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That means that the 100 billion or so galaxies we can now see through our telescopes will zip out of range, one by one. Tens of billions of years from now, the Milky Way will be the only galaxy we're directly aware of (other nearby galaxies, including the Large Magellanic Cloud and the Andromeda galaxy, will have drifted into, and merged with, the Milky Way).
By then the sun will have shrunk to a white dwarf, giving little light and even less heat to whatever is left of Earth, and entered a long, lingering death that could last 100 trillion years--or a thousand times longer than the cosmos has existed to date. The same will happen to most other stars, although a few will end their lives as blazing supernovas. Finally, though, all that will be left in the cosmos will be black holes, the burnt-out cinders of stars and the dead husks of planets. The universe will be cold and black.
But that's not the end, according to University of Michigan astrophysicist Fred Adams. An expert on the fate of the cosmos and co-author with Greg Laughlin of The Five Ages of the Universe (Touchstone Books; 2000), Adams predicts that all this dead matter will eventually collapse into black holes. By the time the universe is 1 trillion trillion trillion trillion trillion trillion years old, the black holes themselves will disintegrate into stray particles, which will bind loosely to form individual "atoms" larger than the size of today's universe. Eventually, even these will decay, leaving a featureless, infinitely large void. And that will be that--unless, of course, whatever inconceivable event that launched the original Big Bang should recur, and the ultimate free lunch is served once more.
Astronomers and physicists are a cautious crew, and they insist that the mind-bending discoveries about dark matter, dark energy and the flatness of space-time must be confirmed before they are accepted without reservation. "We're really living dangerously," says Chicago's Turner. "We've got this absurd, wonderful picture of the universe, and now we've got to test it." There could be surprises to come: an Einstein-style cosmological constant, for example, is the leading candidate for dark energy, but it could in principle be something subtly different--a force that could even change directions someday, to reinforce rather than oppose gravity.
In any case, new tests of these bizarre ideas will not be too long in coming. Next week a satellite will launch from Cape Canaveral to make the most sensitive observations ever of the cosmic background radiation. Supernova watchers, meanwhile, are lobbying NASA for a dedicated telescope so they won't have to queue up for time on the badly oversubscribed Hubble. And lower-tech telescopes and microwave detectors, both on the ground and lofted into the air aboard balloons, will continue to refine their measurements.