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-$\ell$, as well as the low-$\ell$,
likelihood. To understand the effect of all these changes, we apply our new
techniques to WMAP data. We find no evidence for anisotropy, constraining shear
in the vector mode to $(\sigma_V/H)_0 < 1.7 \times 10^{-10}$ (95% CL). 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 $(\sigma_{T,\rm reg}/H)_0 < 2.4 \times 10^{-7}$ (95% CL).
