The universe might be full of tiny, ancient black holes. And researchers might be able to prove it.
These mini black holes from the beginning of time, or primordial black holes (PBHs), were first dreamed up decades ago. Researchers proposed them as an explanation for dark matter, an unseen substance that exerts a gravitational pull throughout space. Most explanations for dark matter involve hypothetical particles with special properties that help them evade detection. But some researchers think swarms of little black holes moving like clouds through space offer a cleaner explanation. Now, a new study explains where these PBHs might have come from, and how astronomers could detect the aftershocks of their birth.
Where did the little black holes come from?
A black hole is a singularity, an infinitely dense point in space packed with matter. It forms when that matter gets so tightly packed that the force of gravity overwhelms everything else, and the matter collapses. It warps space-time and surrounds itself with an “event horizon,” a spherical boundary region beyond which no light can escape.
The laws of general relativity allow black holes to exist at any scale; crush an ant hard enough and it will collapse into a black hole just like a star; it’ll just be incredibly tiny.
Most PBH theories assume these objects have masses like small planets, with event horizons as small as grapefruits. It’s an outlandish idea, still on the fringe of black hole and dark matter physics, said Joey Neilsen, a physicist at Villanova University who was not involved in the new study. But recently, as other dark matter theories have turned up empty, some researchers have given the PBH notion a second look.
If PBHs are out there though, they have to be very old. In the modern universe, there are only two known methods for creating new black holes from normal matter: stars much heavier than the sun colliding or exploding. So every known black hole weighs more than the entire solar system (sometimes much more).
Related: Is our solar system’s mysterious ‘Planet 9’ really a grapefruit-size black hole?
Making small black holes requires a whole other set of mechanisms and ingredients.
Those ingredients would be “the stuff of the Big Bang, the same stuff that makes the stars and galaxies,” Neilsen told Live Science.
Right after the Big Bang, the newly expanding universe was full of hot, dense largely-undifferentiated matter expanding in all directions. There were small pockets of turbulence in this morass — still visible as fluctuations in the Cosmic Microwave Background (CMB), the afterglow of the Big Bang — and those fluctuations gave the universe structure.
“If it’s a little more dense at point A, then stuff is gravitationally attracted to point A,” Neilsen said. “And over the history of the universe, that attraction causes gas and dust to fall inwards, coalesce, collapse and form stars, galaxies, and all the structures in the universe that we know of.”