“Projectiles” from primordial black holes, traveling at speeds greater than 7,000 times the speed of sound, could puncture our solar system and could cause a small wobble in the motion of Mars. Ultimately, measuring this wobble could help solve one of science’s most pressing mysteries: the true nature of dark matter.
Primordial Black Holes
Scientists believe that primordial black holes were created at the beginning of time. These objects are very different from so-called “astrophysical black holes” like Sagittarius A* (Sgr A*) at the heart of the Milky Way. For example, while Sgr A* has a mass of about 4.3 million times that of the sun, primordial black holes are thought to be about as massive as an asteroid or a small moon and much smaller than atoms.
Some scientists also theorize that clusters of these tiny black holes could explain dark matter, the most mysterious “stuff” in the universe. Others, however, argue that primordial black holes evaporated long ago via the emission of so-called “Hawking radiation,” meaning they are no suspect dark matter in the modern universe. Now, a team of physicists proposes a way to settle the debate: with detailed observations of Mars.
Scientists suggest that if primordial black holes are indeed dark matter, they should pass through the solar system at least once every decade. This flyby could cause a “wobble” in Mars’s orbit that is detectable using current technology, thanks to our precise documentation of the Red Planet’s orbit.
“Thanks to decades of precision telemetry, scientists know the distance between Earth and Mars to within about 10 centimeters. [3,9 pollici]“,” said David Kaiser, a team member and professor of the history of science at the Massachusetts Institute of Technology in Germeshausen, in a statement. “We’re using this highly instrumented region of space to try to spot a small effect.
“If we saw that, it would be a real reason to continue pursuing this delicious idea that all dark matter is made up of black holes that were created less than a second after the Big Bang and have been circling the universe for 14 billion years.”
To understand why dark matter is such a big problem for physicists, consider that it outweighs “everyday” particles by a factor of five to one. That means that every star, planet, moon, asteroid, gas cloud, rocket, satellite, and spaceship you observe through your backyard telescope represents less than 20 percent of the total mass of the universe.
Scientists know that dark matter cannot be made of atoms, which are made of protons, neutrons, and electrons. That’s because those particles interact with light, or more precisely, electromagnetic radiation. Dark matter does not interact with light, or if it does, it interacts so weakly that it is undetectable. This makes dark matter effectively invisible to us.
Rather, scientists can only infer the existence of dark matter through its interaction with gravity and how this interaction affects light and ordinary matter, such as the 642 million trillion metric tons of matter that make up Mars!
Until now, the search for dark matter candidates has focused on previously unknown particles. But as these searches become more sophisticated (but still come up empty-handed), scientists are increasingly turning to an idea first posed in the 1970s: Dark matter may not be particles at all, but rather tiny black holes left over from the Big Bang.
These primordial black holes would not form from the collapse of massive stars as stellar-mass black holes do, nor would they form a chain of merging pairs of increasingly massive black holes 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 spreading them out into space.
The reflection on the gravitational impact of such a primordial black hole began with an idle reflection.
“I think someone asked me what would happen if a primordial black hole went through a human body,” team leader Tung Tran, a graduate 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 he claims would push that person 20 feet (6 meters) in just one second. So why, fortunately, does this never happen?
Moreover, Tran also found that the chances of a primordial black hole passing somewhere near a person on Earth are infinitesimally small.
Intrigued, Tran reasoned that to increase the likelihood of such an interaction occurring, an object much larger and wider than a person would be needed. But the more massive the body, the smaller the effect.
By calculating the speed of the primordial black holes’ passages and taking into account the asteroid-like mass of these dark matter candidates, the team then deduced that these tiny black holes would have been speeding through the solar system at a staggering 8.5 million kilometers per hour, or about 7,000 times faster than the speed of sound.
By focusing on “close encounters” between these racing black holes and solar system bodies, the team found that Mars actually makes 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 black hole primordial planet 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 over 140 million miles from Earth, a distance that is a full 225 trillion times greater than the proposed effect.
Even so, the team believes that the instruments currently monitoring Mars would indeed be able to spot such a small deviation.
Even if this deviation were to be detected in the next few decades, scientists would still have to confirm that it was indeed caused by a primordial black hole and not a passing asteroid with the same mass.
“We need as much clarity as possible about the predicted backgrounds, such as the typical velocities and distributions of boring space rocks, relative to these primordial black holes,” Kaiser said. “Fortunately for us, astronomers have been tracking ordinary space rocks as they pass through our solar system for decades, so we can calculate the typical properties of their trajectories and begin to compare them to the very different types of paths and velocities that primordial black holes should follow.”
“This is a very interesting 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 would like to point out that there is also a bit of luck involved. Whether or not a search finds a strong, clear signal depends on the exact path a wandering black hole takes through the solar system.
“Now that they’ve verified this idea with simulations, they need to do the hard part: verifying real data.” The team’s research was published Tuesday (September 17) in the journal Physical Review D.
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