ABSTRACT
The distribution of matter and radiation in the observable Universe is homogeneous and isotropic. While this by no means guarantees that the entire Universe is smooth, it does imply that a region at least as large as physicists' present Hubble volume is smooth. So long as the Universe is spatially homogeneous and isotropic on scales as large as the Hubble volume, for purposes of description of physicists' local Hubble volume physicists may assume the entire Universe is homogeneous and isotropic. While a homogeneous and isotropic region within an otherwise inhomogeneous and anisotropic Universe will not remain so forever, causality implies that such a region will remain smooth for a time comparable to its light-crossing time. This time corresponds to the Hubble time, about 10 Gyr. The metric for a space with homogeneous and isotropic spatial sections is the maximally-symmetric Robertson-Walker (RW) metric.
