Probing a QCD String Axion with Precision Cosmological Measurements

TL;DR

The paper demonstrates that in string and M-theory compactifications, a compact modulus cannot serve as a QCD axion to solve the strong CP problem if high-scale inflation with Hubble constant above 10^{13} GeV is observed, due to induced isocurvature and non-Gaussian fluctuations.

Contribution

It provides a no-go result showing that such compact moduli cannot be the QCD axion under high-scale inflation, independent of initial conditions or late entropy release.

Findings

High-scale inflation implies observable isocurvature/non-Gaussian signatures.

Compact moduli cannot solve the strong CP problem as QCD axions in this scenario.

Result is robust against assumptions about initial misalignment or dark matter composition.

Abstract

String and M-theory compactifications generically have compact moduli which can potentially act as the QCD axion. However, as demonstrated here, such a compact modulus can not play the role of a QCD axion and solve the strong CP problem if gravitational waves interpreted as arising from inflation with Hubble constant $H_inf \gsim 10^{13}$ GeV are observed by the PLANCK polarimetry experiment. In this case axion fluctuations generated during inflation would leave a measurable isocurvature and/or non-Gaussian imprint in the spectrum of primordial temperature fluctuations. This conclusion is independent of any assumptions about the initial axion misalignment angle, how much of the dark matter is relic axions, or possible entropy release by a late decaying particle such as the saxion; it relies only on the mild assumption that the Peccei-Quinn symmetry remains unbroken in the early universe.