Alignment and signed‐intensity anomalies in Wilkinson Microwave Anisotropy Probe data

P. Vielva 1 Y. Wiaux 0 P. Vandergheynst 0 0 Signal Processing Institute , Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland 1 Instituto de Fsica de Cantabria (CSIC - UC) , 39005 Santander, Spain A B S T R A C T Significant alignment and signed-intensity anomalies of local features of the cosmic microwave background (CMB) are detected on the three-year Wilkinson Microwave Anisotropy Probe data, through a decomposition of the signal with steerable wavelets on the sphere. In addition to identifying local features of a signal at specific scales, steerable wavelets allow one to determine their local orientation and signed intensity. First, an alignment analysis identifies two mean preferred planes in the sky, both with normal axes close to the CMB dipole axis. The first plane is defined by the directions towards which local CMB features are anomalously aligned. A mean preferred axis is also identified in this plane, located very close to the ecliptic poles axis. The second plane is defined by the directions anomalously avoided by local CMB features. This alignment anomaly provides further insight on recent results. Secondly, a signedintensity analysis identifies three mean preferred directions in the southern Galactic hemisphere with anomalously high or low temperature of local CMB features: a cold spot essentially identified with a known cold spot, a second cold spot lying very close to the southern end of the CMB dipole axis, and a hotspot lying close to the southern end of the ecliptic poles axis. In both analyses, the anomalies are observed at wavelet scales corresponding to angular sizes around 10 on the celestial sphere, with global significance levels around 1 per cent. Further investigation reveals that the alignment and signed-intensity anomalies are only very partially related. Instrumental noise, foreground emissions and some form of other systematics are strongly rejected as possible origins of the detections. An explanation might still be envisaged in terms of a global violation of the isotropy of the Universe, inducing an intrinsic statistical anisotropy of the CMB. - E. Martnez-Gonz alez1 In the light of the results obtained from the analysis of several recent high-precision data sets, a wide consensus was reached in the cosmology community on a concordance model for a flat Lambda cold dark matter ( CDM) Universe with a primordial phase of inflation. In this framework, the Universe is flat and filled in with CDM and dark energy in the form of a cosmological constant (), in addition to the standard baryonic and electromagnetic components. The cosmological principle assumption states that the Universe is globally homogeneous and isotropic. The large-scale structure and the cosmic microwave background (CMB) radiation find their origin in E-mail: (PV); (YW); (EM-G); (PV) Gaussian quantum energy density fluctuations developed around the homogeneous and isotropic background, as predicted by the standard inflationary scenario taking place in the primordial Universe. The CMB data, and specifically the data provided by the NASA Wilkinson Microwave Anisotropy Probe (WMAP) satellite experiment, have played a leading role in defining this concordance model (Bennett et al. 2003a; Spergel et al. 2003; Hinshaw et al. 2007; Spergel et al. 2007). However, anomalies have been reported. They naturally question the basic hypotheses on which the model relies, in particular the Gaussianity (assumed by the standard inflationary scenario) and statistical isotropy (postulated by the cosmological principle) of the statistical distribution from which the CMB temperature fluctuations arise in the primordial Universe. Departures from Gaussianity of the one-year and three-year WMAP data have been reported in terms of various statistics. First, non-Gaussianity was notably detected through the use of a genus-based statistic (Park 2004). Secondly, wavelet analyses have also reported non-Gaussian deviations. An excess in the kurtosis of the wavelet coefficients of the axisymmetric Mexican hat wavelet on the sphere was found at wavelet scales corresponding to angular sizes on the celestial sphere around 10 (Vielva et al. 2004). This deviation is located in the southern galactic hemisphere, where a very cold spot was identified at ( , ) = (147, 209), with [0, ] and [0, 2), respectively, standing for the colatitude and longitude in galactic spherical coordinates. This detection was confirmed with various wavelets and various statistics (Mukherjee & Wang 2004; Cay on, Jin & Treaster 2005; Cruz et al. 2005; McEwen et al. 2005; Cruz et al. 2006; McEwen et al. 2006; Cruz et al. 2007). Note that the cold spot identified not only bears non-Gaussianity, but also represents a departure from statistical isotropy, in terms of a asymmetry in galactic coordinates. Departures from statistical isotropy of the one-year and three-year WMAP data have also been reported in terms of various statistics. First, a northsouth asymmetry in ecliptic coordinates has been detected through the use of N-point correlation functions (Eriksen et al. 2004a, 2005), Minkowski functionals (Eriksen et al. 2004b), local power spectra (Hansen, Banday & G orski 2004b; Donoghue & Donoghue 2005), local bispectra (Land & Magueijo 2005a) and local curvature (Hansen et al. 2004a). The asymmetry is maximum in a coordinate system with the north pole lying at ( , ) = (80, 57), close to the northern end of the ecliptic poles axis at ( , ) = (60, 96). This northsouth asymmetry was confirmed through the application of a pair angular separation histogram method (Bernui et al. 2006, 2007). More recently, it was again observed through the search for a best-fitting dipolar modulation of the three-year WMAP data (Eriksen et al. 2007; Spergel et al. 2007). The corresponding dipolar axis has a northern end at ( , ) = (63, 45). Secondly, an anomalous alignment of the lowest multipoles of the WMAP data was reported, especially at = 2, 3 (Copi, Huterer & Starkman 2004; de Oliveira-Costa et al. 2004; Katz & Weeks 2004; Schwarz et al. 2004; Bielewicz et al. 2005), but also up to = 5 (Land & Magueijo 2005b; Abramo et al. 2006), and for = 6, 7 (Freeman et al. 2006). This alignment highlights the so-called axis of evil, with northern end at ( , ) = (30, 260), very close to the CMB dipole axis, with northern end at ( , ) = (42, 264). Very recently, a new statistic to analyse the alignment of multipoles that is robust against treatments of the galactic plane was proposed (Land & Magueijo 2006). The corresponding analysis highlights a weaker detection for these alignments. The large-scale fluctuations of the CMB were also pointed out to fit with the expected pattern of an anisotropic Bianchi VIIh Universe (Jaffe et al. 2006a,c; Bridges et al. 2007), although the cosmological parameters derived from this hypothesis are ruled out (Jaffe et al. 2006b; Bridges et al. 2007). Note on the contrary that bipolar power spectra (...truncated)