Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: SUNDAY, January 17, 1993 TAG: 9301170017
SECTION: NATIONAL/INTERNATIONAL PAGE: A-13 EDITION: METRO
SOURCE: The Baltimore Sun
DATELINE: BALTIMORE LENGTH: Medium
FINDINGS SUPPORT `BIG BANG'
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.Moreover, their findings support a leading theory about the birth, and fate, of the universe.
Not bad for a couple of graduate students still working on their doctorates.
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.
"We wanted to get it done because it looked like a simple result that we could get quickly," said Davis, who works at the Goddard Space Flight Center near Baltimore. "And it was. But it also turned out to be extremely exciting."
Mulchaey works at the Space Telescope Science Institute at Johns Hopkins University in Baltimore.
At a gathering of astronomers in Phoenix this month, Davis and 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.
"Dark matter has been one of the biggest problems in astronomy for decades now," said Richard Griffiths, a professor of astronomy at 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."
Astronomers have long predicted the existence of dark matter, using it to explain why galaxies swirl through the sky as though there is 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 theorists 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.
Like ordinary matter, WIMPS particles create gravitational fields. Unlike most ordinary matter, they can pass undisturbed through people, planets or even stars.
"Scientists don't like dark matter," 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 many times less dark matter than predicted by Big Bang theorists.
Dark matter wasn't on 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 like it was squashed slightly on one side.
"The idea we had was that it was bumping into something you couldn't see in the visible light pictures, that it was probably running into a cloud of hot gas," Mulchaey said. But when they got ROSAT pictures, the researchers were astonished to find a gigantic gas cloud enveloping the puny galaxies.
They also found that the ratio of dark matter to ordinary stars in NGC 2300 fit neatly into predictions of the ratio of dark matter to ordinary matter made by the "inflationary" Big Bang theory of the universe's creation.
The inflation theory says that for an unimaginably brief period during the first second of the universe's existence, it suddenly expanded a million, trillion trillion times. It also predicts that there is precisely enough matter floating in the universe to "close" it, or slow and ultimately stop its expansion. With any less matter, the universe would expand forever.