Let us begin with a few preliminaries.1 A (relativistic) model of the universe is an ordered pair(M,g) where M is a smooth four-dimensional “manifold” representing the shape of the universe and g is a smooth relativistic “metric” encoding the geometry of the universe. Each point in the manifold represents a possible event in space and time. Experience seems to tell us that any event (e.g., the moon landing) can be characterized by four numbers – one temporal and three spatial coordinates. Accordingly, the local structure of a manifold “looks like” a four-dimensional Cartesian coordinate system. But the global structure can be quite different. Many two-dimensional manifolds are familiar to us: the plane, the sphere, the torus, and so on.