Great Bubbles in the Cosmos

By MICHAEL D. LEMONICK

For all their skill at finding and analyzing such bizarre objects as black holes, neutron stars and quasars, astronomers have so far failed to solve one of the most basic mysteries of the cosmos: What does the universe look like? The heavens appear just as two dimensional through powerful modern telescopes as they did to the eyes of the ancient Greeks, and until recently, no one could say for sure whether the myriad galaxies were organized in some meaningful way. Astrophysicists are fiercely competing to discover how the universe evolved into its present structure, but they cannot test their theories until they know what that structure is.

Now astronomy's ignorance is rapidly being dispelled, thanks in large part to two researchers at the Harvard-Smithsonian Center for Astrophysics (CfA). Since 1985, Margaret Geller and John Huchra have been meticulously crafting a three-dimensional map that charts the positions of thousands of galaxies. Last week, in the journal Science, they presented their latest map of one small chunk of the visible universe, and the findings are startling.

Far from being a uniformly distributed collection of galaxies, as the textbooks have long assumed, the cosmos seems to be organized into immense bubbles, each of them about 150 million light-years across. The walls of the bubbles are galaxies, and the interiors appear to be virtually empty. Most surprising of all is a feature Geller and Huchra call the "Great Wall" -- a sheet of galaxies at least 200 million light-years wide, 500 million long and perhaps 15 million thick. It looks like a single structure, but the scientists say it may instead be made up of the walls of adjacent bubbles. Says Geller: "Because it runs off the edge of our survey, we don't know how big it really is."

The CfA study is not the first to see dark voids and large conglomerations of galaxies, but it is by far the most comprehensive. The reason no one had done such a search earlier, says Huchra, is that galaxy mapping is extremely time consuming. Their survey of 4,000 galaxies took about 1,000 hours of telescope time.

Huchra, who made the telescopic observations for the Harvard-Smithsonian team, used an instrument called a spectrograph to break down each galaxy's light into its constituent colors. Within the spectrum he could see lines representing various elements in and around the galaxy's stars. These lines appear to be shifted toward the red end of the spectrum, depending on how fast the galaxy is moving and thus how far away from earth it is. By carefully measuring the degree of red shift, Huchra and Geller calculated the relative positions of the galaxies.

The results are posing something of a problem for theorists. Says Jeremiah Ostriker, chairman of Princeton's astrophysics department: "There is no theory using conventional physics that can explain these structures without causing other inconsistencies." Ostriker has coauthored a quite unconventional scenario involving hypothetical objects called cosmic strings. These strings, he believes, could generate explosive bursts of energy that would in turn create the bubbles.

But another idea, called the cold dark matter theory, has gathered more support. This theory postulates an as yet undiscovered form of exotic subatomic particle that pervades the universe. The presence of this mysterious "dark matter" could explain why most galaxies -- including our Milky Way -- seem, judging from measurements of gravitational forces, to contain about ten times as much invisible matter as they do visible stars, gas and dust. The existence of dark matter is needed to fill the gaps in some of the Grand Unified Theories that physicists have concocted to account for the fundamental structure of matter and energy.

In particular, some scientists speculate that cold dark matter caused galaxies to form into the kind of bubbles Geller and Huchra have found. The process supposedly got under way 10 billion to 20 billion years ago, when the universe began with the Big Bang and the energy from that explosion started to condense into matter. Since then, ordinary visible matter, by itself, has probably not had time to gather into enormous structures. But cold dark matter may have condensed first, and its gravitational force could have helped pull visible matter into bubbles and galaxies. In fact, recent computer simulations at Princeton of a universe dominated by cold dark matter look remarkably like the real one.

But that theory received a jolt from another astronomical discovery announced this week. Scientists from Caltech, Princeton and the Institute for Advanced Study have detected the most distant quasar (an exceptionally bright starlike object) ever spotted. It is billions of light-years away, and the researchers estimate that it existed when the universe was only 7% of its present age. It is hard to explain how a quasar could be formed that early, even under the influence of cold dark matter.

Another major mystery is the fact that the faint glow of microwaves left over from the Big Bang is almost completely uniform. The presence of large bubbles in the universe suggests that this microwave radiation should be much more uneven. More clues may come from the new Cosmic Background Explorer satellite, which is designed to measure radiation intensities as it orbits the earth in the coming year.

In the meantime, the CfA study will go on, and other mapping efforts are in the works. "Big as it is," Geller explains, "our survey area compared with the visible universe is like Rhode Island compared with the surface of the earth." The bubbles and walls could be isolated phenomena. But, notes Geller: "Every survey ever done has contained structures as big as the survey could contain." If that trend continues, then there are larger objects yet to be found, which will give theorists even worse headaches. "These surveys test in the most acute way our conceptions of how structure developed in the universe," says Ostriker, "and for that reason they are possibly the most important studies in extragalactic astrophysics now. This is an exciting time to be in this field."

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CREDIT: M.J. GELLER, J.P. HUCHRA, E. FALCO, R.K. MCMAHAN

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