Few of us can look into the starry sky on a clear night without experiencing a sense of awe. We can see that our world—the planet Earth with all its mountains, oceans, forests, cities, and villages—is part of a vastly larger universe. Gazing at the faraway stars, many barely visible, we think we can feel the immensity of the universe, but in fact, we have no real sense of its size. We cannot see that our sun and all the stars within our view are part of a giant pinwheel of more than 100 billion stars, a galaxy known as the Milky Way. And we have no reason even to suspect that other galaxies exist. Astronomers have found, however, that space is teeming with galaxies, about 100 billion of them within the reach of our most powerful telescopes.
How did this vast and magnificent universe come to be? That is a question that cosmology, the branch of science that studies the universe as a whole, seeks to answer. Cosmologists have made tremendous strides since the early 1900's. We have advanced from the belief that the Milky Way is an “island universe,” a solitary galaxy surrounded by a sea of emptiness, to the picture of a universe filled with galaxies. We have also progressed from the belief that the universe is motionless to the view that it has expanded from a dramatic explosion, the big bang, which astronomers think occurred 10 billion to 15 billion years ago. But despite these advances, the most fundamental questions about the universe remain unanswered.
For example, the traditional form of the big bang scenario does not attempt to address questions about the very beginning of the universe. Although it is called the big bang theory, it is really only a theory of what happened after the big bang explosion. The theory describes how the early universe expanded and cooled and how a nearly uniform cloud of gaseous matter condensed into clumps to form stars and galaxies. But the theory gives not even a clue about what the universe emerged from, or what caused it to explode into being.
Today many cosmologists are looking for ways to go beyond the big bang theory, to describe the very moment of creation in terms of the laws of nature. Although our understanding of these laws is still incomplete, there has been much progress in constructing a theory of how the universe may have come into existence. If the new ideas are correct, then the universe is far older than 15 billion years—and vastly larger than had been previously thought. Furthermore, the new ideas imply that the big bang occurred not just once, but countless times. Each big bang produced a huge expanse of space, often called a “bubble universe.” These bubble universes are similar to the observable universe (the region within the reach of our telescopes), but they are separate from it, and possibly much larger.