Dark matter could be made of primordial black holes, study finds

Scientists are becoming more and more convinced that dark matter, the elusive, nearly invisible material which makes up the majority of the universe, could be made up of black holes that were formed during the very first moments after the Big Bang, according to new reports.

Last month, research published in the Astrophysical Journal Letters proposed that these black holes could help explain the gravitational waves detected last year by the Laser Interferometer Gravitational-Wave Observatory (LIGO), as well as other observations of the early universe.

If that study, which was authored by Kashlinsky, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, is correct, it would suggest that dark matter may be made up of black holes formed within the first second of the universe’s existence, and that thing may have evolved quite differently in those earliest moments than experts initially thought.

Investigating if the LIGO black holes were primordial

On Monday, Kashlinsky, who used the Spitzer Space Telescope to examine the background glow of infrared light in the universe (known as the cosmic infrared background, or CIB) explained his findings to Space.com, telling the website that he and his colleagues wanted to focus on the early universe, at a point beyond that which telescopes can detect individual galaxies.

“Suppose you look at New York from afar. You cannot see individual lampposts or buildings, but you can see this cumulative diffuse light that they produce,” he said. By removing the light from the known galaxies in the universe, his team was still able to detect light originating from the background glow of light sources from more than 13 billion years ago.

That was in 2007. Six years later, Kashlinsky and his colleagues used the NASA Chandra X-ray Observatory to re-examine this background glow in the X-ray portion of the spectrum, and found that the patterns within the two different sets of observations matched perfectly. Only one source would be capable of producing this kind of light in both infrared and X-ray spectrums, he noted: black holes. At the time, however, he did not realize that they could be very ancient.

LIGO’s detection of the first ever gravitational waves – cosmic ripples in the very fabric of space-time – in September 2015 changed that, Space.com said. Those waves were produced by two black holes colliding, and while that alone was significant because it marked the first time scientists were able to directly detect black holes, Johns Hopkins University astronomer Simeon Bird suggested that the black holes observed by LIGO might be primordial.

Primordial black hole proposal would explain excess background radiation

Unlike most black holes, primordial ones are not formed when a dead star collapses, the website explained. Rather, they formed shortly after the Big Bang, during a time when sound waves were prevalent throughout the universe. Regions where those sound waves were at their densest could have collapsed to form these black holes, which are currently still just hypothetical.

Inspired by Bird’s suggestion regarding the LIGO black holes, Kashlinsky investigated how these primordial black holes would influence the evolution of the universe. During the first 500 million years after the Big Bang, dark matter collapsed into masses known as halos, which then went on to act as the gravitational seeds that allowed matter to accumulate and form into stars and galaxies.

If that dark matter was made up of primordial black holes, however, there would have been far more halos, which Kashlinsky told Space.com would help explain by the excess cosmic infrared background and the excess cosmic X-ray background his team observed during their research. The first stars that formed within the halos would give off the infrared glow, while the gas that helped create those stars falling onto the black holes would have emitted the X-rays after they became heated.

“Everything fits together remarkably well,” Kashlinsky told the website on Monday. Previously, in a statement, he said, “Depending on the mechanism at work, primordial black holes could have properties very similar to what LIGO detected. 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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Image credit: NASA

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