Cosmological window onto the string axiverse and the supersymmetry breaking scale

In the simplest picture, the masses of string axions populating the axiverse depend on two parameters: the supersymmetry breaking scale $M_{\rm susy}$ and the action $S$ of the string instantons responsible for breaking the axion shift symmetry. In this work, we explore whether cosmological data can be used to probe these two parameters. Adopting string-inspired flat priors on $\log_{10}M_{\rm susy}$ and $S$, and imposing that $M_{\rm susy}$ be sub-Planckian, we find $S=198\pm28$. These bounds suggest that cosmological data complemented with string-inspired priors select a quite narrow axion mass range within the axiverse, $\log_{10}(m_a/{\rm eV}) = -21.5^{+1.3}_{-2.3}$. We find that $M_{\rm susy}$ remains unconstrained due to a fundamental parameter degeneracy with $S$. We explore the significant impact of other choices of priors on the results, and we comment on similar findings in recent previous literature.