The Friedmann–Lemaître–Robertson–Walker

The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is an exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic expanding or contracting universe that may be simply connected or multiply connected[1][2][3]. (If multiply connected, then each event in spacetime will be represented by more than one tuple of coordinates.)