How isotropic is the Universe?
  A fundamental assumption in the standard model of cosmology is that the
Universe is isotropic on large scales. Breaking this assumption leads to a set
of solutions to Einstein's field equations, known as Bianchi cosmologies, only
a subset of which have ever been tested against data. For the first time, we
consider all degrees of freedom in these solutions to conduct a general test of
isotropy using cosmic microwave background temperature and polarization data
from Planck. For the vector mode (associated with vorticity), we obtain a limit
on the anisotropic expansion of $(\sigma_V/H)_0 < 4.7 \times 10^{-11}$ (95%
CI), which is an order of magnitude tighter than previous Planck results that
used CMB temperature only. We also place upper limits on other modes of
anisotropic expansion, with the weakest limit arising from the regular tensor
mode, $(\sigma_{T,\rm reg}/H)_0<1.0 \times 10^{-6}$ (95% CI). Including all
degrees of freedom simultaneously for the first time, anisotropic expansion of
the Universe is strongly disfavoured, with odds of 121,000:1 against.
