Cosmic lace. By analyzing the clustering of galaxies (dots on map at left), SDSS astronomers confirmed that dark energy is expanding the universe.

The Universe Is Still Exploding

Just half a decade ago, astronomers came to the shocking conclusion that the universe is being pulled apart by a mysterious force called dark energy. That strange picture became even clearer earlier this year, when the Wilkinson Microwave Anisotropy Probe (WMAP) revealed the most detailed pictures of the infant universe (ScienceNOW, 22 July). Now, two papers released by a consortium of cosmologists with the Sloan Digital Sky Survey (SDSS) have provided an independent means of estimating how much dark energy the universe holds and how fast the cosmos is expanding.

The SDSS is gathering evidence by mapping galaxies in a large section of the sky. The survey, which aims to chart a million by 2006, was designed to give astronomers a handle on the way galaxies cluster in space, a pattern that reveals the hidden influences (such as dark matter and dark energy) that drive cluster formation. Because galaxy surveys such as SDSS analyze clusters of galaxies (which are fairly young, cosmologically speaking), they provide a much more recent snapshot than microwave-background fluctuations do. But it takes a lot of galaxy-cluster observations to develop a decent picture.

Broadly speaking, the SDSS team uses a mathematical technique similar to the method microwave astronomers use: They generate what is known as a "power spectrum," a bumpy graph that represents the abundance of different-sized features in the sky (Science, 31 May 2002, p. 1588). And just as the WMAP team did, the SDSS team can use its power spectrum to derive "cosmological parameters" such as the amounts of ordinary matter, exotic matter, and dark energy in the universe. Two papers, both available on the arXiv preprint server (, do just that with the first quarter-million galaxies. "I think the news here is that there has been a convergence," says Sloan team member Michael Strauss, a physicist at Princeton University. "It's all pointing in the same direction."

Even after the researchers exclude all of WMAP's data from the joint analysis of cosmological parameters, they get the same results that the WMAP team has been getting: The universe is about 14 billion years old, flat, and dominated by dark energy. "The cosmic model stands tall," says University of Pennsylvania physicist and Sloan team member Max Tegmark. "It's depressing if you would like to see everything go down in flames."

Related sites
Sloan Digital Sky Survey
Wilkinson Microwave Anisotropy Probe