arXiv subjects

A. Bernui

Publications and source records attributed to A. Bernui.

45 records · Page 3Linked to original sources

arXiv2007

On the large-scale angular distribution of short-Gamma ray bursts

We investigate the large-scale angular distribution of the short-Gamma ray bursts (SGRBs) from BATSE experiment, using a new coordinates-free method. The analyses performed take into account the angular correlations induced by the non-uniform sky exposure during the experiment, and the uncertainty in the measured angular coordinates. Comparising the large-scale angular correlations from the data with those expected from simulations using the exposure function we find similar features. Additionally, confronting the large-angle correlations computed from the data with those obtained from simulated maps produced under the assumption of statistical isotropy we found that they are incompatible at 95% confidence level. However, such differences are restricted to the angular scales 36o - 45o, which are likely to be due to the non-uniform sky exposure. This result strongly suggests that the set of SGRBs from BATSE are intrinsically isotropic. Moreover, we also investigated a possible large-angle correlation of these data with the supergalactic plane. No evidence for such large-scale anisotropy was found.

astro-ph↗

arXiv2007

A note on the large-angle anisotropies in the WMAP cut-sky maps

Recent analyses of the WMAP data seem to indicate the possible presence of large-angle anisotropy in the Universe. If confirmed, these can have important consequences for our understanding of the Universe. A number of attempts have recently been made to establish the reality and nature of such anisotropies in the CMB data. Among these is a directional indicator recently proposed by the authors. A distinctive feature of this indicator is that it can be used to generate a sky map of the large-scale anisotropies of the CMB maps. Applying this indicator to full-sky temperature maps we found a statistically significant preferred direction. The full-sky maps used in these analyses are known to have residual foreground contamination as well as complicated noise properties. Thus, here we performed the same analysis for a map where regions with high foreground contamination were removed. We find that the main feature of the full-sky analysis, namely the presence of a significant axis of asymmetry, is robust with respect to this masking procedure. Other subtler anomalies of the full-sky are on the other hand no longer present.

astro-ph↗

arXiv2006

Mapping the large-scale anisotropy in the WMAP data

Analyses of recent cosmic microwave background (CMB) observations have provided increasing indications for the existence of large scale anisotropy in the universe. Given the 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 further information that may be helpful for identifying its causes. Here we propose a new directional indicator, based on angular-separation histograms of pairs of pixels with similar temperatures in the CMB map, which provides a measure of departure from statistical isotropy. The main advantage of this indicator is that it can be used to generate a sky map of large-scale anisotropies in the CMB temperature map, thus allowing a possible additional window into their causes. Using this indicator, we find a statistically significant (at 95% CL) preferred direction in the CMB data and discuss how it compares with other such axes recently reported. We also show that our findings are robust with respect to both the details of the method used, and the choice of the WMAP CMB maps employed, including the three-year CMB data released recently.

astro-ph↗

arXiv2004

A Note on the Robustness of Pair Separations Methods in Cosmic Topology

The pair separations statistical methods devised to detect the topology of the universe rely on the accurate knowledge of the three-dimensional positions of the cosmic sources. The determination of these positions, however, involves inevitable observational uncertainties. The only significant (measurable) sign of a nontrivial topology in PSH's was shown to be spikes. We briefly report our results concerning the sensitivity of the topological spikes in the presence of the uncertainties in the positions of the cosmic sources, which arise from uncertainties in the values of the density parameters.

astro-ph↗

arXiv2002

Spikes in Cosmic Crystallography

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 PSH's 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 PSH's 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 PSH's 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 PSH's built from simulated catalogues.

gr-qc↗

arXiv2000

Topological Reverberations in Flat Space-times

We study the role played by multiply-connectedness in the time evolution of the energy E(t) of a radiating system that lies in static flat space-time manifolds M_4 whose t=const spacelike sections M_3 are compact in at least one spatial direction. The radiation reaction equation of the radiating source is derived for the case where M_3 has any non-trivial flat topology, and an exact solution is obtained. We also show that when the spacelike sections are multiply-connected flat 3-manifolds the energy E(t) exhibits a reverberation pattern with discontinuities in the derivative of E(t) and a set of relative minima and maxima, followed by a growth of E(t). It emerges from this result that the compactness in at least one spatial direction of Minkowski space-time is sufficient to induce this type of topological reverberation, making clear that our radiating system is topologically fragile. An explicit solution of the radiation reaction equation for the case where M_3 = R^2 x S^1 is discussed, and graphs which reveal how the energy varies with the time are presented and analyzed.

gr-qc↗

arXiv2000

Cosmic crystallography: the euclidean isometries

Exact expressions for probability densities of conjugate pair separation in euclidean isometries are obtained, for the cosmic crystallography.These are the theoretical counterparts of the mean histograms arising from computer simulation of the isometries. For completeness, also the isometries with fixed points are examined, as well as the orientation reversing isometries.

gr-qc↗

arXiv1999

Cosmic crystallography: three multi-purpose functions

A solid sphere is considered, with a uniformly distributed infinity of points. Two points being pseudorandomly chosen, the analytical probability density that their separation have a given value is computed, for three types of the underlying geometry: $E^3, H^3$ and $S^3$. Figures, graphs and histograms to complement this short note are given.

astro-ph↗

arXiv1998

Radiation Damping in FRW Space-times with Different Topologies

We study the role played by the compactness and the degree of connectedness in the time evolution of the energy of a radiating system in the Friedmann-Robertson-Walker (FRW) space-times whose $t=const $ spacelike sections are the Euclidean 3-manifold ${\cal R}^3$ and six topologically non-equivalent flat orientable compact multiply connected Riemannian 3-manifolds. An exponential damping of the energy $E(t)$ is present in the ${\cal R}^3$ case, whereas for the six compact flat 3-spaces it is found basically the same pattern for the evolution of the energy, namely relative minima and maxima occurring at different times (depending on the degree of connectedness) followed by a growth of $E(t)$. Likely reasons for this divergent behavior of $E(t)$ in these compact flat 3-manifolds are discussed and further developments are indicated. A misinterpretation of Wolf's results regarding one of the six orientable compact flat 3-manifolds is also indicated and rectified.

gr-qc↗