Measuring the speed of light with ultra-compact radio quasars

Abstract

In this paper, based on a 2.29 GHz VLBI all-sky survey of 613 milliarcsecond ultra-compact radio sources with 0.0035<z<3.787, we describe a method of identifying the sub-sample which can serve as individual standard rulers in cosmology. If the linear size of the compact structure is assumed to depend on source luminosity and redshift as lm=l Lβ (1+z)n, only intermediate-luminosity quasars (1027 W/Hz<L< 1028 W/Hz) show negligible dependence (|n|≃ 10−3, |β|≃ 10−4), and thus represent a population of such rulers with fixed characteristic length l=11.42 pc. With a sample of 120 such sources covering the redshift range 00.46<z<2.8, we confirm the existence of dark energy in the Universe with high significance under the assumption of a flat universe, and obtain stringent constraints on both the matter density Ωm=0.323+0.245−0.145 and the Hubble constant H0=66.30+7.00−8.50 km sec−1 Mpc−1. Finally, with the angular diameter distances DA measured for quasars extending to high redshifts (0z∼ 3.), we reconstruct the DA(z) function using the technique of Gaussian processes. This allows us to identify the redshift corresponding to the maximum of the DA(z) function: 0zm=1.7 and the corresponding angular diameter distance DA(zm)=1719.01±43.46 Mpc. Similar reconstruction of the expansion rate function H(z) based on the data from cosmic chronometers and BAO gives us H(zm)=176.77±6.11 km sec−1 Mpc−1. These measurements are used to estimate the speed of light: c=3.039(±0.180)× 105 km/s. This is the first measurement of the speed of light in a cosmological setting referring to the distant past.