As they try to figure out how this strange behavior could be happening to the universe today, astronomers say the ultimate prize from all the new observing projects could be as simple as a single number.

That number, known as w, is the ratio between the pressure and density of dark energy. Knowing this number and how it changes with time -- if it does -- might help scientists pick through different explanations of dark energy and thus the future of the universe -- ''whether it's gonna lead to a Big Rip, a Big Collapse or just a Big Fizzle,'' as Dr. Adam Riess of the Space Telescope Science Institute in Baltimore put it in an e-mail message.

One possible explanation for dark energy, perhaps the sentimental favorite among astronomers, is a force known as the cosmological constant, caused by the energy residing in empty space. It was first postulated back by Einstein in 1917. A universe under its influence would accelerate forever.

While the density of energy in space would remain the same over the eons, as the universe grows there would be more space and thus more repulsion. Within a few billion years, most galaxies would be moving away from our own faster than the speed of light and so would disappear from the sky; the edge of the observable universe would shrink around our descendants like a black hole.

But attempts to calculate the cosmological constant using the most high-powered modern theories of gravity and particle physics result in numbers 1060 times as great as the dark energy astronomers have observed -- big enough, in fact, to have blown the universe apart in the first second, long before even atoms had time for form. Theorists admit they are at a loss. Perhaps, some of them now say, Einstein's theory of gravity, the general theory of relativity, needs to be modified.

Another possibility comes from string theory, the putative theory of everything, which allows that space could be laced with other energy fields, associated with particles or forces as yet undiscovered. Those fields, collectively called quintessence, could have an antigravity effect. Quintessence could change with time -- for example, getting weaker and eventually disappearing as the universe expanded and diluted the field -- or could even change from a repulsive force to an attractive one, which could set off a big crunch.

Recently, in a variation on the quintessence idea, Dr. Leonard Parker of the University of Wisconsin at Milwaukee and various colleagues, including Dr. Caldwell, have suggested that the field associated with some unknown very light particle could get tangled up with gravity and cause the universe to accelerate. That would alter Einstein's equations, said Dr. Caldwell. He added, ''Our calculations show, however, that galaxies reside in a bubble of old-fashioned Einstein gravity, whereas gravity has changed outside and between galaxies.''