How Anisotropic is Our Universe?

Large-scale cosmic microwave background anisotropies in homogeneous, globally anisotropic cosmologies are investigated. We perform a statistical analysis in which the 4-yr data from the Cosmic Background Explorer satellite is searched for the specific anisotropy patterns predicted by these models and thereby set definitive upper limits on the amount of shear, $(\sigma /H{)}_0$, and vorticity, $(\omega /H{)}_0$, which are orders of magnitude stronger than previous constraints. We find that primordial anisotropy should have been fine tuned to be less than ${10}^{-3\mathrm{}}$ of its natural value in the Planck era.