Cosmological Consequences of String Axions

TL;DR

This paper explores how measurements of cosmic microwave background fluctuations can constrain the existence and properties of string axions, especially in relation to inflationary gravitational waves and their implications for solving the strong CP problem.

Contribution

It analyzes the cosmological consequences of two specific string axions under inflationary conditions, providing constraints based on potential future observations by the PLANCK experiment.

Findings

Detection of inflationary gravitational waves would rule out the first axion.

The second axion cannot follow a scale-independent mass formula if such gravitational waves are observed.

The analysis constrains axion models based on upcoming cosmological measurements.

Abstract

Axion fluctuations generated during inflation lead to isocurvature and non-Gaussian temperature fluctuations in the cosmic microwave background radiation. Following a previous analysis for the model independent string axion we consider the consequences of a measurement of these fluctuations for two additional string axions. We do so independent of any cosmological assumptions except for the axions being massless during inflation. The first axion has been shown to solve the strong CP problem for most compactifications of the heterotic string while the second axion, which does not solve the strong CP problem, obeys a mass formula which is independent of the axion scale. We find that if gravitational waves interpreted as arising from inflation are observed by the PLANCK polarimetry experiment with a Hubble constant during inflation of H_inf \apprge 10^13 GeV the existence of the first axion is ruled out and the second axion cannot obey the scale independent mass formula. In an appendix we quantitatively justify the often held assumption that temperature corrections to the zero temperature QCD axion mass may be ignored for temperatures T \apprle \Lambda_QCD.