Black Holes from the Universe's Infancy express
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.
Original abstract
as written by the authors arXiv:2606.23846
Primordial Black Holes (PBHs) have emerged as a leading non-particulate candidate for dark matter and a unique cosmological probe, a paradigm shift accelerated by the detection of anomalous binary mergers by the LIGO-Virgo-KAGRA (LVK) collaboration. While the literature is rich with phenomenological constraints, the fundamental quantum and relativistic underpinnings governing PBH genesis and evolution often receive comparatively less emphasis. This review aims to bridge that gap by systematically detailing the physics of PBH formation and their subsequent evolutionary trajectory. We critically examine the hydrodynamic complexity of the early universe, establishing the relativistic thresholds for collapse, the non-linear \emph{race against sound} in the primordial plasma, and the rigorous mathematical utility of the compaction function. Furthermore, by incorporating the dynamic nature of FLRW backgrounds, higher curvature corrections, and quantum backreaction via the memory burden effect, we challenge the standard hawking evaporation and show that extreme-curvature environments halt evaporation entirely, leaving Planck-scale relics that evade current extragalactic bounds. Finally, we map the multimessenger observational landscape, highlighting how the imminent search for sub-solar mass inspirals by next-generation gravitational wave observatories such as the Einstein Telescope and Cosmic Explorer could yield smoking-gun evidence for the PBH paradigm, ultimately transforming these primordial relics into unparalleled laboratories for high-energy physics.
A primordial black hole with the mass of an asteroid would be smaller than an atomic nucleus, yet it could tug a small satellite into orbit around Earth.
In Bruce Sterling's novel Schismatrix, intelligent beings use primordial black holes as a source of energy and as weapons.