The low quadrupole of the cosmic microwave background ( CMB ) , measured by COBE and confirmed by 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 { \cal C } _ { \ell } ^ { TE } amplitude on the largest scales is discrepant at the \sim 99.8 \% level with the observed { \cal C } _ { \ell } ^ { TT } for the concordance \Lambda CDM model with \tau = 0.10 .
Using \tau = 0.17 , the preferred WMAP model-independent value , the discrepancy is at the level of 98.5 % .