LIGO’s black holes may be dark matter

The black holes that produced the first detected gravitational waves may have exotic origins in the early universe.

When the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, glimpsed gravitational waves from two merging black holes, scientists were surprised at how large the black holes were — about 30 times the mass of the sun (SN: 3/5/16, p. 6). Inspired by this unusual finding, two papers published in Physical Review Letters propose that the hefty black holes were born in the universe’s infancy.

Unlike run-of-the-mill black holes that form from collapsing stars, such primordial black holes could have formed when dense regions of the very early universe collapsed under their own gravity, some theories suggest. If they exist, primordial black holes could also solve another puzzle: the identity of dark matter, the unknown source of mass in the universe that holds galaxies and galaxy clusters together. Primordial black holes could make up the universe’s missing mass, an idea that counters the more popular theory that dark matter is made up of undetected particles.

A Japanese team of astrophysicists reported August 2 that LIGO’s black holes may be primordial, and that, if so, they could make up some portion of the universe’s dark matter. Johns Hopkins University scientists reported May 19 that LIGO’s estimated rate of black hole mergers matches with that expected from primordial black hole dark matter.

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LIGO’s black holes’ bigger than expected mass had astrophysicist Simeon Bird and colleagues at Johns Hopkins University wondering, “Gosh — it’s unexpected — what else could it be?” Bird says. Previous research had ruled out primordial black hole dark matter for all but a narrow range of masses. But that allowed range happens to overlap with the masses of the black holes LIGO found.

Drawing on scientists’ knowledge of dark matter’s properties, Bird and colleagues estimated how often LIGO would expect to see merging primordial black holes, assuming they were the source of dark matter. This rate matched LIGO’s estimated detection rate, made by assuming the one unexpectedly massive black hole merger LIGO has seen so far wasn’t a fluke. Although both estimates have large errors, their agreement indicates that dark matter may be composed of primordial black holes.

Likewise, the Japanese team reported that LIGO could have detected primordial black holes. But the researchers found that such primordial black holes could explain only a small fraction of dark matter. This disparity boils down to differing assumptions about how primordial black holes group into pairs before merging.

“The important thing is that this can be tested,” says astrophysicist Misao Sasaki of Kyoto University in Japan. More data from LIGO or further studies of the cosmic microwave background — remnant light from the aftermath of the Big Bang — could exclude primordial black holes as a possibility.

To better understand LIGO’s black holes, “we’re going to need to make more detections,” says LIGO scientist Chad Hanna of Penn State University. (LIGO also detected a second black hole merger (SN: 7/9/16, p. 8), but those black holes were smaller, indicating that they formed from stars.)

Eventually, subtle signs of primordial black holes may appear in gravitational wave data, says Bernard Carr of Queen Mary University of London. The eccentricity of the black holes’ orbits around one another — how elliptical their paths are — could indicate whether the black holes are primordial or standard black holes, Carr says. “It’s a bit more exciting to my mind if they turn out to be primordial black holes.”

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