look the same everywhere, with no beginning and no end. As galaxies drifted apart from each other, new matter slowly arose out of empty space to keep the overall density of the universe the same. The steady-state theory was pleasing to the mind, avoided the question of a single "origin," and was consistent with observations of the universe for many years. Then, in the 1960s, astronomers discovered quasars . These bright beacons of light were all far away from Earth; none were nearby. Thus, they were more common when the universe was younger. This violated the perfect cosmological principle because it indicated that the universe had clearly changed with time--more evidence for the Big Bang .

Since then, data from a series of studies have continually supported the Big Bang . These studies have forced scientists and nonscientists alike to grapple with the theory's implications. Let's face it: The Big Bang is bizarre. It suggests that an explosion roughly 13 billion years ago created all space and matter and energy within a fireball that initially could have passed through the eye of a needle. By taking a hard look at the details, however, we will see why astrophysicists are willing to put their stock in the Big Bang .

Evidence in favor of the Big Bang rests on all three pillars of our approach to understanding our universe: motion, matter, and energy. Hubble's research revealed that distant galaxies recede from us more quickly than closer ones in direct proportion to how far away they are. We now recognize this as the expansion of space itself, launched by the Big Bang .Einstein's general theory of relativity predated Hubble's work by 13 years. Even so, solutions to one of the theory's many equations predicted a universe that expands precisely according to the pattern found by Hubble. Today, as we peer more deeply into space, we still observe that galaxies appear to flee from us more quickly as the distances grow larger--just as Hubble would have predicted.

How do we know? One clue comes from gravitational lenses . The gravitational field of a massive object can noticeably bend light. Sir Arthur Eddington proved that in 1919 by observing the warped paths of starlight passing close to the Sun during a total eclipse . If light from a distant quasar travels near a galaxy or a cluster of galaxies on its way to Earth, the gravitational lens can create three or more images of the quasar. The resulting optical effect is like looking at a warped mirror and seeing several images of your face. They're all you, but the light rays have traveled different paths to get to your eyes. We have observed such optical antics for dozens of quasars . Red shifts in the spectral lines of the objects reveal how quickly they are moving away from us. In each case the more distant "lensed" object is always traveling faster than the object whose gravity serves as the lens. We never see gravitational lenses in which the distant quasars move more slowly than the closer lensing galaxies. In other words, gravitational lenses support Hubble's contention that the expansion of the universe grows faster and faster with increasing distance.

Einstein's special theory of relativity provides another clue. Think back to the hyperkinetic unicyclist pedaling past you at close to the speed of light. His mass increased, his length shrank, and his clock slowed down relative to yours. We have identified "clocks" in distant galaxies that display this effect in space. The clocks are supernova explosions , which behave uniformly from one galaxy to the next. The most distant supernovas take more time to explode and to decline in brightness than comparable ones in nearby galaxies. That's just what should happen if the distant galaxies stream away at a fair fraction of the speed of light. A "week" for us might appear to last eight days, nine days, or longer for supernovas in such galaxies.

These objects in motion all trace backward to a time when the universe was much smaller than it is today. The matter within these objects--the stuff of stars and galaxies--holds another important test. The Big Bang theory maintains that the fires of the early universe raged at trillions of degrees , too hot for atoms to exist. Instead, matter consisted of a soup of quarks , electrons , and other subatomic particles . It took about 100 seconds for temperatures to drop to a billion degrees, "cool" enough for the first atomic nuclei to fuse. We can take an educated guess at calculating the mixture of atoms that resulted. To do so, we combine all that we know about quantum mechanics with all we have learned about smashing atoms to smithereens in particle accelerators . The outcome predicts a universe with an original mix of about 75 percent hydrogen ; 25 percent helium ; and a smattering of other ingredients such as lithium , the third element in the periodic table, and deuterium , a type of heavy hydrogen with an extra neutron. This matches what we see in the universe to a satisfying degree.

The most convincing evidence for the Big Bang comes not from motion or matter but from the energy of the universe itself. If an unimaginably hot explosion formed the (continued)