Variograms of the cosmic microwave background temperature fluctuations: confirmation of deviations from statistical isotropy

The standard inflationary model predicts an isotropic distribution of the cosmic microwave background temperature fluctuations. Detection of deviations from statistical isotropy would call for a revision of the physics of the early Universe. This paper introduces the variogram as a powerful tool for detecting and characterizing deviations from statistical isotropy in cosmic microwave background maps. Application to the Wilkinson Microwave Anisotropy Probe data clearly shows differences between the Northern and the Southern hemispheres. The sill and range of the Northern hemisphere's variogram are lower than those of the Southern hemisphere. Moreover, the variogram for the Northern hemisphere lies outside the 99 per cent confidence limit for scales of above 10 degrees. Differences between the Northern and Southern hemispheres in the functional dependence of the variogram with the scale can be used as a validation benchmark for proposed anisotropic cosmological models.