vast event horizon. Here's another favorite topic: What came before the Big Bang? To answer that the laws of physics did not work before the Big Bang may sound as if we're dodging the issue. However, time and space were meaningless before the Big Bang ; they simply did not exist. Just as you cannot go any farther north on Earth if you are standing at the North Pole, you cannot go farther back in time from the birth of the universe.

This does not stop some theorists from speculating. The cosmic microwave background tells us that the space-time fabric of our infant universe was roiled by fluctuations on the smallest possible scale--a quantum foam , if you will. One class of inflationary hypotheses describes a mega-universe with endless bubbles of expansion arising from this foam. Each bubble looks like a Big Bang universe from within, and it may sustain laws of physics that differ from the ones we know. Inhabitants of those other bubbles would face the same impossible mysteries, forever confined to their part of the "multiverse " (page 178). Testing this wild scenario will require some scientific advance that we can't envision today. At this point it's safe to say that our laws of physics don't exclude the existence of other universes ; they simply can't explain them.

It's tempting to regard the Big Bang as little more than science fiction. However, our belief in the theory is bolstered by an impressive set of successful predictions--far more than most theories in progress enjoy. Indeed, nearly everyone in the community of astrophysicists now accepts the Big Bang . But we all recognize that, as cosmology progresses, it one day may become the core idea of something even bigger.