String Axiverse
Asimina Arvanitaki, Savas Dimopoulos, Sergei Dubovsky, Nemanja, Kaloper, John March-Russell

TL;DR
This paper explores how upcoming astrophysical experiments can detect a multitude of ultralight axions predicted by string theory, through effects on cosmic microwave background polarization, galaxy power spectra, and black hole superradiance.
Contribution
It provides a comprehensive analysis of observational signatures of the string axiverse across a wide mass range, linking theoretical predictions with potential astrophysical tests.
Findings
CMB polarization rotation of order 1/137 for axions between 10^-33eV and 10^-28eV.
Detection of multiple steps in matter power spectrum from axions in 10^-28eV to 10^-18eV.
Mass gaps in black hole spin distributions due to axion superradiance, constraining axion decay constants.
Abstract
String theory suggests the simultaneous presence of many ultralight axions possibly populating each decade of mass down to the Hubble scale 10^-33eV. 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 upcoming astrophysical experiments will explore the existence of such axions over a vast mass range from 10^-33eV to 10^-10eV. Axions with masses between 10^-33eV to 10^-28eV cause a rotation of the CMB polarization that is constant throughout the sky. The predicted rotation angle is of order \alpha~1/137. Axions in the mass range 10^-28eV to 10^-18eV give rise to multiple steps in the matter power spectrum, that will be probed by upcoming galaxy surveys. Axions in the…
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