Science: Great Bubbles in the Cosmos

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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.