WMAP 7 year constraints on CPT violation from large angle CMB anisotropies

We constrain the rotation angle $\alpha$ of the linear polarization of CMB photons using the large angular scale (up to $\sim 4^\circ$) signal in WMAP 7 year data. At these scales, the CMB polarization pattern probes mostly the reionization era. A genuine rotation may be interpreted as cosmological birefringence, which is a well known tracer of new physics, through the breakdown of fundamental symmetries. Our analysis provides $\alpha = -1.6^{\circ} \pm 1.7^{\circ} (3.4^{\circ})$ at 68% (95%) C.L. for the multipoles range $\Delta \ell = 2-47$ not including an instrumental systematic uncertainty that the WMAP team estimates at $\pm 1.5^\circ$. This improves the bound obtained by WMAP team (Komatsu et al., 2010). Moreover we show, for the first time at low multipoles, the angular power spectrum $\alpha_\ell$ in search of a possible scale dependence of the birefringence effect. Our findings are compatible with no detection at all angular scales probed here. We finally forecast the capabilities of Planck in tightening the present constraints.