[Cosmo-torun] cosmo workshop Fri 15:00 12.03.2010: Li and Liu arXiv:1003.1073: WMAP quadrupole = antenna direction interpolation error

[Boud Roukema]

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