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It all began not with emptiness, but with a boiling ocean of plasma. In that primordial broth, sound waves raced through the fiery matter, colliding and compressing it. Sometimes a clump grew so dense that light could no longer escape, and right out of the scorching gas, a primordial black hole was born. Unlike the giants that form when stars die, these "fossils" could have been tiny — the size of a mountain or even a grain of sand.

Calculating such a birth is no simple task. The review's authors trace the entire chain: from the relativistic threshold of collapse to the race between gravity and sound in the plasma. The key tool here is the so-called compaction function, a mathematical "thermometer" that shows whether a clump will collapse or not. Then quantum effects come into play: the bending of spacetime at the limit of possibility, corrections from higher-order curvatures, and the backreaction of radiation. Together, they lead to a surprising conclusion: in extremely strong fields, the evaporation predicted by Hawking stops.

Such relics are invisible, don't participate in reactions, and are practically eternal. They are ideal candidates for dark matter. Today's telescopes can't see them directly, but future gravitational-wave observatories, like the Einstein Telescope and the Cosmic Explorer, could pick up the "chirp" of two such tiny holes merging. Such a signal would be undeniable proof that primordial black holes are not just a hypothesis, but real building blocks of the cosmos.

So the ocean of plasma that raged in the first second after the Big Bang may have left us a legacy we are only beginning to discern. And perhaps the darkest corners of space are filled not with ghostly particles, but with tiny black holes — silent witnesses to the birth of everything.