The cosmological deceleration parameter estimated from the angular-size/redshift relation for compact radio sources

IN cosmological models based on the standard Friedmann-Robertson-Walker geometry, the apparent flux density or angular size of standard candles or standard rods varies with redshift in a way that depends on the deceleration parameter q ₀. (Open universes have q ₀ < 0.5; closed universes have q ₀ > 0.5.) At low redshift, however, observational errors are much greater than the differences in q ₀ expected for different cosmological models, while at high redshift observational uncertainties, particularly at optical wavelengths, and apparent systematic evolutionary changes in sources obscure the expected geometrical effects. Here I show that measurements by very-long-baseline interferometry (VLBI) of compact radio sources associated with active galaxies and quasars may be largely free of evolutionary effects even at substantial redshifts. The relation between angular size and redshift for a sample of these sources indicates a value of q ₀ close to 0.5, corresponding to cosmological density near the critical value.