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

Abstract

We re-examine the evidence for 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 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 a hundred corresponding 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 foreground 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 a bipolar modulation field, exclude the field amplitude valuemore » of the isotropic model, A = 0, at the confidence level of {approx}99.5% ({approx}99.4%) in the multipole range l element of [7,19] (l element of [7,79]) for the V5 data, and at the confidence level of {approx}99.9% in the multipole range l element of [7,39] for the ILC5 data, with best-fit (modal probability distribution function) 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 to only about 94% (95%) for the V5 data, in the range l element of [7,19] (l element of [7,79])« less

Authors:
 [1]
  1. Department of Physics and Astrophysics, Nagoya University, Nagoya 464-8602 (Japan)
Publication Date:
OSTI Identifier:
22156753
Resource Type:
Journal Article
Journal Name:
Journal of Cosmology and Astroparticle Physics
Additional Journal Information:
Journal Volume: 2008; Journal Issue: 09; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1475-7516
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; AMPLITUDES; ANISOTROPY; ASYMMETRY; DISTRIBUTION FUNCTIONS; FIELD EQUATIONS; GAUSSIAN PROCESSES; MODULATION; MULTIPOLES; POWER DISTRIBUTION; PROBABILITY; PROBES; RELICT RADIATION; SIMULATION; SKY

Citation Formats

Lew, Bartosz, and Torun Center for Astronomy, Nicolaus Copernicus University, ulica Gagarina 11, 87-100 Torun. Hemispherical power asymmetry: parameter estimation from cosmic microwave background WMAP5 data. United States: N. p., 2008. Web. doi:10.1088/1475-7516/2008/09/023.
Lew, Bartosz, & Torun Center for Astronomy, Nicolaus Copernicus University, ulica Gagarina 11, 87-100 Torun. Hemispherical power asymmetry: parameter estimation from cosmic microwave background WMAP5 data. United States. https://doi.org/10.1088/1475-7516/2008/09/023
Lew, Bartosz, and Torun Center for Astronomy, Nicolaus Copernicus University, ulica Gagarina 11, 87-100 Torun. 2008. "Hemispherical power asymmetry: parameter estimation from cosmic microwave background WMAP5 data". United States. https://doi.org/10.1088/1475-7516/2008/09/023.
@article{osti_22156753,
title = {Hemispherical power asymmetry: parameter estimation from cosmic microwave background WMAP5 data},
author = {Lew, Bartosz and Torun Center for Astronomy, Nicolaus Copernicus University, ulica Gagarina 11, 87-100 Torun},
abstractNote = {We re-examine the evidence for 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 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 a hundred corresponding 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 foreground 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 a bipolar modulation field, exclude the field amplitude value of the isotropic model, A = 0, at the confidence level of {approx}99.5% ({approx}99.4%) in the multipole range l element of [7,19] (l element of [7,79]) for the V5 data, and at the confidence level of {approx}99.9% in the multipole range l element of [7,39] for the ILC5 data, with best-fit (modal probability distribution function) 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 to only about 94% (95%) for the V5 data, in the range l element of [7,19] (l element of [7,79])},
doi = {10.1088/1475-7516/2008/09/023},
url = {https://www.osti.gov/biblio/22156753}, journal = {Journal of Cosmology and Astroparticle Physics},
issn = {1475-7516},
number = 09,
volume = 2008,
place = {United States},
year = {Mon Sep 15 00:00:00 EDT 2008},
month = {Mon Sep 15 00:00:00 EDT 2008}
}