A matter of gravity.

Will the universe expand forever or eventually collapse? The answer wouldn't seem to depend on the detailed appearance of an individual quasar system. But astronomers say the true nature of a double quasar may shed light on the question.

The double quasar Q2345+007 has intrigued scientists since its 1982 discovery. Debate continues about whether these twin beacons of light are two distinct quasars with similar spectra and locations or a cosmic mirage that originates from a single quasar. In the latter case, a massive object in front of the quasar would act as a gravitational lens, splitting or distorting the single beacon's light into two images.

Previous searchers for a cluster of galaxies that would constitute the lens came up empty-handed. But now astronomers using data from three telescopes have examined the double quasar and its surroundings with a sensitivity unmatched by surveys anyplace else in the sky. They report that they have found several massive candidates for the gravitational lens. If their circumstantial evidence proves correct, it suggests that massive, compact clumps of matter existed earlier in the universe than several cosmological models can easily explain.

The team of astronomers gathered some 22 hours of observations from telescopes in Hawaii, Chile, and Arizona. They say two findings support the notion that one or more clusters of galaxies act as a gravitational lens for the quasar.

Philippe Fischer and J. Anthony Tyson of AT&T Bell Labs in Murray Hill, N.J., and their colleagues found a higher than average density of faint galaxies in the vicinity of Q2345+007. They also detected an individual faint galaxy that lies near one of the two quasar images.

This galaxy may be responsible for the absorption of a narrow band of wavelengths in the spectra of the double quasar. If so, the galaxy would have to reside an estimated 9 billion light-years from Earth and would have formed when the cosmos was less than 40 percent of its current age. And if it is indeed part of a gravitational lens, the galaxy would belong to a cluster containing as much mass as 10 trillion suns.

But there's a cosmological rub. Several models for the universe's evolution can't explain massive clusters forming only a few billion years after the Big Bang. "It takes a long time to accumulate this much matter into so small a region," Fischer says.

In one popular cosmological model, the universe contains just enough matter that mutual gravitational attraction will eventually halt the expansion of the universe. The cosmos would teeter between an open universe, in which the expansion continues forever, and a closed universe, in which complete collapse is inevitable.

In this model, much of the unseen, or dark, matter that astronomers believe makes up most of the mass of the universe would take the form of exotic material rather than ordinary atoms. But even hypothetical material known as cold dark matter, which moves more slowly and clumps more easily than other exotic types, couldn't explain such early formation of galaxy clusters.

Instead, concludes the team, an open universe in which ordinary neutrons and protons make up most of the dark matter may best account for the team's preliminary findings.