Nonetheless, the inflationary theory has led to new speculation about the nature of our universe. One of the most fascinating new ideas is the possibility that inflation never ends.
To understand how this could be so, recall that inflation is driven by a false vacuum and ends when the false vacuum decays. The false vacuum, however, does not decay all at once. In most versions of the inflationary theory, it decays with a fixed half-life, like a radioactive material. The half-life is the time it takes for half of a radioactive substance or a false vacuum to decay. Imagine that we could sprinkle a region of false vacuum uniformly with hundreds of tiny probes. After one half-life, half of the probes, on average, would be in regions that still contain false vacuum. The other half would be in regions in which the false vacuum has decayed into a hot soup of particles. The decay of the false vacuum would look just like the big bang in our own past, so each region in which the false vacuum decayed would become a new universe. Cosmologists call these universes “bubble universes,” and many think our own universe is such a bubble.
Although a radioactive material will eventually decay completely, a false vacuum behaves differently. While the false vacuum is decaying into bubble universes, it is also exponentially expanding. The time it takes for the false vacuum to double in size—perhaps 10 to the minus 37th power second—is much shorter than the false vacuum's half-life. Thus, despite continually losing regions of itself to decay, the remaining false vacuum grows ever larger, and inflation never stops. The result is the eternal creation of an infinite number of bubble universes as the false vacuum goes on inflating and decaying. Each bubble universe continues to expand at an ever-decreasing rate for billions of years as it evolves. The fate of each bubble universe is determined by how much mass it contains. Some bubble universes will eventually collapse by the force of gravity into a big crunch; others will expand forever, becoming cold and dark. According to the theory, however, the overall universe—the false vacuum and all the bubbles that it spawns—continues forever.
The theory of eternal inflation invites us to ask further questions about the origin of the universe. For example, if the universe includes an infinite number of bubble universes, are the laws of physics the same within all of them? If inflation can continue without end, is it possible that it is also without beginning? If not, then how did the universe begin? To describe what cosmologists are thinking about these questions, we need to summarize what physicists have learned about the fundamental laws of nature.