Preheating and Entropy Perturbations in Axion Monodromy Inflation

TL;DR

This paper investigates how gauge field preheating in axion monodromy inflation affects entropy and curvature perturbations, finding a blue spectrum that is consistent with observations but could influence primordial black hole formation.

Contribution

It provides a detailed calculation of entropy perturbations from gauge field preheating in axion monodromy inflation and assesses their impact on the curvature spectrum.

Findings

The induced curvature perturbation spectrum has a blue tilt with index 5/2.

The entropy perturbations are negligible for the observed universe.

Constraints on primordial black hole production are satisfied.

Abstract

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.