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Galaxy map reveals the limits of cosmic structure

Science News, August 12, 2000 by Ron Cowen

Stars congregate in galaxies, galaxies in clusters, and clusters in gargantuan assemblages called superclusters. This cosmic hierarchy prompts a question that astronomers have pondered for nearly 2 decades: Have we already discerned the largest structures in the universe, or are there greater things to come?

A new map of the heavens--the largest galaxy survey to date--reveals that superclusters are indeed the biggest objects in the universe. Instead of being disappointed, cosmologists say they're thrilled to find evidence that structure in the cosmos maxes out.

The finding offers graphic proof that the universe has evolved according to a prescription widely accepted by astronomers, says one of the cosmic cartographers, Carlos S. Frenk of the University of Durham in England.

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According to that model, the universe began as an almost perfectly smooth soup of elementary particles. The soup had some tiny lumps, however--places where the density was slightly higher than average. Other regions had a slightly lower density than average. Over time, notes Frenk, gravity amplified the lumps and they became the seeds from which galaxies, clusters of galaxies, and superclusters assembled.

The largest structures that gravity could have built since the birth of the universe some 13 billion years ago are indeed the largest collections of galaxies depicted by the survey.

In fact, the new survey reveals that galaxies never come in any cosmic package larger than about 250 million light-years in length, the size of a few superclusters. In comparison, the Milky Way is about 100,000 light-years across. Had astronomers found larger structures, "we would have had to conclude that the standard picture that we have, ... that the seeds grow by gravity, was substantially in error," says Frenk.

Frenk's colleagues, including Karl Glaze-brook of Johns Hopkins University in Baltimore, reported the findings in June at a meeting of the American Astronomical Society in Rochester, N.Y. The researchers base their conclusions on an ongoing heavenly census known as the 2dF Galaxy Redshift Survey, named for the 2 [degrees] fields of view being examined with a pair of spectrographs at the Anglo-Australian Telescope in Coonabarabran, Australia. The survey encompasses about one-twentieth the area of the entire sky.

To record galaxies, a robot arm at the telescope precisely positions up to 400 optical fibers at a time. Simultaneously, each fiber collects light from a portion of the telescope's mirror corresponding to a different galaxy. The fibers shunt the light into the spectrographs, which on a clear night map the position of hundreds of galaxies in a single hour.

As of July, the 2dF survey had mapped more than 100,000 galaxies--four times any previous survey--out to a depth of 4 billion light-years (SN: 6/10/00, p. 374). Astronomers expect the census to encompass 250,000 galaxies by the time researchers complete it next year.

The map, like other large-scale galaxy surveys, reveals a cosmos similar to a mammoth web. Superclusters of galaxies lie bunched in walls or form long filaments. Each of these structures is separated by vast voids.

For years, galaxy mappers kept finding bigger and bigger pieces of the cosmic web, notes David N. Spergel of Princeton University. The larger the survey, the larger the structure.

During the 1980s, cosmologists concentrated on the most extraordinary features they could find in galaxy surveys. In 1981, for example, Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his colleagues announced they had found the biggest empty region ever detected in space. They dubbed the region, located in the constellation Bootes, the Great Void.

A few years later, a research team led by Margaret J. Geller and John P. Huchra of Harvard-Smithsonian found something with more substance: the Great Wall. The length of this collection of galaxies measures several hundred million light-years (SN: 11/25/89, p. 340).

The first survey with a volume big enough to contain several walls and voids was the Las Campanas Redshift Survey, which astronomers completed in the mid-1990s. Analyses of data from the survey, which mapped about 25,000 galaxies, left the impression that there were no structures larger than the Great Wall and Great Void, Kirshner says.

"I took to calling this `the end of greatness'--by which I meant that we had finally done an adequate survey to find the largest structures in the universe," says Kirshner. He realized, however, that the Las Campanas survey still encompassed too small a sample to prove that astronomers had found the limits of cosmic structure.

The 2dF survey, which covers a bigger swath of sky, is the first map large enough to definitively demonstrate that the voids and walls have a limited size, says Spergel.

"Hats off to 2dF!" says cosmologist Michael S. Turner of the University of Chicago. "We can be quite confident now that we have seen the largest structures."

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Galaxy map reveals the limits of cosmic structure

Science News, August 12, 2000 by Ron Cowen

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