All or at least a significant part of dark matter is made up of so-called primordial black holes, according to Dr. Alexander Kashlinsky, an astrophysicist at NASA’s Goddard Space Flight Center.
Collision of two black holes is seen in this still from a computer simulation. Image credit: SXS.
Dark matter is currently one of the greatest mysteries of science. Most astrophysicists think that it is composed of non-baryonic matter.
An alternative view is that dark matter is made of primordial black holes. Now Dr. Kashlinsky suggests that this interpretation aligns with our knowledge of cosmic infrared and X-ray background glows, and may explain high masses of two merging black holes discovered by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015.
“If this is correct, then all galaxies, including our own, are embedded within a vast sphere of black holes each about 30 solar masses,” said Dr. Kashlinsky, an author of a paper recently published in the Astrophysical Journal Letters (arXiv.org preprint).
In September 2015, gravitational waves produced 1.3 billion light-years away were captured by the LIGO facilities in Hanford, Washington, and Livingston, Louisiana. This event marked the first-ever detection of gravitational waves as well as the first direct detection of black holes.
The signal provided LIGO scientists with information about the masses of the individual black holes, which were 29 and 36 times the Sun’s mass. These values were both unexpectedly large and surprisingly similar.
“Depending on the mechanism at work, primordial black holes could have properties very similar to what LIGO detected,” Dr. Kashlinsky said.
“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.”
In his paper, Dr. Kashlinsky analyzes what might have happened if dark matter consisted of a population of black holes similar to those detected by LIGO.
The black holes distort the distribution of mass in the early Universe, adding a small fluctuation that has consequences hundreds of millions of years later, when the first stars begin to form.
For much of the Universe’s first 500 million years, normal matter remained too hot to coalesce into the first stars.
Dark matter was unaffected by the high temperature because it primarily interacts through gravity.
Aggregating by mutual attraction, dark matter first collapsed into clumps called minihaloes, which provided a gravitational seed enabling normal matter to accumulate. Hot gas collapsed toward the minihaloes, resulting in pockets of gas dense enough to further collapse on their own into the first stars.
Dr. Kashlinsky shows that if black holes play the part of dark matter, this process occurs more rapidly and easily produces the lumpiness of the cosmic infrared background (CIB) detected in the data from NASA’s Spitzer Space Telescope even if only a small fraction of minihaloes manage to produce stars.
As cosmic gas fell into the minihaloes, their constituent black holes would naturally capture some of it too. Matter falling toward a black hole heats up and ultimately produces X-rays.
Together, infrared light from the first stars and X-rays from gas falling into dark matter black holes can account for the observed agreement between the patchiness of the CIB and the cosmic X-ray background (detected by NASA’s Chandra X-ray Observatory).
Occasionally, some primordial black holes will pass close enough to be gravitationally captured into binary systems.
The black holes in each of these binaries will, over eons, emit gravitational radiation, lose orbital energy and spiral inward, ultimately merging into a larger black hole like the event LIGO observed.
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A. Kashlinsky et al. 2016. LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies. ApJ 823, L25; doi: 10.3847/2041-8205/823/2/L25
