Axion Dark Matter in the Time of Primordial Black Holes

TL;DR

This paper explores how primordial black holes influence QCD axion dark matter production, enlarging the viable parameter space for axion masses and decay constants, with implications for future detection experiments.

Contribution

It demonstrates that PBH evaporation and domination can significantly modify axion relic abundance, expanding the viable axion parameter space without fine-tuning.

Findings

Enlarged axion mass window down to ~10^{-8} eV.

Allowed decay constants up to ~10^{14} GeV.

Potential detectable signals in future experiments.

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.