But did LIGO detect black holes?
There’s a slight problem: It’s not 100 percent certain that the merger in the GW150914 event that LIGO detected were caused by primordial black holes.
Lionel London, a professor of physics at Cardiff University and a research associate with LIGO, says in an email that it is just as likely that GW150914 was caused by more traditional black holes formed by the first generation of massive, low-metallicity stars exploding at the end of their short lives. He says that with more LIGO detections, “the work presented by Kashlinsky may be either bolstered or ruled out.”
Others in the physics community are urging similar caution.
“I think the paper is pursuing an interesting idea, which is to understand how LIGO's results affect the theory that primordial black holes constitute the dark matter,” says Chanda Prescod-Weinstein, a dark matter expert and professor of physics at the University of Washington. “It is certainly the case that one reason primordial black holes were not a popular dark matter candidate is because we hadn't observed many of the scale that LIGO detected. Rumors suggest LIGO is regularly finding candidates in this range, which could indicate that they are common.”
For now, she calls the theory a “very interesting line of reasoning that should indeed be thought carefully through.”
For Kashlinsky, it’s all or nothing: All the dark matter is primordial black holes or none of it is. But he says he feels bolstered by the LIGO results.
Not everyone is buying it.
“It's very speculative,” Priyamvada Natarajan, a professor of astronomy and physics at Yale University who researches dark matter. “I don't believe in it at all, but it's an interesting idea to pursue.”
Natarajan’s research also focuses on the early population black holes, particularly direct collapse models (those formed from clouds of gas and not the end states of stars). She believes that they could be the culprits behind soft X-ray excess, removing one of Kashlinsky’s required assertions for his theory.
And indeed, members of this direct collapse class of black holes may have been spotted. There are candidates derived by combining observations from the Chandra X-ray Observatory, Hubble Space Telescope, and the Spitzer Telescope.The entire theory rests, Natarajan says, on the soft X-ray excess coming from primordial black holes — those that formed very early in the universe even prior to the direct collapse black holes . Otherwise it falls apart.
Natarajan still believes dark matter is a particle, and she leans toward a single class of particles called neutralinos. They are a bit different from other forms of hypothetical dark matter called axions, with neutralinos predicted by “supersymmetry” as a particle related to z-bosons, photons, and the Higgs boson, all known and detected. Supersymmetry is a theoretical part of physics dealing with particles of similar ilk but different spins.
“It interacts very, very weakly with itself, and with other particles,” she says.
Still, there is that nagging problem with the particle theory of dark matter.
“We haven’t detected the particle yet, which is the conundrum, which is why the field is open for speculation,” Natarajan says. She points to promising — but not yet replicated — results from an Italian agency called DAMA. It found a weak detection of dark matter passing through a sodium iodide filter, but four follow-up experiments generated null results.
Scientists need more experiments to try to replicate the results. After all, if it exists, dark matter barely interacts, so it could be two or three years, or, like gravitational waves, it could be a decades-long endeavor of waiting for the right detector technology.