  In homogeneous isotropic cosmological models the angular size theta of a
standard measuring rod changes with redshift z in a manner that 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 ultracompact 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 cold dark matter 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.
