Cosmic opacity to CMB photons and polarization measurements

Anisotropy data analysis leaves a significant degeneracy between primeval spectral index ( n s ) and cosmic opacity to CMB photons ( τ). Low- l polarization measures, in principle, can remove it. We perform a likelihood analysis to see how cosmic variance possibly affects such a problem. We find that, for a sufficiently low noise level ( σ P pix) and if τ is not negligibly low, the degeneracy is greatly reduced, while the residual impact of cosmic variance on n s and τ determinations is under control. On the contrary, if σ P pix is too high, cosmic variance effects appear to be magnified. We apply general results to specific experiments and find that, if favorable conditions occur, it is possible that a 2- σ detection of a lower limit on τ is provided by the SPOrt experiment. Furthermore, if the PLANCK experiment will measure polarization with the expected precision, the error on low- l harmonics is adequate to determine τ, without significant magnification of the cosmic variance. This however indicates that high sensitivity might be more important than high resolution in τ determinations. We also outline that a determination of τ is critical to perform detailed analyses on the nature of dark energy and/or on the presence of primeval gravitational waves.