Ragnarok. Armageddon. Doomsday. Since the dawn of time, man has wondered how the world would end. In "The Last Three Minutes, " Paul Davies reveals the latest theories. It might end in a whimper, slowly scattering into the infinite void. Then again, it might be yanked back by its own gravity and end in a catastrophic " Big Crunch." There are other, more frightening possibilities. We may be seconds away from doom at this very moment.