Legacy data and cosmological constraints from the angular-size/redshift relation for ultracompact radio sources

We have re-examined an ancient VLBI survey of ultracompact radio sources at 2.29 GHz, which gave fringe amplitudes for 917 such objects with total flux density {\gtrsim }\,

0.5 Jy. A number of cosmological investigations based upon this survey have been published in recent years. We have updated the sample with respect to both redshift and radio information, and now have full data for 613 objects, significantly larger than the number (337) used in earlier investigations. The corresponding angular-size/redshift diagram gives Ω_m = 0.25+0.04/-0.03, Ω_Λ = 0.97+0.09/-0.13 and K = 0.22+0.07/-0.10 (68% confidence limits). From this in combination with supernova data and a simple-minded approach to CMB data based upon the angular size of the acoustic horizon, our best figures are Ω_m = 0.304+0.024/-0.023, Ω_Λ = 0.693+0.034/-0.035 and K = -0.003+0.021/-0.019. We have examined a simple model of vacuum energy, based upon a scalar potential V (phi) = ω_C²phi²/2, to test the possibility that the vacuum is dynamical; whereas the data favour a value ω_C = 0, they are compatible with values in excess of the Hubble rate.