Testing isotropy of light propagation with cmb polarization data

Lorentz symmetry violations are expected to emerge when spacetime is probed on very short distance scales, of the order of the Planck length (10ô35m), and can produce anomalous light propagation. CMB photons provide a way to test spacetime on these very small scales thanks to their long propagation time, during which propagation anomalies can accumulate. Here we concentrate on a quite generic model for Lorentz-violating electrodynamics, leading to energy-dependent birefringence, whose amount varies with the propagation direction of radiation if space isotropy is also violated. We present the current constraints on isotropic birefringence and show that data gathered by the PLANCK satellite will reach the sensitivity required to test spacetime symmetries up to the Planck-scale. Moreover, we show that the availability of an almost full-sky coverage can allow to perform also accurate tests on non-isotropic birefringence effects. This report is mainly based on works by the author and collaborators. 1;2;3.