A tiny black hole, no larger than an atomic nucleus, hovers in a cloud of hydrogen. It doesn't swallow everything around it; instead, it slowly radiates away, losing mass with every moment. Just as a pot on the stove doesn't boil all at once (first bubbles race across the bottom, then the water churns more vigorously) so the universe after the Big Bang might have heated up not in a sudden burst but smoothly, like milk simmering on low heat.
Physicists used to think that all primordial black holes were born with the same mass. In that case, their evaporation would occur nearly simultaneously, filling space with powerful gravitational waves — ripples that travel through time itself. But general relativity suggests a different scenario: the masses of the holes are spread out, with many more light ones. This tail of the distribution, predicted by Einstein, changes the picture. Evaporation stretches out, as if hundreds of kettles were coming to a boil not in unison but one after another.
Such a gradual heating dramatically weakens the gravitational waves — by orders of magnitude. Detectors like LISA simply won't see them. But this opens a window for ultralight black holes: they could have evaporated quietly, without disturbing the orderly picture of the early universe. The final paragraph returns to the image: the pot on the stove — only now it's clear that the bubbles didn't appear all at once but gradually, and so the bubbling was quieter.
as written by the authors arXiv:2605.21477
The hot Big Bang may have emerged from evaporation of primordial black holes (PBHs) lighter than $$10^9$$g. Standard monochromatic treatments predict nearly simultaneous evaporation, abrupt reheating, and a large Poltergeist scalar-induced gravitational wave signal. We confront this expectation with the irreducible collapse mass tail predicted by general relativity, $$df_{\rm PBH}/d\ln M\propto M^{3.78}$$, which smooths reheating, suppresses the signal by orders of magnitude, and reopens the ultra-light PBH window.
A black hole with a mass of a thousand tons would evaporate in a second, while one with a million tons would take a year. But primordial holes lighter than ten thousand tons couldn't have survived to the present day.
In Paul McAuley's novel "Anima," ultralight black holes are used as weapons capable of destroying an entire planet, but in reality, they simply melt away quietly.