Systematic effects in the radio source proper motion
  Some models of the expanding Universe predict that the astrometric proper
motion of distant radio sources embedded in space-time are non-zero as radial
distance from observer to the source grows. Systematic effects due to this
proper motion can even increase with distance making possible to measure them
with high precision astrometric techniques like VLBI. We analyzed a large set
of geodetic VLBI data spanning from 1979 till 2008 to estimate the dipole and
the quadrupole harmonics in the expansion of the vector field of the proper
motions of quasars in the sky. We estimated the vector spherical harmonics
(three parameters for the dipole and ten - for the quadrupole systematic)
directly from the VLBI group delays without intermediate calculation of the
individual proper motion. The estimates have been obtained separately for
different red shift zones. It was shown that the dipole harmonic does not vary
significantly, whereas the amplitude of the quadrupole gradually increases with
the red shift. This quadrupole pattern can be interpreted either as an
anisotropic Hubble expansion, or as an indication of the primordial
gravitational waves in the early Universe. However, more prosaic explanations
may also be possible.
