Inhomogeneous universes in observational coordinates

Abstract

Isotropic inhomogeneous dust universes are analysed via observational coordinates based on the past light cones of the observer's galactic worldline. The field equations are reduced to a single first-order ODE in observational variables on the past light cone, completing the observational integration scheme. This leads naturally to an explicit exact solution which is locally nearly homogeneous (i.e. FRW), but at larger redshift develops an inhomogeneity. New observational characterizations of homogeneity (FRW universes) are also given.