Robertson-Walker metric

A metric or formula for calculating proper intervals in a homogeneous and isotropic universe. The Robertson-Walker metric is the mathematical formula for time and distance calibration generally adopted for models of the large-scale structure of the universe. It applies to an idealized model, the Friedmann universe, that is perfectly homogeneous and perfectly isotropic, i.e. satisfying the cosmological principle. Robertson and Walker showed that, in such an idealized model, spacetime could be separated into two concepts common to all observers: curved space and cosmic time. Although this conceptual separation of space and time might seem obvious, it does in fact relate to a highly restricted geometry for spacetime in general. In the vicinity of black holes, for example, the distinction between space and time is blurred and the Kerr metric or Schwarzschild metric is used instead.