Sorry, Stephen, Dark Matter Isn’t Tiny Black Holes

Famed physicist Stephen Hawking made a prediction in 1974 that dark matter — the mysterious “something” that makes up 85 percent of everything in the Universe — would turn out to be tiny black holes formed during the earliest age of matter. A new study conducted by astronomers at the Subaru telescope in Hawaii, coming on the eve of the release of the first-ever detailed photograph of the region around a black hole, showed he was, uncharacteristically, wrong.

Hawking postulated that black holes less than a millimeter (1/25th of an inch) in diameter would account for the gravity which holds together groups of galaxies, as well altering the rotational rates of galaxies, including the Milky Way.

The Andromeda Galaxy, like our own Milky Way, is home to dark matter — we just don’t know what it is, but now we have a better idea of what it is not. Image credit: Kavli IPMU

All the Things Dark Matter Could Be

Dark matter describes — something — which can not be seen, gives off no radiation, yet provides enough gravity to keep galaxies together in groups, and drive rotational rates of objects on the outskirts of these stellar families. Evidence for dark matter between galaxies was first observed by astronomer Fritz Zwicky in 1933, and in the 1970’s, the presence of dark matter within galaxies was detected by Vera Rubin. Since then, astronomers and astrophysicists have puzzled over the nature of dark matter, and we know less about what it is than what it is not.

“Dark matter possibly could be brown dwarfs, ‘failed’ stars that never ignited because they lacked the mass needed to start burning. Dark matter could be white dwarfs, the remnants of cores of dead small- to medium-size stars. Or dark matter could be neutron stars or black holes, the remnants of large stars after they explode,” NASA explains in a description of dark matter.

But, Probably Not…

However, problems exist with each of these ideas. There are likely not enough white or brown dwarfs to account for the tremendous gravity dark matter exerts on visible objects. Neutron stars and black holes are exceedingly rare. It is possible that dark matter may be exotic subatomic particles, but careful experiments to detect these particles have, so far, turned up empty. The Fermi space telescope should be able to detect gamma ray emissions resulting from the collision of exotic dark matter particles, but that search has also returned fruitless.

If these miniature black holes do exist, then they are expected to bend space around them, causing light from distant stars to curve, and brighten, like a lens focusing the Sun on a hot sidewalk. As the black hole moves, the distant star would then become dim. Researchers used the Subaru Telescope to look at light coming from stars in the Andromeda Galaxy, searching for this brightening and dimming, but did not see the predicted effect, showing such primordial black holes do not exist in the quantities Hawking predicted.

A diagram showing how gravitational lensing from stars in the Andromeda Galaxy were expected to reveal the presence of primordial black holes. Image credit: Kavli IPMU

In order to view one of these gravitational lensing events, a star and primordial black hole must be aligned relative to the Earth — a rare alignment, only expected to last for periods lasting between a few minutes to a few few hours. The Hyper Suprime-Cam on the Subaru Telescope, capable of imaging the entire Andromeda Galaxy at one time, was used to maximize the chances of seeing these events.

The Hyper Suprime-Cam (HSC) mounted on the prime focus of the Subaru Telescope. Image credit: Subaru Telescope

The Experiment

“From 190 consecutive images of the Andromeda galaxy taken over seven hours during one clear night, the team scoured the data for potential gravitational lensing events. If dark matter consists of primordial black holes of a given mass, in this case masses lighter than the moon, the researchers expected to find about 1000 events. But after careful analyses, they could only identify one case,” researchers reported in a press release from the Subaru Telescope.

These results suggest that primordial black holes may only may up 0.1 percent of all dark matter. Whatever dark matter is, the answer does not seem to be miniature black holes, leaving this great mystery of physics intact.

The Cosmic Companion

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Writing about space since I was 10, still not Carl Sagan. Mailing List/Podcast:

The Cosmic Companion

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