This the way the world ends

THIS IS THE WAY THE WORLD ENDS Our universe began with a Bang, but will end with a fiery Crunch or a

The Old Testament opens with the first great question: Where didwe come from? and the New Testament closes with the second: Where are we going? Between Genesis and Apocalypse other ques- tions are incidental. Throughout history these two great riddles have confronted anyone who has found a few momnts in which to put aside the daily task of survival. Incurable romantics, like artists and scientists, are frequent victims of cosmic meditation. Artists have been inspired by our destiny more often than by our origins. For every master who has taken Genesis as his subject, I'd wager ten have chosen to create apocalyptic visions. Yes, there's Michelangelo's Creation of Adam on the Sistine ceiling. But there are also his Last Judgment above the altar, Hubert and Jan van Eyck's Ghent altarpiece, the famous fourteenth century Angers tapestries, and Durer's woodcuts, not to mention the nightmares of Bosch, Goya, Redon, and Landau, as well as innumerable icons, requiem masses, and dirges.

Paradoxically, scientists, who have at their disposal the predictive power of physics and mathematics, have been far more concerned with origins than destiny. This is especially true of cosmology, the branch of physics that deals with the large-scale structure of the universe. While thousands of papers appear annually about the birth of the cosmos, probably fewer than a dozen in the past twenty years have speculated on its fate.

The paradox is easily explained. Unlike most sciences, cosmologyhas only one laboratory (the universe) and one experiment (its evolution), and at present there's no way to alter the experimental parameters. When we look at distant objects in the sky, such as quasars and Seyfert galaxies, we're peering back in time, because the light from these objects has traveled millions or billions of years to reach us. So it's easier for cosmologists to study the universe's past than its future.

Technical obstacles to foretelling the future are only part of the problem. Scientists also gen- uinely fear making predictions. A leaf falling from a tree on a windblown day is a phenomenon too complicated for today's physics to describe completely. Physicists know the leaf will eventually hit the ground, but they can't say exactly where or when. To predict the future of the entire universe is a task that conservative scientists would prefer to leave to Madame So sostris and her bad cold.

Nevertheless, certain daring physicists have used the crystal ball of science to try to pene- trate the shrouded fate of the universe. They're perhaps not quite as headstrong as they appear. The Big Bang models that describe the birth of the universe are considerably simpler than those that describe a leaf falling on a windy day, and the scenarios of the future that derive from them aren't difficult to limn. In fact, the problem is that the models are so simplified that they may bear no more resemblance to the real universe than a Brancusi bird does to a real one. Any forecast based on them will undoubtedly be wrong in many details. But we can hope that our theories, like a Brancusi bird, are at least correct in broad outline. We can also expect that the laws of physics, valid up to now, will continue to hold indefinitely. Dangerous assumptions? If so, then what follows is purest fantasy. If not, then here is a glimpse of apocalypse.

According to Einstein's theory of relativity, the standard Big Bang model has two possible destinies: continued expansion or collapse. The nature of the final catastrophe (if catastrophe indeed awaits us) depends on which comes to pass. If matter and radiation exceed a critical density of about 10- 2 9 grams per cubic centimeter, the universe is ''closed'' and will eventually collapse; otherwise it's ''open'' and will expand forever.

A count of the visible stars, galaxies, dust, and so forth suggests the actual density is only about ten per cent of critical, so the universe should keep expanding. But since you can't be sure you've counted everything, the ten per cent may be an underestimate. Further more, many theoreticians subscribe to the inflationary model -- a cosmological theory that says the universe expanded suddenly right after birth to a state just on the border between open and closed -- and therefore think the universe should be almost precisely at critical density. If it is, it may still collapse, but only after an essentially infinite time. I'll treat this as equivalent to continued expansion, and posit that if the universe collapses, it will begin to do so in the uncomfortably short time of 15 billion to 20 billion years.

Given those assumptions, let's consider our possible destinies -- expansion and col lapse -- separately, beginning with collapse. In a short paper written in 1969, Martin Rees, an astrophysicist at Cam bridge, drew the first sketch of the End in a closed universe. The universe today is filled with stars, galaxies, clusters of galaxies, and even superclus ters. Virgo, the nearest and most famous cluster, contains about 2,500 galaxies and lies at a distance of nearly 20 megaparsecs (65 million light-years). By cosmological standards that's close -- the radius of the observable universe is roughly 5,000 megaparsecs. Clusters are usually separated by tens or hundreds of megaparsecs, and fill very roughly one per cent of all space.