eventualities the "open" and "closed" universes, respectively. An open universe continues onward much as our universe today for many billions of years, until everything simply burns out. The cosmic fuel tank of hydrogen, after all, is exceedingly large but not infinite. A closed universe ends in an all-encompassing smash-up of matter--an inverse Big Bang that we could name the "big squeeze." Such an event might even lead to a new Big Bang, but nothing you recognize today would survive the transition from one universe to the next.

We'll eliminate the suspense: It looks more and more likely that we live in an open universe. For years the debate raged about whether there is enough mass to slow and eventually halt the Hubble expansion. The latest tallies have shown that the answer is no. All the visible matter and dark matter in the universe appear to add up to no more than 30 to 40 percent of the amount needed. Whatever process forged our supply of matter in the Big Bang clearly did not do so with the intent of making the universe fall back into a fiery point.

Recently, another curious finding made it appear even more likely that our cosmos faces an expansive future. Certain supernova explosions serve as excellent "standard candles" in the universe. Like a succession of 100-watt lightbulbs on a row of porches, these supernovas nearly match one another in luminosity no matter where they pop off. It's then straightforward to figure out how far away they are. If one supernova is nine times fainter than another, it's about three times farther away, and so on--the simple "distance squared" formula for brightness that also applies to shining objects on Earth. In this way researchers found that they could gauge the distances to many galaxies scattered throughout the universe by watching for supernova blasts. (One goes off somewhere in the universe every second, so large telescopes detect supernovas regularly.) Then, they could measure the speed at which each supernova's host galaxy is moving away from us by studying the galaxy's spectral lines.

When combined, these observations reveal a chronology of the universe's expansion. Because of the effects of gravity, the astronomers had expected to see that the expansion rate is gradually slowing as time passes, but not enough to ever stop completely. Instead, they discovered that expansion of the universe seems to be slowing less quickly than gravity should allow--and may even be speeding up with time. It's as if some strange force is counteracting gravity. What's going on?

This may ring a bell for Einstein aficionados. When Einstein wrote his general theory of relativity in 1916, scientists presumed the universe was static. From his equations Einstein derived the existence of a repulsive energy that acts like an antigravity force. Such a force, he said, would balance out gravity to keep the universe at a stable size. But when Hubble discovered the outward motions of galaxies a decade later, that need vanished. Einstein quickly discarded the antigravity addition to his equations and called it his greatest mistake. Now it appears that he may have been right all along. Many astrophysicists favor the repulsive energy, called the cosmological constant, as the best explanation of the supernova data. It is indeed tempting to think of it as a literal "antigravity." But in fact the cosmological constant has nothing whatsoever to do with matter. It is a springiness inherent in space itself, an outward pressure that grows as space expands. The more space there is, the springier it becomes. In other words, if the cosmological constant is real, it will force the universe to expand faster and faster without limit. Quantum mechanics may explain the force as a "vacuum energy" present throughout the void of space, but a consensus does not yet exist.

All of this research allows us to paint a portrait of our future in the cosmos. Let's optimistically assume that our civilization endures for a billion years. If our descendants haven't found a way to colonize other planetary systems by then, they'll be out of luck. The Sun will gradually start to brighten, sterilizing Earth with increasing radiation. In about 5 billion years the Sun will use up its hydrogen fuel and swell into a red giant. Earth may escape being swallowed by the Sun's outer atmosphere, a 3,000-degree plasma, because the Sun will shed a great deal of mass. This will force Earth's orbit to slowly move outward in the solar system. Even so, the oceans will boil off, the atmosphere will evaporate, and the crust itself may melt. Earth will be a charred ember.

Another event shortly thereafter could stir up havoc elsewhere in the Milky Way. The Milky Way and our sister galaxy in Andromeda are moving toward each other at the leisurely clip of 250,000 miles per hour. We may collide in 6 billion years or so. We don't yet know enough about the sideways motion of the Andromeda galaxy relative to that of the Milky Way, but it could be a direct hit. Individual stars are so far apart that they aren't likely to collide in such an encounter. However, close flybys between stars would disturb their giant clouds of comets, scattering many of them like(continued)