Study finds that Mars rumblings could be a sign of dark matter

In a new study, MIT physicists propose that if most of the dark matter in the universe is composed of microscopic primordial black holes — an idea first proposed in the 1970s — then these gravitational dwarfs should pass by our solar system in as little as a decade. Such a flyby, the researchers predict, would cause a wobble in Mars' orbit to an extent that today's technology can actually detect.

Such a discovery could support the idea that primordial black holes are the primary source of dark matter throughout the universe.

“Based on decades of precise telemetry, scientists know the distance between Earth and Mars to an accuracy of about 10 centimeters,” says study author David Kaiser, a professor of physics and the Germeshausen Professor of the History of Science at MIT.

“We're trying to look at a tiny effect by taking advantage of this highly instrumented region of space. If we can see it, it will become a real reason to pursue the pleasant idea that all dark matter is made up of black holes that arose less than a second after the Big Bang and have been streaming through the universe for 14 billion years.”

Kaiser and his colleagues report their findings in the journal Physical Review DCo-authors of the study are lead author Tung Tran, now a graduate student at Stanford University; Sarah Geller Ph.D., now a postdoctoral student at the University of California at Santa Cruz; and MIT Pappalardo Fellow Benjamin Lehman.

Beyond the particles

Less than 20% of all physical matter is made up of visible matter, such as stars and planets, down to the kitchen sink. The rest is made up of dark matter, a hypothetical form of matter that is invisible across the electromagnetic spectrum, yet is believed to pervade the universe and exert enough gravitational force to affect the motion of stars and galaxies.

Physicists have placed detectors on Earth to find dark matter and explore its properties. For the most part, these experiments assume that dark matter exists as an exotic particle that might disintegrate and decay into observable particles when it passes through a given experiment. But so far, such particle-based searches have come up empty-handed.

In recent years, another possibility, first proposed in the 1970s, has gained traction again: Instead of taking the form of particles, dark matter might exist as microscopic, primordial black holes that formed in the earliest moments after the Big Bang.

Unlike astrophysical black holes, which form from the collapse of old stars, primordial black holes may have formed from the collapse of dense pockets of gas in the very early universe and spread throughout the universe as it expanded and cooled.

These primordial black holes would have packed a very large amount of mass into a tiny space. Most of these primordial black holes could be as small as an atom and as heavy as the largest asteroids. It is then conceivable that such small giants could exert gravitational forces that could explain at least a portion of dark matter. For the MIT team, this possibility initially raised a trivial question.

“I think someone asked me what would happen if a primordial black hole passed through a human body,” Tung recalls. He did a quick calculation with pencil and paper and found that if such a black hole passed within 1 meter of a person, the force of the black hole would push that person about 6 meters, or about 20 feet, away in a second. Tung also found that the likelihood of a primordial black hole passing through a person on Earth was astronomically unlikely.

Their interest piqued, and the researchers took Tung's calculations a step further, to estimate the impact the passing black hole might have on larger bodies, such as the Earth and Moon.

“We did projections to see what would happen if a black hole passed by Earth and shook the moon a little bit,” says Tung. “The numbers we got weren't very clear. There are a lot of other dynamics in the solar system that could act as some kind of friction that could dampen the vibrations.”

Close encounters

To get a clearer picture, the team created a relatively simple simulation of the solar system, including the orbits and gravitational interactions between all the planets and some of the largest moons.

“State-of-the-art simulations of the solar system involve over a million objects, each of which has a small residual effect,” Lehman said. “But even with a couple dozen objects carefully modeled in the simulation, we could see that there was a real effect that we could detect.”

The team found the rate at which a primordial black hole should pass through the solar system, based on the amount of dark matter estimated to exist in a given region of space and the mass of the passing black hole, which, in this case, they estimated to be comparable to the largest asteroids in the solar system, consistent with other astrophysical constraints.

“Ancient black holes don't reside in the solar system. Rather, they drift out into the universe, doing their thing,” says co-author Sarah Geller. “And they likely pass through the inner solar system at some angle about once every 10 years.”

Given this rate, the researchers simulated various asteroid-mass black holes flying into the solar system from different angles, and at speeds of about 150 miles per second. (Directions and speeds are taken from other studies of the distribution of dark matter in our galaxy.)

They focused on flybys that appeared to be “close encounters,” or events that appeared to have some kind of effect on nearby objects. They quickly found that any effects on Earth or the moon were too uncertain to be linked to a particular black hole. But Mars seemed to offer a clearer picture.

The researchers found that if a primordial black hole passed within a few hundred million miles of Mars, the encounter would cause a “wobble,” or slight deviation, in Mars' orbit. Within a few years of such an encounter, Mars' orbit should shift by about one meter — an incredibly small change, considering that the planet is more than 140 million miles from Earth. And yet, this wobble could be detected by a variety of high-precision instruments monitoring Mars today.

If this type of vibration is detected in the next few decades, the researchers believe there will still be a lot of work to confirm that the push came from a passing black hole, rather than an ordinary asteroid.

“We need greater clarity about the expected background, such as the typical speeds and distributions of the rocks piercing space, and about these primordial black holes,” Kaiser said.

“Luckily for us, astronomers have been tracking ordinary space rocks for decades as they pass through our solar system, so we can calculate the specific properties of their trajectories and compare them to the different types of paths and speeds that primordial black holes should follow.”

To aid in this, the researchers are exploring the possibility of a new collaboration with a group that has extensive expertise in simulating many other objects in the Solar System.

“We are now working on simulating a much larger number of objects, from planets to moons and rocks, and how they are all moving over longer time scales,” says Geller. “We want to incorporate close encounter scenarios, and look at their effects with greater accuracy.”

“This is a really cool test of what they've proposed, and it could tell us whether the nearest black hole is closer than we think it is,” says Matt Kaplan, an associate professor of physics at Illinois State University who was not involved in the study.

“I would like to stress that there is a bit of luck involved. Whether the search will yield a strong and clear signal depends on the exact path the wandering black hole takes through the solar system. Now that they have tested this idea with simulations, they have to do the hard part – testing it with real data.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation, and teaching.