The Cosmic Microwave Background Quadrupole in a Polarized Light

The low quadrupole of the cosmic microwave background (CMB), measured by COBEand confirmed by WMAP, has generated much discussion recently. We point outthat the well-known correlation between temperature and polarizationanisotropies of the CMB further constrains the low multipole anisotropy data.This correlation originates from the fact that the low-multipole polarizationsignal is sourced by the CMB quadrupole as seen by free electrons during therelatively recent cosmic history. Consequently, the large-angle temperatureanisotropy data make restrictive predictions for the large-angle polarizationanisotropy, which depend primarily on the optical depth for electron scatteringafter cosmological recombination, tau. We show that if current cosmologicalmodels for the generation of large angle anisotropy are correct and theCOBE/WMAP data are not significantly contaminated by non-CMB signals, then theobserved C_te amplitude on the largest scales is discrepant at the 99.8% levelwith the observed C_tt for the concordance LCDM model with tau=0.10. Usingtau=0.17, the preferred WMAP model-independent value, the discrepancy is at thelevel of 98.5%.Comment: 6 pages, 6 figures, ApJ in pres