The False Vacuum Theory and the Big Bang
This expanding hot soup of particles, produced by the expansion and subsequent decay of the false vacuum, is exactly the form of matter that the traditional big bang theory had assumed made up the primordial fireball. Thus, the inflationary theory provides the detailed description of the initial explosion that was absent from the original form of the big bang theory. The description of the universe after the decay of the false vacuum is the same in both the theory of inflation and the older form of the big bang theory.
The inflationary universe theory can explain the nearly uniform distribution of matter throughout the universe, because the enormous burst of expansion makes it possible for the universe to have started out much smaller than had been previously thought. Just before inflation began, the region of space that we call the observable universe would have been more than a billion times smaller than a proton. Since the region was so small, both the temperature and the distribution of matter had a chance to become uniform before inflation kicked in. This uniformity would have been preserved as inflation expanded this tiny region to an immensely larger volume.
The inflationary theory can also explain why the average mass density in the universe is so close to the critical value. As the universe evolves, both the mass density and its critical value change, since the critical density is determined by the rate of expansion. (If the rate of expansion were high, for example, then a strong gravitational force would be needed to reverse it, and the critical density would be high.) During inflation, the expansion rate of the universe was controlled by the gravitational repulsion of the false vacuum, which according to calculations had exactly the needed strength. Whatever the conditions were before inflation—whether the average mass density was greater or less than the critical value–the inflationary expansion would have adjusted the balance between the rate of expansion and the mass density. When the period of inflation ended, after 100 or more doublings in the size of the universe, the mass density would have been extraordinarily close to the critical value.
Because inflation is the only known explanation for the uniformity of the universe and the fact that the universe's mass density is very close to the critical value, cosmologists are fairly certain that the universe did indeed go through a period of inflation. Nonetheless, the theory of inflation is not the final word in cosmology, for at least three reasons. First, although inflation describes the universe at an extraordinarily early moment in time, it does not explain the actual origin of the universe—time zero. Second, inflation is not a unique theory, but rather a class of theories. Most of the versions present the same basic scenario of the early universe, but they differ in their details. There was still much work to be done in 1997 to determine which version of the theory, if any, is correct. And third, though inflation is a persuasive theory, decisive tests of its predictions were still needed.