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April 2010

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cosmo workshop Fri 15:00 12.03.2010: Li and Liu arXiv:1003.1073: WMAP quadrupole = antenna direction interpolation error
by Boud Roukema 02 Aug '10

02 Aug '10

witam cosmo-torun Cosmo workshop WHEN: Fri 15:00 12.03.2010 WHO: boud TITLE: Liu and Li arXiv:1003.1073 ("The origin of the WMAP quadrupole") ABSTRACT: The WMAP quadrupole is apparently mostly a pointing error. The true quadrupole is much closer to zero. The missing fluctuations problem has just got much worse for the infinite-flat-universe hypothesis. Liu and Li arXiv:1003.1073 ("The origin of the WMAP quadrupole") have posted a very interesting article (presumably submitted to Nature, guessing from the style and length). The best estimate of the CMB quadrupole is apparently... zero! They claim that the WMAP quadrupole comes from a single error - an antenna direction representation error by half of an observational angular interval. The discussion concerns quaternions, but don't be frightened - it presumably uses the imaginary part only, to represent X, Y, Z directions. You can think of them as vectors in R^3 if you like. Order of magnitude check of their calculation: Liu and Li say that the error is by 7' and that this causes incorrect subtraction of the dipole in the time-ordered-date (TOD). The direction of the error varies as the direction of observation varies, so it's reasonable that it doesn't give a simple offset detectable by post-processing in the analysis pipeline, IMHO (though Bartek may have another opinion). The dipole is about 3.3mK (e.g. section 7 Bennett et al 2003). sin(7') * 3.3mK = 0.002 * 3.3mK = 6.7 microK Liu and Li say 10-20 microK, just slightly higher, but their Eq.(1) is a motivational equation - it is not used as an entry to their data analysis pipeline. Figures 1 and 2 should the effect very dramatically. Rephrasing what they've said: Figure 1 left: difference between Liu Li analysis, using the same wrong method that the WMAP team used. This is a check that their pipeline does the same thing that the WMAP team does. |Difference| < 2 or so microK. Figure 1 right: Liu Li using correct antenna directions on WMAP Q1 3yr data. They get a strong quadrupole! Figure 2 left: The effect of using a wrong dipole, calculated using *only the directional information* from the time-ordered-data from the spacecraft, with *no CMB data*. See paragraph 2, page 4: "only the spacecraft attitude information is used to compute d' ." Figure 2 right: official WMAP5 V+W quadrupole. Figures 1 right, and 2 left look very, very similar to Figure 2 right. There seems to be a slight difference in angular position visible by eye, but the coincidence is striking. Can we really believe that the CMB quadrupole just by chance happens to be very strongly aligned with and of nearly identical amplitude to a map made using only the time-ordered-data of the spacecraft direction ("attitude") and the spacecraft orbit around the Sun, and the Sun's motion with respect to the CMB? Since this is a Nature-type article, the authors are forced to exclude almost all interesting details from the article, but info like the spacecraft attitude quaternions and interpolation method should not be too difficult to find. * Bennett et al 2003: arXiv:astro-ph/0302207 http://arXiv.org/abs/astro-ph/0302207 * Liu, Li 2010: arXiv:1003.1073 http://arXiv.org/abs/1003.1073 The good thing about this is that it shows that a lot of very intelligent people - those in a mega-collaboration plus many outside of the official group - can spend 7 years looking closely at an important set of observational data without finding what appears to be a very elementary error in the analysis details, with a very fundamental consequence. The fundamental consequence is that a lower quadrupole makes Infeld's (1949) prediction of the equivalent of the low l cutoff due to the shape of the Universe even stronger! Liu & Li suggest an inflationary argument, but their interpretation is not what is important - they don't want the referee to make a fuss about interpretations, since their observational argument is so strong. pozdr boud

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