Friedman-Lemaître-Robertson-WalkerThe Friedman-Lemaître-Robertson-Walker (FLRW) metric describes a homogeneous isotropic expanding/contracting universe. Depending on geographical/historical preferences, this may be referred to under the names of a preferred subset of the four scientists Alexander Friedman, Georges Lemaître, Robertson and Walker, e.g. Friedman-Robertson-Walker (FRW) or Robertson-Walker (RW).
Its metric is given by
Most cosmologists agree that the observable part of the Universe is well approximated by an almost FLRW model, that is, a model which follows the FLRW metic apart from primordial density fluctuations. In a strictly FLRW model, there are no clusters of galaxies, stars or people, since these are objects much denser than a typical part of the Universe.
However, for brevity, the almost FLRW model is often referred to simply as the FLRW model (or the FRW model).