Ultra-compact radio sources and the isotropy and homogeneity of the
  Universe
  A 2.29 GHz VLBI all-sky survey of ultra-compact radio sources has formed the
basis of a number of cosmological investigations, which examine the
relationship between angular-size and redshift. Here I use a sample of 468 such
sources with 0.5<z<=3.787, to investigate the isotropy of the Universe. The
sample is divided into hemispherical sub-samples, over an all-sky 5 degree x 5
degree array, each of which is allowed to determine a value of Omega_m,
assuming that we are living in a spatially flat homogeneous isotropic LambdaCDM
model. If we regard the latter as a null hypothesis, then it fails the test --
the results show significant anisotropy, the smallest value of Omega_m being
towards (l,b)=(253.9,24.1) degrees, the largest in the opposite direction. This
is close to the CMB dipole axis, but in the obverse sense. This is interpreted
as meaning that the Universe is not spatially homogeneous on the largest
scales, and is better represented at late times by a spherically symmetric
model with a density enhancement at its centre.
