 Remarkably, much of cosmology is based on an extremely simple set of solutions found within a decade of Einstein’s discovery of GR. These Friedman-Lemaître-Robertson-Walker (FLRW) solutions have, in a precise sense, the most symmetry possible. The spacetime geometry is constrained to be uniform, so that there are no preferred locations or  directions.[5]  They have a simple geometric structure, consisting of a “stack” of three-dimensional spatial surfaces \(\Sigma(t)\) labeled by values of the cosmic time \(t\) (topologically, \(\Sigma \times \mathbb{R}\)). The surfaces \(\Sigma(t)\) are three-dimensional spaces (Riemannian manifolds) of constant curvature, with three possibilities: (1) spherical space, for the case of positive curvature; (2) Euclidean space, for zero curvature; and (3) hyperbolic space, for negative  curvature.[6] 