But others aren’t sure that black holes are as prevalent as they would need to be to account for dark matter. “I think it’s probably unlikely,” says the cosmologist Anne Green of the University of Nottingham, in the U.K. One problem with the theory is that having large numbers of multi-solar-mass black holes throughout the cosmos would have all sorts of visible effects that have never been spotted. As such objects consume gas and dust, they should be shooting out large numbers of radio waves and X-rays that could give away their presence, she adds.
In regard to dark matter, the theoretical models of the early universe also require a great deal of tweaking to get them to spit out the right number of black holes to match the amount of dark matter we know is out there. “It turns out to be quite difficult to come up with models that make the right amount of black holes,” Green says.
Even some of the bigger fans of primordial black holes are no longer as optimistic about the prospect that the types of black holes detected by LIGO could account for all dark matter in the universe. If many of those black holes were lurking throughout space, astronomers would have seen more of their effects by now, Kovetz says. He still thinks that they may account for some dark matter and, more generally, that including more sizes of primordial black holes beyond what LIGO has detected could add up to enough to explain dark matter. And yet, “personally, I’ve lost some of my motivation.”
The good news is that new instruments may be able to help physicists get to the bottom of the question very soon. LIGO and Virgo are currently being upgraded and have now been joined by a Japanese gravitational-wave detector named KAGRA. An Indian instrument will also turn on in the next few years.
Observations from these facilities may finally tip the scales one way or the other. Should the observatories spot a small black hole with one solar mass or less—something impossible to create from stellar evolution—it would provide exciting and definitive evidence of at least one type of primordial black hole, making them a much more appealing explanation for dark matter and galaxy formation.
In addition to looking for very small black holes, scientists could also seal the deal by finding black holes that formed before stars even existed. This may be beyond the capability of the existing observatories, but the European Space Agency is planning to launch a new, highly sensitive space probe called the Laser Interferometer Space Antenna (LISA) in the 2030s, which may be up to the task.
García-Bellido and others are planning to use yet another new instrument slated to be fully operational by 2023, the Vera C. Rubin Observatory in Chile, to hunt for stars that brighten over multiyear timescales, which could be evidence of clusters of black holes drifting amid the heavens. At least a few researchers expect that in three or four years’ time, they might finally have an actual, definitive answer to whether primordial black holes exist or not.
Until then, scientists will be sitting on the edge of their seats, trying to keep an open mind about dark matter. Perhaps the mysterious substance will turn out to be made of many things, including both exotic particles and black holes. “The universe is messy, and it has a lot of stuff in it,” says Bird. “I kind of believe that the universe likes to make things hard for physicists.”
This post appears courtesy of Knowable Magazine.