Is Dark Matter Just Black Holes Made During the Big Bang?

Is Dark Matter Just Black Holes Made During the Big Bang?

Originally published on Wired on 2020 09 27 by Joshua Sokol

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.

Upcoming observations might eventually settle that question, too. The European Space Agency has recently agreed to contribute a key extra feature to NASA’s upcoming Nancy Grace Roman Space Telescope, one that would allow it to do groundbreaking microlensing studies.

The addition came at the behest of Günther Hasinger, ESA’s science director, who made the case that primordial black holes could explain multiple mysteries. To Hasinger, the idea is appealing because it doesn’t invoke new particles or new physics theories. It just repurposes old elements.

“I believe maybe some of the puzzles which are still out there could actually solve themselves,” he said, “when you look with different eyes.”

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.

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