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.
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.