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2004MNRAS.354..641H - Mon. Not. R. Astron. Soc., 354, 641-665 (2004/November-1)
Testing the cosmological principle of isotropy: local power-spectrum estimates of the WMAP data.
HANSEN F.K., BANDAY A.J. and GORSKI K.M.
Abstract (from CDS):
Finally, we probe the isotropy of the largest angular scales by estimating the power spectrum on hemispheres and reconfirm strong indications of a north-south asymmetry previously reported by other authors. Indeed, there is a remarkable lack of power in a region associated with the North ecliptic Pole. With the greater fidelity in ℓ-space allowed by this larger sky coverage, we find tentative evidence for residual foregrounds in the range ℓ= 2-4, which could be associated with the low measured quadrupole amplitudes and other anomalies on these angular scales (e.g. planarity and alignment). However, over the range ℓ= 5-40 the observed asymmetry is much harder to explain in terms of residual foregrounds and known systematic effects.
By reorienting the coordinate axes, we partition the sky into different hemispheres and search for the reference frame which maximizes the asymmetric distribution of power. The North Pole for this coordinate frame is found to intersect the sphere at (80°, 57°) in Galactic colatitude and longitude over almost the entire multipole range ℓ= 5-40. Furthermore, the strong negative outlier at ℓ= 21 and the strong positive outlier at ℓ= 39, as determined from the global power spectrum by the WMAP team, are found to be associated with the Northern and Southern hemispheres, respectively (in this frame of maximum asymmetry). Thus, these two outliers follow the general tendency of the multipoles ℓ= 5-40 to be of systematically lower amplitude in the north and higher in the south. Such asymmetric distributions of power on the sky provide a serious test for the cosmological principle of isotropy.
Abstract Copyright: 2004 RAS
Journal keyword(s): methods: data analysis - methods: statistical - techniques: image processing - cosmological parameters - cosmology: observations
Simbad objects: 2
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