Weak lensing B-modes on all scales as a probe of local isotropy
  This article derives a multipolar hierarchy for the propagation of the weak-lensing shear and convergence in a general spacetime. The origin of B-modes, in particular on large angular scales, is related to the local isotropy of space. Known results assuming a Friedmann-Lema\^itre background are naturally recovered. The example of a Bianchi I spacetime illustrates our formalism and its implications for future observations are stressed. 
Cyril Pitrou, Jean-Philippe Uzan, Thiago S. Pereira
astro-ph.CO gr-qc