Deceleration without dark matter

Abstract

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.

Although published (MNRAS 285, 806, 1997, submitted 1996 May 3), this paper has not previously appeared on the arXive. Despite its title, a prominent conclusion is that if the Universe is spatially flat, then the best cosmological parameters are Omega_m=0.2, Omega_Lambda=0.8, with probable range 0.1<Omega_m<0.3. It is the first in a series, the second being JCAP 0411(2004)007, astro-ph/0309390; the third is a recent preprint, astro-ph/0605065