BLACK HOLES AND DARK MATTER

The nature of dark matter remains one of the most important unresolved issues in astrophysics. This mysterious and unseen substance comprises almost 80% of matter in the Universe and 26% of all the mass/energy. By comparison, baryonic matter, the stuff that makes up you and I and all the things we see in the entire cosmos, comprise only 20% of the matter and 5% of the mass/energy.

credit: NASA, ESA, M. J. Jee and H. Ford et al (John Hopkins University)
A Dark Matter “Ring” indicating location of invisible matter lensing background galaxies.

Science has been concentrating on hypothetical, sub-atomic exotic particles (such as WIMPs) to explain the presence of, and the effect of dark matter. However, researchers have failed to turn up evidence these hypothetical particles actually exist. NASA is currently investigating this issue as part of its Alpha Magnetic Spectrometer and Fermi Gamma-ray Space Telescope missions.

An alternative theory to dark matter being made of exotic and hypothetical particles is that dark matter is made of black holes formed during the first nano-second of our Universe’s existence. These are known as primordial “black holes”. Primordial black holes were formed during the so-called radiation dominated era. The essential condition for a primordial black hole to form is a fluctuation in the density of a small section of the Universe, inducing the small section’s gravitational collapse. This contrasts with black holes that were formed as a result of the collapse of a star after going nova, a much later occurrence in universal cosmology.

What is of interest is the possibility that black holes of all kinds — and particularly primordial black holes — are so commonplace that they may be all that’s required to explain the effects of “dark matter.” Dark matter, which, according to current observational models, has been firmly established as real, both by calculation of the gravity necessary to hold spiral galaxies like our own together, and by direct observation of gravitational lensing.

The Laser Interferometer Gravitational-Wave Observatory, or LIGO detected gravitational waves (for the first time) from the collision of 2 black holes. In addition, LIGO observations (of several events) have suggested that the masses of these colliding black holes are around 30 times the mass of the Sun. Black holes were once ruled out as being the component of dark matter, because of a number of the observational constraints when a wide range of black hole masses were considered. For example, strong constraints on some black hole masses result from the way they would distort the spectrum of the cosmic microwave background radiation, the remnant light of the Big Bang.  However, the case of 30 solar masses (as detected by LIGO) lies within the range where observation constraints are lacking and there is the possibility that this class of primordial black holes (at least) is a good candidate for dark matter.

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. If his analysis is correct, then “all galaxies, including our own, are embedded within a vast sphere of black holes.” So, do black holes make up the major portion of dark matter? Other astronomers are not so sure. “It’s a nutty idea,” says Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, whose team discussed the subject of black hole/dark matter here last week at a meeting of the American Physical Society. “But every idea of what dark matter might be is a nutty idea.” Others are also skeptical, and new studies add to the doubts. For the idea to hold up, “I think you need some miracles,” says Daniel Holz, a theorist at the University of Chicago in Illinois.

credit: LIGO
Merging Black Holes

All this contention doesn’t even address the issue of dark energy, which makes up 70% of the mass/energy of the Universe, and how it relates (if it does) to dark matter and black holes.

Sources:

NASA —“Scientist Suggests Possible Link Between Primordial Black Holes and Dark Matter“ Alexander Kashlinsky, May 24, 2016

“SCIENCE” Adrian Cho Is dark matter made of black holes” Feb. 9, 2017

“ASTROBITES” Did LIGO detect dark matter Simeon Bird, Ilias Cholis, Julian B. Muñoz, Yacine Ali-Haïmoud, Marc Kamionkowski, Ely D. Kovetz, Alvise Raccanelli, Adam G. Riess 1/9/18

NOVA AAS “Revisiting Black Holes as Dark Matter Susan Kohler Feb 17

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