Galaxy map reveals the limits of cosmic structure

Science News,  August 12, 2000  by Ron Cowen

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The same fluctuations in the density of the early universe that are thought to have produced the largest collections of galaxies also made their mark on the cosmic microwave background. This whisper of radiation left over from the Big Bang has a nearly uniform temperature of 2.7 kelvins. However, it contains hot spots and cold spots that reveal the seeds of cosmic structure (SN: 4/29/00 p. 276).

The cosmic microwave Background provides a snapshot of the early universe. Large-scale galaxy maps, such as the 2dF, provide a more recent look at the cosmos. Both enable astronomers to study the blueprint for the universe generated in the Big Bang, says Kirshner. The two approaches both indicate that gravity alone created the large-scale structure of the cosmos.

"It would be a big surprise and a great challenge to our understanding of the growth of structure through gravitation if there were a real feature in the galaxy distribution that corresponds to scales of a billion light-years," says Kirshner. Structures of that size would imply that something other than gravitation was at work to create the universe's features. "So the end of greatness--and the beginning of `dullness'--is important."

An even larger map of the cosmos, the Sloan Digital Sky Survey, is now under way and should provide further evidence for the end of greatness, says Turner, a member of the Sloan team. When that project is completed in about 5 years, it will have surveyed 1 million galaxies over 25 percent of the sky.

Even after Sloan, the universe will harbor its share of mysteries, Turner adds. To explain how the universe evolved from an almost smooth soup to a lumpy patchwork, for example, astronomers hypothesize that more than 90 percent of the matter in the cosmos is made of some exotic substance that doesn't emit light but exerts a gravitational pull.

In their models, theorists often employ a hypothetical version of this stuff--known as cold dark matter--which moves slowly and readily coalesces under the influence of gravity. However, even though computer simulations suggest that cold dark matter forms filaments, the resulting structures end up fatter than those observed with telescopes, Turner says.

As the universe continues aging and gravity keeps pulling material together, the largest structures in the cosmos should grow bigger, Frenk notes. Superclusters will remain the largest structures in the universe, but the superclusters of the future will make today's look downright diminutive.

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