Tight cosmological constraints from the angular-size/redshift relation for ultra-compact radio sources

Some years ago (Jackson and Dodgson 1997) analysis of the angular-size/redshift relationship for ultra-compact radio sources indicted that for spatially flat universes the best choice of cosmological parameters was Ωm = 0.2 and ΩΛ = 0.8. Here I present an astrophysical model of these sources, based upon the idea that for those with redshift z > 0.5 each measured angular size corresponds to a single compact component which is moving relativistically towards the observer; this model gives a reasonable account of their behaviour as standard measuring rods. A new analysis of the original data set (Gurvits 1994), taking into account possible selection effects which bias against large objects, gives Ωm = 0.24 + 0.09/− 0.07 for flat universes. The data points match the corresponding theoretical curve very accurately out to z ∼ 3, and there is clear and sustained indication of the switch from acceleration to deceleration, which occurs at z = 0.85.