# Friedmann-Lemaître-Robertson-Walker universe

The simplest assumptions one can make about a universe are that it
is homogeneous and isotropic. Homogeneity means that the properties of matter and of the geometry of spacetime are the same at every point in space.
Isotropy means that all spatial directions are on the same footing, and
that to a hypothetical observer, such a universe looks exactly the
same, in whatever direction he or she might be looking. These
assumptions are quite restrictive; in fact, it is possible to write
down an expression characterizing the spacetime geometry of all
homogeneous and isotropic solutions of Einstein's equations. The result is a family of spacetimes known as *Friedmann-Lemaître-Robertson-Walker universes*.
Typically, these universes are either in a state of expansion or a
state of collapse. The best-known example is the expanding universe
described by big bang cosmology.

Sometimes, these model universes are also referred to as Friedmann-Lemaître universes, Robertson-Walker universes or Friedmann-Robertson-Walker universes.