Mapping the universe then and now
Cosmic cartography gets a boost by looking at ever-more distant galaxies ever further back in time.
It's a slow, palm-moistening drive up the one-lane dirt road that clings to the steep contours of Mt. Hopkins, 35 miles south of Tucson, Ariz.
Creeping along in a small car, it's hard to imagine how a tractor-trailer truck bearing a fragile, 21-foot-diameter mirror could ever clear the hairpin turns to reach the glistening white telescope perched atop the 8,500-foot summit.
With its new, larger mirror to gather more light and some new instruments, the refurbished telescope is slated to become a powerful tool in one of modern astronomy's most ambitious efforts: to build 3-D maps of the observable universe. Using mountaintop telescopes worldwide, teams of astronomers are picking slices of the night sky and mapping as many galaxies as weather, instruments, and funding allow. Underlying these efforts is a "drive to explore and write your address in the long form. The first step in knowing or understanding is to have a map. Nearly everything we do in science, we first need a picture," explains Margaret Geller, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CFA) in Cambridge, Mass., which oversees the observatory on Mt. Hopkins. It has been a center for cosmic cartography for more than 20 years.
Looking at ever-more distant objects in the universe means looking back in time, so the maps, and the information needed to build them, contain a wealth of clues about the structure and evolution of the universe.
Last week, for example, a team of astronomers using the 3.9-meter Anglo-Australian Telescope in New South Wales, Australia, released a 3-D map containing more than 100,000 nearby galaxies, a population four times larger than any previous survey. Their goal is to gather data on 250,000 galaxies.
Like its predecessors, the new survey shows galaxies gathered into enormous superclusters stretched out into thin corridors of light separated by vast, empty regions as much as 200 million light-years across.
"To see the scales of structure is interesting, but it's not telling us anything new. The real objective is to measure clusters of galaxies at high precision and in a lot of detail," says collaborator David Weinberg, an associate professor of astronomy at Ohio State University.
Such data, he continues, can provide critical tests of existing theories or open up new views of what the early universe was like, how galaxies and their stars form and evolve, and help refine values for such critical cosmological factors as the density of matter in the universe.
Using data from the new survey, the project's researchers concluded that all of the matter in the universe - the "normal" matter we can see because it gives off radiation, and the "dark" matter we don't see, but which keeps galaxies and clusters of galaxies from flying apart as they rotate - falls far short of what's needed to halt the universe's expansion.
The results, according to Johns Hopkins University astrophysicist Karl Glazebrook, point to a universe in which two-thirds of the energy driving the expansion come from "vacuum energy," a form of energy predicted by quantum physics. Under this scenario, he notes, the universe will expand forever, and at an increasing speed.
Efforts to map the distribution of galaxies began in the 1950s, when it was still fashionable to call galaxies nebulae. Follow-up surveys found clusters of galaxies and the local supercluster, but the surveys weren't large enough or deep enough to reveal anything larger, Dr. Geller explains.
Then in the 1970s, a team including Robert Kirschner, also of the Center for Astrophysics, conducted a survey that uncovered something odd: an empty region larger than the area covered by the largest survey to date.
"Most people, with the great wisdom of knowing the answer before they begin, thought that there must be something wrong" with what Dr. Kirschner and company did, Geller recalls. "It turned out that the reason they found this is not that they were wrong, it was that these big regions are all over the place."
Discovery of the void led Geller, colleague John Huckra, and graduate student Valeria de Lapparent to mount another, large survey over slices of the sky and encompassing 1,700 galaxies. "We were prejudiced against finding a pattern, so much so that we didn't urge Valerie to plot her data before they were all in hand; we were pretty sure there wouldn't be anything remarkable," Geller recalls. "Fortunately, nature was not subtle. We saw this amazing 'stickman' pattern. We saw that galaxies outline vast dark regions, as though they're sitting on the surface of enormous bubbles."
Since then, the CFA has extended its survey to cover 15,000 galaxies in the northern sky, and "several thousand" in the southern sky, giving us a good picture of how galaxies are distributed nearby, she says.
The CFA survey also spawned succeeding generations of maps and mapping teams. The Sloan Digital Sky Survey, which began taking data two years ago and is run by a consortium of five US universities for collaborators in the US, Japan, and Europe, aims to map one-fourth of the night sky, gathering detailed data on more than 100 million celestial objects, including 1 million of the nearest galaxies. While many of these objects reside in our own galaxy, the 1-million-galaxy survey will cover a volume of space 100-times larger than any explored to date.
Another consortium is combining the light-gathering capability of the 10-meter Keck telescope in Hawaii and the above-the-atmosphere clarity of the Hubble Space Telescope to build maps of the universe at its earliest stages of galaxy formation.
With the universe's current epoch increasingly well covered, and the universe's infancy about to come under scrutiny, Geller and colleagues are now taking aim at the great gap in the middle, what Geller terms the universe's adolescence. Based on what researchers think they've gleaned so far about the universe's evolution, this should be a transition period from the early, relatively even distribution of matter implied by the nearly smooth distribution of the cosmic microwave background radiation to the bubbles and voids we see today.
One obvious question: If the CFA survey uncovered larger structures than astronomers have ever seen, might there be anything larger laying in wait to be discovered? Based on the Australian 3-D map, the answer seems to be no. But the intervening epochs are likely to be rich in information on how galaxies form and how that formation was shaped by the environments they inhabited.
"Humans are wired to appreciate natural structures and to want to understand what they mean and how they originate," Geller says, adding that if technology continues to improve at its current rate, astronomers "will have a map of the visible universe in about the year 2100."
(c) Copyright 2000. The Christian Science Publishing Society