The proposal that we are in no special place and that the Universe looks much the same to all observers, wherever they are. In technical terms, this means that the Universe must be homogeneous and isotropic. This principle is not strictly true as the Universe is clearly irregular on small scales. There is good evidence, however, that on very large scales the Universe is quite smooth and regular. A consequence of the cosmological principle is that the spacetime geometry of the Universe must be described by the Robertson–Walker metric, which drastically restricts the set of cosmological models compatible with general relativity. See also perfect cosmological principle.