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Register a new userHemispherical power asymmetry in the three-year Wilkinson Microwave Anisotropy Probe sky maps
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Added by
Hans Kristian Eriksen
Publication date
2007
fields
Physics
and research's language is
English
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We consider the issue of hemispherical power asymmetry in the three-year WMAP data, adopting a previously introduced modulation framework. Computing both frequentist probabilities and Bayesian evidences, we find that the model consisiting of an isotropic CMB sky modulated by a dipole field, gives a substantially better fit to the observations than the purely isotropic model, even when accounting for the larger prior volume. For the ILC map, the Bayesian log-evidence difference is ~1.8 in favour of the modulated model, and the raw improvement in maximum log-likelihood is 6.1. The best-fit modulation dipole axis points toward (l,b) = (225 deg,-27 deg), and the modulation amplitude is 0.114, in excellent agreement with the results from the first-year analyses. The frequentist probability of obtaining such a high modulation amplitude in an isotropic universe is ~1%. These results are not sensitive to data set or sky cut. Thus, the statistical evidence for a power asymmetry anomaly is both substantial and robust, although not decisive, for the currently available data. Increased sky coverage through better foreground handling and full-sky and high-sensitivity polarization maps may shed further light on this issue.
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We present a full-sky model of polarized Galactic microwave emission based on three years of observations by the Wilkinson Microwave Anisotropy Probe (WMAP) at frequencies from 23 to 94 GHz. The model compares maps of the Stokes Q and U components from each of the 5 WMAP frequency bands in order to separate synchrotron from dust emission, taking into account the spatial and frequency dependence of the synchrotron and dust components. This simple two-component model of the interstellar medium accounts for at least 97% of the polarized emission in the WMAP maps of the microwave sky. Synchrotron emission dominates the polarized foregrounds at frequencies below 50 GHz, and is comparable to the dust contribution at 65 GHz. The spectral index of the synchrotron component, derived solely from polarization data, is -3.2 averaged over the full sky, with a modestly flatter index on the Galactic plane. The synchrotron emission has mean polarization fraction 2--4% in the Galactic plane and rising to over 20% at high latitude, with prominent features such as the North Galactic Spur more polarized than the diffuse component. Thermal dust emission has polarization fraction 1% near the Galactic center, rising to 6% at the anti-center. Diffuse emission from high-latitude dust is also polarized with mean fractional polarization 0.036 +/- 0.011.
The Wilkinson Microwave Anisotropy Probe WMAP has mapped the entire sky in five frequency bands between 23 and 94 GHz with polarization sensitive radiometers. We present three-year full-sky maps of the polarization and analyze them for foreground emission and cosmological implications. These observations open up a new window for understanding the universe. WMAP observes significant levels of polarized foreground emission due to both Galactic synchrotron radiation and thermal dust emission. The least contaminated channel is at 61 GHz. Informed by a model of the Galactic foreground emission, we subtract the foreground emission from the maps. In the foreground corrected maps, for l=2-6, we detect l(l+1) C_l^{EE} / (2 pi) = 0.086 +-0.029 microkelvin^2. This is interpreted as the result of rescattering of the CMB by free electrons released during reionization and corresponds to an optical depth of tau = 0.10 +- 0.03. We see no evidence for B-modes, limiting them to l(l+1) C_l^{BB} / (2 pi) = -0.04 +- 0.03 microkelvin^2. We find that the limit from the polarization signals alone is r<2.2 (95% CL) corresponding to a limit on the cosmic density of gravitational waves of Omega_{GW}h^2 < 5 times 10^{-12}. From the full WMAP analysis, we find r<0.55 (95% CL) corresponding to a limit of Omega_{GW}h^2 < 10^{-12} (95% CL).
We present new full-sky temperature maps in five frequency bands from 23 to 94 GHz, based on the first three years of the WMAP sky survey. The new maps, which are consistent with the first-year maps and more sensitive, incorporate improvements in data processing made possible by the additional years of data and by a more complete analysis of the polarization signal. These include refinements in the gain calibration and beam response models. We employ two forms of multi-frequency analysis to separate astrophysical foreground signals from the CMB, each of which improves on our first-year analyses. First, we form an improved Internal Linear Combination map, based solely on WMAP data, by adding a bias correction step and by quantifying residual uncertainties in the resulting map. Second, we fit and subtract new spatial templates that trace Galactic emission; in particular, we now use low-frequency WMAP data to trace synchrotron emission. The WMAP point source catalog is updated to include 115 new sources. We derive the angular power spectrum of the temperature anisotropy using a hybrid approach that combines a maximum likelihood estimate at low l (large angular scales) with a quadratic cross-power estimate for l>30. Our best estimate of the CMB power spectrum is derived by averaging cross-power spectra from 153 statistically independent channel pairs. The combined spectrum is cosmic variance limited to l=400, and the signal-to-noise ratio per l-mode exceeds unity up to l=850. The first two acoustic peaks are seen at l=220.8 +- 0.7 and l=530.9 +- 3.8, respectively, while the first two troughs are seen at l=412.4 +- 1.9 and l=675.1 +- 11.1, respectively. The rise to the third peak is unambiguous; when the WMAP data are combined with higher resolution CMB measurements, the existence of a third acoustic peak is well established.
(Abridged) New full sky temperature and polarization maps based on seven years of data from WMAP are presented. The new results are consistent with previous results, but have improved due to reduced noise from the additional integration time, improved knowledge of the instrument performance, and improved data analysis procedures. The improvements are described in detail. The seven year data set is well fit by a minimal six-parameter flat Lambda-CDM model. The parameters for this model, using the WMAP data in conjunction with baryon acoustic oscillation data from the Sloan Digital Sky Survey and priors on H_0 from Hubble Space Telescope observations, are: Omega_bh^2 = 0.02260 +-0.00053, Omega_ch^2 = 0.1123 +-0.0035, Omega_Lambda = 0.728 +0.015 -0.016, n_s = 0.963 +-0.012, tau = 0.087 +-0.014 and sigma_8 = 0.809 +-0.024 (68 % CL uncertainties). The temperature power spectrum signal-to-noise ratio per multipole is greater that unity for multipoles < 919, allowing a robust measurement of the third acoustic peak. This measurement results in improved constraints on the matter density, Omega_mh^2 = 0.1334 +0.0056 -0.0055, and the epoch of matter- radiation equality, z_eq = 3196 +134 -133, using WMAP data alone. The new WMAP data, when combined with smaller angular scale microwave background anisotropy data, results in a 3 sigma detection of the abundance of primordial Helium, Y_He = 0.326 +-0.075.The power-law index of the primordial power spectrum is now determined to be n_s = 0.963 +-0.012, excluding the Harrison-Zeldovich-Peebles spectrum by >3 sigma. These new WMAP measurements provide important tests of Big Bang cosmology.
We present new full-sky temperature and polarization maps in five frequency bands from 23 to 94 GHz, based on data from the first five years of the WMAP sky survey. The five-year maps incorporate several improvements in data processing made possible by the additional years of data and by a more complete analysis of the instrument calibration and in-flight beam response. We present several new tests for systematic errors in the polarization data and conclude that Ka band data (33 GHz) is suitable for use in cosmological analysis, after foreground cleaning. This significantly reduces the overall polarization uncertainty. With the 5 year WMAP data, we detect no convincing deviations from the minimal 6-parameter LCDM model: a flat universe dominated by a cosmological constant, with adiabatic and nearly scale-invariant Gaussian fluctuations. Using WMAP data combined with measurements of Type Ia supernovae and Baryon Acoustic Oscillations, we find (68% CL uncertainties): Omega_bh^2 = 0.02267 pm 0.00059, Omega_ch^2 = 0.1131 pm 0.0034, Omega_Lambda = 0.726 pm 0.015, n_s = 0.960 pm 0.013, tau = 0.084 pm 0.016, and Delta_R^2 = (2.445 pm 0.096) x 10^-9. From these we derive: sigma_8 = 0.812 pm 0.026, H_0 = 70.5 pm 1.3 km/s/Mpc, z_{reion} = 10.9 pm 1.4, and t_0 = 13.72 pm 0.12 Gyr. The new limit on the tensor-to-scalar ratio is r < 0.22 (95% CL). We obtain tight, simultaneous limits on the (constant) dark energy equation of state and spatial curvature: -0.14 < 1+w < 0.12 and -0.0179 < Omega_k < 0.0081 (both 95% CL). The number of relativistic degrees of freedom (e.g. neutrinos) is found to be N_{eff} = 4.4 pm 1.5, consistent with the standard value of 3.04. Models with N_{eff} = 0 are disfavored at >99.5% confidence.
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