Testing the isotropy of the Universe with type Ia supernovae

Abstract

We analyze the magnitude-redshift data of type Ia supernovae included in the Union and Union2 compilations in the framework of an anisotropic Bianchi type I cosmological model and in the presence of a dark-energy fluid with anisotropic equation of state. We find that the amount of deviation from isotropy of the equation of state of dark energy, the skewness $\ensuremath{\delta}$, and the present level of anisotropy of the large-scale geometry of the Universe, the actual shear ${\ensuremath{\Sigma}}_{0}$, are constrained in the ranges $\ensuremath{-}0.16\ensuremath{\lesssim}\ensuremath{\delta}\ensuremath{\lesssim}0.12$ and $\ensuremath{-}0.012\ensuremath{\lesssim}{\ensuremath{\Sigma}}_{0}\ensuremath{\lesssim}0.012$ ($1\ensuremath{\sigma}$ C.L.) by Union2 data. Supernova data are then compatible with a standard isotropic universe ($\ensuremath{\delta}={\ensuremath{\Sigma}}_{0}=0$), but a large level of anisotropy, both in the geometry of the Universe and in the equation of state of dark energy, is allowed.