An M theory solution to the strong CP-problem, and constraints on the axiverse

We give an explicit realization of the “StringAxiverse” discussed in Arvanitaki et. al [ 1 ] by extending our previous results on moduli stabilization in M theory to include axions. We extend the analysis of [ 1 ] 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 × 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 [ 2 ], we note that 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 × 10 14 GeV require no (anthropic) fine-tuning, though standard unification at 10 16 GeV is difficult to accommodate.