A study of angular size-redshift relation for models in which Lambda decays as the energy density

By modifying the Chen and Wu ansatz, we have investigated some Friedmann mo dels in which Lambda varies as rho. In order to test the consistency of the models with observations, we study the angular size-redshift relation for 256 ultracompact radio sources selected by Jackson and Dodgson. The angular sizes of these sources were determined by using very long-baseline interfe rometry in order to avoid any evolutionary effects. The models fit the data very well and require an accelerating universe with a positive cosmologica l constant. Open, flat and closed models are almost equally probable, thoug h the open model provides a comparatively better fit to the data. The model s are found to have intermediate density and imply the existence of dark ma tter, though not as much as in the canonical Einstein-de Sitter model.