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Bosons in the Sky with Black Holes

Title: Discovering the QCD Axion with Black Holes and Gravitational WavesAuthors: Asimina Arvanitaki, Masha Baryakhtar, Xinlu HuangFirst Author’s Institution: Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, CanadaStatus: : Published in Phys. Rev. D 91, 084011 (2015), open access on arXivThe success of the LIGO/VIRGO collaboration has opened a window into many previously gravitationally unprobed phenomena, from tiny dark matter candidates to supermassive black holes. One such example is black hole superradiance: when a wave is scattered off a rotating black hole, it can exit with a larger amplitude than it had before, carrying with it some of the angular momentum of the black hole. This effect is particularly interesting in the case of bosonic waves, whose quantum numbers are not suppressed by Pauli’s Exclusion Principle. This interaction leads to the formation of high occupation bound states around the black hole – creating, in essence, a gravitational atom, with the black hole as its nucleus.Thus, black hole superradiance can be used to detect signals of bosonic fields which cannot be observed in a detector. In this paper, Arvanitaki et al. consider in particular the QCD axion, a robust bosonic dark matter candidate, and the signatures it might produce through black hole superradiance. They estimate its gravitational signatures, both from transitions between “orbitals” of the gravitational atom and from annihilation, as well as its bounds from black hole spin measurements. Both of these criteria are measurable by advanced LIGO (aLIGO), and future observations will serve to constrain axion masses in the event of no detection. Gravitational AtomsJust like the familiar and beloved Hydrogen atom, the gravitational atom has a...

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