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

TL;DR

This study examines Friedmann cosmological models with a decaying cosmological constant and tests their consistency with angular size-redshift data from ultracompact radio sources, supporting an accelerating universe with dark matter.

Contribution

It modifies the Chen and Wu ansatz to explore models where Lambda varies with density and tests these models against observational data, showing they fit well and imply an accelerating universe.

Findings

Models fit the data well and suggest an accelerating universe.

Open models provide a better fit among different geometries.

Models imply the presence of dark matter, but less than in Einstein-de Sitter.

Abstract

By modifying the Chen and Wu ansatz, we have investigated some Friedmann models 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 interferometry in order to avoid any evolutionary effects. The models fit the data very well and require an accelerating universe with a positive cosmological constant. Open, flat and closed models are almost equally probable, though the open model provides a comparatively better fit to the data. The models are found to have intermediate density and imply the existence of dark matter, though not as much as in the canonical Einstein-de Sitter model.