Introduction

Stars are not scattered at random throughout the universe but instead occur in immense swarms called galaxies. Each galaxy is separated from the others by vast reaches of nearly empty space. The stars of the galaxy to which our sun belongs appears in the sky as the Milky Way.

Of the many remarkable properties galaxies have, one stands out: the are moving apart from one another, so that the universe as a whole is expanding at a steady rate. If we project this expansion backward, we find that it began about 15 billion years ago. Can it be that the entire universe was born in a cosmic big bang at that time and has been evolving ever since into the galaxies of today? As we shall see in this chapter, several lines of evidence support such a picture, which has been filled out in considerable detail.