Study of the angular size - redshift relation for models in which decays as the energy density

Abstract

By modifying the Chen and Wu ansatz, we have investigate some Friedmann models in which Λ varies as ρ. In order to test the con sistency of the models with observations, we study the angular size - red shift relation for 256 ultracompact radio sources selected by Jackson an Dodgson. The angular sizes of these sources were determined by using ver long-baseline interferometry in order to avoid any evolutionary effects. Th models fit the data very well and require an accelerating universe with a po itive cosmological constant. Open, flat and closed models are almost equall probable, though the open model provides a comparatively better fit to th data. The models are found to have intermediate density and imply the ex istence of dark matter, though not as much as in the canonical Einstein-d Sitter model.