We are now in a position to talk meaningfully about the earliest stages of our universe's history. Over the last few decades a scenario has emerged that indicates that the universe began life as a rapidly expanding and intensely hot fireball. This picture is referred to as the big bang model. Most cosmologists agree that this model represents an accurate description of the very early universe, at least for times after about one second.

In this chapter we will trace the history of the universe from the big bang through to the present day. Let us begin by discussing some of the key principles that governed the behaviour of the universe during the big bang.

The temperature of the universe at any given time is directly related to the size and age of the universe. It often proves convenient to measure the age of the universe directly in terms of its temperature. A higher temperature then corresponds to an earlier time. For example, when the universe was one second old its temperature was about ten billion degrees. It will be useful to keep this temperature in mind for comparative purposes in our forthcoming discussion.

The very early universe was considerably hotter than ten billion degrees. Matter in the form of atoms would not have been present. Indeed, atoms did not come into existence until the universe was about three hundred thousand years old. Moreover, nuclei did not become stable until a few minutes had elapsed.