A sponge thrown into a bucket of water eagerly soaks up moisture and swells before your eyes. But if you keep adding water and squeeze and release the sponge, absorption becomes harder and takes longer. That's roughly how tiny black holes behaved at the very beginning of the universe.
These holes are not remnants of stars but clumps of superdense matter that arose from random fluctuations in the first moments after the Big Bang. They are called primordial black holes. They appeared before the universe cooled enough for the first atomic nuclei to form (the epoch of nucleosynthesis). Around them at that time raged a special field, the inflaton, which oscillated and gradually decayed, turning into a hot 'soup' of particles and radiation. This period is called reheating.
The authors of the study built a complete picture of the life of such a hole in this changing environment. They linked the initial conditions directly to observational data from the cosmic microwave background (the 'echo' of the Big Bang that satellites detect). Then they calculated how the hole sucks in matter from the oscillating field and the growing hot cloud around it. It turned out that the combined effect of the field and radiation leads to a very sharp, nonlinear growth in the hole's mass. In simpler terms, the hole inflates much more than if it were simply absorbing radiation.
This extra growth dramatically changes the hole's fate. Tiny black holes evaporate by emitting energy (this is called Hawking radiation). The lifetime of such a hole depends steeply on its mass: double the mass, and it lives eight times longer. Holes that were originally doomed to disappear quickly, after being 'fed' by the field and radiation, survive to much later times, when the universe is already filled with ordinary matter and light. And since they live longer, they emit longer, creating ripples in spacetime — gravitational waves. This stochastic (random) gravitational-wave background, according to calculations, is amplified by hundreds and thousands of times. Its traces could be picked up by existing and future gravitational-wave detectors. Thus, the story of a tiny hole that survived the universe's youth can tell us about the very first moments of the universe.
If a primordial black hole with the mass of a small asteroid were near Earth, it would evaporate in a fraction of a second, but it would release energy comparable to the explosion of millions of hydrogen bombs.
In the series 'Star Trek: Voyager', the crew uses quantum singularities (essentially microscopic black holes) as an energy source for their engines.