The Milky Way may be home to strange black holes from the first moments of the universe, and the best candidates are the three closest black holes to Earth.
The closest black holes to Earth may date back to the first moments after the big bang. New models of how these so-called primordial black holes would orbit other cosmic objects seem to match the unusual properties of the nearest black holes we have ever seen.
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| Primordial black holes could be closer than we thought SpaceEngine / Cosmographic Software LLC. |
Regular, or astrophysical, black holes form when massive stars collapse in on themselves. But before stars existed at all, in the very early universe, tiny black holes could have formed from extraordinarily dense pockets of matter. Such primordial black holes have never been seen, but if they exist – and if there are enough of them – they could explain the pervasive gravitational effects that are generally attributed to a mysterious substance called dark matter.
Black holes, like dark matter, are impossible to spot directly, so to find them we must look at their influence on the objects around them. To that end, Benjamin Lehmann at the Massachusetts Institute of Technology and his colleagues performed a series of simulations of exchange processes – common scenarios where one of two objects that are circling each other gets kicked out of its orbit and replaced by a primordial black hole.
Because primordial black holes aren’t produced by stellar collapse, they could, in theory, be much less massive than regular black holes. The researchers simulated black holes with similar masses to asteroids, planets and stars to see if any of these would be detectable.
They found that if primordial black holes exist, asteroid-mass ones should orbit asteroids and planet-mass and stellar-mass ones should orbit stars, all in our cosmic neighbourhood. But the smaller black holes would be so rare and so hard to spot that the only ones that we have any hope of detecting are those of stellar mass.
The problem with this is that black holes the mass of stars can be produced the typical way, and it is tough to discern whether such a black hole is astrophysical or primordial. Telling the difference between these would be “very difficult” unless we see a black hole much smaller than a star, says Juan García-Bellido at the Autonomous University of Madrid in Spain. But we may be able to use the characteristics of its orbit, such as the width or speed, to figure out whether a black hole was born from a star in a binary or if it is a captured primordial black hole, he says.
Tantalisingly, the researchers did identify a few black holes with strange orbital properties: Gaia BH1, BH2 and BH3. These are the three closest known black holes to Earth, and all three of them seem to have formed through some sort of exchange process. They were all discovered within the past two years.
“It’s a chin scratcher,” says Lehmann. “Right now, the issue is that the systems are so new newly observed that there hasn’t really been time for the astrophysics community to come to a clear consensus on what the possible origin scenarios for these objects are.”
If any of these black holes are primordial, it would turn the study of dark matter on its head by essentially ruling out weakly interacting massive particles (WIMPs), which have been the favoured candidate to explain dark matter for decades. “The moment you have even a small fraction of dark matter in primordial black holes, it’s basically impossible to have the rest in WIMPs, at least in traditional standard WIMPs,” says Lehmann. He says we will need to find many more systems that formed through exchange processes to be sure that any of them contain primordial black holes, though.
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