The merger of unexpectedly large black holes has led to speculation that they could be dark matter.

Mark Garlick/Science Source

Is dark matter made of black holes?

Could dark matter consist of primordial black holes, as numerous as the stars? It’s an old, improbable idea, but it made a Lazarus-like comeback a year ago, when the discovery of gravitational waves suggested that the cosmos abounds with unexpectedly heavy black holes. With decades-long searches failing to find the hypothetical dark matter particles that theorists have favored, physicists are turning to more radical ways of explaining the universe’s missing mass.

“It’s a nutty idea,” says Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, Maryland, whose team made the case for black hole dark matter here last week at a meeting of the American Physical Society. “But every idea of what dark matter might be is a nutty idea.” Others are skeptical, and new studies add to the doubts. For the idea to hold up, “I think you need some miracles,” says Daniel Holz, a theorist at the University of Chicago in Illinois.

Ordinary black holes form when individual stars collapse, and were thought to top out at about 15 times the mass of the sun. And the supermassive black holes that lurk in galactic centers swallow billions of stars. But astrophysicists didn’t see how collapsing stars could form black holes of intermediate masses. That’s why it was a surprise when physicists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced in February 2016 that they had detected ripples in space from the violent merger of two black holes 29 and 36 times as massive as our sun.

Theorists say there is a way to form such heavy black holes even before the first stars: through the direct collapse of dense spots in the seething plasma of particles that filled the cosmos right after the big bang. If LIGO’s discovery wasn’t a statistical burp, space could teem with these primordial black holes, says Kamionkowski—enough to account for the 85% of the universe’s matter that is missing.

They should also have left a mark on the cosmic microwave background (CMB). X-rays from matter swirling into the black holes should have ionized some of the first atoms, which would have altered the CMB’s mottled appearance. Kamionkowski and colleagues calculate black holes between 20 and 100 solar masses could be consistent with CMB measurements. But Massimo Ricotti, a cosmologist at the University of Maryland in College Park, who did an earlier calculation with different assumptions, thinks “it would be very difficult to have all the dark matter in 30-solar-mass black holes.”

Observations of galaxies today cast a different doubt on black hole dark matter, reports Timothy Brandt, an astrophysicist at the Institute for Advanced Study in Princeton, New Jersey. Black holes heavier than 10 solar masses should have long ago settled to the centers of small galaxies, churning up stars with their gravity like bowling balls setting the pins flying. That would have puffed up the galaxies. However, Brandt examined five faint dwarf galaxies near the Milky Way, and found them to be compact and unruffled. “That’s a very strong argument against this sort of dark matter,” he says.

Heavy black holes might also betray their presence by occasionally passing in front of more distant stars. Their gravity would magnify the star, causing it to temporarily brighten in an effect called microlensing. Two microlensing surveys in the 1990s ruled out the possibility of swarms of black holes. But because the surveys were short, they were only sensitive to relatively small black holes. Brightening events for 30-solar-mass black holes would stretch out for years, so they haven’t yet been ruled out, says Ely Kovetz, a cosmologist at Johns Hopkins.

Kovetz also hopes that confirmation could come from new radio telescopes coming online, such as the Canadian Hydrogen Intensity Mapping Experiment (CHIME) in Okanagan Falls. Since 2007, astronomers have known of millisecond flashes of radio waves called fast radio bursts (FRBs). Microlensing by a 30-solar-mass black hole should generate a rapid echo of a burst, making the black hole easier to detect. CHIME should spot thousands of FRBs in a few years, Kovetz says, enough to look for telltale echoes.

Katelin Schutz, a theorist at the University of California, Berkeley, says that clarity could come even faster from stellar beacons called millisecond pulsars, which emit exquisitely regular pulses of radio waves. Microlensing by a black hole should slightly slow the pulses, she reports. Those shifts would occur over years, but radio astronomers already have 30 years of data they can search through, she says.

The LIGO discovery sparked this debate, but LIGO is unlikely to end it. Last June, the consortium reported a second black hole merger, but the black holes involved weighed just 8 and 14 solar masses. And last week, LIGO said it had found two “triggers” in new data taken since November 2016—which could also end up being black hole mergers. But Kovetz says LIGO would need to spot about 100 black holes before a true census of their masses emerges. That could take a decade.