Oscillations in the CMB from Axion Monodromy Inflation

TL;DR

This paper analyzes how axion monodromy inflation models predict characteristic oscillations in the CMB power spectrum, with potential for detection in upcoming experiments, based on analytical calculations and string theory constraints.

Contribution

It provides an analytical expression for the scalar power spectrum in axion monodromy inflation and assesses the observational limits and detectability of modulations and resonant non-Gaussianities.

Findings

Detectable modulations are common in well-controlled models.

Resonant bispectrum contributions vary in detectability.

String theory constraints influence the amplitude of observable features.

Abstract

We study the CMB observables in axion monodromy inflation. These well-motivated scenarios for inflation in string theory have monomial potentials over super-Planckian field ranges, with superimposed sinusoidal modulations from instanton effects. Such periodic modulations of the potential can drive resonant enhancements of the correlation functions of cosmological perturbations, with characteristic modulations of the amplitude as a function of wavenumber. We give an analytical result for the scalar power spectrum in this class of models, and we determine the limits that present data places on the amplitude and frequency of modulations. Then, incorporating an improved understanding of the realization of axion monodromy inflation in string theory, we perform a careful study of microphysical constraints in this scenario. We find that detectable modulations of the scalar power spectrum are commonplace in well-controlled examples, while resonant contributions to the bispectrum are undetectable in some classes of examples and detectable in others. We conclude that resonant contributions to the spectrum and bispectrum are a characteristic signature of axion monodromy inflation that, in favorable cases, could be detected in near-future experiments.