Cosmic axion background
Jeff A. Dror, Hitoshi Murayama, and Nicholas L. Rodd
Phys. Rev. D 103, 115004 – Published 7 June 2021
Existing searches for cosmic axions relics have relied heavily on the axion being nonrelativistic and making up dark matter. However, light axions can be copiously produced in the early Universe and remain relativistic today, thereby constituting a Cosmic axion Background (CaB). As prototypical examples of axion sources, we consider thermal production, dark-matter decay, parametric resonance, and topological defect decay. Each of these has a characteristic frequency spectrum that can be searched for in axion direct detection experiments. We focus on the axion-photon coupling and study the sensitivity of current and future versions of ADMX, HAYSTAC, DMRadio, and ABRACADABRA to a CaB, finding that the data collected in search of dark matter can be repurposed to detect axion energy densities well below limits set by measurements of the energy budget of the Universe. In this way, direct detection of relativistic relics offers a powerful new opportunity to learn about the early Universe and, potentially, discover the axion.
A representative depiction of the landscape of the CaB, showing the differential axion energy density, given in (2), as a function of energy. The black dashed curves show four different realizations of the CaB, corresponding to thermal production (with Ta=T0, the CMB temperature), a Gaussian distribution representative of parametric-resonance production (with ρa=ργ, ω¯=0.3 μeV, and σ/ω¯=0.1), dark-matter decay (χ→aa), and cosmic-string production (fa=1015 GeV, Td=1012 GeV). For the dark-matter decay distribution the parameters are set to parameters already accessible to ADMX, as justified later in this work. In particular, we take mDM≃5.4 μeV and τ≃2×103tU, with tU the age of the universe. While the thermal distribution will always peak roughly where shown and the cosmic-string production is dominant at lower frequencies, the parametric resonance and dark-matter decay signals can populate the full energy range shown. In all cases we set the axion photon coupling to the largest allowed value consistent with star-emission bounds over this energy range, gaγγSE=0.66×10−10 GeV−1. The colored regions denote the sensitivity in this same space that could be obtained by reanalyzing existing ADMX and HAYSTAC data, or with the future sensitivities of DMRadio and MADMAX. In determining the sensitivities, we have assumed that the CaB axion-photon coupling saturates star-emission bounds. We show the region of parameter space where the CaB could partially alleviate the Hubble tension, labeled H0 Preferred. Finally, the gray dotted line depicts the approximate boundary, to the left of which the CaB has sufficient number densities to be treated as a classical wave.
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