Dark Matter Could Be Made of Primordial Black Holes

The elusive substance is running out of places to hide.

Dark matter is a very mysterious and elusive substance composing most of the universe. It has been suggested that dark matter could be a massive, exotic particle, or that it is made of black holes formed during the first second of our universe’s existence, known as primordial black holes.

According to NASA, scientists currently favor models that explain dark matter as an exotic, massive particle, but so far no evidence for it actually exists.

"These studies are providing increasingly sensitive results, slowly shrinking the box of parameters where dark matter particles can hide," said Alexander Kashlinsky, an astrophysicist at NASA Goddard, in a press release. "The failure to find them has led to renewed interest in studying how well primordial black holes [...] could work as dark matter."

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Kashlinsky suggests that primordial black holes align with our knowledge of cosmic infrared and X-ray background glow, and that they may explain the unexpectedly high number of merging black holes detected last year.

"This study is an effort to bring together a broad set of ideas and observations to test how well they fit, and the fit is surprisingly good," said Kashlinsky. "If this is correct, then all galaxies, including our own, are embedded within a vast sphere of black holes each about 30 times the sun's mass."

The older the universe is, the larger black holes can be, but because primordial black holes form in just a fraction of a second, scientists expect them to have a restricted range of masses. "Depending on the mechanism at work, primordial black holes could have properties very similar to what LIGO detected," Kashlinsky explained.

In Kashlinsky’s new paper, published May 24 in The Astrophysical Journal Letters, he analyzed what would happen if dark matter consisted of a population of black holes similar to those detected by LIGO.

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According to Kashlinsky, for much of the universe’s first 500 million years, normal matter remained too hot to coalesce into the first stars. On the other hand, dark matter was unaffected by the high temperature because, for whatever reason, it primarily interacts through gravity.

Due to this mutual attraction, dark matter first collapsed into clumps called minihaloes, providing a gravitational center around which normal matter could accumulate. This hot gas then collapsed toward the minihaloes, resulting in pockets of gas dense enough to further collapse on their own into the first stars.

^{Photo credit: NASA/JPL-Caltech/A. Kashlinsky (Goddard). Top: Image from NASA's Spitzer Space Telescope showing an infrared view of an area in the constellation Ursa Major (CXB). Bottom: After masking out all known stars, galaxies, and artifacts and enhancing what's left, an irregular background glow appears (CIB). Scientists say it likely originated from the first luminous objects to form in the universe, which includes both the first stars and black holes.}

Now, if primordial black holes play the part of dark matter, this process occurs much faster and produces the lumpiness seen in the CIB image (lower). As cosmic gas fell into the minihaloes, the constituent black holes would capture some of it too. Since matter falling toward a black hole heats up and produces X-rays, the infrared light from the first stars and the X-rays from gas falling into dark matter black holes can account for the observed similarities between the patchiness of the CIB and the CXB (upper image).

Occasionally, some primordial black holes pass close enough to be captured into binary systems, and over eons, they emit gravitational radiation, spiral inward, and eventually merge into a larger black hole — like the event LIGO observed.

"Future LIGO observing runs will tell us much more about the universe's population of black holes, and it won't be long before we'll know if the scenario I outline is either supported or ruled out," concluded Kashlinsky.

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