A recent study explores how the emission of string axions by light primordial black holes (PBHs) could boost dark matter production via superradiance. Researchers have shown that Hawking emission of a large number of light axion species, predicted in realistic string theory constructions (on the order of 100 to 10^5), can significantly increase the efficiency of superradiance. This enhancement is due to the associated increase in the PBH spin, suggesting a more effective mechanism for the formation of micro-boson stars, which are self-gravitating remnants of superradiant dark matter clouds.

The concept of a "string axiverse" expands the parametric regions (dark matter mass, and PBH mass and spin) where a sizeable fraction of dark matter might exist in the form of these micro-boson stars. However, the study also points out a limitation: if the number of axion species is too large, PBHs evaporate too quickly, preventing superradiant clouds from attaining their maximum mass. This establishes a delicate balance in the contribution of axions to dark matter production.

Assuming that all dark matter is produced by PBHs, through both superradiance and Hawking emission, the authors conclude that the axions emitted during PBH evaporation make an immeasurably small contribution to the relativistic degrees of freedom at recombination. This implies that while axions may play a crucial role in dark matter production, their direct impact on early cosmology, in terms of relativistic radiation, would be negligible and unobservable with current techniques.