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May 25, 2016

Scientists find link between dark matter and primordial black holes

While most scientists believe that the elusive substance known as dark matter is comprised of extremely massive exotic particles, one NASA scientist believes that it might actually be made up of black holes formed during the very first moments of the universe’s existence.

In a new study published Tuesday in the Astrophysical Journal Letters, Alexander Kashlinsky, an astrophysicist at the Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues believe that the link between dark matter and primordial black holes correlates with what experts know about the cosmic infrared background (CIB) and cosmic X-ray background (CXB).

Furthermore, it could help to explain the unexpectedly high masses of merging black holes first detected in 2015, the study authors said. The research 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,” Kashlinsky explained in a statement. “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.”

Dark Matter is one of the greatest scientific mysteries of our time. Credit: NASA Goddard

Dark Matter is one of the greatest scientific mysteries of our time. Credit: NASA Goddard

According to NASA, dark matter is “one of the most important unresolved issues” in the field of astrophysics, and while the preeminent theoretical models used to explain its existence believe it exists as an exotic massive particle, thus far researchers have been unsuccessful in detecting any concrete evidence to support this possibility.

While studies investigating dark matter “are providing increasingly sensitive results, slowly shrinking the box of parameters where dark matter particles can hide," Kashlinsky noted that “the failure to find them has led to renewed interest in studying how well primordial black holes – black holes formed in the universe's first fraction of a second – could work as dark matter.”

How gravitational wave observations provided important clues

The Goddard astrophysicist was a member of a team that, in 2005, used NASA's Spitzer Space Telescope to explore the background glow of infrared light in one portion of the sky, detecting a pattern of patchiness in the glow and determining that it was aggregate light coming the cosmic infrared background, a 13 billion year old light source dating from the origins of the universe.

Eight years later, another study compared the cosmic X-ray background detected by the Chandra X-ray Observatory to the CIB in the same part of the sky, and found that the irregular glow of the low-energy X-rays in the CXB were a close match to the patchiness of the CIB, according to the US space agency. Since the only object capable of being this luminous across such a wide energy range is a black hole, the researcher team determined that primordial black holes made up at least one-fifth of all of sources contributing to the CIB.

Physicists have come up with multiple hypothesis that could explain how the early universe may have produced primordial black holes during the first milliseconds following the Big Bang, noted NASA. The older the universe was when these mechanisms occurred, the largest the black holes can be, and since there was such a narrow window during which these black holes might have been produced, experts believe that they would exhibit a narrow range of masses.

Last September, the Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities in Washington, and Louisiana detected the first ever gravitational waves produced by two merging black holes located 1.3 billion light years from Earth. The signal also provided scientists with measurements of the masses of each black hole, which at 29 times and 36 times the mass of the sun, respectively, were “unexpectedly large and surprisingly similar,” according to NASA.

Assuming that primordial black holes have properties very similar those of the black holes that LIGO detected, Kashlinsky said that the scientists could have detected “a merger of black holes formed in the early universe,” which he said may have an impact of “our understanding of how the cosmos ultimately evolved.” He and his colleagues set out to determine what could have taken place if dark matter was comprised of black holes similar to those detected by LIGO.

Post-Big Bang heat would not have affected dark matter

The researchers explained that these black holes would have caused small fluctuations in the distribution of mass in the early universe, which would have had an impact on the development of the first stars several hundred-million years later. While normal matter would have been far too hot to form stars during the first 500 million years after the Big Bang, the heat would have no impact on dark matter, as the mysterious matter primarily interacts through gravity.

By coalescing through mutual attraction, dark matter would have formed into a series of clumps known as minihaloes, which would have helped enable normal matter to begin accumulating as hot gas collapsed towards them. This phenomenon would have created gas pockets dense enough to further collapse into the very first stars, and if black holes play the role of dark matter, then the entire process would have occurred more rapidly, according to Kashlinsky.

Ultimately, this would produce the so-called lumpiness found in the Spitzer observations of the CIB, even if only a small percentage of minihaloes successfully produced stars. As cosmic gases collapsed into these dark matter clumps, some of it would also have been captured by the black holes that comprised them, causing that matter to become heated and produce X-rays. Together, the infrared light from the early stars and the X-rays produced by the gas falling into dark matter black holes would account for the similar irregularities observed in the CIB and the CXB.

“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,” added Kashlinsky.

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Image credit: NASA