The angular size-redshift relation as a cosmological tool

The angular size - redshift relation is important for observational cosmology as it provides valuable information on the evolution with epoch of the linear sizes of extragalactic radio sources and can be used for discriminating against some models of the Universe. The physics of the evolution of individual sources and the various factors responsible for the inferred evolution with epoch are, however, far from being understood. There is therefore little hope at this stage of using the relation as a test of the geometry of the Universe.