Galaxy map reveals dark business as usual.

The most precise map of galaxies has confirmed that much of the cosmos is in the dark. The map, which covers 6 percent of the sky and includes 200,000 galaxies recorded by the Sloan Digital Sky Survey, supports previous evidence that most of the universe's matter is invisible. Without the gravity provided by this unseen material, dubbed dark matter, galaxies wouldn't have clustered as tightly as the Sloan map indicates they do.

By combining the galaxy map with the newest images of the cosmic microwave background, the relic radiation left over from the Big Bang, astronomers say they have also confirmed the existence of something even stranger. That stuff, known as dark energy, opposes gravity's usual tug, pushing objects apart and causing the universe to expand at an ever-faster rate (SN: 10/11/03, p. 227).

Temperature variations within the cosmic microwave background, as recorded by the Wilkinson Microwave Anisotropy Probe (SN: 2/15/03, p. 99), represent tiny lumps in the otherwise smooth soup of the infant universe. These lumps were the seeds from which galaxies arose. By comparing the size of these lumps with that of the vast clusters of galaxies in the Sloan map, astronomers have pinned down several key measures of the universe to an unprecedented accuracy.

Max Tegmark of the University of Pennsylvania in Philadelphia and a large team of collaborators find that the universe consists of 5 percent ordinary matter, 25 percent dark matter, and 70 percent dark energy. The new analysis puts the age of the universe at 14.1 billion years, give or take a billion years. The researchers announced the results Oct. 28 in two articles posted on the Internet (http://xxx.lanl.gov/abs/astro-ph/0310725, http://xxx.lanl.gov/abs/astro-ph/0310723).

"I've always felt very uneasy about dark energy and dark matter, despite all the papers I've written about them," says Tegmark. "Now, I feel I have to accept them."

Comments David N. Spergel of Princeton University: "I think that this is an important test of the emerging standard model," in which dark matter and dark energy are both required to explain the evolution of the cosmos.

The map's most eye-catching feature is the Sloan Great Wall of galaxies, a clustering of galaxies that stretches 1.37 billion light-years across the sky and is the largest cosmic structure ever found. Astronomers worried that such a humongous structure, 80 percent bigger than the famous Great Wall of galaxies first discerned in a sky survey 2 decades ago, might violate the accepted model of galaxy evolution. But modeling by Tegmark, J. Richard Gott of Princeton, and their coworkers reveals that such structures arise in about 15 percent of the computer simulations.

"It's a rare feature, but not embarrassingly so," says Gott, whose team recently posted its study's results on the Internet at http://xxx.lanl.gov/abs/astro-ph/ 0310571.