Weak-lensing $B$-modes as a probe of the isotropy of the universe
  We compute the angular power spectrum of the $B$-modes of the weak-lensing
shear in a spatially anisotropic spacetime. We find that there must also exist
off-diagonal correlations between the $E$-modes, $B$-modes, and convergence
that allow one to reconstruct the eigendirections of expansion. Focusing on
future surveys such as Euclid and SKA, we show that observations can constrain
the geometrical shear in units of the Hubble rate at the percent level, or even
better, offering a new and powerful method to probe our cosmological model.
