Asymmetries in the CMB anisotropy field
Abstract
We report on the results from two independent but complementary statistical analyses of the WMAP data. In the first, the data are subjected to an N-point correlation function analysis, and in the second to a power spectrum analysis. We focus on large and intermediate scales (larger than about 3 degrees), and compare the observed data on these scales against Monte Carlo ensembles with WMAP-like properties. On the largest scales we find that the WMAP data contain too little structure overall; both the two-point and three-point correlation functions reject the model at the 3-sigma level. We also find that there is a clear asymmetry between the northern and the southern galactic hemispheres (also seen in ecliptic coordinates); large regions of the southern galactic hemisphere show the similar amplitudes of fluctuations as the simulated maps, whereas the northern galactic hemisphere is almost completely devoid of large-scale structure. The WMAP maps also lack power on intermediate scales (as measured by correlation functions at 3-5 degrees), and while this effect is visible over the full sky, the north-south asymmetry is evident even in this case. The same asymmetry is found by comparing the power spectra for the northern and southern ecliptic (and galactic) hemispheres. The ratio of the northern to the southern power spectrum is found to be low in the multipole range l~2-35 at the 98% confidence level. Similar asymmetries are found also for the eastern and western galactic hemispheres. Finally, in order to check for systematics, we compute the power spectra also from the COBE-DMR maps, and find the same asymmetries in those data.Keywords
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