Could Dark Matter Actually Just Be Black Holes?

Artist's depiction of matter swirling around a black hole. NASA/Dana Berry/SkyWorks Digital

Most of the matter in the universe is invisible to us. Everything we can see and touch is made of matter, but there is another substance called dark matter that doesn’t interact with light that's responsible for keeping galaxies together.

There’s a lot of speculation about the nature of dark matter, from exotic particles to weird gravitational effects, so physicists are following different leads. A team at NASA, though, has another idea: Dark matter is actually primordial black holes.

These objects could have formed in the first instants after the Big Bang, according to certain scenarios. If they are used in simulations as a substitute for dark matter, they not only reproduce the general dark matter scenario well, they can also explain curious phenomena like the "lumpiness" of the cosmic infrared background.    

"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 Alexander Kashlinsky, lead author of the study, in a statement.

This is the cosmic infrared background (CIB); lighter colors indicate brighter areas and nearby sources are masked in grey. The CIB glow is more irregular than can be explained by distant unresolved galaxies, and this excess structure is thought to be light emitted when the universe was less than a billion years old. NASA/JPL-Caltech/A. Kashlinsky (Goddard)

Kashlinsky previously led a team to look at the background glow of infrared in the sky, during which they discovered an excess in glow patchiness. A similar distribution in sources was observed in the cosmic X-ray background, and that made Kashlinsky think that the two might be related.

There’s only one type of source that can emit enough energy in both infrared and X-ray: a black hole. The distribution of light in the infrared and X-ray background can be produced by having a large number of black holes in and around galaxies, a larger number than previously thought.  

"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," said Kashlinsky.

The mass of these black holes is important. In the paper, which is published in the Astrophysical Journal Letters, the researchers linked them to the discovery of gravitational waves by LIGO, which was announced last February.  

"Depending on the mechanism at work, primordial black holes could have properties very similar to what LIGO detected," Kashlinsky explained. "If we assume this is the case, that LIGO caught a merger of black holes formed in the early universe, we can look at the consequences this has on our understanding of how the cosmos ultimately evolved."

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