Phantom energy would trigger the Big Rip. (Cosmic Doomsday Scenario).

Cosmologists have long speculated about the fate of the universe. Will it expand forever or collapse in a Big Crunch? In the latest model, published online last week, the universe instead ends with a Big Rip--every galaxy, star, planet, molecule, and atom torn asunder--21 billion years from now.

The cosmos killer in this scenario is dark energy, an invisible substance suspected of pervading the universe and exerting a force opposite to gravity's usual pull. Albert Einstein first proposed the notion of antigravity in 1917 and later abandoned it. Scientists have resurrected the idea of antigravity several times. Observations of distant supernovas reported in 1998 and more recently suggest that the universe is not merely expanding but doing so at an ever faster rate (SN: 4/7/01, p. 218). Because dark energy can turn gravity into a repulsive force, it could account for this acceleration.

If the density of dark energy is constant or slowly declining, the fate of the universe is simple to chronicle. As dark energy stretches space-time and pushes galaxies ever farther apart, it would create a diluted universe in which stargazers in the Milky Way billions of years from now would be too distant from other galaxies to view any of them. They would see only a desolate canvas (SN: 8/31/02, p. 139).

But if dark energy were dense enough, as considered in the recent analysis (http:/xxx.lanl.gov/abs/astro-ph/0302506) by Robert R. Caldwell of Dartmouth College in Hanover, N.H.; and his collaborators, the universe would have a far different fate. In this model, which Caldwell claims is consistent with the latest observations of the early universe, the density of dark energy grows as the universe does and constantly increases its repulsive force.

The end result would be a runaway expansion, or Big Rip, in which the universe would become infinitely large and time would effectively cease at age 35 billion years, according to calculations by Caldwell and Marc Kamionkowski and Nevin N. Weinberg of the California Institute of Technology in Pasadena.

A billion years before the very end, dark energy, which Caldwell calls phantom energy, would strip apart clusters of galaxies. The Milky Way would succumb about 60 million years before the Big Rip. A few months before the end of time, the dark energy content of the empty space between Earth and the sun would overwhelm the sun's pull, and Earth would float off into space. A half hour before the end, Earth itself would fall apart. Finally, one ten-thousandth of a femtosecond ([10.sup.-19] second) before the Big Rip, molecules and atoms would break up.

"The model is a logical possibility," says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. However, he says, it's more likely that dark energy density is constant. If so, galaxies would remain intact even as they rapidly recede from one another.