ABSTRACT
The universe has existed for only 10 or 15 billion years. This chapter discusses the beginning of the uni verse (the big bang), the ensuing expansion, and the relationship between the shape of the universe and the matter it contains. The chapter is organized around the following questions:
1. What do we know about the universe? 2. In what sense is the universe expanding? 3. How is the density of matter related to the
curvature of space?
