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 < \delta < 0.12 and -0.012 <
\Sigma_0 < 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.Comment: 12 pages, 7 figures, 2 tables. Union2 analysis added. New references
added. To appear in Phys. Rev.
