Deceleration without dark matter

In homogeneous isotropic cosmological models the angular size θ 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 (Ω 0 and Λ 0). The canonical inflationary cold dark matter model (Λ 0 = 1, Λ 0 = 0) appears to be ruled out by the observed relationship, whereas low-density models with a cosmological constant of either sign are favoured.