A black hole is not an eternal vacuum cleaner. Left alone in the void, it slowly disappears, radiating energy like a puddle drying on asphalt. This process, predicted by Stephen Hawking, is called evaporation. The smaller the hole, the faster it melts, and the hotter its "breath."
The authors of the study looked not at a single hole but at a whole cloud of such objects — for example, primordial black holes that might have formed in the early universe. Their masses are not identical but spread over a range. It turns out that over time, this cloud inevitably changes its shape according to a single scenario dictated by evaporation. It's like a puddle leaving behind smaller puddles of different sizes, but all of them dry up according to a common law, and in the end only the smallest ones remain, vanishing last.
This redistribution of masses leaves a trace in gravitational waves — ripples in spacetime that such holes could have generated in the distant past. The spectrum of these waves (the frequency distribution) acquires a characteristic power-law decline at low frequencies — a universal signature of evaporation. Importantly, this signature does not depend on how the masses were initially distributed, whether by critical collapse or any other mechanism. Nature seems to stamp a label: "Hawking evaporation was at work here."
A black hole with the mass of a small asteroid would evaporate in a time billions of times shorter than the age of the universe, while a supermassive black hole at the center of a galaxy would take longer to melt than all the stars put together have existed.
In Dan Simmons's novel "Hyperion," evaporating black holes are used as weapons capable of destroying entire star systems — here, the same process, but on cosmological scales.