Towards a Heterotic Axiverse

In this paper we initiate a broad study of some central properties of the string axiverse arising from Calabi-Yau compactifications of the perturbative heterotic $E_8\times E_8$ theory. Along this road toward a heterotic axiverse, we characterize the generic structure of the axion mass spectrum and the effective couplings of the non-QCD heterotic axions to Abelian and non-Abelian gauge fields and discuss their implications for cosmology, particle phenomenology, and the QCD axion quality problem. We also provide arguments that the heterotic axion masses are bounded from below much more strongly than, for example, the spectrum in type IIB compactifications.