Some General Characteristics of an Expanding Universe:
Have you ever pumped up a bicycle with a hand pump? If you have, you may have noticed that after you have run the pump for a while, the barrel gets very hot. All matter heats up when it is compressed.
The universe is no exception to this rule. A universe that is more compressed and denser that the one in which we live would also be hotter on average. In such a universe, the cosmic radiation background would correspond to a temperature much higher than 2.7K(where it is today) and the wavelength of the background radiation would be shorter that 7.35cm.
When the universe was younger, it must have been much hotter and denser that it is today. The cardinal principle guides our understanding of how the universe evolved. In fact, the big bang theory we have been discussing is often called the "hot big bang" to emphasize the fact that the universe began in a very hot, dense state and has been expanding and cooling ever since.
Remember that changes in temperature may correspond to changes of state in matter. If you cool water, for example, it eventually turns into ice at the freezing point. In just the same way, modern theories claim, as the universe cooled from its hot origins it went through changes of state very much like the freezing of water. We will refer to these dramatic changes in the fabric of the universe as freezings, even though they are not actually changes from liquid to a solid state.
A succession of freezing dominates the history of the universe. Six distinct episodes occurred, and each had its own unique effect on the universe that we live in. Between each pair of freezings was a relatively long period of steady and rather uneventful expansion. Once we understand these crucial transitions in the history of the universe, we will have come a long way toward understanding why the universe is the way it is.
