7. Dark Matters

Astronomers unveil a new map of the mysterious invisible stuff that makes up 90 percent of the universe.

By Tim Folger
Dec 12, 2007 6:00 AMNov 12, 2019 5:48 AM


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This magazine is made from some of the most exotic particles in the universe. So are you. The matter that makes up everything we can see or touch, either on Earth or beyond, is exceedingly rare, cosmically speaking. Most of the material in the universe is something called dark matter, mysterious stuff that doesn’t emit or reflect light and doesn’t interact with what we think of as ordinary matter. It reveals its presence only by its gravitational effects, guiding the evolution of the early universe and still affecting the motion of galaxies. Earth-based experiments have attempted to detect dark matter particles, but so far they have drawn a blank.

Astronomers, however, have had a better year, continuing to find evidence of the crucial role dark matter plays in shaping the visible cosmos. Thanks to about a thousand hours of observation by the Hubble Space Telescope, scientists have compiled a dark matter map of a tiny slice of the sky, about two square degrees of the entire sky’s 40,000-square-degree span. The map, which was published in the journal Nature last January, confirmed a central prediction of modern astrophysics: Galaxies formed in, and remain bound to, enormous clouds of dark matter.

In the early universe, astronomers believe, dark matter provided the gravitational scaffolding on which ordinary matter coalesced and grew into galaxies. According to these dark matter theories, as the visible galaxies formed, some of the matter surrounding them should have clumped together into hundreds of small satellite galaxies, most of which should survive today. But the observed number of satellite galaxies is only a fraction of what the theory predicts. “We should see about a hundred to a thousand, but up to 2005, there were only 12,” says Marla Geha, an astrophysicist at Yale University. Astronomers call it the missing satellite problem.

Astronomers had speculated that the existence of small, dark matter–dominated satellite galaxies might solve the problem, but there was no evidence that any such galaxies existed.

Last spring, Geha and Josh Simon, a colleague at Caltech, used the 10-meter Keck II telescope on Hawaii’s Mauna Kea to study the mass of eight newly discovered satellite galaxies, detected over the last two years by the Sloan Digital Sky Survey, an ongoing effort to make a detailed map of a million galaxies and quasars. Geha and Simon found that these satellite galaxies were much fainter and smaller in mass than the other known satellites—and 99 percent of their mass was in the form of dark matter. Given that the galaxies found by Geha and Simon have such high concentrations of dark matter, it’s likely that many other satellite galaxies could be 100 percent dark matter.

“We expect some to be undetectable, with no stars or gas,” says Geha. “There are indirect ways of finding the dark matter satellites, but it will take more work.”

Some astrophysicists believe that dark matter particles may occasionally annihilate each other, producing bursts of high-energy gamma rays. If the Milky Way has dark matter satellites, and if they do emit gamma rays, the Gamma-Ray Large Area Space Telescope, scheduled for launch in February, might detect them.

Dark matter may also be responsible for creating the most awesome objects in the universe: the enormous black holes believed to lurk in the center of nearly every large galaxy. Tom Theuns and Liang Gao, astronomers at Durham University in England, used a computer model last year to study how two types of dark matter, known as warm and cold, may have influenced the formation of the very first stars in the universe—and the first giant black holes.

In their simulations, Gao and Theuns found that within clumps of cold dark matter, single massive stars formed, but warm dark matter formed filaments about a quarter the width of the Milky Way, attracting enough ordinary matter to create some 10 million stars—and some of these very first stars could still be around. “You could potentially form low-mass stars,” says Theuns. “And they live very much longer. They could live for 13 billion years and could be in the Milky Way today. Maybe we’ve seen them already. Who knows?”

But the most unexpected result of the model was that the filaments could catastrophically collapse, warping space-time to form a huge black hole.

The model suggested that collapsing dark matter could warp space-time to form a huge black hole.

“Even if only 1 percent of the mass in a filament takes part in the collapse, that’s already 100,000 times the mass of the sun, a very good start to making one of these supermassive black holes,” Theuns says. “We know that the formation of these supermassive black holes has to be very rapid because we can see very bright quasars very soon after the Big Bang, not much later than the epoch of the first star formation.”

Is there any chance that astronomers could detect an echo of the primordial cataclysms that birthed these black holes?

“You would think it’s such a violent process that something would be left over from that,” Theuns says. “I don’t have any predictions, but you would think there would be something.”

See the related Web-exclusive feature: A (Dark) Matter of Time

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