Mars wobble could be sign of dark matter, study finds

In a new study, MIT physicists suggest that if most of the dark matter in the universe is made up of microscopic primordial black holes (an idea first proposed in the 1970s), then these gravitational dwarfs should pass through our solar system at least once a decade. A flyby like this would introduce a wobble into Mars’ orbit, the researchers say, to a degree that current technology could detect.

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

“Thanks to decades of precision telemetry, scientists know the distance between Earth and Mars to within about 10 centimeters,” said study author David Kaiser, a professor of physics and the Germeshausen Professor of the History of Science at MIT.

“We’re taking advantage of this highly instrumented region of space to try to look for a small effect. If we see it, it will be a real reason to continue to pursue this delightful idea that all dark matter is black holes that were spawned in less than a second after the Big Bang and have been circulating in the universe for 14 billion years.”

Kaiser and colleagues report their findings in the journal Physical examination DThe study’s co-authors are lead author Tung Tran, now a graduate student at Stanford University; Sarah Geller, Ph.D., now a postdoctoral fellow at the University of California, Santa Cruz; and Benjamin Lehmann, an MIT Pappalardo Fellow.

Beyond particles

Less than 20 percent of physical matter is visible material, from stars and planets to the kitchen sink. The rest is dark matter, a hypothetical form of matter that is invisible across the electromagnetic spectrum but is thought to permeate the universe and exert a gravitational force strong enough to affect the motion of stars and galaxies.

Physicists have set up detectors on Earth to try to spot dark matter and determine its properties. Most of these experiments assume that dark matter exists as an exotic particle that could disperse and decay into observable particles as it passes through a given experiment. But so far, these particle-based searches have yielded no results.

In recent years, another possibility, first introduced in the 1970s, has gained ground: rather than taking the form of particles, dark matter could exist in the form of microscopic, primordial black holes that formed in the first moments after the Big Bang.

Unlike astrophysical black holes that form from the collapse of old stars, primordial black holes are thought to have formed from the collapse of dense pockets of gas in the early universe and scattered across the cosmos as the universe expanded and cooled.

These primordial black holes would have collapsed a huge amount of mass into a tiny space. Most of these primordial black holes could be as small as a single atom and as heavy as the largest asteroids. It would therefore be conceivable that such tiny giants could exert a gravitational force capable of explaining at least some of the dark matter. For the MIT team, this possibility raised an initially frivolous question.

“I think someone asked me what would happen if a primordial black hole passed through a human body,” recalls Tung, who did a quick calculation with pencil and paper to discover that if such a black hole came within three feet of a person, the force of the black hole would push the person back 20 feet, or about 20 feet, in a single second. Tung also discovered that the chances of a primordial black hole passing close to a person on Earth were astronomically small.

Their interest piqued, the researchers took Tung’s calculations a step further, to estimate how a black hole flyby might affect much larger bodies like Earth and the Moon.

“We extrapolated to see what would happen if a black hole passed near Earth and caused the moon to wobble a little bit,” Tung says. “The numbers we got weren’t very clear. There are a lot of other dynamics in the solar system that could act as a kind of friction and dampen the wobble.”

Close Encounters

To get a clearer picture, the team generated a relatively simple simulation of the solar system that incorporates the orbits and gravitational interactions between all the planets and some of the larger moons.

“The most recent simulations of the solar system include over a million objects, each of which has a minimal residual effect,” Lehmann notes. “But even by modeling two dozen objects in a careful simulation, we could see that there was a real effect that we could analyze.”

The team calculated the speed at which a primordial black hole would be expected to pass through the solar system, based on the amount of dark matter estimated to be in a given region of space and the mass of a passing black hole, which in this case they assumed to be as massive as the largest asteroids in the solar system, consistent with other astrophysical constraints.

“Primordial black holes don’t live in the solar system. They just move around the universe as they please,” said Sarah Geller, a co-author of the study. “And they probably pass through the inner solar system at some angle once every decade or so.”

Given this speed, the researchers simulated several asteroid-mass black holes passing through the solar system, from different angles and at speeds of about 240 km per second. (The directions and speeds come from other studies of the distribution of dark matter in our galaxy.)

They focused on flybys that appeared to be “close encounters”—cases that caused an effect on nearby objects. They quickly discovered that any effect on Earth or the moon was too uncertain to be attributed to a particular black hole. But Mars seemed to offer a clearer picture.

The researchers found that if a primordial black hole were to pass within a few hundred million kilometers of Mars, the encounter would cause a “wobble,” or a slight deviation in Mars’ orbit. In the few years following such an encounter, Mars’ orbit would shift by about a meter—an incredibly small wobble, given that the planet is more than 225 million kilometers from Earth. And yet this wobble could be detected by the various high-precision instruments that monitor Mars today.

If such a wobble were detected in the coming decades, researchers acknowledge that much work would still need to be done to confirm that the thrust came from a passing black hole rather than an ordinary asteroid.

“We need as much clarity as possible about expected backgrounds, such as typical velocities and distributions of boring space rocks, relative to these primordial black holes,” Kaiser notes.

“Luckily for us, astronomers have been tracking ordinary space rocks as they pass through our solar system for decades, so we were able to calculate the typical properties of their trajectories and begin to compare them to the very different types of trajectories and speeds that primordial black holes would be expected to follow.”

To achieve this, the researchers are exploring the possibility of a new collaboration with a group with extensive expertise in simulating many other objects in the solar system.

“We’re currently working on simulating a large number of objects, from planets to moons to rocks, and how they move over long time scales,” Geller says. “We want to inject close encounter scenarios and observe their effects with greater precision.”

“It’s a very interesting test that they came up with, and it could tell us whether the nearest black hole is closer than we think,” says Matt Caplan, an associate professor of physics at Illinois State University, who was not involved in the study.

“I want to emphasize that there is also an element of luck. Whether or not a strong, clear signal is detected depends on the exact path taken by a black hole wandering through the solar system. Now that they have verified this idea using simulations, they must tackle the hard part: verifying the real data.”

This article is republished with kind permission from MIT News (web.mit.edu/newsoffice/), a popular site covering the latest research, innovation, and teaching at MIT.