On the existence of a standard rod in the Universe
  Using standard cosmological model we show that the cores of ultra-compact
radio sources observed with ground-based Very Long Baseline Interferometry
(VLBI) on the angular scales of milliarcseconds cannot be used as a reasonable
standard unit of linear size. "Luminosity - linear size" correlation obtained
by many authors for ultra compact radio sources has different origin than that
for the radio galaxies and quasars on the angular scales of arcminutes. It is
just the manifestation of the fact that ground-based VLBI networks are unable
to resolve VLBI-cores and the Malmquist bias presents. Thus, the cores of
compact radio sources can't be used as "standard rods" at least with resolution
offered by ground-based VLBI. This conclusion is illustrated on 15 GHz VLBA
sample of radio sources.
