Swarms of 'primordial' black holes might fill our universe

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."

Most PBH theories involve very intense fluctuations in the early universe, stronger than the ones that formed galaxies.

In this new paper, the researchers place those intense fluctuations during a period known as "inflation." In the first thousand billion billion billionths of a second after the Big Bang, the universe expanded exponentially fast. That rapid early expansion gave space-time its current "flat" shape, researchers believe, and it likely prevented space from ending up curved, as Live Science has previously reported.

In a new paper published Nov. 20 to the arXiv database, researchers propose that during inflation, there might have been moments where all of space-time was intensely curved, before eventually flattening out. Those brief curvatures, however, would have produced fluctuations in the expanding universe intense enough to eventually form a large population of Earth-mass black holes.

How to find the tiny black holes

The easiest way to prove this theory correct is to look for "secondary gravitational waves" (SGWs) echoing around the universe, the researchers wrote.

These waves, much weaker than gravitational waves produced by colliding black holes, would ring out from the same perturbations that formed the PBHs. They'd be subtle vibrations in the universe, inaudible to current detectors. But two future methods might be able to find them.

One approach: pulsar timing arrays. Space is full of whirling neutron stars known as pulsars that send flashes of energy toward Earth as they spin. Pulsars are like precise, predictable ticking clocks in the sky, but their signals can get distorted by gravitational waves. A secondary gravitational wave passing between Earth and a pulsar would warp space-time, causing the pulsar's tick to arrive a bit early or late in ways a pulsar timing array could detect.

There's a problem with this plan though: Pulsar timing arrays would rely on precisely detecting the ticks of pulsars that emit radio waves. And one of the world's most important radio detectors, the giant Arecibo Telescope in Puerto Rico, has essentially been destroyed, as Live Science sister site Space.com reported.

But even if a high-quality pulsar timing experiment doesn't work out in the next 15 years, the next generation of gravitational wave detectors should be sensitive enough to pick up these secondary gravitational waves, the authors wrote.

Right now, gravitational wave detectors are buried underground, looking for fluctuations in space-time by measuring changes in the travel time of light across long distances. But other effects — minor earthquakes, waves pounding against distant shores and even rabbits hopping around overhead — can muddy the signal. In 2034, the European Space Agency plans to launch the Laser Interferometer Space Antenna (LISA), a far more sensitive space-based gravitational wave detector that avoids those pitfalls. And LISA, the authors wrote, should be able to pick up secondary gravitational waves.

Such a detection, they wrote, would prove that PBHs account for most (if not all) of the dark matter in the universe.

Originally published on Live Science.