2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/36389We show that significant anisotropy in electromagnetic propagation generates a distinctive signature in the microwave background. The anisotropy may be determined by looking at the cross correlator of the $E$-mode and $B$-mode polarisation spectrum.4 pagesGeneral Relativity and Quantum Cosmologytext