  The sphere has the same degree of metric homogeneity as the world of  the special theory of relativity, which can be conceived as a  four-dimensional “plane” in the same space. The plane,  however, has only one connected infinitely distant  “seam,” while it is the most prominent topological  property of the sphere to be endowed with two—the infinitely  distant past and the infinitely distant future. In this sense one may say that space is closed in de Sitter’s solution. On the other hand,  however, it is distinguished from the well-known Einstein solution,  which is based on a homogeneous distribution of mass, by the fact  that the null cone of future belonging to a world-point does not  overlap with itself; in this causal sense, the de Sitter space is  open.