A mind-blowing hypothesis is gaining ground among scientists: the universe could be filled with microscopic black holes the size of an atom, but with the mass of an asteroid the size of a city.
Created just a fraction of a second after the Big Bang, these hypothetical black holes would zip silently through the solar system about once every few years, traveling a hundred times faster than a speeding bullet.
Some have even argued that a massive explosion that ripped through a Siberian forest in 1908 could have been the result of one of these micro black holes hitting Earth.
Now, researchers say they've discovered a way to test whether these cosmic bullets really exist.
In a study published Tuesday in the journal Physical Review D, MIT physicists say the presence of a small black hole hurtling through the solar system could be identified by the gentle gravitational tug it exerts on Earth and other planets, altering their orbital paths by no more than a few feet.
The possibility of proving the existence of micro black holes is generating excitement among some astrophysicists because it could help explain a mystery that has puzzled them for nearly a century: the nature and composition of dark matter.
In the 1930s, astronomers began to notice anomalies in the motion of galaxies. Something lurked in the dark, empty expanse of intergalactic space, generating enormous amounts of gravity that tugged at galaxies but seemingly refused to interact with light or any other force.
Scientists found this mysterious gravitational pull everywhere. To explain it, they hypothesized that the cause was an invisible mass, or dark matter, which made up about 85% of all the matter in the universe.
Some physicists have suggested that dark matter may be made of exotic, yet-undiscovered particles. Others, such as researchers at MIT, think dark matter is probably just regular matter — extremely difficult to detect. And black holes, researchers say, are a prime example of dark matter's properties.
“It's fantastic that the conceptually most conservative answer is to say, 'They're just super-small black holes that formed a fraction of a second after the Big Bang,'” said David Kaiser, a physics professor at MIT and an author on the paper.
“It's not about inventing new forms of matter that have not yet been detected. It's not about changing the laws of gravity,” he said.
Still, black holes are not the only potential culprits, and there is still much debate in this field.
In their search for dark matter, physicists have looked for new exotic particles as well as common matter that might have been overlooked, such as black holes of various sizes. So far, they have found nothing.
Until now, astronomers haven't been sure how to look for black holes of a particularly pesky size: those that are too small for their gravity to bend starlight.
Using modeling, MIT researchers determined that these tiny black holes could have formed from pockets of dense matter that collapsed in on themselves immediately after the Big Bang.
The researchers simulated what might happen if one of these primordial black holes were to pass through Jupiter's orbit. They found that the orbits of Earth, Mars, Venus and Mercury could deviate from their original course by up to 90 centimeters within a decade.
The researchers said they would expect to detect a push from a black hole between once a year and once a century, depending on the abundance and masses of black holes.
To reassure themselves, the researchers also calculated the probability of one of these tiny black holes hitting Earth and found that it would only happen about once every billion years.
Even then, the black hole would not lead to an apocalypse.
Instead, it would pass straight through Earth, leaving the planet relatively untouched.
In the 1970s, scientists even showed that a black hole impact would closely resemble a flash of light and explosion that occurred over Russia 116 years ago and which scientists believe was caused by a small asteroid or comet (although a black hole would also leave an “exit wound”).
Detecting the existence of mini black holes will require extremely precise measurements of the planets' locations and models of where they are supposed to be. Fortunately, scientists have the tools to do just that.
NASA's Jet Propulsion Laboratory in La Cañada Flintridge, for example, He has created a detailed model of the solar system. which uses Albert Einstein's theory of general relativity of gravity to calculate the expected orbits of planets and account for hundreds of asteroids in excruciating detail (they even calculated how Earth's ocean tides affect the Moon's orbit).
NASA scientists have also developed an extremely precise method for determining the distance between Earth and Mars. By measuring the time it takes for radio signals to travel from Earth to spacecraft orbiting Mars or to rovers on its surface, scientists can calculate the distance between the Red Planet and Earth to within 60 centimeters (2 feet).
“It’s really only been a few decades since we’ve had that level of precision,” Kaiser said. “From a number of missions in the space program, we can worry about whether Mars is 50 centimeters below where we expect it to be.”
To convince skeptics, scientists would also have to prove that the jolt was not caused by a passing asteroid.
Researchers say the speed of the black holes — which would travel more than twice as fast as anything else in our solar system — would create an unmistakably unique wobble in the planets' orbits.
And astronomers are pretty good at spotting objects with a mass similar to that of hypothetical black holes. In 2017, researchers identified the first object from another star to enter our solar system, which had a mass much lower than that of a microscopic black hole.
Scientists say that whether or not they detect a passing black hole, it will advance humanity's understanding of dark matter.
“Of course I would love to discover dark matter in the solar system,” said Benjamin Lehmann, a postdoctoral fellow at MIT and an author on the paper. However, “if this kind of observation is what helps us close this window and say that dark matter is not in the shape of these primordial black holes, that’s really important information.”
By proposing a method to easily test for this possibility, “they have done … exactly what we should be doing in dark matter searches,” said Vera Gluscevic, a USC cosmology professor who was not involved in the study. “We should leave no stone unturned.”
While scientists plan to continue refining models of planetary motion and dig into historical observations from recent decades for signs of black holes, the main test will be to simply watch and wait.
