Testing the cosmological principle of isotropy: Local power spectrum estimates of the WMAP data

By F. Hansen, A. Banday, K. G'orski. 2004

302 citations recorded by the source corpus.

We apply the Gabor transform methodology proposed in (Hansen et al. 2002, 2003) to the WMAP data in order to test the statistical properties of the CMB fluctuation field and specifically to evaluate the fundamental assumption of cosmological isotropy. In particular, we apply the transform with several apodisation scales, thus allowing the determination of the positional dependence of the angular power spectrum with either high spatial localisation or high angular resolution (ie. narrow bins in multipole space). Practically, this implies that we estimate the angular power spectrum locally in discs of various sizes positioned in different directions: small discs allow the greatest sensitivity to positional dependence, whereas larger discs allow greater sensitivity to variations over different angular scales. In addition, we determine whether the spatial position of a few outliers in the angular power spectrum could suggest the presence of residual foregrounds or systematic effects. For multipoles close to the first peak, the most deviant local estimates from the best fit WMAP model are associated with a few particular areas close to the Galactic plane. Such deviations also include the “dent” in the spectrum just shortward of the first peak which was remarked upon by the WMAP team. Estimating the angular power spectrum excluding these areas gives a slightly higher first Doppler peak amplitude. 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 (eg. 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 maximises the asymmetric distribution of power. The north pole for this coordinate frame is found to intersect the sphere at (80 ◦ ,57 ◦ ) in Galactic co-latitude 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/9b918b5e98d47c66b0f8860149e6c7ea66ba8f7d

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In Astronomy and Astrophysics

1. H. Eriksen
2. A. Banday
3. K. G'orski
4. F. Hansen
5. P. Lilje
6. Jiro Soda
7. J. Hoftuft
8. A. Maleknejad

Citations in the corpus

• Hemispherical Power Asymmetry in the Third-Year Wilkinson Microwave Anisotropy Probe Sky Maps (2007, background): “…multipoles (de Oliveira-Costa et al. 2004; Eriksen et al. 2004a), 2) an apparent asymmetry in the distribution of fluctuation power in two opposing hemispheres (Eriksen et al. 2004b; Hansen et al. 2004), and 3) a peculiar cold spot in the southern hemisphere (Vielva et al. 2004; Cruz et al. 2005).”
• Evidence of Vorticity and Shear at Large Angular Scales in the WMAP Data: A Violation of Cosmological Isotropy? (2005, result): “(Eriksen et al. 2004a; Hansen et al. 2004a,b).”
• POWER ASYMMETRY IN WMAP AND PLANCK TEMPERATURE SKY MAPS AS MEASURED BY A LOCAL VARIANCE ESTIMATOR (2014, background): “Among these was one suggesting a directional dependency of the CMB angular power spectrum (Eriksen et al. 2004; Hansen et al. 2004), often referred to as a “hemispherical power asymmetry.””
• Detection of a non‐Gaussian spot in WMAP (2004, background): “Thereafter several other groups have found non-Gaussian signatures or asymmetries in the WMAP data (Park 2003, Eriksen et al. 2003, 2004a, Hansen et al. 2004a, b).”
• Gauge fields and inflation (2012, background): “These “anomalies” include non-Gaussianity which we already discussed, a low power in the quadrapole moment [35, 36], the alignment of the lower multipoles [37], a five-degree scale cold spot with suppressed power [38], an asymmetry in power between the Northern and Southern ecliptic hemisphere [39],”
• FIVE-YEAR WILKINSON MICROWAVE ANISOTROPY PROBE OBSERVATIONS: COSMOLOGICAL INTERPRETATION (2008, unknown): “…et al. 2007; Land & Magueijo 2005b,a, 2007; Rakić & Schwarz 2007; Park et al. 2007; Bernui et al. 2007; Hajian & Souradeep 2003; Hajian et al. 2005; Hajian & Souradeep 2006; Prunet et al. 2005; Hansen et al. 2004; Hansen et al. 2004), many of which are related to the large-scale features at l .”
• The Axis of Evil revisited (2006, unknown): “Reports of something awry have been obtained using a variety of techniques (e.g., Park (2004); Eriksen et al. (2004a); Hansen et al. (2004); Donoghue & Donoghue (2005); Land & Magueijo (2005a); Hansen et al. (2004); Eriksen et al. (2004b); Vielva et al. (2004)).”
• INCREASING EVIDENCE FOR HEMISPHERICAL POWER ASYMMETRY IN THE FIVE-YEAR WMAP DATA (2009, background): “Of particular interest to us is the question of hemispherical distribution of power in the WMAP data, first reported by Eriksen et al. (2004a) and later confirmed by, e.g., Hansen et al. (2004) and Eriksen et al. (2005).”
• POWER ASYMMETRY IN COSMIC MICROWAVE BACKGROUND FLUCTUATIONS FROM FULL SKY TO SUB-DEGREE SCALES: IS THE UNIVERSE ISOTROPIC? (2008, methodology): “…as well as other statistical anomalies were confirmed by several other authors using other methods (de Oliveira-Costa et al. 2004; Tegmark et al. 2003; Park 2004; Vielva et al. 2004b; Eriksen et al. 2004b; Land & Magueijo 2005a,b; Larson & Wandelt 2004; McEwen et al. 2005; Hansen et al. 2004b,c).”
• Fundamental physics with the Square Kilometre Array (2018, methodology): “…was reported based on analysis of the off-diagonal components of angular correlations of CMB anisotropies with WMAP and Planck data on large scales (Hansen et al., 2004; Gordon et al., 2005; Eriksen et al., 2007; Gordon, 2007; Planck Collaboration et al., 2014b; Akrami et al., 2014; Planck…”