In the early Universe, tiny black holes could have been born — mountain-mass but atomic-nucleus-sized. Today they could serve as factories for dark matter. There are two mechanisms: the familiar evaporation (Hawking radiation) and the lesser-known superradiance. This is a process where the black hole's rotation amplifies the surrounding field, siphoning energy and turning it into a cloud of particles.
The authors added an important detail from string theory — the so-called "string axiverse." According to it, nature may host not one but hundreds or thousands of ultra-light particle species — axions. When a black hole emits many such species at once, its rotation is amplified, and superradiance becomes much more efficient. The particle cloud around the hole grows faster and reaches higher densities.
The finale could be a "micro-boson star"—a self-gravitating clump of dark matter left after the black hole stops pumping. Such objects do not shine and barely interact with ordinary matter, but they have mass and exert gravity. Calculations showed that this scenario works for a wide range of black hole and axion masses. It significantly expands the parameter space where primordial black holes could explain all observed dark matter.
Interestingly, the axions themselves emitted during evaporation contribute almost nothing to the early Universe's radiation: their imprint on the cosmic microwave background is vanishingly small. Thus, the main channel for producing dark matter remains superradiance, not direct particle emission.
as written by the authors arXiv:2606.20524
We study the effects of string axion emission on dark matter production by light primordial black holes (PBHs), through both evaporation and superradiance. We show, in particular, that the Hawking emission of $$\mathcal{O}(100-10^5)$$ light axion species predicted in realistic string theory constructions can significantly enhance the efficiency of superradiance, given the associated increase in the PBH spin. The string axiverse thus significantly expands the parametric regions (dark matter mass and PBH mass and spin) for which a sizeable fraction of dark matter may presently be in the form of "micro-boson stars": the self-gravitating remnants of superradiant dark matter clouds. Conversely, for too large a number of axion species PBHs evaporate too quickly for superradiant clouds to attain their maximum mass. Finally, assuming that all dark matter is produced by PBHs (through both superradiance and Hawking emission), we show that the axions emitted during PBH evaporation give an immeasurably small contribution to the relativistic degrees of freedom at recombination.
Superradiance works like a cosmic analog of a laser, but for gravitational waves: the black hole's rotation amplifies the particle field, causing them to multiply in an avalanche.
In Fred Hoyle's novel "The Black Cloud," an intelligent gas cloud turns out to be a life form — the micro-boson stars from this paper would also be invisible clumps, but without any hint of intelligence.