An M Theory Solution to the Strong CP Problem and Constraints on the Axiverse

TL;DR

This paper presents an M theory-based model that naturally produces axions solving the strong CP problem, explores cosmological constraints on the Axiverse, and discusses how future observations could falsify the model.

Contribution

It provides an explicit realization of the String Axiverse within M theory, including moduli stabilization and high-scale inflation, with implications for axion properties and cosmological constraints.

Findings

Axions solving the strong-CP problem naturally arise in the model.

Tensor mode observations at Planck could falsify the Axiverse.

Certain light axion models require no fine-tuning, but standard GUT scale unification is challenging.

Abstract

We give an explicit realization of the "String Axiverse" discussed in Arvanitaki et. al \cite{Arvanitaki:2009fg} by extending our previous results on moduli stabilization in $M$ theory to include axions. We extend the analysis of \cite{Arvanitaki:2009fg} to allow for high scale inflation that leads to a moduli dominated pre-BBN Universe. We demonstrate that an axion which solves the strong-CP problem naturally arises and that both the axion decay constants and GUT scale can consistently be around $2\times 10^{16}$ GeV with a much smaller fine tuning than is usually expected. Constraints on the Axiverse from cosmological observations, namely isocurvature perturbations and tensor modes are described. Extending work of Fox et. al \cite{Fox:2004kb}, we note that {\it the observation of tensor modes at Planck will falsify the Axiverse completely.} Finally we note that Axiverse models whose lightest axion has mass of order $10^{-15}$ eV and with decay constants of order $5\times 10^{14}$ GeV require no (anthropic) fine-tuning, though standard unification at $10^{16}$ GeV is difficult to accommodate.