UM students' curiosity yields clue to birth of universe Work backs theory of 'dark matter'

January 28, 1993|By Douglas Birch | Douglas Birch,Staff Writer

They parlayed curiosity about an oddly shaped galaxy into fresh evidence that up to 96 percent of the universe is made of a mysterious, invisible material called dark matter. And their findings support a leading theory about the birth, and ultimate fate, of the universe.

Not bad for a couple of College Park students still working on their Ph.Ds.

David S. Davis, 34, and John S. Mulchaey, 25, didn't expect to find anything more than a small gas cloud floating about 150 million light years from Earth: an interesting, though certainly not world-shaking, discovery.

"We wanted to get it done because it looked like a simple result that we could get quickly," said Mr. Davis, a Birmingham, Ala., native now living in Baltimore and working at the Goddard Space Flight Center in Greenbelt. "And it was. But it also turned out to be extremely exciting."

At a gathering of astronomers in Phoenix this month, Mr. Davis and Mr. Mulchaey reported that x-ray photographs from the European Roentgen Satellite, or ROSAT, found a massive cloud of hot gas -- 16 times wider than our Milky Way galaxy and weighing as much as 500 billion suns.

More important, the cloud was far too compact and too hot to be held together by the three galaxies it nearly envelops. So, the astronomers reasoned, the gas must be mired in a Sargasso Sea of dark matter 12 to 25 times heavier than the trio of galaxies nearby.

The discovery was welcomed by many astrophysicists.

"For the first time there is a large-scale measurement of dark matter where that ratio is in agreement with the prediction for the Big Bang," said Joseph Silk, an astronomer with the Center for Particle Astrophysics at the University of California at Berkeley.

"No astronomer should get too excited until they've measured several other galaxy groups," he cautioned. But if the same ratio of missing mass is found in other galaxy groups, he said, it will show that the cosmos is almost entirely made of a so-far unknown, invisible substance.

"Dark matter has been one of the biggest problems in astronomy for decades now," said Richard Griffiths, a professor of astronomy at the Johns Hopkins University. "A piece of evidence like the ROSAT observations is very important because it shows us that the dark matter can be very massive just around very normal-looking local galaxies. One implication of that is that there could be a lot of it just in our local group of galaxies, for instance."

Mr. Mulchaey, a lanky Californian who writes pop music in his spare time, lives in Baltimore and works at the Space Telescope Science Institute on the Johns Hopkins University campus.

He says the discovery came because he, Mr. Davis and two senior research partners looked hard for what the data from their observation actually showed -- rather than for what they expected them to show. (The senior astronomers are Dr. Richard F. Mushotzky of Goddard and Dr. David Burstein, a professor at Arizona State University.)

Dr. Burstein, Mr. Mulchaey recalled, kept telling him: "John, keep your eyes open. Look at the data."

"This discovery is an example of what he meant," Mr. Mulchaey said. "It was just a matter of looking."

Astronomers have long predicted the existence of dark matter, using it to explain why galaxies swirl through the sky as though there were a lot more gravity-producing matter out there than anyone sees with optical, radio, X-ray and other telescopes.

Some of this invisible stuff is probably just more or less ordinary matter too cool or compact to emit much radiation. These everyday quarks are locked up in black holes, or drifting around in brown dwarf stars, planets and debris.

But theoriticians figure most of the universe -- perhaps 96 percent -- is made up of stuff never before detected, given the droll name of "weakly interactive massive particles," or WIMPS for short. Candidate particles have exotic names, like "neutralino," "axion" and "photino."

An elementary question

Like ordinary matter, these fat particles create gravitational fields. Unlike most ordinary matter, they can pass undisturbed through people, planets or even stars as easily as a bowling ball through an open window.

"Scientists don't like dark matter," Mr. Mulchaey said. "But they've been forced to accept it because no other explanation works. How did Sherlock Holmes put it? 'When you have eliminated the impossible, whatever remains, however improbable, must be the truth.' "

Researchers using the Hubble Space Telescope and other instruments have weighed dark matter before, by watching its gravity bend the light from distant stars.

But most places they looked, scientists found more than 20 to 30 times less dark matter than predicted by Big Bang theorists.

Dark matter wasn't on Mr. Mulchaey's mind two years ago when he became curious about a galaxy group called NGC 2300, beyond the constellation Cepheus. He was intrigued by the shape of one galaxy that looked as if it were squashed slightly on one side -- like a cinnamon bun hurled into an invisible air bag.

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