Constraints on anisotropic cosmic expansion from supernovae
  We test the isotropy of the expansion of the Universe by estimating the
hemispherical anisotropy of supernova type Ia (SN Ia) Hubble diagrams at low
redshifts (z<0.2).
  We compare the best fit Hubble diagrams in pairs of hemispheres and search
for the maximal asymmetric orientation. For an isotropic Universe, we expect
only a small asymmetry due to noise and the presence of nearby structures. This
test does not depend on the assumed content of the Universe, the assumed model
of gravity, or the spatial curvature of the Universe. The expectation for
possible fluctuations due to large scale structure is evaluated for the \Lambda
cold dark matter (\Lambda CDM) model and is compared to the supernova data from
the Constitution set for four different light curve fitters, thus allowing a
study of the systematic effects.
  The expected order of magnitude of the hemispherical asymmetry of the Hubble
expansion agrees with the observed one. The direction of the Hubble asymmetry
is established at 95% confidence level (C.L.) using both, the MLCS2k2 and the
SALT II light curve fitter. The highest expansion rate is found towards (l, b)
~ (-35{\deg},-19{\deg}), which agrees with directions reported by other
studies. Its amplitude is not in contradiction to expectations from the \Lambda
CDM model. The measured Hubble anisotropy is \Delta H/H ~ 0.026. With 95% C.L.
the expansion asymmetry is \Delta H/H<0.038.
