Bayesian Limits on Primordial Isotropy Breaking

It is often assumed that primordial perturbations are statistically isotropic, which implies, among other properties, that their power spectrum is invariant under rotations. In this article, we test this assumption by placing bounds on deviations from rotational invariance of the primordial spectrum. Using five-year Wilkinson Microwave Anisotropy Probe cosmic microwave anisotropy maps, we set limits on the overall norm and the amplitude of individual components of the primordial spectrum quadrupole and hexadecapole. We find that there is no significant evidence for primordial isotropy breaking, and constrain the relative contribution of the quadrupole and hexadecapole to be less than, respectively, 23% and 34% at 95% confidence level.