A proposal by Carnegie Mellon University to use X-ray telescopes to measure the dark energy that makes up most of the universe was selected yesterday by NASA as one of five candidate missions for the space agency's Explorer program.

The proposal by Carnegie Mellon astrophysicist Richard Griffiths, called the Dark Universe Observatory, would include a spacecraft equipped with seven identical X-ray telescopes and would be launched in 2007. Griffiths estimates the mission would cost $153 million.

By NASA's standards, that's a low-cost mission that can be rapidly mounted -- just the sort of project targeted by its Explorer program. Griffiths will receive $450,000 to develop the mission plans over the next five months.

Next summer, NASA will select two of the five candidates for development as Small Explorer missions.

The Dark Universe Observatory mission proposed by Griffiths is designed to explore further the most dramatic discovery in astrophysics of the past decade: that the expansion of the universe is speeding up.

The universe has been expanding ever since the Big Bang that most physicists believe began its existence. Astrophysicists used to debate whether gravity would eventually slow the expansion, or whether it would continue to infinity. But measurements based on the distances between exploding stars, called supernovae, have shown that the rate of expansion is accelerating, suggesting some previously unsuspected force, dubbed dark energy, is actually countering gravity's pull.

The Dark Universe Observatory mission would use X-ray telescopes to detect 10,000 clusters of galaxies in the universe and to plot their positions precisely, creating a three-dimensional map of the universe. This map would provide an independent measurement of dark energy, Griffiths said.

The mission also would provide information on the rate at which galactic clusters are being formed, Griffiths said. That, in turn, might provide some hints about the nature of dark energy -- what physicists call its "equation of state."

Scientists simply don't know how dark energy behaves, he explained, "because we don't know what it is." Gauging its effects on the formation of clusters of galaxies might provide some hints, he suggested.

The mission would make use of an X-ray telescope design originally developed for a German satellite called ABRAXIS. That spacecraft was launched several years ago, but a faulty power supply caused it to fail before making any observations.

The design and other components remain, however, so it would be relatively cheap to build duplicate telescopes. The Dark Universe Observatory mission also would use X-ray detectors originally designed for the European Space Agency's X-ray Multi-Mirror telescope. And because Germany's Max Planck Institute for Extraterrestrial Physics would pick up some of the cost, the $153 million mission would cost NASA just $132 million, Griffiths said.

Other Explorer candidate missions were proposed by investigators at NASA's Goddard Space Flight Center in Greenbelt, Md.; the Southwest Research Institute in San Antonio, Texas; the California Institute of Technology; and the University of Colorado.