Dark matter black hole may zip through the solar system once every decade

Dark matter may be hiding as atomic-sized black holes

By Clara Moskowitz

Scientists say that black holes the size of atoms and the mass of asteroids may pass through the inner solar system about once every decade. In theory, these so-called primordial black holes, created just after the Big Bang, could explain the disappearance of dark matter thought to dominate the universe. And new research suggests that if a black hole were to pass by the Moon or Mars, scientists should be able to detect it.

Such black holes could easily have arisen just after the beginning of the universe, when space is thought to have expanded enormously in a split second. During this expansion, small quantum fluctuations in the density of space could have grown so large that some spots became too dense and collapsed into black holes, scattering them throughout the universe. If dark matter can be fully explained by such black holes, some theories suggest that their most likely masses would be in the range of 10 to 10 million kilograms.¹⁷ Up to 10^{twenty three} grams, or about the weight of a large asteroid.

If primordial black holes are the source of dark matter, they probably pass through the solar system about every 10 years, a new study finds. If one of these black holes comes close to a planet or a large moon, that object should be knocked out of orbit enough to be measurable by current instruments. “When a black hole passes by, the planet starts to wobble,” says Sarah R. Geller, a theoretical physicist at the Massachusetts Institute of Technology and co-author of the study published September 17. Physics Review D“The fluctuations will grow over the course of a few years, but will eventually subside and return to zero.”

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Research team member Tung X. Tran, then an undergraduate at MIT, built a computer model of the solar system to see how the distance between Earth and nearby objects in the solar system would change after a close pass by a black hole. He found that such effects would be most pronounced on Mars, which scientists know is within about 10 centimeters of distance. For black holes in the middle of the mass range, “we found that the signal would grow to one to three meters after three years,” Tran said. “That’s way beyond the threshold of precision that we can measure.” The distance between Earth and Mars is particularly well tracked, as scientists have sent probes and landers to the Red Planet for generations.

If scientists detect a disturbance, they must determine whether the planet was pushed by a black hole or simply an asteroid. By tracking the pattern of the wobble over time, they can trace the object’s trajectory and predict where it might go in the future. “The disturbance pattern gives us a really rich amount of information,” says MIT study co-author Benjamin V. Lehman. “We have to convince ourselves that it’s really a black hole by telling observers where to look.” If the object were an asteroid, we should be able to see it with a telescope. Moreover, most asteroids come from within the solar system, so they orbit in the same plane as planets. Primordial black holes, on the other hand, come from far away, so they’re likely to follow a different orbit than asteroids.

Another way to look for primordial black holes in the solar system is to analyze data from asteroids, especially the asteroid Bennu, which is tracked with great precision by the ongoing space mission OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer). “After reading (the team’s) paper, I think we can take a closer look at the OSIRIS-REx data and try to see if we can confirm this effect,” says Yu-Dai Tsai, an astrophysicist at Los Alamos National Laboratory. “I think this is a promising direction.” Tsai and his colleagues used the probe’s measurements of Bennu to study how to look for other forms of dark matter. The paper was published in the journal Nature on September 20. Communication Physics.

Primordial black holes are an increasingly attractive solution to the mystery of dark matter, the invisible form of mass that physicists believe makes up most of the matter in the universe. Because physicists can only “see” this matter through its gravitational effects on ordinary matter, its true identity remains elusive. Many leading theories about its composition have failed to work out. For decades, physicists have thought dark matter was likely to take the form of so-called weakly interacting massive particles (WIMPs). But generations of increasingly sensitive experiments to find these particles have come up empty, and particle accelerators have not picked up any signs of them either. “WIMPs have been pushed into a corner, and because they’ve been the dominant paradigm for decades, everything is on the table,” says Kevork Abazazian, an astrophysicist at the University of California, Irvine, who was not involved in the study. Physics Review D Research. “Primordial black holes are really gaining popularity.”

Physicists also recognize the possibility that dark matter may not interact with normal matter through forces other than gravity. Unlike WIMPs, which can also touch normal matter through the weak force, black holes can only be detected by gravity. “Given that we’re still trying to figure out the right way to detect dark matter interacting with normal matter, it’s especially important that we have probes searching for it based on the gravity it produces. This is the only dark matter interaction whose strength we already know, and the only one whose existence we’re certain of,” says Tim M. P. Tait, a theoretical physicist at the University of California, Irvine, who was also not involved in the MIT team’s new research. “This is a really interesting idea, and a timely one.”

Coincidentally, an independent team of researchers published a paper in the same issue on the search for signs of primordial black holes flying close to Earth. Physics Review DThe researchers’ simulations showed that such a signal could potentially be detected not only by gravimeters, which measure changes in Earth’s gravitational field, but also by orbital data from the Global Navigation Satellite System. David I. Kaiser of MIT, co-author of the Earth-Mars distance measurement study, said the two papers are complementary.

These black holes can pass relatively close by, but the chances of them passing right through the human body are extremely low. But if that were to happen to you, it wouldn’t be any fun. If a tiny black hole passed through your body, it would pull everything in and crush your cells in a fatal way. But because its volume is so tiny, at least you wouldn’t be sucked in.