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