Hemispherical power asymmetry: parameter estimation from cosmic microwave background WMAP5 data

Abstract

We re-examine the evidence of hemispherical power asymmetry, detected in the cosmic microwave background (CMB) WMAP (Wilkinson Microwave Anisotropy Probe) data using a new method. We use a data filtering, preprocessing , and a statistical approach different from those used previously, and pursue an independent method of parameter estimation. First, we analyze the hemispherical variance ratios and compare these with simulated distributions. Secondly, working within a previously proposed CMB bipolar modulation model, we constrain the model parameters: the amplitude and the orientation of the modulation field as a function of various multipole bins. Finally, we select three ranges of multipoles leading to the most anomalous signals, and we process corresponding 100 Gaussian, random field (GRF) simulations, treated as observational data, to further test the statistical significance and robustness of the hemispherical power asymmetry. For our analysis we use the Internally-Linearly-Coadded (ILC) full sky map, and the KQ75 cut sky V channel foregrounds reduced map of the WMAP five year data (V5). We constrain the modulation parameters using a generic maximum a posteriori method.In particular, we find differences in hemispherical power distribution, which when described in terms of a model with bipolar modulation field, exclude the field amplitude value of the isotropic model A = 0 at confidence level of ∼ 99.5% ( ∼ 99.4%) in the multipole range ℓ ∈ [7, 19] (ℓ ∈ [7, 79]) in the V5 data, and at the confidence level ∼ 99.9% in the multipole range ℓ ∈ [7, 39] in the ILC5 data, with the best fit (modal PDF) values in these particular multipole ranges of A = 0.21 (A = 0.21) and A = 0.15 respectively.However, we also point out that similar or larger significances (in terms of rejecting the isotropic model), and large best-fit modulation amplitudes are obtained in GRF simulations as well, which reduces the overall significance of the CMB power asymmetry down to only about 94% (95%) in the V5 data, in the range ℓ ∈ [7, 19] (ℓ ∈ [7, 79]).

Needlet bispectrum asymmetries in the WMAP 5-year data

“…The presence of these anomalies has been tested against mask effect and foreground contamination by Bernui et al (2007). Lew (2008) constrains the direction of the anomaly axis using a generic maximum a posteriori method. Very recently, Hansen et al (2009) reported that the power asymmetry spans a very large range of angular scales (corresponding to multipoles 2 ≤ℓ≤ 600): this result is based on an angular power spectrum analysis of the WMAP sky maps.…”

Signals of statistical anisotropy inWMAPforeground-cleaned maps

“…The CMB studies indicate an alignment of the low‐ l multipoles (de Oliveira‐Costa et al 2004; Ralston & Jain 2004; Schwarz et al 2004) and a hemispherical anisotropy (Eriksen et al 2004). The indications of violation of isotropy in CMB data has prompted substantial activity with varying outcomes (Bielewicz, Górski & Banday 2004; Hansen, Banday & Górski 2004; Katz & Weeks 2004; Bielewicz et al 2005; Prunet et al 2005; Bernui et al 2006; Copi et al 2006; de Oliveira‐Costa & Tegmark 2006; Freeman et al 2006; Wiaux et al 2006; Bernui et al 2007; Copi et al 2007; Eriksen et al 2007b; Helling, Schupp & Tesileanu 2007; Land & Magueijo 2007; Magueijo & Sorkin 2007; Bernui 2008; Lew 2008). Differences arise due to different tests being used by different authors (Efstathiou 2003; Hajian, Souradeep & Cornish 2004; Donoghue & Donoghue 2005; Hajian & Souradeep 2006) and radio (Bietenholz & Kronberg 1984) in CMB data.…”

Can residuals of the solar system foreground explain low multipole anomalies of the CMB?

“…This assumption, leads to the following properties of the foreground: f l,m = f (l)δ m,0 δ l,2n , n = 1, 2..., clearly demonstrating that only even multipoles can be affected by the KBO-foreground. At the same time, at the level of 3σ [4], the ILC 7 map has dipole modulation, which will provide a coupling between the even and odd components of the KBO emission, depending on the orientation of the dipole term and the corresponding amplitude of modulation. Here we assume that this kind of modulation has a non-cosmological (systematic) origin, leading to a distortion of the CMB and the foregrounds as well.…”

“…More importantly, due to the high degree of symmetry, the morphology of the KBO signal could be remarkably similar to the morphology of the MNO, for instance at the V-band, and at least at the low multipole range l ≤ 20 − 30, where the power is comparable to the CMB power spectrum. In [4], the concept of dipole modulation of the CMB is presented. The dipole modulation is effectively an addition of a term in harmonic space, with an amplitude A.…”

“…Effectively this introduces a dipole component in the measured map. See [4] for further details. The critical point of our analysis is the assumption, that the dipole modulation of the CMB, is associated with an unknown systematic effect.…”