beams of radiation from a pulsar's poles but narrower and vastly more energetic. If this model holds up, it means that viewers off to the side would not see the gamma-ray burst. Instead, they might see something resembling an "ordinary" supernova. A hypernova may reveal its true nature to about 1 percent of the universe--the portion that lies along the line of sight of either of its two beams.

In the unlikely event that one of these bursts went off in our Milky Way, it probably would sterilize any planets in the path of its beams. Gamma rays are the most energetic form of light. An intense blast of them could wipe out the protective ozone in a planet's atmosphere and expose the planet to deadly radiation from its own star as well as from space. Astronomers learned recently that Earth is not immune to such influences. A lone neutron star in the Milky Way flared up in 1998, spitting out a relatively small burst of gamma rays. By the time the radiation reached Earth it had dwindled to the strength of a dental x-ray. Still, measurements with radio signals showed that the radiation zapped electrons away from atoms in the ionosphere--the uppermost, very tenuous part of the atmosphere. This energetic link between our isolated planet and the rest of the cosmos is fascinating but also vaguely unsettling. When and where will the next powder keg explode?

Where Does the UNIVERSE Go from Here?

To extend motion, matter, and energy to their limits, we must ponder the future of the cosmos. We know that space itself is expanding, carrying galaxies ever farther apart from each other. Will this pattern continue forever? We know that the matter around us is not as solid as it seems and that elements can decay even after billions of years. Can the ultimate constituents of matter survive an infinitely long time? We know that the background energy of the cosmos left over from the Big Bang is slowly winding down, even as colossal bursts still flare up from time to time. Will these displays become ever rarer, until the universe is black and silent?

Existentialists might ponder these questions with delight. They truly put us in our place as inconsequential motes in a vast universe, in terms of both space and time. We would like to know the answers as well for somewhat more practical reasons. Studying the future of the universe requires us to have a firm grasp on its present. To do so we must comprehend many aspects of today's cosmos that elude our limited vision.

One crucial puzzle goes by the name of "dark matter." Simply put, this is stuff we can't see, yet it exerts a gravitational pull like visible matter. Objects that shine may dominate our images of the cosmos, but they hardly make a difference in the big picture of mass in the universe. At least 90 percent of all mass out there is invisible in any wavelength of light--perhaps as much as 99 percent. Indeed, if our telescopes observed gravity rather than light, the cherished galaxies in galaxy clusters would appear as insignificant blips amid giant gravitational fields.

These statements rest on a secure body of observational and theoretical evidence. Inferring the existence of hidden things by looking only at what is visible may sound dubious, but you do it when you see distant headlights coming toward you on a dark road. Even though you cannot see the car itself, you know one is coming. You are familiar with cars, and when you see headlights coming, you know they are attached to a car. In fact, if you are a car buff, you might even deduce how large the car is, what model it is, and how fast it's moving. We also are well aware of the physics of floating ice. A little peak sticking out of the ocean is part of a jagged mass 10 times bigger hidden beneath the waves. Similarly, if we see luminous matter behaving exactly as it would if more mass were present, we confidently predict the existence of that extra mass. Astrophysics buffs can deduce how much mass there is, where it is located, and how it moves.

Among the many signs for dark matter, several convincing ones stand out. Observations of spiral galaxies such as our Milky Way show that their outer stars revolve around the galaxy too quickly for the amount of matter we see. They appear to violate Kepler's laws of orbital motion. The best explanation points to a huge halo of unseen material, contributing mass but no light. The halo extends beyond a galaxy's luminous edge and accelerates the visible matter with its gravitational pull. On larger scales a rich cluster of galaxies must hold tremendous amounts of stars, gas, and dust together with its gravity. But after tallying up all the visible matter in the cluster, we see only a tiny fraction of the mass needed to generate the necessary force. This was discovered in 1936 for the Coma cluster of galaxies by the Swiss-American astronomer Fritz Zwicky. Since then we have confirmed it for every cluster we have observed.