Axion Cosmology and the Energy Scale of Inflation

TL;DR

This paper explores how axion physics constrains the energy scale of inflation, highlighting the interplay between axion parameters, isocurvature fluctuations, and inflationary models, with implications for gravitational wave detection.

Contribution

It maps out observational constraints on axion and inflation parameters, identifying allowed windows and deriving bounds on inflation energy scale from isocurvature fluctuation limits.

Findings

High Peccei-Quinn scale excludes high-energy inflation models

Current bounds imply negligible gravitational waves from inflation

Isocurvature fluctuations are significant in inflationary axion cosmology

Abstract

We survey observational constraints on the parameter space of inflation and axions and map out two allowed windows: the classic window and the inflationary anthropic window. The cosmology of the latter is particularly interesting; inflationary axion cosmology predicts the existence of isocurvature fluctuations in the CMB, with an amplitude that grows with both the energy scale of inflation and the fraction of dark matter in axions. Statistical arguments favor a substantial value for the latter, and so current bounds on isocurvature fluctuations imply tight constraints on inflation. For example, an axion Peccei-Quinn scale of 10^16 GeV excludes any inflation model with energy scale > 3.8*10^14 GeV (r > 2*10^(-9)) at 95% confidence, and so implies negligible gravitational waves from inflation, but suggests appreciable isocurvature fluctuations.