Searching for non-Gaussianity in the WMAP data

Abstract

Some analyses of recent cosmic microwave background (CMB) data have provided hints that there are deviations from Gaussianity in the WMAP CMB temperature fluctuations. Given the far reaching consequences of such a non-Gaussianity for our understanding of the physics of the early universe, it is important to employ alternative indicators in order to determine whether the reported non-Gaussianity is of cosmological origin, and/or extract further information that may be helpful for identifying its causes. We propose two new non-Gaussianity indicators, based on skewness and kurtosis of large-angle patches of CMB maps, which provide a measure of departure from Gaussianity on large angular scales. A distinctive feature of these indicators is that they provide sky maps of non-Gaussianity of the CMB temperature data, thus allowing a possible additional window into their origins. Using these indicators, we find no significant deviation from Gaussianity in the three and five-year WMAP ILC map with KQ75 mask, while the ILC unmasked map exhibit deviation from Gaussianity, quantifying therefore the WMAP team recommendation to employ the new mask KQ75 for tests of Gaussianity. We also use our indicators to test for Gaussianity the single frequency foreground unremoved WMAP three and five-year maps, and show that the K and Ka maps exhibit clear indication of deviation from Gaussianity even with the KQ75 mask. We show that our findings are robust with respect to the details of the method.

Non-Gaussianity in the foreground-reduced CMB maps

“…Our primary objective in this paper is to address this question by extending the analysis of Ref. [11] in three different ways. First, we use the same statistical indicators to carry out a new analysis of Gaussianity of the available full-sky foreground-reduced five-year and sevenyear CMB maps [12][13][14][15].…”

“…Thus, a pertinent question that arises is how the analysis of Gaussianity made in Ref. [11] is modified if whole-sky foreground-reduced CMB maps are used. Our primary objective in this paper is to address this question by extending the analysis of Ref.…”

“…[9]). In a recent paper [11] we proposed two new large-angle nonGaussianity indicators, based on skewness and kurtosis of large-angle patches of CMB maps, which provide measures of the departure from Gaussianity on large angular scales. We used these indicators to search for the largeangle deviation from Gaussianity in the three and fiveyear single frequency maps with a KQ75 mask, and found that while the deviation for the Q, V, and W masked maps are within the 95% expected values of Monte-Carlo (MC) statistically Gaussian CMB maps, there is a strong indication of deviation from Gaussianity (≫ 95% off the MC) in the K and Ka masked maps.…”

Large-Angle Non-Gaussianity in Simulated High-Resolution CMB Maps

“…We have used these indicators to carry out analyses of large-angle deviation from Gaussianity in both band and foreground-reduced WMAP CMB maps with and without a KQ75 mask. 1,2 We found that while non-Gaussianity of the Q, V, and W masked maps are consistent with Gaussianity, there is a strong indication of deviation from Gaussianity in the K and Ka masked maps. We have also shown that the full-sky five-year foreground-reduced internal linear combination (ILC) 17,18 as well as the harmonic ILC (HILC) 19 and the needlet ILC (NILC) 20 maps present a significant deviation from Gaussianity.…”

“…The steps of a constructive way of defining our non-Gaussianity indicators S and K, and the associated maps (discrete functions defined on S 2 ) from input CMB (simulated or real data) maps are the following: 1,2 i. Take a discrete finite set of points {j = 1, .…”