Inflationary Expansion

The Universe grew by 10⁵⁰ or more in size over a a tiny fraction of a second during inflation. This, for example, means that the entire volume of the Universe that we have been able to see so far expanded from a volume that was far to small to even see with a microscope when inflation began! (If these statements leave you somewhat dizzy, you are not alone. Even astronomers have difficulty comprehending such stupendous numbers, but that is what the solutions to the equations suggest happened.)

The Scale Factor and Temperature

This astonishing exponential expansion was driven by the vacuum energy density. Then, when the brief period of inflation was complete, the Universe settled down into the big bang evolution that we have discussed prior to this point in a phase with either zero or very tiny vacuum energy density. The top right figure illustrates the behavior of the scale factor and the temperature in highly schematic fashion during inflation and the following big bang evolution. During inflation the Universe expanded at a much higher rate than in normal big bang evolution. At the same time, the temperature dropped rapidly in the exponentially expanding Universe. Finally, when the period of inflation halted, the Universe first rapidly reheated and then began to decrease in temperature according to the standard scenario we have already outlined in the radiation-dominated big bang. The question marks represent our substantial lack of knowledge concerning the Universe prior to the inflationary period.

History of the Universe with Inflation

The overall history of the Universe, including the inflationary period, is summarized in the following diagram. Note that the epoch of inflation was far too short to even represent on this graph, so the width of the region labeled "Inflation" is completely unrealistic.

A Modified Big Bang

As observed in the right panel, inflation does not modify the successful features of the standard big bang theory, which depend primarily on events happening after inflation. However, the inflationary modification of the Universe's earliest moments has potentially favorable implications for the three problems of the big bang that we discussed earlier.