Abstract
We investigate the production of QCD axion dark matter in a nonstandard cosmological era triggered by primordial black holes (PBHs) that fully evaporate before the onset of BBN. Even if PBHs cannot emit the whole axion cold dark matter abundance through Hawking radiation, they can have a strong impact on the dark matter produced via the misalignment mechanism. First, the oscillation temperature of axions reduces if there is a PBH dominated era, and second, PBH evaporation injects entropy to the standard model, diluting the axion relic abundance originally produced. The axion window is therefore enlarged, reaching masses as light as $\sim 10^{-8}$ eV and decay constants as large as $f_{a}\sim 10^{14}$ GeV without fine tuning the misalignment angle. Such small masses are in the reach of future detectors as ABRACADABRA, KLASH, and ADMX, if the axion couples to photons. Additionally, the axions radiated by PBHs contribute to $\Delta N_\text{eff}$ within the projected reach of the future CMB Stage 4 experiment.
Highlights
QCD axion is a pseudo-Nambu-Goldstone boson arising from the spontaneous breaking of a global Peccei-Quinn symmetry [1]
We investigate the production of QCD axion dark matter in a nonstandard cosmological era triggered by primordial black holes (PBHs) that fully evaporate before the onset of big bang nucleosynthesis (BBN)
PBHs modify the standard axion production in different ways: (i) axions are produced inevitably from Hawking radiation by PBH evaporation, (ii) the oscillation temperature of axions reduces if there is a PBH dominated era, and (iii) PBH evaporation injects entropy to the SM, diluting the axion relic abundance originally produced by the misalignment mechanism
Summary
QCD axion is a pseudo-Nambu-Goldstone boson arising from the spontaneous breaking of a global Peccei-Quinn symmetry [1]. PBHs, and PBH dominated eras, can have a number of phenomenological consequences in the early universe They can trigger baryogenesis [36,37,38,39], or radiate DM or dark radiation via their Hawking evaporation [40,41,42,43,44,45,46,47,48,49,50,51], or even source DM, baryogenesis and density perturbations simultaneously [52].
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