simple axion monodromy model and compute the induced entropy perturbations and their effect on the curvature fluctuations. We find that the correction to the spectrum of curvature perturbations has a blue spectrum with index $n_s = 5/2$. Hence, these induced modes are harmless for the observed structure of the universe. Since the spectrum is blue, there is the danger of overproduction of primordial black holes. However, we show that the observational constraints are easily satisfied. 
Evan McDonough, Hossein Bazrafshan Moghaddam and Robert H.   Brandenberger
hep-th astro-ph.CO gr-qc hep-ph