In homogeneous isotropic cosmological models the angular size of a standard measuring rod changes with redshift z in a manner which depends upon the parameters of the model .
It has been argued that as a population ultra­compact ( milliarcsecond ) radio-sources measured by very long-baseline interferometry ( VLBI ) do not evolve with cosmic epoch , and thus comprise a set of standard objects , at least in a statistical sense .
Here we examine the angular-size/redshift relation for 256 ultra-compact sources with z in the range 0.5 to 3.8 , for cosmological models with two degrees of freedom ( \Omega _ { 0 } and \Lambda _ { 0 } ) .
The canonical inflationary CDM model ( \Omega _ { 0 } = 1 , \Lambda _ { 0 } = 0 ) appears to be ruled out by the observed relationship , whereas low-density models with a cosmological constant of either sign are favoured .

Key words : cosmology – observations – theory – dark matter .