Evidence for a distant galaxy cluster.

Astronomers say they may have found the most distant cluster of galaxies ever observed in the universe. Because-a telescope acts like a time machine, detecting distant objects as they appeared when the universe was young, the researchers could see what these faraway galaxies looked like when the universe was roughly one-tenth its present age.

The tentative finding has profound implications for theories about the evolution of the universe. If galaxy clusters did exist in the infant universe, then galactic structure may have begun developing earlier and matured over a longer period than some popular models suggest. Instead of assembling galaxies and galaxy clusters from small, primordial subunits, the universe may have evolved from the breakup of immense, pancake-like structures a thousand trillion times the mass of the sun. And astronomers might have to revise their notion of dark matter -- hypothetical, invisible material that makes its presence known through its gravitational tug.

Two teams of scientists contributed to the new report. With the 4-meter telescope at Cerro Tololo Inter-American Observatory in La Serena, Chile, one team used light from the distant quasar QSO 0000-263 as a beacon to search for primeval galaxies. In analyzing the spectrum of the quasar light, Charles C. Steidel of the Massachusetts Institute of Technology and Donald Hamilton, then at the California Institute of Technology in Pasadena, found that an object in front of the quasar -- a candidate primeval galaxy -- absorbed particular wavelengths of the light. The absorption pattern indicated that this foreground galaxy has a redshift of 3.4. This means that the galaxy -- according to one cosmological model -- lies more than 14 billion light-years from Earth.

Using special color filters that reveal galaxies located 14 to 14.5 billion light-years from Earth, Steidel and Hamilton imaged the primeval galaxy. The filters also enabled them to find 15 other galaxies residing somewhere in the same distance range. If these galaxies actually lie close to one another, rather than at varying distances in the same patch of sky, then they form the most distant cluster ever found, Steidel says.

In an independent study, E Duccio Macchetto and Mauro Giavalisco of the Space Telescope Science Institute in Baltimore examined the vicinity of the same quasar with the European Southern Observatory's 3.6-meter telescope in La Serena. Rather than look for absorption features, this team searched for a telltale hydrogen emission from a galaxy near the quasar. They found one such galaxy. In recording the galaxy's emission spectra, Macchetto and Giavalisco discovered that it is distinct from, but has the same redshift as, the light-absorbing galaxy found by Steidel and Hamilton.

These two galaxies thus lie at the same distance from Earth and in the same patch of sky, proving that they are closely associated. But two galaxies do not a cluster make. It remains uncertain whether the other galaxies found by Steidel and Hamilton are part of the same grouping. Borrowing a statistical technique developed by Alexander S. Szalay of Johns Hopkins University in Baltimore, Macchetto and Giavalisco calculate that there is only a 1.2 percent chance that the other galaxies could occupy the same patch of sky without being in close proximity.

However, Stanislav G. Djorgovski of the California Institute of Technology notes that the astronomers have only measured the exact redshifts, and thus the actual distances, of two galaxies. The filters used to estimate the distance of the other 14 galaxies allow too much leeway to even suggest that these galaxies are associated, he adds. "I personally tend to think there is a cluster there, but there's no evidence [right now] to support this," he says. Steidel says the findings, though tentative, "are certainly intriguing enough to alert astronomers."