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The Boston Globe OnlineBoston.com
Boston Globe Online / Health | Science
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Shedding light on dark matter

By Philip Plait, Globe Correspondent, 12/31/2002

Scientists, as a rule, enjoy mysteries. But perhaps the biggest mystery in astronomy has been more frustrating than fun: Astronomers are unable to detect most of the matter in the universe. ''This is embarrassing,'' said Claude Canizares, a professor of physics at the Massachusetts Institute of Technology who is researching the problem. ''Ninety percent of the universe is dark and cannot be seen!''

When we gaze upon the night sky, we see bright stars by the thousands, and many millions or even billions more through telescopes. We can find gas clouds, planets, even whole galaxies glowing brightly. But this visible stuff is only a tiny fraction of the total amount of matter in the universe: The rest of the cosmos is made up of ''dark matter'' that gives off either very little or no detectable forms of energy, including visible light.

Dark matter has been sought after, puzzled over, and vehemently argued about for the past 70 years; but, just last summer, a breakthrough was made, giving scientists strong evidence of its presence. The answer to one of the oldest standing riddles in astronomy finally may be revealing itself.

It was long assumed that the majority of matter in the universe is inside galaxies, which are made mostly of stars and luminous gas. Astronomers would ''weigh'' a galaxy by counting up all the light it emitted and assuming that its stars were like our own sun. If the galaxy gave off a billion times as much light as the sun, then the galaxy's mass must be one billion times that of the sun.

Observations of a cluster of galaxies in the 1930s, however, revealed a problem: The galaxies were moving more rapidly than could be explained by the calculated mass of the cluster. The only way the cluster could still be together after billions of years was if either the cluster had far more mass than could be seen, or if there were some fundamental problem with our understanding of gravity. Since the physics of gravity was thought to be well understood, astronomers decided that there must be matter that couldn't be seen - dark matter.

By the 1970s, the invisible reach of dark matter was seen in so many different cosmic venues that few professional astronomers seriously doubted its existence. But the dark matter itself was maddeningly shy, never showing its face directly.

That is until last summer. In July 2002, four teams of astronomers, all using NASA's orbiting Chandra X-ray observatory, announced that they had found strong evidence for the existence of dark matter. These teams, from MIT, the Harvard-Smithsonian Center for Astrophysics, Ohio State University, and the University of Michigan, observed the effects of dark matter on gas flows throughout the universe.

For years, well-tested theories and computer models strongly indicated that dark matter should form vast, thin filaments throughout the universe, like arteries permeating the human body. According to Canizares, a member of the MIT team that made the discoveries, ''The bulk of normal matter is in these filaments.'' It was distributed this way as matter condensed from the primordial fireball of the Big Bang, the explosive event from which the universe was created.

These filaments of matter possess a strong gravity field, and draw in the gas that fills intergalactic space. Like a fog bank filling in a low-lying valley, the gas eventually accumulates along the tendrils of dark matter.

Also like fog, the dark matter filaments and their attendant gas affect the light that comes from even more distant X-ray-emitting galaxies, absorbing it and making the galaxies look dimmer. By carefully measuring the amount of dimming of these distant galaxies in the Chandra observations, astronomers can determine the composition and temperature of the hot gas in the filaments. The hot gas, in turn, maps out the location of the dark matter component of those filaments, allowing astronomers for the first time to ''see'' where the dark matter is.

''The Chandra observations ... are a major advance in our understanding of how the universe evolved over the last 10 billion years,'' Fabrizio Nicastro, leader of the Harvard-Smithsonian Center for Astrophysics, stated in a press release.

There have been challenges to dark matter theory. In 1983, an alternative theory was proposed by Mordechai Milgrom of the Weizman Institute of Israel. Called MOND, for Modified Newtonian Dynamics, this theory supposes that scientists' understanding of gravity is incomplete. MOND assumes that at very low levels - some 100 billionth the strength at the Earth's surface - gravity behaves oddly, weakening more slowly than it does in standard theories.

But yet another new Chandra result published in 2002 has weakened the MOND theory considerably. A team of astronomers led by David Buote at the University of California at Irvine used Chandra to observe a hot halo of gas surrounding the galaxy NGC 720. Both the dark matter and MOND theories can explain why the gravity of the galaxy is stronger than it appears. But a critical difference between the two theories is their predictions for the shape of the halo: MOND predicts it to have the same shape as the galaxy, while dark matter theory allows the halo to be shaped differently.

What Buote's team found was that the shape of the halo was significantly different than the shape of the galaxy, contrary to the MOND predictions. This gives astronomers more confidence that, in fact, most of the universe is indeed comprised of invisible stuff.

The effects of dark matter go beyond keeping galaxies from flying apart and painting the universe with tendrils of gas. The dark matter coalesced into a network of filaments shortly after the universe formed. This makes that network possibly the oldest structure in the universe. By investigating the dark matter and hot gas in the filaments, astronomers are like cosmic arch eologists, looking at ancient remains. As they continue studying dark matter, they hope to learn more about the time when the universe was young, and why the universe looks the way it does today.

This story ran on page D17 of the Boston Globe on 12/31/2002.
© Copyright 2002 Globe Newspaper Company.

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