Searching for a Cosmological Preferred Axis: Union2 Data Analysis and
  Comparison with Other Probes
  We review, compare and extend recent studies searching for evidence for a
preferred cosmological axis. We start from the Union2 SnIa dataset and use the
hemisphere comparison method to search for a preferred axis in the data. We
find that the hemisphere of maximum accelerating expansion rate is in the
direction $(l,b)=({309^\circ}^{+23^\circ}_{-3^\circ},
{18^\circ}^{+11^\circ}_{-10^\circ})$ ($\omm=0.19$) while the hemisphere of
minimum acceleration is in the opposite direction
$(l,b)=({129^\circ}^{+23^\circ}_{-3^\circ},{-18^\circ}^{+10^\circ}_{-11^\circ})$
($\omm=0.30$). The level of anisotropy is described by the normalized
difference of the best fit values of $\omm$ between the two hemispheres in the
context of \lcdm fits. We find a maximum anisotropy level in the Union2 data of
$\frac{\Delta \ommax}{\bomm}=0.43\pm 0.06$. Such a level does not necessarily
correspond to statistically significant anisotropy because it is reproduced by
about $30%$ of simulated isotropic data mimicking the best fit Union2 dataset.
However, when combined with the axes directions of other cosmological
observations (bulk velocity flow axis, three axes of CMB low multipole moments
and quasar optical polarization alignment axis), the statistical evidence for a
cosmological anisotropy increases dramatically. We estimate the probability
that the above independent six axes directions would be so close in the sky to
be less than $1%$. Thus either the relative coincidence of these six axes is a
very large statistical fluctuation or there is an underlying physical or
systematic reason that leads to their correlation.
