How we might finally find black holes from the cosmic dawn

An undiscovered population of ancient black holes may be lurking throughout the universe. These bottomless cosmic pits would have a lot in common with more familiar black holes; in some cases, the two may be indistinguishable. But unlike their kin, these undiscovered black holes wouldn’t have formed from a massive star collapsing in on itself, nor would they be peers of the supermassive black holes that feed at the centers of galaxies.

Instead, these black holes would have been born in the earliest epochs after the Big Bang — before stars and galaxies even appeared.

Called primordial black holes, these hypothetical objects have attracted interest since the 1960s. Stephen Hawking wrote one of the earliest papers about their potential existence. Just a few years later, his investigation of primordial black holes led him to perhaps his most famous idea, that black holes leak energy — now called Hawking radiation — in a way that slowly robs them of their mass.

Now, after decades of pondering primordial black holes, scientists sound genuinely optimistic about the possibility of detecting them. There’s been a surge of interest in the field. Researchers new to primordial black holes are teaming up with longtime investigators to pin down the data that could prove these black holes exist. If they do linger across the universe, they’d be emitting Hawking radiation, bending starlight, colliding with other cosmic objects and each other, perhaps even gobbling up stars from the inside out.

In other words, they’d be shaping the cosmos in observable ways.

In 2023, a team that includes cosmologist Bernard Carr, who coauthored a pivotal early paper with Hawking on the subject, outlined more than 20 lines of evidence that might support the existence of primordial black holes. In a recent historical review, Carr predicted we’ll have an answer within the next decade.

“I would bet you, say, 70 percent — maybe 60 or 70 percent — that they exist,” says Carr, a professor emeritus at Queen Mary University of London. “And that’s partly wishful thinking because I prefer them to exist, but it’s something that’s trying to be objective.”

If primordial black holes are out there, they could help solve one of the biggest mysteries in cosmology: What is dark matter? This elusive substance is six times as abundant as all the ordinary stuff we’re familiar with, from people to planets to pickleballs. Its gravitational influence is credited with holding galaxies together and scaffolding all the cosmic substance we can see. But despite decades of searching, no one yet knows what it is.

Primordial black holes could account for some of the dark matter out there. Some researchers believe that these black holes may account for all of it. But their existence isn’t a given. Their formation requires new physics, some critics point out. Among the researchers now studying these black holes are true believers, those hoping to disprove the idea and everyone in between.

“There’s certainly more people who are excited now,” says Anne Green, an astroparticle physicist at the University of Nottingham in England who coauthored the historical review with Carr but considers herself agnostic on the question of existence. “And it probably is that there is more cause for excitement.”

Black holes everywhere

The recent surge in interest traces its origins to 2016. That year, scientists reported that gravitational waves, ripples in spacetime predicted by Einstein’s general theory of relativity, had been detected from a pair of merging black holes. The discovery, recognized with the Nobel Prize in physics the next year, opened a new window on black holes.

“Once we knew we could directly observe black hole mergers with gravitational waves, this became a probe,” says cosmologist Will Kinney of the University at Buffalo in New York. “Every time you create a new tool, a new way to observe the universe, then you start asking questions differently.” He says the current interest in primordial black holes is a good example of that.

Until the gravitational wave data started pouring in, there were two types of black holes known to exist in abundance. The first, “stellar” black holes, forms when a very massive star runs out of fuel and its core collapses in on itself. These black holes generally have masses between five and 10 times the mass of the sun, and sometimes as much as 20 times or more.

The second abundant type, supermassive black holes, sits at the centers of galaxies and can weigh in at billions of times the mass of the sun. Perhaps these ones formed early in galactic history from the direct collapse of gas, or through successive mergers of stellar black holes. In either case, they grew as they fed on anything in their grip.

But when the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, reported those first colliding black holes in 2016, the objects were more massive than many people expected, with each one in the pair weighing as much as 30 suns.

Simeon Bird, a cosmologist at Johns Hopkins University at the time, recalls puzzling over the masses with his adviser shortly before the results became public. Why would LIGO’s first detections be of what are thought to be relatively rare black holes rather than something more common?

“Maybe it’s a primordial black hole,” Bird recalls saying with a laugh. “What a silly idea.” But that supposedly silly idea quickly turned into a paper making the case that LIGO may have detected dark matter in the form of primordial black holes. Reports from other teams pointed to the same possibility. In the years since, LIGO in the United States has been joined by Virgo in Italy and KAGRA in Japan. So far, the collaboration has detected more than 80 black hole mergers.

In addition to the surprising black hole masses that Bird and others puzzled over, some scientists are intrigued by the slow spins of the black holes, the number of mergers between black holes of dramatically different masses and how often black holes seem to be merging across cosmic time.

“There are many properties that are bizarre,” says Sébastien Clesse, a cosmologist at the Université Libre de Bruxelles in Belgium. Primordial black holes could help explain the unexpected findings.

Making a primordial black hole

In the tiniest of the tiniest fractions of a second after the Big Bang, when the universe was nothing but a hot, compact ball of energy, scientists believe it expanded exponentially, growing by a factor of at least 10²⁵ in less than a trillionth of a trillionth of a trillionth of a second — a period known as inflation. During this time, quantum fluctuations would have generated extreme changes in energy density. Some pockets may have become so dense that they could have collapsed in on themselves, popping off primordial black holes.

This is just one picture researchers have come up with to explain how primordial black holes could have formed just after the Big Bang, some 13.8 billion years ago. There are other proposed mechanisms across the earliest moments of the universe, including cosmic string loops or colliding bubbles.

“The only ingredient you really need is a large energy density,” says theoretical physicist Florian Kühnel of the Max Planck Institute for Physics in Munich.

No matter how and when these primordial black holes formed, though, they would have appeared in a range of masses far more diverse than what we see today. There’d be black holes about the mass of a wildebeest (a couple hundred kilograms), as well as ones with the mass of Mount Everest (tens to hundreds of trillions of kilo­grams). Black holes with roughly the masses of asteroids would still be microscopic. And there’d be black holes with the masses of planets and stars, perhaps all the way past a million solar masses.

Some of those primordial black holes might account for the unexpected gravitational wave findings, Carr, Clesse, Kühnel and Juan García-Bellido of the Universidad Autónoma de Madrid argue in a 2021 paper in Physics of the Dark Universe.

The largest of the primordial black holes might resolve another open question: how supermassive black holes, especially those detected early in the universe, could have grown so big so quickly. Clesse and García-Bellido suggested as early as 2015 that if primordial black holes exist, they could have served as seeds for today’s supermassive ones.

The 2021 paper sets out a plausible picture, says Bird, now at the University of California, Riverside. But more ordinary astrophysics may still explain the puzzling black hole observations.

Part of the challenge is that scientists don’t know enough about black holes in general. There’s not yet a clear picture of how they are distributed, how commonly they merge or how their surroundings influence feeding, growth or evaporation due to Hawking radiation. Studying the physics of black holes surrounded by gas and dust, as they are in the universe, is tricky. Many models simply don’t account for that.

“We have gorgeous, gorgeous theorems that have rightly earned our colleagues Nobel Prizes,” says David Kaiser, a physicist and historian of science at MIT, “and these results are almost entirely studying black holes and nothing else, speaking loosely.”

Detecting primordial black holes

Despite all that’s unknown, there are two observations that most scientists agree would definitively point to a primordial black hole, and much of the recent excitement is about how to spot such signs.

The first would be a black hole detected from before the first stars formed, perhaps within the first hundred million years after the Big Bang. Since it couldn’t have formed from stars, it must be primordial, the thinking goes. Existing gravitational wave detectors can’t look back that far, but future ones might. The space-based LISA gravitational wave observatory, planned for launch in the 2030s, and the Einstein Telescope and the Cosmic Explorer, both in the planning phases, could reach this ultra-ancient epoch.

The second possible certain sign, which could perhaps be found with existing observatories, would be a black hole about the mass of the sun or less. That would be hard to understand through typical formation mechanisms, leaving primordial black holes as the most plausible explanation.

García-Bellido is leading a group looking for these black holes in the current gravitational wave data, and the team is already studying some candidates.

“If LIGO found a one-solar-mass black hole, then everyone would be convinced primordial black holes are real,” agrees stellar astrophysicist Earl Bellinger of Yale University. He says he can think of no other reasonable process that would yield that mass.

“And if it is less than one solar mass, even better.”

The Einstein Telescope and the Cosmic Explorer would boost the search for black holes roughly the mass of the sun or less. And some teams are eyeing radically different types of detectors that would look for gravitational waves from black holes the mass of a planet, an asteroid or less.

But gravitational waves might not be the only way to detect such a black hole. Some researchers have other ideas.

Bellinger, for example, recently asked the question: What would happen if a small primordial black hole lurked inside a star? A lot of eminent physicists, including Hawking, have explored this question before. But there’s not a solid understanding of how fast a black hole with a mass similar to the moon or an asteroid would feed and grow within a star, and thus whether the star’s light would escape the black hole’s pull.

“The black hole has this all-you-can-eat buffet, which is the stellar plasma, and you might think the star just falls into it, which might happen,” Bellinger says. “But if it falls in almost at an angle, you expect everything around that to get heated up. If it gets heated up, it exerts some pressure and some luminosity flows out.”

Bellinger, Kühnel and colleagues decided to investigate several scenarios for stars with black holes inside, dubbed Hawking stars. The team reported the results in December 2023 in the Astrophysical Journal.

“The most fun scenario is if the energy does get out,” Bellinger says. In that case, you’d see a type of red giant known as a red straggler. Such stars (for which there are other, perhaps more plausible explanations) have been found in abundance in dwarf galaxies near the Milky Way that are thought to be dominated by dark matter. Bellinger and colleagues note that studies of how the intensity of light from these stars oscillates could distinguish a Hawking star from a red straggler that formed in another way, thus offering evidence for primordial black holes.

What about closer to home? In September 2024, two separate teams of researchers suggested how a primordial black hole passing through our solar system might be detected. Clesse and others argued that a black hole the mass of an asteroid would be hefty enough to tweak the orbits of satellites, including those used for GPS navigation. The other team, which included Kaiser and colleagues from MIT, described how such a primordial black hole might disrupt the orbit of Mars.

Kaiser and theoretical physicist Elba Alonso-Monsalve, also of MIT, have even suggested that there might be a way to detect a long-gone population of ultra-tiny primordial black holes.

In a recent study, the team investigated the formation of primordial black holes slightly after inflation but still only around 10⁻²⁰ seconds after the Big Bang. At that time, subatomic particles known as quarks and gluons floated freely, not yet bound up in protons and neutrons.

As black holes formed across a range of masses, they would have swallowed up these quarks and gluons, along with a quantum property called color charge that the particles possess. For big enough black holes, the amalgamation of color charges would cancel out, leaving no net color charge. But that wouldn’t be true for the measliest black holes.

Any primordial black holes small enough to have color charge would have evaporated via Hawking radiation by now, but they could have left a calling card that scientists can look for, Alonso-Monsalve and Kaiser reported in June in Physical Review Letters. As just one example, the evaporation of color-charge black holes could have affected the ratios of the light elements hydrogen, helium and lithium that formed from the plasma of the early universe.

If clear signs of color-charge black holes are discovered, they’d point to the existence of larger primordial black holes without color charge still around today. And it’s today’s primordial black holes that have the potential to resolve the dark matter question.

Can primordial black holes explain dark matter?

As an explanation for dark matter, primordial black holes have long been in the shadow of another popular candidate: hypothetical subatomic particles. Half a century ago, there were compelling reasons for particle physicists to believe a lot of new and exotic particles would soon be discovered, Kaiser says. With high hopes, scientists went out en masse to find them.

When the Large Hadron Collider, the world’s most powerful particle accelerator, turned on in 2008 near Geneva, it was expected to find proof of such particles, notably WIMPs, short for weakly interacting massive particles. Others sought particle dark matter high and low with massive detectors in underground laboratories and even a compact detector on the International Space Station. But so far, there’s no evidence.

Green, of the University of Nottingham, compares the search for WIMPs to looking for a needle in a haystack — now we’re most of the way through the haystack with no needle found. It doesn’t mean the particles aren’t there, but confidence that they’ll be found is starting to dwindle.

“There’s now probably thousands of people working on the WIMP detection experiments, and they absolutely shouldn’t stop,” Green notes. “But I’ve not necessarily got the champagne on ice.”

In one way of viewing it, primordial black holes aren’t a far-fetched dark matter candidate. Because we know that black holes generally exist, some researchers argue, primordial black holes are a simpler explanation than new and exotic particles. On the flip side, primordial black holes do require new physics in the form of adjustments to current models. Standard pictures of inflation, for example, don’t generate extreme enough fluctuations in energy densities on their own.

“If I just write down a model for inflation, it doesn’t produce any black holes at all,” Kinney says. “I have to really do some violence to that in order to get it to make black holes in the first place.”

Regardless of what sounds more plausible, if a primordial black hole is found, it is by definition dark matter. Primordial black holes, like other black holes, are dark; they don’t interact with other matter much except through gravity. And they have two other important properties that would align with our current understanding of dark matter: They are cold (meaning they move slowly) and considered nonbaryonic (since they formed before protons and neutrons dominated the universe).

But just because primordial black holes are dark matter, that doesn’t mean their existence would fully resolve the mystery. To do so, they’d have to be abundant enough to explain all the universe’s missing mass. Much of the past primordial black hole research has focused on putting limits on how abundant they could be.

Consider the itty-bittiest ones. They can’t account for dark matter because they’re long gone. At the other extreme, the gravitational influence of the most massive primordial black holes, and the radiation given off as they feed, would’ve already given them away.

Various types of measurements made over several decades have suggested that for all but one mass range, primordial black holes can’t be abundant enough to account for more than a small portion of dark matter. Those measurements come from hunts for what are called massive compact halo objects, or MACHOs, in the Milky Way, as well as observations of the relic light left over from the Big Bang, gamma-ray studies and more.

One effort reported in 2024 in Nature, for example, looked at 20 years’ worth of data for the telltale magnification of distant starlight that primordial black holes or other massive objects would cause. The researchers concluded that primordial black holes from about four times the mass of the Earth up to 860 solar masses could make up no more than 10 percent of the universe’s dark matter.

Based on such observations, most scientists believe that only black holes with masses somewhere around the mass of asteroids (say about 10²⁰ grams, or roughly one trillionth the mass of the sun) could make up the majority of dark matter. Still, a small group of primordial black hole enthusiasts, including Carr, Clesse and others, don’t think existing evidence is strong enough to rule out primordial black holes around the mass of the sun as dark matter candidates.

“Considering the idea that primordial black holes might contribute to dark matter in some way is not too much of a stretch,” Kinney says. “The fact that the black holes might be all of the dark matter, that’s a tougher sell for me.”

It’s always possible that dark matter isn’t just one thing, but a mix of types of things. Ordinary matter is a mix of particles, after all. Or it could be that neither primordial black holes nor WIMPs are part of the answer. Though dark matter is a widely accepted idea, it’s even possible that it doesn’t exist at all, and instead scientists need to revisit ideas about how gravity works.

Theoretical physicist Marek Abramowicz of the University of Gothenburg in Sweden believes that some modifications to the theory of gravity can explain away the dark matter puzzle. He acknowledges being among a minority, but he adds: “Fortunately in physics, we are proving things either by calculation or observation, and not by voting.” Abramowicz bets that primordial black holes will be ruled out as an explanation for dark matter in the next few years.

Even if primordial black holes turn out not to exist, working on them won’t have been for nothing, Clesse says. The concept of Hawking radiation, for example, which is considered a scientific triumph, was born from research on the topic. Plus, he notes, all the scientists studying primordial black holes are gaining a ton of insights into the physics of the early universe.

“It is not useless,” Clesse says. “It is science.”