Are There Dark Matter Black Holes?

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The Coma Cluster of Galaxies, where Zwicky first found evidence for dark matter. Image Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

The question: Dark matter is supposed to interact with gravity. Does that mean dark matter is sucked into black holes? And are there dark matter black holes?

A complete answer requires an explanation of why astronomers think there is dark matter in the universe, and what astronomers think constitutes the dark matter.

Why Astronomers Think Dark Matter Exists

The first evidence for the existence of dark matter came from Fritz Zwicky in 1933.

Zwicky studied the orbits of galaxies in the Coma cluster of galaxies. These galaxies orbit the center of the cluster, however the total mass of the cluster required to explain the orbital properties of individual galaxies in the cluster is about 10 times the mass of the galaxies visible in the cluster.

Hence, 90 percent of the gravitationally-detected mass in the Coma cluster is matter that is too dark to be visible. At the time, astronomers widely ignored Zwicky’s work.

Starting in the late 1960s, Vera Rubin, using a sensitive spectrometer built by Kent Ford, studied the orbits of individual stars in spiral galaxies. Rubin found that the orbital properties of the individual stars in a galaxy required a total mass of the galaxy that is roughly 10 times the visible mass of the galaxy. Again 90% of the gravitationally-detected mass in galaxies is unseen dark matter.

Since these early observations, evidence for the existence of dark matter has continued to grow. It seems that about 90% of the matter in the universe is invisible dark matter that astronomers can only detect by its gravitational effect on visible matter.

What Is Dark Matter?

Astronomers do not yet know what dark matter is made of, but they have plenty of theories. Two of the most likely possibilities are MACHOs and WIMPs.

WIMPs, Weakly Interacting Massive Particle, are postulated elementary subatomic particles that interact primarily via the weak nuclear force. As all elementary particles, WIMPs have microscopic masses, but if enough exist they might account for the dark matter. Because they do not interact significantly with the electromagnetic force or less exotic subatomic particles, WIMPs do not emit light or other electromagnetic radiation. Hence we don’t see them. WIMPS are also extremely difficult to detect, so particle physicists searching for possible WIMPs with particle accelerators have yet to find particles that might explain dark matter.

Artist’s conception of the distribution of dark matter in the halo surrounding the Milky Way Galaxy. Image Credit: ESO/L. Calçada

MACHOs, Massive Astrophysical Compact Halo Objects, are celestial objects that might be distributed in spherical halos around spiral galaxies, but are too faint to be observed directly. Possible MACHOs include faint white dwarfs, neutron stars, isolated black holes, and substellar objects such as brown dwarfs or free floating giant planets. Astronomers are trying to indirectly detect MACHOs from their gravitational lensing effects. Current data suggest that MACHOs might explain some but not all of the dark matter.

The mystery of what constitutes the dark matter remains unsolved.

Black Holes and Dark Matter

The fact that astronomers detect dark matter via its gravitational effects means that dark matter does indeed interact gravitationally with ordinary matter.

Dark matter in the form of either WIMPs or MACHOs could quite easily fall into black holes. Black holes do not emit light, but material falling into black holes usually emits X-rays, so such events might be detected with X-ray telescopes.

Because one possibility for MACHOs is isolated black holes, it is quite possible that black holes, originally formed from ordinary matter, constitute at least some of the dark matter in the universe.

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