It was an old idea of Stephen Hawking’s: Unseen “primordial” black holes might be the hidden dark matter. It fell out of favor for decades, but a new series of studies has shown how the theory can work.
Primordial black holes would cluster in distinct clumps throughout the universe. Relatively large black holes would be surrounded by much smaller ones.
Black holes are like sharks. Elegant, simple, scarier in the popular imagination than they deserve, and possibly lurking in deep, dark places all around us.
Their very blackness makes it hard to estimate how many black holes inhabit the cosmos and how big they are. So it was a genuine surprise when the first gravitational waves thrummed through detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO) in September 2015. Previously, the largest star-size black holes had topped out at around 20 times the mass of the sun. These new ones were about 30 solar masses each — not inconceivable, but odd. Moreover, once LIGO turned on and immediately started hearing these sorts of objects merge with each other, astrophysicists realized that there must be more black holes lurking out there than they had thought. Maybe a lot more.
The discovery of these strange specimens breathed new life into an old idea — one that had, in recent years, been relegated to the fringe. We know that dying stars can make black holes. But perhaps black holes were also born during the Big Bang itself. A hidden population of such “primordial” black holes could conceivably constitute dark matter, a hidden thumb on the cosmic scale. After all, no dark matter particle has shown itself, despite decades of searching. What if the ingredients we really needed — black holes — were under our noses the whole time?
“Yes, it was a crazy idea,” said Marc Kamionkowski, a cosmologist at Johns Hopkins University whose group came out with one of the many eye-catching papers that explored the possibility in 2016. “But it wasn’t necessarily crazier than anything else.”
Alas, the flirtation with primordial black holes soured in 2017, after a paper by Yacine Ali-Haïmoud, an astrophysicist at New York University who had previously been on the optimistic Kamionkowski team, examined how this type of black hole should affect LIGO’s detection rate. He calculated that if the baby universe spawned enough black holes to account for dark matter, then over time, these black holes would settle into binary pairs, orbit each other closer and closer, and merge at rates thousands of times higher than what LIGO observes. He urged other researchers to continue to investigate the idea using alternate approaches. But many lost hope. The argument was so damning that Kamionkowski said it quenched his own interest in the hypothesis.
Now, however, following a flurry of recent papers, the primordial black hole idea appears to have come back to life. In one of the latest, published last week in the Journal of Cosmology and Astroparticle Physics, Karsten Jedamzik, a cosmologist at the University of Montpellier, showed how a large population of primordial black holes could result in collisions that perfectly match what LIGO observes. “If his results are correct — and it seems to be a careful calculation he’s done — that would put the last nail in the coffin of our own calculation,” said Ali-Haïmoud, who has continued to play with the primordial black hole idea in subsequent papers too. “It would mean that in fact they could be all the dark matter.”
“It’s exciting,” said Christian Byrnes, a cosmologist at the University of Sussex who helped inspire some of Jedamzik’s arguments. “He’s gone further than anyone has gone before.”
The original idea dates back to the 1970s with the work of Stephen Hawking and Bernard Carr. Hawking and Carr reasoned that in the universe’s first fractions of a second, small fluctuations in its density could have endowed lucky — or unlucky — regions with too much mass. Each of these regions would collapse into a black hole. The size of the black hole would be dictated by the region’s horizon — the parcel of space around any point reachable at the speed of light. Any matter within the horizon would feel the black hole’s gravity and fall in. Hawking’s rough calculations showed that if the black holes were bigger than small asteroids, they could plausibly still be lurking in the universe today.
More progress came in the 1990s. By then, theorists also had the theory of cosmic inflation, which holds that the universe experienced a burst of extreme expansion right after the Big Bang. Inflation could explain where the initial density fluctuations would have come from.
On top of those density fluctuations, physicists also considered a key transition that would coax along the collapse.
When the universe was new, all of its matter and energy seethed in an unthinkably hot plasma. After the first hundred-thousandth of a second or so, the universe cooled a little, and the plasma’s loose quarks and gluons could bind together into heavier particles. With some of the lightning-fast particles now straitjacketed together, the pressure dropped. That might have helped more regions collapse into black holes.
But back in the 1990s, nobody understood the physics of a fluid of quarks and gluons well enough to make precise predictions about how this transition would affect black hole production. Theorists couldn’t say how massive primordial black holes should be, or how many to expect.
Moreover, cosmologists didn’t really seem to need primordial black holes. Astronomical surveys scanned patches of sky hoping to find a sea of dense, dark objects like black holes floating on the outskirts of the Milky Way, but they didn’t find many. Instead, most cosmologists came to believe that dark matter was made of ultra-shy particles called WIMPs. And hopes simmered that either purpose-built WIMP detectors or the upcoming Large Hadron Collider would soon find hard evidence of them.
With the dark matter problem about to wrap itself up with a bow and no observations suggesting otherwise, primordial black holes became an academic backwater. “One senior cosmologist kind of ridiculed me for working on that,” said Jedamzik, who traces his own interest back to the 1990s. “So I stopped that, because I needed to have a permanent position.”
Of course, no WIMPs have been found in the decades since then, nor any new particles (save the long-predicted Higgs boson). Dark matter remains dark.
Yet much more is known today about the environment that could have spawned primordial black holes. Physicists can now calculate how pressure and density would have evolved from the quark-gluon plasma at the beginning of the universe. “It took the community really decades to work this out,” said Byrnes. With that information in hand, theorists such as Byrnes and Juan García-Bellido at the Autonomous University of Madrid have spent the last few years publishing studies predicting that the early universe could have spawned not just one size of black hole, but a range of them.
First, the quarks and gluons were glued together into protons and neutrons. That caused a pressure drop and could have spawned one set of primordial black holes. As the universe kept cooling, particles such as pions formed, creating another pressure plunge and possible black hole burst.
Between these epochs, space itself expanded. The first black holes could suck in about one solar mass of material from the horizon around themselves. The second round could grab perhaps about 30 solar masses’ worth — just like the strange objects first seen by LIGO. “Gravitational waves came to our rescue,” said García-Bellido.
Within weeks of the first gravitational wave announcement from LIGO in 2016, the primordial black hole hypothesis roared back to life. But the following year, Ali-Haïmoud came out with his argument that primordial black holes would be colliding far too often, which gave proponents a major hurdle to overcome.
Jedamzik took up the challenge. During a long vacation in Costa Rica, he went after Ali-Haïmoud’s argument. Ali-Haïmoud had done his work analytically, through equations. But when Jedamzik created numerical simulations of the same problem, he found a twist.
Primordial black holes would indeed form binaries. But Jedamzik concluded that in a universe teeming with black holes, a third black hole would often approach the initial pair and change places with one of them. This process would repeat again and again.
Over time, this swinging from partner to partner would leave binary black holes with almost circular orbits. These partners would be incredibly slow to collide. Even a huge population of primordial black holes would merge so infrequently that the entire hypothesis would still fit within LIGO’s observed merger rate.
Advocates of the primordial black hole hypothesis still have a lot of convincing to do. Most physicists still believe that dark matter is made of some kind of elementary particle, one that’s devilishly hard to detect. Moreover, the LIGO black holes aren’t too different from what we would expect if they came from ordinary stars. “It sort of fills a hole in the theory that isn’t actually there,” said Carl Rodriguez, an astrophysicist at Carnegie Mellon University. “There are things that are weird about some of the LIGO sources, but we can explain everything that we’ve seen so far through normal stellar evolutionary process.”
Selma de Mink, an astrophysicist at Harvard University who has sketched out theories for how stars alone can produce the heavy black hole binaries seen by LIGO, is more blunt: “I think astronomers can laugh a bit about it.”
Finding just one black hole of sub-solar mass — which should be common, according to the primordial black hole scenario, and which can’t form from stars — would transform this entire debate. And with every subsequent observing run, LIGO has increased its sensitivity, allowing it to eventually either find such small black holes or set strict limits on how many can exist. “This is not one of these stories like string theory, where in a decade or three decades we might still be discussing if it’s correct,” Byrnes said.
In the meantime, other astrophysicists are probing different aspects of the theory. For example, perhaps the strongest constraints on primordial black holes come from microlensing searches — those same surveys that began in the 1990s. In these efforts, astronomers monitor bright but distant sources, waiting to see if a dark object passes in front of them. These searches have long ruled out an evenly dispersed population of small black holes.
But if primordial black holes exist at a range of masses, and if they’re packed into dense, massive clusters, those results could be less significant than researchers thought, García-Bellido said.