A. Bernui

Mapping the large-scale anisotropy in the WMAP data

Aims. Analyses of recent cosmic microwave background (CMB) observations have provided increasing hints that there are deviations in the universe from statistical isotropy on large scales. Given t he far reaching consequences of such an anisotropy for our understanding of the universe, it is important to employ alternative indicators in order to determine whether the reported anisotropy is cosmological in origin and, if so, extract information that may be helpful for identifying its causes. Methods. Here we propose a new directional indicator, based on separation histograms of pairs of pixels, which provides a measure of departure from statistical isotropy. The main advantage of this indicator is that it generates a sky map of large-scale a nisotropies in the CMB temperature map, thus allowing a possible additional window into their causes. Results. Using this indicator, we find statistically significant exce ss of large-scale anisotropy at well over the 95% confidence l evel. This anisotropy defines a preferred direction in the CMB data. We d iscuss the statistical significance of this direction compa red to Monte Carlo data obtained under the statistical isotropy hypothesis, and al so compare it with other such axes recently reported in the literature. In addition we show that our findings are robust with respect to the details o f the method used, so long as the statistical noise is kept und er control; and they remain unchanged compared to the foreground cleaning algorithms used in CMB maps. We also find that the application of our method to the first and three-year WMAP data produces similar results.

Alignment Tests for low CMB multipoles

We investigate the large scale anomalies in the angular distribution of the cosmic microwave background radiation as measured by WMAP using several tests. These tests, based on the multipole vector expansion, measure correlations between the phases of the multipoles as expressed by the directions of the multipole vectors and their associated normal planes. We have computed the probability distribution functions for 46 such tests, for the multipoles l=2-5. We confirm earlier findings that point to a high level of alignment between l=2 (quadrupole) and l=3 (octopole), but with our tests we do not find significant planarity in the octopole. In addition, we have found other possible anomalies in the alignment between the octopole and the l=4 (hexadecupole) components, as well as in the planarity of l=4 and l=5. We introduce the notion of a global anomaly statistic to estimate the relevance of the low-multipoles tests of non-Gaussianity. We show that, as a result of these tests, the CMB maps which are most widely used for cosmological analysis lie within the {approx}10% of randomly generated maps with lowest global anomaly statistics.

On the cosmic microwave background large-scale angular correlations

We study the large-scale angular correlation signatures of the Cosmic Microwave Background (CMB) temperature fluctuations from WMAP data in several spherical cap regions of the celestial sphere, outside the Kp0 or Kp2 cut-sky masks. We applied a recently proposed method to CMB temperature maps, which permits an accurate analysis of their angular correlations in the celestial sphere through the use of normalized histograms of the number of pairs of such objects with a given angular separation versus their angular separation. The method allows for a better comparison of the results from observational data with the expected CMB angular correlations of a statistically isotropic Universe, computed from Monte Carlo maps according to the WMAP best-fit Lambda CDM model. We found that the, already known, anomalous lack of large-scale power in full-sky CMB maps are mainly due to missing angular correlations of quadrupole-like signature. This result is robust with respect to frequency CMB maps and cut-sky masks. Moreover, we also confirm previous results regarding the unevenly distribution in the sky of the large-scale power of WMAP data. In a bin-to-bin correlations analyses, measured by the full covariance matrix chi^2 statistic, we found that the angular correlations signatures in opposite Galactic hemispheres are anomalous at the 98%-99% confidence level.Aims. We study the large-scale angular correlation signatures of the cosmic microwave background (CMB) temperature fluctuations from WMAP data in several spherical cap regions of the celestial sphere, outside the Kp0 or Kp2 cut-sky masks. Methods. We applied a recently proposed method to CMB temperature maps to permit an accurate analysis of their angular correlations in the celestial sphere through the use of normalized histograms of the number of pairs of such objects with a given angular separation versus their angular separation. The method allows for a better comparison of the results from observational data with the expected CMB angular correlations of a statistically isotropic universe, computed from Monte-Carlo maps according to the WMAP best-fit Λ CDM model. Results. We found that the already known, anomalous lack of large-scale power in full-sky CMB maps is mainly due to deficient angular correlations of a quadrupole-like signature. This result is robust with respect to frequency CMB maps and cut-sky masks. Moreover, we also confirm previous results regarding the uneven distribution in the sky of the large-scale power of WMAP data. In a bin-to-bin correlation analyses, measured by the full covariance matrix

PROBING COSMOLOGICAL ISOTROPY WITH TYPE Ia SUPERNOVAE

\chi^2

Spikes in cosmic crystallography

statistic, we found that the angular correlation signatures in opposite Galactic hemispheres are anomalous at a 98%–99% confidence level.

Temperature fluctuations of the cosmic microwave background radiation: A case of non-extensivity?

We investigate the validity of the cosmological principle by mapping the cosmological parameters H0 and q0 through the celestial sphere. In our analysis, which is performed in a low-redshift regime to follow a model-independent approach, we use two compilations of type Ia Supernovae (SNe Ia), namely the Union2.1 and the JLA data sets. First, we show that the angular distributions for both SNe Ia data sets are statistically anisotropic at a high confidence level (p-value < 0.0001), particularly the JLA sample. Then we find that the cosmic expansion and acceleration are mainly of dipolar type, with maximal anisotropic expansion [acceleration] pointing toward [], and [] for the Union2.1 and JLA data, respectively. Second, we use a geometrical method to test the hypothesis that the non-uniformly distributed SNe Ia events could introduce anisotropic imprints on the cosmological expansion and acceleration. For the JLA compilation, we found significant correlations between the celestial distribution of data points and the directional studies of H0 and q0, suggesting that these results can be attributed to the intrinsic anisotropy of the sample. In the case of the Union2.1 data, nonetheless, these correlations are less pronounced, and we verify that the dipole asymmetry found in the H0 analyses coincides with the well-known bulk-flow motion of our local group. From these analyses, we conclude that the directional asymmetry on the cosmological parameter maps are mainly either of local origin or due to celestial incompleteness of current SNe Ia samples.

Deviation from Gaussianity in the cosmic microwave background temperature fluctuations

If the universe is multiply connected and small the sky shows multiple images of cosmic objects, correlated by the covering group of the 3-manifold used to model it. These correlations were originally thought to manifest as spikes in pair separation histograms (PSH) built from suitable catalogues. Using probability theory we derive an expression for the expected pair separation histogram (EPSH) in a rather general topological-geometrical-observational setting. As a major consequence we show that the spikes of topological origin in PSHs are due to translations, whereas other isometries manifest as tiny deformations of the PSH corresponding to the simply connected case. This result holds for all Robertson–Walker spacetimes and gives rise to two basic corollaries: (i) that PSHs of Euclidean manifolds that have the same translations in their covering groups exhibit identical spike spectra of topological origin, making clear that even if the universe is flat the topological spikes alone are not sufficient for determining its topology; and (ii) that PSHs of hyperbolic 3-manifolds exhibit no spikes of topological origin. These corollaries ensure that cosmic crystallography, as originally formulated, is not a conclusive method for unveiling the shape of the universe. We also present a method that reduces the statistical fluctuations in PSHs built from simulated catalogues.

Is the cold spot responsible for the CMB North-South asymmetry?

Abstract Temperature maps of the cosmic microwave background (CMB) radiation, as those obtained by the Wilkinson microwave anisotropy probe (WMAP), provide one of the most precise data sets to test fundamental hypotheses of modern cosmology. One of these issues is related to the statistical properties of the CMB temperature fluctuations. We analysed here the WMAP data and found that the distribution of the CMB temperature fluctuations P CMB ( Δ T ) can be quite well fitted by the anomalous temperature distribution emerging within non-extensive statistical mechanics. This theory is based on the non-extensive entropy S q ≡ k { 1 − ∫ d x [ P q ( x ) ] q } / ( q − 1 ) , with the Boltzmann–Gibbs expression as the limit case q → 1 . For the frequencies investigated ( ν = 40.7 , 60.8, and 93.5 GHz), we found that P CMB ( Δ T ) is well described by P q ( Δ T ) ∝ 1 / [ 1 + ( q − 1 ) B ( ν ) Δ T 2 ] 1 / ( q − 1 ) , with q = 1.045 ± 0.005 , which indicate, at the 99% confidence level, that Gaussian temperature distributions P Gauss ( Δ T ) ∝ e − B ( ν ) Δ T 2 , corresponding to the q → 1 limit, do not properly represent the CMB temperature fluctuations measured by WMAP.

ON THE LARGE-SCALE ANGULAR DISTRIBUTION OF SHORT GAMMA-RAY BURSTS

Recent measurements of the temperature fluctuations of the cosmic microwave background (CMB) radiation from the WMAP satellite provide indication of a non-Gaussian behavior. Although the observed feature is small, it is detectable and analyzable. Indeed, the temperature distribution PCMB(?T) of these data can be quite well fitted by the anomalous probability distribution emerging within nonextensive statistical mechanics, based on the entropy Sq???k{1????dx[P(x)]q}/(q?1) (S1????k???dxP(x)ln[P(x)]). For the CMB frequencies analysed, ??=?40.7, 60.8, and 93.5?GHz, PCMB(?T) is well described by Pq(?T)??1/[1+(q?1) B(?) (?T)2]1/(q?1), with q?=?1.04???0.01, the strongest non-Gaussian contribution coming from the South-East sector of the celestial sphere. Moreover, Monte Carlo simulations exclude, at the 99% confidence level, P1(?T)??e?B(?)(?T)2 to fit the three-year data.

Is there evidence for anomalous dipole anisotropy in the large-scale structure?

Several intriguing phenomena, unlikely within the standard inflationary cosmology, were reported in the cosmic microwave background (CMB) data from the Wilkinson Microwave Anisotropy Probe and appear to be uncorrelated. Two of these phenomena, termed CMB anomalies, are representative of their disparate nature: the North-South asymmetry in the CMB angular-correlation strength, inconsistent with an isotropic universe, and the cold spot, producing a significant deviation from Gaussianity. We find a correlation between them, at medium angular scales (