 Space, in this context, is characterized by its curvature (taken to be constant) and its topology. Let us consider curvature first. In these models, there are three possible types of spaces depending on whether the curvature of the space is negative, null, or positive. The spaces corresponding to such curvatures are called hyperbolic, Euclidean, and elliptical. A spherical space (constant positive curvature) is always of finite extension, no matter what its topology is. This explains, at least in part, why many early cosmologists (including Einstein, de Sitter, Friedmann, Lemaître and others) opted for this solution. Indeed, for a long time issues concerning the topology of space were not brought to the fore due to the implicit assumption that the topology of space was a simply connected topology (in a simply connected topology every loop on the surface can be continuously contracted to a point). Under that assumption, spaces of positive constant curvature are finite and those of null and negative constant curvature are infinite. The issue then of the finiteness vs. infiniteness of the universe rests on the mean density of matter and energy and on the value of a parameter \(\lambda\) introduced by Einstein in 1917, called the cosmological constant (measuring a sort of anti-gravitational force). With most cosmologists (but not Einstein in 1917) assuming \(\lambda = 0\), and with the assumption that space is simply connected, determining curvature (and hence resolving the finiteness vs infinity issue) depends only on a critical value for the mean density of matter—equivalently, on a density parameter \(\Omega\). Thus, under those assumptions, it would be in principle possible to determine the curvature of space experimentally.