A framework for testing isotropy with the cosmic microwave background

We present a new framework for testing the isotropy of the Universe using cosmic microwave background data, building on the nested-sampling ANICOSMO code. Uniquely, we are able to constrain the scalar, vector and tensor degrees of freedom alike; previous studies only considered the vector mode (linked to vorticity). We employ Bianchi type VII_h cosmologies to model the anisotropic Universe, from which other types may be obtained by taking suitable limits. In a separate development, we improve the statistical analysis by including the effect of Bianchi power in the high-ℓ, as well as the low-ℓ, likelihood. To understand the effect of all these changes, we apply our new techniques to Wilkinson Microwave Anisotropy Probe data. We find no evidence for anisotropy, constraining shear in the vector mode to (σ_V/H)₀ < 1.7 × 10^-10 (95 per cent confidence level). For the first time, we place limits on the tensor mode; unlike other modes, the tensor shear can grow from a near-isotropic early Universe. The limit on this type of shear is (σ_{T, reg}/H)₀ < 2.4 × 10^-7 (95 per cent confidence level).