Evidence of Vorticity and Shear at Large Angular Scales in the WMAP Data: A Violation of Cosmological Isotropy?

Motivated by the large-scale asymmetry observed in the cosmic microwave background sky, we consider a specific class of anisotropic cosmological models -- Bianchi type VIIₕ -- and compare them to the WMAP first-year data on large angular scales. Remarkably, we find evidence of a correlation which is ruled out as a chance alignment at the 3sigma level. The best fit Bianchi model corresponds to x=0. 55, Omega₀=0. 5, a rotation axis in the direction (l, b) = (222degr, -62degr), shear (sigma/H) ₀=2. 4e-10 and a right--handed vorticity (omega/H) ₀=6. 1e-10. Correcting for this component greatly reduces the significance of the large-scale power asymmetry, resolves several anomalies detected on large angular scales (ie. the low quadrupole amplitude and quadrupole/octopole planarity and alignment), and can account for a non--Gaussian "cold spot" on the sky. Despite the apparent inconsistency with the best-fit parameters required in inflationary models to account for the acoustic peaks, we consider the results sufficiently provocative to merit further consideration.