Axion cosmology and the energy scale of inflation

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 cosmic microwave background, 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 ${10}^{16}\text{ }\text{ }\mathrm{GeV}$ excludes any inflation model with energy scale $>3.8\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }\mathrm{GeV}$ ($r>2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$) at 95% confidence, and so implies negligible gravitational waves from inflation, but suggests appreciable isocurvature fluctuations.