[1103.3505] Bias in low-multipole CMB reconstructions
Posted: March 25 2011
The most conspicuous anomalies in the CMB are the deviations from statistical homogeneity and isotropy on large angles. There has been debate about their significance and the best method for evaluating them. (See http://cosmocoffee.info/viewtopic.php?t=1582. and http://cosmocoffee.info/viewtopic.php?t=1601&highlight= ).
In particular, the masked CMB sky shows almost zero large angle correlation function, at a level which is here quoted as 99.975% unlikely. However, if you do the analysis on the ILC map, the unlikeliness is only at the 95% level, which is not significant. It seems to me that this result alone shows that something funny is going on: the regions inside and outside the mask seem to behave rather differently!
The present paper argues that the procedures for reconstructing the full sky from a masked sky in fact involve assumptions about what lies behind the mask to such an extent that the reconstruction is significantly biased by the contents of the cut region. The authors therefore conclude (in agreement with their previous work) that the large-angle properties should be studied on the cut sky, not on a reconstructed full sky.
In particular, the masked CMB sky shows almost zero large angle correlation function, at a level which is here quoted as 99.975% unlikely. However, if you do the analysis on the ILC map, the unlikeliness is only at the 95% level, which is not significant. It seems to me that this result alone shows that something funny is going on: the regions inside and outside the mask seem to behave rather differently!
The present paper argues that the procedures for reconstructing the full sky from a masked sky in fact involve assumptions about what lies behind the mask to such an extent that the reconstruction is significantly biased by the contents of the cut region. The authors therefore conclude (in agreement with their previous work) that the large-angle properties should be studied on the cut sky, not on a reconstructed full sky.