Black holes slamming into the moon could end the dark matter debate
Circa 14 billion years ago, when the universe's clock began to tick, space was still a tight, blazing hot, frenzied packet of cosmic stuff. Stars were yet to shine, planets hadn't been born, and jittery particles of every shape and size were zipping around at random.
It was chaos.
But somewhere amid the lawlessness, in between spirals of stardust, a few minuscule, unstable and hyper-dense pockets of flaming matter might have collapsed. And if they did, scientists believe they would've dotted the early universe with clusters of black holes even smaller than atoms.
Don't let these petite spheres of doom fool you. A black hole half the size of a golf ball would have a mass equivalent to Earth's. Even microscopic black holes, with masses comparable to asteroids, would've unceasingly sucked in and destroyed everything along their path.
Slowly, as the universe progressed, swarms of them would have seen planetary systems rise and fall, and billions of years ago there's a fair chance they'd have even whizzed through our corner of the cosmos. Eventually, these mini black holes would've sailed away from each other. But if they did exist, experts think they'd still be roaming in and around the galaxies right this second.
They are, scientists believe, our newest lead on dark matter -- perhaps the greatest mystery of the universe.
Dark matter quests that hope to unveil the strange, invisible particle or force that somehow binds the cosmos together often reach a wall. Solving the puzzle requires, well, actually… finding dark matter.
So to ensure this innovative hypothesis isn't a dead end, we'd need to locate unseen, miniature versions of black holes. But how? We have enough trouble finding supermassive, visible ones with high-tech equipment tailored to the search.
That's where the moon comes in.
"There's this funny estimate that you can do," says Matt Caplan, an assistant professor of physics at Illinois State University and one of the theorists behind the research published in March. Caplan contends that if dark matter can indeed be explained by these tiny black holes, then at some point, they would have punctured the moon.
Yes, you read that correctly: The moon might've been bombarded by atomic-sized black holes. Taking it a step further, the wounds they inflicted should still be up there; if these mini-abysses are proven to exist, dark matter may no longer be an everlasting enigma.
Oh, and while we're on the topic, Earth might've been hit by them, too.
But really, what is dark matter?
"A more accurate description would be transparent matter," says Almog Yalinewich, a theoretical physicist at the Canadian Institute for Theoretical Astrophysics, who co-authored the paper with Caplan. "It doesn't interact with light, it doesn't reflect light, it doesn't produce light."
Dark energy, responsible for speeding up the universe's expansion, accounts for 68% of the cosmos. Dark matter, which slows it down, holds 27%. That means less than 5% of the universe is visible, standard energy and matter we're used to on Earth.
But even though we can't see dark matter, it isn't sly enough to disguise its effects. That's how scientists discovered the hidden material exists in the first place. Sarah Shandera, associate professor of physics at Pennsylvania State University and director of its Institute for Gravitation and the Cosmos, says the way astrophysical bodies move within the universe proves dark matter lurks out there.
"You look at the motions of stars and galaxies, or clusters of galaxies, and you realize what we're able to infer about the mass that's there, that's giving off visible light or any kind of electromagnetic radiation," she said. "It's not enough to account for the motion of the objects -- it looks like there's a lot more matter there."
In other words, astronomers suspect dark matter is making the universe stretch way faster than their calculations predict it should. Shandera also emphasizes the overwhelming amount of further evidence to support the force's omnipresence.
Its invisibility is the only issue.
But remember, the sneaky stuff leaves fingerprints -- if a micro black hole really did hit the moon, as Caplan and Yalinewich propose, we'd notice the collision's effects. The smash, they say, would produce small craters a few meters wide with different shapes and properties than classic asteroid impacts.
Thus, Caplan and Yalinewich argue that if we study the moon and find craters fitting the description of an impact by a miniature black hole, we'd prove the existence of these tiny, early universe ("primordial") black holes.
One caveat to the theory, according to Shandera, is that astronomers aren't yet studying the type of super small, early universe fluctuations needed to create microscopic, primordial black holes. Instead, they study such perturbations on larger scales, because there's more research to support the greater size.
She also notes that the notion of primordial black holes, microscopic or not, has a few gaps yet to be filled.
"They sound really cool, but you're not sort of relieved from the burden of saying, 'Where did these things come from, why are they there?'" she says. "And that's a really hard and interesting question that I don't think has a very good answer." Although, as dark matter studies are all rather preliminary, she admits that Caplan and Yalinewich's theory has some footing.
"It doesn't seem to fit easily in the framework that we've got," she said. "But maybe we've got the wrong framework."
Exploring the science of microscopic black holes
The ambitious theory was conceived three years ago when Caplan asked Yalinewich a simple question: Can you tell, from just the shape of a crater, whether it was formed from a regular impact or from a compact object, like a black hole?
"At the time I didn't have an answer," Yalinewich said. "Two years after that, it suddenly dawned on me."
He realized that the variation lies in how matter splashes around as something falls into it. If we threw a penny onto a table of baby powder, particles of dust would plume upward, then land around the coin's edge. Craters work the same way -- asteroids that barrel into the moon make a hill that tapers off around the impact site, giving the resulting crater its characteristic valleylike appearance.
"Because black holes punch through, they don't deposit all of their energy right at the surface -- they deposit energy along a stripe," Caplan said. "This would be like drilling a borehole, filling it with a column of dynamite and then setting that off."
Though they'd still kind of look like the round craters we're familiar with, the hill around the crater rim changes. Called the ejecta blanket, it would be steeper and taller. The black hole would also create an exit wound, like a bullet, somewhere on the other side of the moon.
According to Yalinwiche, the team calculates the magnitude of these black holes to range from 10^17 to 10^23 grams. They say black holes a bit smaller would emit detectable X-ray waves -- or they'd vanish altogether. "This is why our paper is significant," Yalinewich said. "We prove a range that can't be proven by other methods."
But it's not just the moon that could've been hit.
"In principle, there's nothing special about the moon -- the only reason we invoke the moon is because it's well studied," Yalinewich says. "Some of the moons of Neptune and Jupiter, or Mercury, could be good candidates."
While these eerily small black holes could've also slammed into the early Earth, long before humans were around, our planet's atmosphere would've protected it from the brunt of the impact. Erosion on Earth's surface would've likely erased any data from a possible collision, according to Caplan.
That means, as our atmosphere is still intact -- unless climate change has another future in store for the planet -- Earth continues to be safe from puny black holes. Either way, the researchers suggest that these baby astrophysical bodies would be so spread out by now that the likelihood one of them impacts us isn't of concern.
A trip to the moon
Caplan and Yalinewich urge a backup measure to bolster their unique theory: revisiting the moon.
They say because these black holes have a hyper-intense gravitational impact -- incomprehensible by the human mind -- they'd have hit the moon forcefully enough to alter the properties of matter around it. Nuclear bombs behave similarly. The first one notoriously turned all the sand near its detonation site into glass.
"You could look for dust of different quartz phases and silicates that you wouldn't be able to produce [otherwise]," Caplan said. "Rock smashing into rock doesn't get that hot."
But finding these altered materials would require getting astronauts to return samples from the lunar surface or sending a probe to the moon that can sample rocks, similar to the way Mars rovers work. NASA hasn't sent anyone to the moon since the '70s but has been attempting to go again this decade with the Artemis missions.
Before any of that can be set into motion, however, Caplan says the first step is to use supercomputers and analyze crater structures on the moon from here on Earth. Even so, Yalinewich stresses that he aims to invite a refreshing angle to the scientific community.
"When people think about dark matter, they're usually fixated about … trying to extend existing methods, for the most part," he says. "It's very rare that people try to think outside the box."
Existing dark matter theories and beyond
Welcome to the dark matter party.
Caplan and Yalinewich's theory is just one among the unending slew of ideas physicists have come up with to explain the phenomenon. One can only imagine what novel ideas are yet to amend and footnote the epic chronicle.
For instance, astrophysicist Sergio Martin-Alvarez, from the Kavli Institute for Astrophysics at the University of Cambridge, highlights a few candidate particles often employed to explain the shifty phenomenon.
Besides small astrophysical objects, including the microscopic black holes proposed by Caplan's research, he says there's been talk of something called WIMPS, or weakly interacting massive particles thought to be up to 1,000 times heavier than protons.
Noting that the hype around WIMPS has died down over the years because they still haven't been detected, he also presents two other contenders: axions and neutrinos. Unfortunately, neutrinos, according to Martin-Alvarez, might be too energetic to explain dark matter, but axions are presently at the search's forefront.
"Although people advocate separately for each of these different candidates, every single one appears to be a bit of a stretch individually," he says. "I am personally a fan of either all of them in different degrees ... or alternatively that we are missing some big piece of the picture."
Shandera has also been working on novel candidates for dark matter. She believes studying unique properties of the black holes we can detect might reveal some were actually formed by the secretive stuff.
Her theory relies on dark matter having its own chemical model, similar to the widely accepted Standard Model for normal matter. If that's true, dark matter would be able to combine to form atoms, cool down and take part in other reactions; it could theoretically collapse into a black hole, too.
"If dark matter can cool,'' she says, "if it has something like hydrogen in it, then dark matter can also, in some regions, form compact objects."
Black holes that arise from dark matter would be smaller than the lower limit scientists place on their size. It's called the Chandrashekar limit. But because that limit typically relies on the mass of a proton, Shandera says, "if you have a dark matter model that looks a lot like the Standard Model -- but the proton is heavier -- the black holes can be smaller; it's inversely related."
Though finding a submass black hole would be the holy grail for her work, she says finding implications of a potential dark matter black hole would suffice, too. That could be the consequences of dark cooling processes.
"What's interesting about this," she says, "is if you continue not to find something, it's a complementary way to constrain the nature of dark matter."
Essentially, it's a process of elimination.
But even if we can never find it, and it's not atomic-sized black holes slamming into the moon, dark matter's purpose will live on until the end of the universe. Until then, the cosmos will continue to tick along its linear timeline.
And one day, long after humanity perishes, all of its stars will die; every planet will cease to exist; and black holes will no longer terrorize the vacuum of space. And those ticks will become just as invisible as the elusive material itself.
CNET's Robert Rodriguez created the illustration at the top of this story.