String axiverse

String theory suggests the simultaneous presence of many ultralight axions, possibly populating each decade of mass down to the Hubble scale ${10}^{\ensuremath{-}33}\text{ }\text{ }\mathrm{eV}$. Conversely the presence of such a plenitude of axions (an axiverse) would be evidence for string theory, since it arises due to the topological complexity of the extra-dimensional manifold and is ad hoc in a theory with just the four familiar dimensions. We investigate how several upcoming astrophysical experiments will be observationally exploring the possible existence of such axions over a vast mass range from ${10}^{\ensuremath{-}33}\text{ }\text{ }\mathrm{eV}$ to ${10}^{\ensuremath{-}10}\text{ }\text{…