The Wilkinson Microwave Anisotropy Probe ( WMAP ) has mapped the full sky in Stokes I , Q , and U parameters at frequencies 23 , 33 , 41 , 61 , and 94 GHz .
We detect correlations between the temperature and polarization maps significant at more than 10 standard deviations .
The correlations are inconsistent with instrument noise and are significantly larger than the upper limits established for potential systematic errors .
The correlations are present in all WMAP frequency bands with similar amplitude from 23 to 94 GHz , and are consistent with a superposition of a CMB signal with a weak foreground .
The fitted CMB component is robust against different data combinations and fitting techniques .
On small angular scales ( \theta < 5 \arcdeg ) , the WMAP data show the temperature-polarization correlation expected from adiabatic perturbations in the temperature power spectrum .
The data for \ell > 20 agree well with the signal predicted solely from the temperature power spectra , with no additional free parameters .
We detect excess power on large angular scales ( \theta > 10 \arcdeg ) compared to predictions based on the temperature power spectra alone .
The excess power is well described by reionization at redshift 11 < z _ { r } < 30 at 95 % confidence , depending on the ionization history .
A model-independent fit to reionization optical depth yields results consistent with the best-fit \Lambda CDM model , with best fit value \tau = 0.17 \pm 0.04 at 68 % confidence , including systematic and foreground uncertainties .
This value is larger than expected given the detection of a Gunn-Peterson trough in the absorption spectra of distant quasars , and implies that the universe has a complex ionization history : WMAP has detected the signal from an early epoch of reionization .