Model selection based on the angular-diameter distance to the compact structure in radio quasars

Of all the distance and temporal measures in cosmology, the angular-diameter distance, d _A(z), uniquely reaches a maximum value at some finite redshift and then decreases to zero towards the Big Bang. This effect has been difficult to observe due to a lack of reliable, standard rulers, though refinements to the identification of the compact structure in radio quasars may have overcome this deficiency. In this letter, we assemble a catalog of 140 such sources with 0 ≲ z ≲ 3 for model selection and the measurement of z_max. In flat ΛCDM, we find that ω_m = 0.24_-0.09 ^+0.1, fully consistent with the Planck optimized value, with z_max = 1.69. Both of these values are associated with a d _A(z) indistinguishable from that predicted by the zero active mass condition, p + 3p = 0, in terms of the total pressure p and total energy density ρ of the cosmic fluid. An expansion driven by this constraint, known as the R_h = ct universe, has z_max = 1.178, which differs from the ΛCDM optimized value by less than ∼ 1.6%. Indeed, the Bayes Information Criterion favours R_h = ct over flat ΛCDM with a likelihood of ∼ 81% vs. 19%, suggesting that the optimized parameters in Planck ΛCDM mimic the constraint p = -p^/3.