Testing the isotropy of the Universe with type Ia supernovae

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 delta, and the present level of anisotropy of the large-scale geometry of the Universe, the actual shear Sigma(0), are constrained in the ranges -0.16 less than or similar to delta less than or similar to 0.12 and -0.012 less than or similar to Sigma(0) less than or similar to 0.012 (1 sigma C.L.) by Union2 data. Supernova data are then compatible with a standard isotropic universe (delta = 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.