Big, Bigger . . . Biggest?

Galaxy map reveals the limits of cosmic structure

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 bird’s-eye view of the cosmos, as mapped by the 2df survey. Matthew Bailes, et al./Swinburne University
The cosmic web revealed by the 2df Galaxy Redshift Survey, which to date has mapped more than 100,000 galaxies. Each slice is about 75 across. Angular scale is measured in hours (h). Peder Norberg and Shaun Cole

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.

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 Glazebrook 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 2º 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: Available to subscribers at Survey confirms composition of the cosmos). 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.”

Fluctuations in density

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: Balloon Sounds Out the Early Universe).

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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