Ultra-compact structure in intermediate-luminosity radio quasars:
  building a sample of standard cosmological rulers and improving the dark
  energy constraints up to $z\sim 3$
  In this paper, we present a new compiled milliarcsecond compact radio data
set of 120 intermediate-luminosity quasars in the redshift range $0.46< z
<2.76$. These quasars show negligible dependence on redshifts and intrinsic
luminosity, and thus represents, in the standard model of cosmology, a fixed
comoving-length of standard ruler. We implement a new cosmology-independent
technique to calibrate the linear size of of this standard ruler as $l_m=
11.03\pm0.25$ pc, which is the typical radius at which AGN jets become opaque
at the observed frequency $\nu\sim 2$ GHz. In the framework of flat
$\Lambda$CDM model, we find a high value of the matter density parameter,
$\Omega_m=0.322^{+0.244}_{-0.141}$, and a low value of the Hubble constant,
$H_0=67.6^{+7.8}_{-7.4}\; \rm{kms}^{-1}\rm{Mpc}^{-1}$, which is in excellent
agreement with the CMB anisotropy measurements by \textit{Planck}. We obtain
${\Omega_m}=0.309^{+0.215}_{-0.151}$, $w=-0.970^{+0.500}_{-1.730}$ at 68.3% CL
for the constant $w$ of a dynamical dark-energy model, which demonstrates no
significant deviation from the concordance $\Lambda$CDM model. Consistent
fitting results are also obtained for other cosmological models explaining the
cosmic acceleration, like Ricci dark energy (RDE) or Dvali-Gabadadze-Porrati
(DGP) brane-world scenario. While no significant change in $w$ with redshift is
detected, there is still considerable room for evolution in $w$ and the
transition redshift at which $w$ departing from -1 is located at $z\sim 2.0$.
Our results demonstrate that the method extensively investigated in our work on
observational radio quasar data can be used to effectively derive cosmological
information. Finally, we find the combination of high-redshift quasars and
low-redshift clusters may provide an important source of angular diameter
distances, considering the redshift coverage of these two astrophysical probes.
