Black hole 'bullets' fired at Mars could reveal more about dark matter

Primordial black hole “bullets,” traveling at speeds greater than 7,000 times the speed of sound, could be punching through our solar system — and they could be causing a tiny wobble in the motion of Mars. Ultimately, measuring this wobble could help unravel one of the most pressing mysteries in science: the true nature of dark matter.

Scientists believe that ancient black holes formed at the beginning of time. These objects are very different from so-called “astrophysical black holes” such as Sagittarius A* (Sgr A*) located at the heart of the Milky Way. For example, while Sgr A* has a mass of about 4.3 million Primordial black holes are thought to be about the size of an asteroid or a small moon, much smaller than atoms, and many times larger than the Sun.

Some scientists also theorize that conglomerates of these small black holes could explain dark matter, the most mysterious “stuff” in the universe. Others, however, argue that ancient black holes would have long since evaporated due to the emission of so-called “Hawking radiation,” meaning they are not suspected dark matter in the modern universe. Now, a team of physicists proposes a way to settle the debate: with detailed observations of Mars.

The scientists suggest that if ancient black holes do indeed form dark matter, they should race through the solar system at least once every decade. This flyby could cause a “wobble” in Mars’ orbit that can be detected with current technology, thanks to our meticulous documentation of the Red Planet’s orbit.

Related: Dark Energy Camera’s new galactic portrait delves into the center of dark matter

“Thanks to decades of precise telemetry, scientists know the distance between Earth and Mars to within about 10 centimeters,” team member David Kaiser, the Germeshausen Professor of the History of Science at the Massachusetts Institute of Technology, said in a statement. “We’re taking advantage of this very well-instrumented region of space to try to find a small effect.

“If we see that, it’s a real reason to continue pursuing the fantastic idea that all dark matter is made of black holes that formed less than a second after the Big Bang and have been moving through the universe for 14 billion years.”

Big problem, small solution

To understand why dark matter is such a big deal to physicists, consider that it outnumbers “everyday” particles by a factor of five to one. That means that every star, planet, moon, asteroid, gas cloud, rocket, satellite, and spacecraft you read about on Space.com or observe from your backyard telescope makes up less than 20 percent of the total mass of the universe.

Scientists know that dark matter can’t be made of atoms, which are made of protons, neutrons, and electrons. That’s because those particles interact with light, or more accurately, electromagnetic radiation. Dark matter doesn’t interact with light — or if it does, it interacts so weakly that it’s undetectable. That makes dark matter essentially invisible to us. Instead, scientists can only infer the existence of dark matter by how it interacts with gravity, and how that interaction affects light and regular matter — like the 642 million trillion tons of matter that makes up Mars!

Orange discs float in a dark area.

An illustration of a procession of primordial black holes. (Image: NASA’s Goddard Space Flight Center)

Until now, the search for dark matter candidates has focused on previously undiscovered particles. But as these searches become more sophisticated (but still come up empty-handed), scientists are increasingly turning to an idea first floated in the 1970s: Dark matter might not be a particle at all, but instead tiny black holes left over from the Big Bang.

These primordial black holes would not form through the collapse of massive stars as stellar-mass black holes do, or through a merging chain of increasingly massive black hole pairs as supermassive black holes do. Instead, primordial black holes, if they exist, would have formed from dense pockets of gas in the early universe, with the rapid expansion of the cosmos scattering them across space.

Thinking about the gravitational influence of such a primordial black hole began with an idle musing.

“I think someone asked me what would happen if a primordial black hole passed through a human body,” team leader Tung Tran, a doctoral student at Stanford University, said in a statement.

From this question, Tran calculated that if a black hole with the mass of an asteroid passed within about 3.2 feet (1 meter) of a person, the force it exerted would push that person 20 feet (6 meters) in one second. So why does this thankfully never happen? Well, Tran also discovered that the chances of a primordial black hole passing anywhere near a person on Earth are infinitesimally small.

Interested, Tran reasoned that to increase the likelihood of such an interaction, you would need an object much larger and wider than a person. But the more massive the body, the smaller the effect would be.

Primordial black hole meets Mars

Tran first focused on our Earth/Moon system.

“We extrapolated to see what would happen if a black hole flew by the Earth and made the moon wobble a little bit,” Tran said. “The numbers we got weren’t very clear. There are a lot of other dynamics in the solar system that could be acting as a kind of friction to dampen the wobble.”

To illustrate this picture, the researchers created a simple simulation of the solar system, taking into account the gravitational interactions between the planets and the largest moons in our cosmic backyard.

“State-of-the-art simulations of the solar system include more than a million objects, each of which has a tiny residual effect,” team member and MIT Pappalardo Fellow Benjamin Lehmann said in a statement. “But even by modeling two dozen objects in a careful simulation, we could see that there was a real effect that we could dig deeper into.”

The team then estimated how often a primordial black hole would theoretically pass through the solar system, based on the estimated amount of dark matter in the region around the sun.

“Primordial black holes don’t live in the solar system. Instead, they’re just streaming through the universe, doing their own thing,” Sarah Geller, a team member and a postdoctoral fellow at the University of California, Santa Cruz, said in the statement. “And chances are good that they pass through the inner solar system at a certain angle once every 10 years or so.”

a graph explaining how black holes evolve.

A diagram explaining how the universe would evolve with the presence of primordial black holes. (Image credit: ESA)

After calculating the speed at which ancient black holes were hurtling through the solar system and accounting for the asteroid-sized mass of these dark matter candidates, the team concluded that these tiny black holes would be hurtling through the solar system at a staggering 5.4 million miles per hour, about 7,000 times faster than the speed of sound.

By focusing on “close encounters” between these roaring black holes and bodies in the solar system, the team found that Mars actually made a better target than Earth or the Moon — at least one that paints a better picture of the interactions the team is interested in.

The team found that if a primordial black hole passed within a few hundred million miles of Mars, it would cause a deviation in the Red Planet’s orbit. To put how small that is into perspective, Mars is more than 140 million miles from Earth, a distance that is a whopping 225 trillion times greater than the proposed effect.

Nevertheless, the team believes that the instruments currently monitoring Mars could indeed detect such a small deviation.

Mars with a purple gradient behind it.

An illustration shows how dark matter causes a ‘wobble’ in the motion of Mars. (Image credit: Robert Lea (created with Canva))

But even if this anomaly were not detected until the next few decades, scientists would still need to confirm that it was indeed a primordial black hole and not a passing asteroid of the same mass.

“We need as much clarity as possible on the expected backgrounds, such as the typical velocities and distributions of boring space rocks, versus these ancient black holes,” Kaiser said. “Luckily for us, astronomers have been tracking ordinary space rocks as they fly through our solar system for decades, so we’ve been able to calculate typical properties of their trajectories and compare them to the very different kinds of paths and velocities that ancient black holes should be traveling on.”

“It’s a really neat test that they’ve proposed, and it could tell us whether the nearest black hole is closer than we think,” Matt Caplan, an associate professor of physics at Illinois State University who was not involved in the study, said in the statement. “I should emphasize that there’s a bit of luck involved. Whether a search finds a loud and clear signal depends on the exact path a wandering black hole takes through the solar system.

“Now that they have checked this idea with simulations, they need to do the hard part: check the real data.”

The team’s research was published Tuesday (Sept. 17) in the journal Physical Review D.

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