The cosmic microwave background quadrupole in a polarized light

The low quadrupole of the cosmic microwave background (CMB), measured by the Cosmic Background Explorer ( COBE) and confirmed by the Wilkinson Microwave Anisotropy Probe (WMAP), has generated much discussion recently. We point out that the well-known correlation between temperature and polarization anisotropies of the CMB further constrains the low-multipole anisotropy data. This correlation originates from the fact that the low-multipole polarization signal is sourced by the CMB quadrupole as seen by free electrons during the relatively recent cosmic history. Consequently, the large-angle temperature anisotropy data make restrictive predictions for the large-angle polarization anisotropy, which depend primarily on the optical depth for electron scattering after cosmological recombination, tau. We show that if current cosmological models for the generation of large-angle anisotropy are correct and the COBE/WMAP data are not significantly contaminated by non-CMB signals, then the observed C-l(TE) amplitude on the largest scales is discrepant at the similar to99.8% level with the observed C-l(TT) for the concordant LambdaCDM model with tau 0.10. Using tau = 0.17, the preferred WMAP model-independent value, the discrepancy is at the level of 98.5%.