Multiple measurements of quasars acing as standard probes: exploring the
  cosmic distance duality relation at higher redshift
  General relativity reproduces main current cosmological observations,
assuming the validity of cosmic distance duality relation (CDDR) at all scales
and epochs. However, CDDR is poorly tested in the redshift interval between the
farthest observed Type Ia supernovae (SN Ia) and that of the Cosmic Microwave
background (CMB). We present a new idea of testing the validity of CDDR,
through the multiple measurements of high-redshift quasars. Luminosity
distances are derived from the relation between the UV and X-ray luminosities
of quasars, while angular diameter distances are obtained from the compact
structure in radio quasars. This will create a valuable opportunity where two
different cosmological distances from the same kind of objects at high
redshifts are compared. Our constraints are more stringent than other currently
available results based on different observational data and show no evidence
for the deviation from CDDR at $z\sim 3$. Such accurate model-independent test
of fundamental cosmological principles can become a milestone in precision
cosmology.
