String axiverse

Abstract

String theory suggests the simultaneous presence of many ultralight axions\npossibly populating each decade of mass down to the Hubble scale 10^-33eV.\nConversely the presence of such a plenitude of axions (an "axiverse") would be\nevidence for string theory, since it arises due to the topological complexity\nof the extra-dimensional manifold and is ad hoc in a theory with just the four\nfamiliar dimensions. We investigate how upcoming astrophysical experiments will\nexplore the existence of such axions over a vast mass range from 10^-33eV to\n10^-10eV. Axions with masses between 10^-33eV to 10^-28eV cause a rotation of\nthe CMB polarization that is constant throughout the sky. The predicted\nrotation angle is of order \\alpha~1/137. Axions in the mass range 10^-28eV to\n10^-18eV give rise to multiple steps in the matter power spectrum, that will be\nprobed by upcoming galaxy surveys. Axions in the mass range 10^-22eV to\n10^-10eV affect the dynamics and gravitational wave emission of rapidly\nrotating astrophysical black holes through the Penrose superradiance process.\nWhen the axion Compton wavelength is of order of the black hole size, the\naxions develop "superradiant" atomic bound states around the black hole\n"nucleus". Their occupation number grows exponentially by extracting rotational\nenergy from the ergosphere, culminating in a rotating Bose-Einstein axion\ncondensate emitting gravitational waves. This mechanism creates mass gaps in\nthe spectrum of rapidly rotating black holes that diagnose the presence of\naxions. The rapidly rotating black hole in the X-ray binary LMC X-1 implies an\nupper limit on the decay constant of the QCD axion f_a<2*10^17GeV, much below\nthe Planck mass. This reach can be improved down to the grand unification scale\nf_a<2*10^16GeV, by observing smaller stellar mass black holes.\n

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Affiliated Institutions

• University of California, Berkeley US
• University of California, Davis US
• Lawrence Berkeley National Laboratory US
• Institute for Nuclear Research RU
• University of Oxford GB
• Stanford University US

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