Circles-in-the-sky searches and observable cosmic topology in a flat Universe
B. Mota, M. J. Reboucas, R. Tavakol
备注：
6 pages. 1 Table. V2: Version to appear in Phys. Rev. D (2010). Presentation improved. Two intermediate steps corrected. Results unchanged. Typos corrected. V3: References added. Typo corrected. Published version
[Abridged] In a Universe with a detectable nontrivial spatial topology the last scattering surface contains pairs of matching circles with the same distribution of temperature fluctuations - the so-called circles-in-the-sky. Searches for nearly antipodal circles in maps of cosmic microwave background have so far been unsuccessful. This negative outcome along with recent theoretical results concerning the detectability of nearly flat compact topologies is sufficient to exclude a detectable nontrivial topology for most observers in very nearly flat positively and negatively curved Universes ($0<|Ω_{tot}-1| \lesssim 10^{-5}$). Here we investigate the consequences of these searches for observable nontrivial topologies if the Universe turns out to be exactly flat ($Ω_{tot}=1$). We demonstrate that in this case the conclusions deduced from such searches can be radically different. We show that for all multiply-connected orientable flat manifolds it is possible to directly study the action of the holonomies in order to obtain a general upper bound on the angle that characterizes the deviation from antipodicity of pairs of matching circles associated with the shortest closed geodesic. This bound is valid for all observers and all possible values of the compactification length parameters. We also show that in a flat Universe there are observers for whom the circles-in-the-sky searches already undertaken are insufficient to exclude the possibility of a detectable nontrivial spatial topology. It is remarkable how such small variations in the spatial curvature of the Universe, which are effectively indistinguishable geometrically, can have such a drastic effect on the detectability of cosmic topology.
The existence of concentric low variance circles in the CMB sky, generated by black-hole encounters in an aeon preceding our big bang, is a prediction of the Conformal Cyclic Cosmology. Detection of three families of such circles in WMAP data was recently reported by Gurzadyan & Penrose (2010). We reassess the statistical significance of those circles by comparing with Monte Carlo simulations of the CMB sky with realistic modeling of the anisotropic noise in WMAP data. We find that the circles are not anomalous and that all three groups are consistent at 3sigma level with a Gaussian CMB sky as…
An important, and potentially detectable, signature of a non-trivial topology for the universe is the presence of so called circles-in-the-sky in the cosmic microwave background (CMB). Recent searches, confined to antipodal and nearly antipodal circles, have however failed to detect any. This outcome, coupled with recent theoretical results concerning the detectability of very nearly flat universes, is sufficient to exclude a detectable non-trivial cosmic topology for most observers in the inflationary limit ($0< |Ω_{tot}-1| \lesssim 10^{-5}$). In a recent paper we have studied the consequence…
[Abridged] An observable signature of a detectable nontrivial spatial topology of the Universe is the circles-in-the-sky in the CMB sky. In the most general search, pairs of circles with deviation from antipodality $0^\circ \leq θ\leq 169^\circ$ and radii $10^\circ \leq λ\leq 90^\circ$ were investigated, but no matching circles were found. Assuming this negative result, we examine the question as to whether there are nearly flat universes with compact topology that would give rise to circles whose observable parameters $λ$ and $θ$ fall o outside the ranges covered by this search. We derive the…
In a Universe with a detectable nontrivial spatial topology the last scattering surface contains pairs of matching circles with the same distribution of temperature fluctuations --- the so-called circles-in-the-sky. Searches undertaken for nearly antipodal pairs of such circles in cosmic microwave background maps have so far been unsuccessful. Previously we had shown that the negative outcome of such searches, if confirmed, should in principle be sufficient to exclude a detectable non-trivial spatial topology for most observers in very nearly flat ($0<\midΩ_{\text{tot}}-1\mid \lesssim10^{-5}$)…
A new analysis of the CMB, using WMAP data, supports earlier indications of non-Gaussian features of concentric circles of low temperature variance. Conformal cyclic cosmology (CCC) predicts such features from supermassive black-hole encounters in an aeon preceding our Big Bang. The significance of individual low-variance circles in the true data has been disputed; yet a recent independent analysis has confirmed CCC's expectation that CMB circles have a non-Gaussian temperature distribution. Here we examine concentric sets of low-variance circular rings in the WMAP data, finding a highly non-i…
In arXiv:1105.1081, Eriksen and Wehus try to criticise the use of Kolmogorov's stochasticity parameter (KSP) for CMB data analysis. Their discussion is based on a serious misunderstanding of the randomness and Arnold's work. Their note includes numerous further inaccuracies and groundless statements. However, this short note is concerned with KSP method only.
In a recent analysis of the 7-year WMAP temperature sky maps, Gurzadyan and Penrose claim to find evidence for violent pre-Big Bang activity in the form of concentric low-variance circles at high statistical significance. In this paper, we perform an independent search for such concentric low-variance circles, employing both chi^2 statistics and matched filters, and compare the results obtained from the 7-year WMAP temperature sky maps with those obtained from LCDM simulations. Our main findings are the following: We do reproduce the claimed ring structures observed in the WMAP data as present…
In a recent paper, Gurzadyan & Penrose claim to have found directions on the sky centred on which are circles of anomalously low variance in the cosmic microwave background (CMB). These features are presented as evidence for a particular picture of the very early Universe. We attempted to repeat the analysis of these authors, and we can indeed confirm that such variations do exist in the temperature variance for annuli around points in the data. However, we find that this variation is entirely expected in a sky which contains the usual CMB anisotropies. In other words, properly simulated Gauss…
New analysis confirms our earlier claim [1], [7] of circles of notably low temperature variance, often in concentric sets, in the cosmic microwave background (CMB), discernable in WMAP data. Their reality can be interpreted as evidence of supermassive black-hole encounters in a previous aeon, as predicted by conformal cyclic cosmology (CCC) [2]. Counter arguments [4-6] pointed out that such circles arise, at similar frequency, also in simulated data using WMAP's CMB power spectrum, plus random input. We responded [7] that if such circles contribute to CMB, this influences the power spectrum, e…
Two groups [3,4] have confirmed the results of our paper concerning the actual existence of low variance circles in the cosmic microwave background (CMB) sky. They also point out that the effect does not contradict the LCDM model - a matter which is not in dispute. We point out two discrepancies between their treatment and ours, however, one technical, the other having to do with the very understanding of what constitutes a Gaussian random signal. Both groups simulate maps using the CMB power spectrum for LCDM, while we simulate a pure Gaussian sky plus the WMAP's noise, which points out the c…
The immediate observational consequence of a non-trivial spatial topology of the Universe is that an observer could potentially detect multiple images of radiating sources. In particular, a non-trivial topology will generate pairs of correlated circles of temperature fluctuations in the anisotropies maps of the cosmic microwave background (CMB), the so-called circles-in-the-sky. In this way, a detectable non-trivial spatial topology may be seen as an observable attribute, which can be probed through the circles-in-the-sky for all locally homogeneous and isotropic universes with no assumptions …
A non-trivial spatial topology of the Universe is a potentially observable attribute, which can be probed through the circles-in-the-sky for all locally homogeneous and isotropic universes with no assumptions on the cosmological parameters. We show how one can use a possible circles-in-the-sky detection of the spatial topology of globally homogeneous universes to set constraints on the dark energy equation of state parameters.
In a recent preprint ("CCC-predicted low-variance circles in the CMB sky and LCDM"), Gurzadyan and Penrose (2011) claim for the second time to find evidence for pre-Big Bang activity in the form of concentric circles of low variance in the WMAP data. The same claim was made in November 2010, but quickly shown to be false by three independent groups. The culprit was simply that Gurzadyan and Penrose's simulations were based on an inappropriate power spectrum. In the most recent paper, they now claim that the significance is indeed low if the simulations are based on the realization-specific WMA…
We examine all-sky cosmic microwave background (CMB) temperature maps on large angular scales to compare their consistency with two scenarios: the standard inflationary quantum picture, and a distribution constrained to have a universal variance of primordial curvature perturbations on great circles. The latter symmetry is not a property of standard quantum inflation, but may be a symmetry of holographic models with causal quantum coherence on null surfaces. Since the variation of great-circle variance is dominated by the largest angular scale modes, in the latter case the amplitude and direct…
High-energetic gamma rays from astrophysical targets constitute a unique probe for annihilation or decay of heavy particle dark matter (DM). After several decades, diverse null detections have resulted in strong constraints for DM particle masses up to the TeV scale. While the gamma-ray signature is expected to be universal from various targets, uncertainties of astrophysical origin strongly affect and weaken the limits. At the same time, spurious signals may originate from non-DM related processes. The many gamma-ray targets in the extragalactic sky being searched for DM play a crucial role t…
We present an update to the search for a non-trivial topology of the universe by searching for matching circle pairs in the cosmic microwave background using the WMAP 7 year data release. We extend the exisiting bounds to encompass a wider range of possible topologies by searching for matching circle pairs with opening angles 10 degree < α< 90 degree and separation angles 11 degree < θ< 180 degree. The extended search reveal two small anomalous regions in the CMB sky. Numerous pairs of well-matched circles are found where both circles pass through one or the other of those regions. As this is …
We investigate for the first time the potential of angular auto- and cross-correlation power spectra in identifying sterile neutrino dark matter in the cosmic X-ray background. We take as reference the performance of the soon-to-be-launched eROSITA satellite. The main astrophysical background sources against sterile neutrino decays are active galactic nuclei, galaxies powered by X-ray binaries, and clusters of galaxies. While sterile neutrino decays are always subdominant in the auto-correlation power spectra, they can be efficiently enhanced when cross-correlating with tracers of the dark mat…
There exists various proposals to detect cosmic strings from Cosmic Microwave Background (CMB) or 21 cm temperature maps. Current proposals do not aim to find the location of strings on sky maps, all of these approaches can be thought of as a statistic on a sky map. We propose a Bayesian interpretation of cosmic string detection and within that framework, we derive a connection between estimates of cosmic string locations and cosmic string tension $Gμ$. We use this Bayesian framework to develop a machine learning framework for detecting strings from sky maps and outline how to implement this f…
The Hantzsche-Wendt space is one of the 17 multiply connected spaces of the three-dimensional Euclidean space E^3. It is a compact and orientable manifold which can serve as a model for a spatial finite universe. Since it possesses much fewer matched back-to-back circle pairs on the cosmic microwave background (CMB) sky than the other compact flat spaces, it can escape the detection by a search for matched circle pairs. The suppression of temperature correlations C(theta) on large angular scales on the CMB sky is studied. It is shown that the large-scale correlations are of the same order as f…
One of the FLRW models that best fits the WMAP sky maps of the CMB is the Poincare dodecahedral space. The optimal fit of this model to WMAP data was recently found using an optimal cross-correlation method, but the systematic error in the estimate of the matched-circle angular radius α, or equivalently, the (comoving) size of the Universe 2\rinj (twice the injectivity radius), might be much higher than the random error. In order to increase the falsifiability of the model, it would be useful to reduce the uncertainty in this estimate and to estimate the fraction of the sky where multiply imag…
We impose constraints on the topology of the Universe determined from a search for matched circles in the temperature anisotropy patterns of the 7-year WMAP data. We pay special attention to the sensitivity of the method to residual foreground contamination of the sky maps, and show that for a full sky estimate of the CMB signal (the ILC map) such residuals introduce a non-negligible effect on the statistics of matched circles. In order to reduce this effect, we perform the analysis on maps for which the most contaminated regions have been removed. A search for pairs of matched back-to-back ci…
Anisotropies in the electromagnetic emission produced by dark matter annihilation or decay in the extragalactic sky are a recent tool in the quest for a particle dark matter evidence. We review the formalism to compute the two-point angular power spectrum in the halo-model approach and discuss the features and the relative size of the various auto- and cross-correlation signals that can be envisaged for anisotropy studies. From the side of particle dark matter signals, we consider the full multi-wavelength spectrum, from the radio emission to X-ray and gamma-ray productions. We discuss the ang…
In order to observe annihilation and decay of dark matter, several types of potential sources should be considered. Some sources, such as dwarf galaxies, are expected to have very low astrophysical backgrounds but fairly small dark matter densities. Other sources, like the Galactic center, are expected to have larger densities of dark matter but also have more complicated backgrounds from other astrophysical sources. To search for signatures of dark matter, the large field-of-view of the HAWC detector, covering 2 sr at a time, especially enables searches from sources of dark matter annihilatio…
The Targeted Standard Siren Cosmology with Pulsar Timing Arrays
Indian Institute of Science Education and Research Bhopal（印度科学教育研究所博帕尔分校）；Max Planck Institute for Gravitational Physics (Albert Einstein Institute)（马克斯·普朗克引力物理研究所（爱因斯坦研究所））；Leibniz Universität Hannover（汉诺威莱布尼茨大学）
In a recent paper, Gurzadyan & Penrose claim to have found directions in the sky around which there are multiple concentric sets of annuli with anomalously low variance in the cosmic microwave background (CMB). These features are presented as evidence for a particular theory of the pre-Big Bang Universe. We are able to reproduce the analysis these authors presented for data from the WMAP satellite and we confirm the existence of these apparently special directions in the newer Planck data. However, we also find that these features are present at the same level of abundance in simulated Gaussia…
We undertook searches for isolated galaxies based on automatic selection of infrared sources from the Two Micron All-Sky Survey (2MASS) followed by a visual inspection of their surrounding. Using a modified criterion by Karachentseva (1973), we compiled a catalog of 3227 isolated galaxies (2MIG) containing 6% of 2MASS Extended Sources (2MASX) brighter than $K_s = 12^m$ with angular diameters $a_K \geq 30\arcsec$. The most isolated 2493 galaxies among them enter in the 2MASS Very Isolated Galaxy Catalog, 2MVIG. Being situated in the regions of extremely low mass density, the 2MVIG galaxies can …
In the last three decades, about 50 radio supernovae have been detected as a result of targeted searches of optically discovered supernovae in the local universe. Despite this relatively small number some diversity among them has already been identified which is an indication of the underlying richness of radio supernovae waiting to be discovered. For example, comparison of star formation and supernova discovery rate imply that as many as half of the supernovae remain undetected in the traditional optical searches, either because of intrinsic dimness or due to dust obscuration. This has far re…
