Wednesday, May 24, 2006

Gary Hinshaw: WMAP 3 year results

EUD colloquium by Gary Hinshaw, data lead on the WMAP mission. He summarized the science justification and history then launched into the the 3 year results. The gain calibration has been improved leading to slightly different results on the temperature power spectrum - particularly at the low l end - and of course the S/N is improved. The big new result is the polarization map which provides a much better measure of the optical depth to free electron scattering - this is significantly less than the best-fit from the 1-year results implying a later reionization epoch (400,000 years after inflation rather than 100,000 years). Since the optical depth is highly correlated with the power-spectrum index this result also enables a more precise measurement of the index which is now inconsistent with Harrison-Zel'dovich n=1 at 2 sigma. The best fit is around n=0.96 consistent with two of the most popular inflation models. In answer to my question Gary also noted that the octopole is now more consistent with LCDM. The quadrapole best fit is still low however examination of the complete likelihood function shows a tail to larger values such that there is a 1/10 probability of getting the observed result. Since precision is limited by cosmic variance (the quadrapole is the variance of 5 independent numbers) we can't do any better.

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