Anisotropic expansion and SNIa: an open issue
  We review the appropriateness of using SNIa observations to detect potential
signatures of anisotropic expansion in the Universe. We focus on Union2 and
SNLS3 SNIa datasets and use the hemispherical comparison method to detect
possible anisotropic features. Unlike some previous works where non-diagonal
elements of the covariance matrix were neglected, we use the full covariance
matrix of the SNIa data, thus obtaining more realistic and not underestimated
errors. As a matter of fact, the significance of previously claimed detections
of a preferred direction in the Union2 dataset completely disappears once we
include the effects of using the full covariance matrix. Moreover, we also find
that such a a preferred direction is aligned with the orthogonal direction of
the SDSS observational plane and this suggests a clear indication that the SDSS
subsample of the Union2 dataset introduces a significant biased, making the
detected preferred direction unphysical. We thus find that current SNIa surveys
are inappropriate to test anisotropic features due to their highly
non-homogeneous angular distribution in the sky. In addition, after removal of
the highest inhomogeneous sub-samples, the number of SNIa is too low. Finally,
we take advantage of the particular distribution of SNLS SNIa sub-sample in the
SNLS3 data set, in which the observations were taken along 4 different
directions. We fit each direction independently and find consistent results at
the 1$\sigma$ level. Although the likelihoods peak at relatively different
values of $\Omega_m$,the low number of data along each direction gives rise to
large errors so that the likelihoods are sufficiently broad as to overlap
within 1$\sigma$.
