The non-Gaussian cold spot found in the WMAP data has created controversy about its origin. Here we calculate the Bayesian posterior probability ratios for three different models that could explain the cold spot. A recent work claimed that the Spot could be caused by a cosmic texture, while other papers suggest that it could be due to the gravitational effect produced by an anomalously large void. Also the Sunyaev-Zeldovich effect caused by a cluster is taken into account as a possible origin. We perform a template fitting on a 20 degrees radius patch centered at Galactic coordinates (b = -57circ, l = 209) and calculate the posterior probability ratios for the void and Sunyaev-Zeldovich models, comparing the results to those obtained with textures. Taking realistic priors for the parameters, the texture interpretation is favored, while the void and Sunyaev-Zeldovich hypotheses are discarded. The temperature decrement produced by voids or clusters is negligible considering realistic values for the parameters.
We present preliminary results of an analysis into the effects of primordial voids on the cosmic microwave background (CMB). We show that an inflationary bubble model of void formation predicts excess power in the CMB angular power spectrum that peaks between 2000 < l < 3000. Therefore, voids that exist on or close to the last scattering surface at the epoch of decoupling can contribute significantly to the apparent rise in power on these scales recently detected by the Cosmic Background Imager (CBI).
In this talk presented at the 5th Rencontres du Vietnam 2004, we discuss our preliminary investigations into voids of primordial origin. We show that if voids in the cold dark matter distribution existed at the epoch of decoupling, they could contribute significantly to the apparent rise in cosmic microwave background (CMB) power on small scales detected by the Cosmic Background Imager (CBI) Deep Field. Here we present the preliminary results of our improved method for predicting the effects of primordial voids on the CMB in which we treat the voids as an external source in the cold dark matte…
We explore the signature of a cold dark matter compensated void in the quasi non-linear regime. We find that this void is entirely a cold spot on the microwave background, in contrast to a non-linear void. On the last scattering surface(LSS), it appears as either a hot or cold spot depending on where this surface cuts the void. In addition, because the usual cancellations do not occur, the void's LSS signature can be very large as it is proportional to $R/H^{-1}$ rather than $(R/H^{-1})^3$. This implies strict limits for voids on the LSS.
If the voids that we see today in the distribution of galaxies existed at recombination, they will leave an imprint on the cosmic microwave background (CMB). On the other hand, if these voids formed much later, their effect on the CMB will be negligible and will not be observed with the current generation of experiments. In this paper presented at the 2004 Annual Scientific Meeting of the Astronomical Society of Australia, we discuss our ongoing investigations into voids of primordial origin. We show that if voids in the cold dark matter distribution existed at the epoch of decoupling, they co…
A database with a high density of accurate distances is used to investigate the contributions to the motion of our Galaxy. It is found that the motion of the Local Group separates remarkably cleanly into 3 components: a large-scale attraction toward structure in the `Great Attractor' sector, a mid-scale attraction toward the Virgo Cluster, and a local-scale `repulsion' from the Local Void. These 3 components cause motions of comparable amplitudes and, conveniently, are directed almost orthogonal to one another.
Using cosmological simulations, we make predictions for the distribution of clusters in a plausible non-gaussian model where primordial voids nucleated during inflation act together with scale-invariant adiabatic gaussian fluctuations as seeds for the formation of large-scale structure. This model agrees with most recent observations of the anisotropies of the cosmic microwave background (CMB) and can account for the excess of power measured on cluster scales by the Cosmic Background Imager (CBI), the large empty regions apparent in nearby galaxy redshift surveys and the number of giant arcs m…
The blackbody nature of the cosmic microwave background (CMB) radiation spectrum is used in a modern test of the Copernican Principle. The reionized universe serves as a mirror to reflect CMB photons, thereby permitting a view of ourselves and the local gravitational potential. By comparing with measurements of the CMB spectrum, a limit is placed on the possibility that we occupy a privileged location, residing at the center of a large void. The Hubble diagram inferred from lines-of-sight originating at the center of the void may be misinterpreted to indicate cosmic acceleration. Current limit…
We study the effect of a void network on the CMB anisotropy in the Einstein-de Sitter background using Thompson &Vishniac's model. We consider comprehensively the Sacks-Wolfe effect, the Rees-Sciama effect and the gravitational lensing effect. Our analysis includes the model of primordial voids existing at recombination, which is realized in some inflationary models associated with a first-order phase transition. If there exist primordial voids whose comoving radius is larger than $\sim10h^{-1}$Mpc at recombination, not only the Sachs-Wolfe effect but also the Rees-Sciama effect is appreciable…
Cosmologists have suggested a number of intriguing hypotheses for the origin of the "WMAP cold spot", the coldest extended region seen in the CMB sky, including a very large void and a collapsing texture. Either hypothesis predicts a distinctive CMB lensing signal. We show that the upcoming generation of high resolution CMB experiments such as ACT and SPT should be able to detect the signatures of either textures or large voids. If either signal is detected, it would have profound implications for cosmology.
Phase transitions taking place during the inflationary epoch give rise to bubbles of true vacuum embedded in the false vacuum. These bubbles can imprint a distinctive signal on the Cosmic Microwave Background (CMB). We evaluate the feasibility of detecting these signatures with waveletsin CMB maps, such as those that will be made available by the European Space Agency's (ESA) Planck mission.
Recent observations of dipole anisotropies show that the velocity of the Local Group ($\Vec v_{\rm G}$) induced by the clustering of IRAS galax ies has an amplitude and direction similar to those of the velocity of Cosmic Microwave Background dipole anisotropy ($\Vec v_{\rm CMB}$), but the difference $| \Vec v_{\rm G} - \Vec v_{\rm CMB} |$ is still $\sim 170$ km/s, which is about 28% of $|\Vec v_{\rm CMB} |$. Here we consider the possibility that the origin of this difference comes from a hypothetical large-scale local void, with which we can account for the accelerating behavior of type Ia su…
We generalize in several ways the results existing in the literature: a) we make use of an exact general relativistic solution for a spherical, nearly empty cavity in the matter dominated era to evaluate the null geodesics and the Sachs-Wolfe effect; b) we evaluate the magnitude of the adiabatic fluctuations of the photon-baryon plasma; c) we study the influence of the shell profile; and d) we take into account the finite thickness of the last scattering surface (LSS) and the influence of its position with respect to the void center. We find empirically an analytic approximation to the Sachs-W…
We explore the large angular scale temperature anisotropies in the cosmic microwave background due to expanding homogeneous local voids at redshift z~1. A compensated spherically symmetric homogeneous dust-filled void with radius \~3*10^2 h^{-1}Mpc, and density contrast ~-0.3 can be observed as a cold spot with a temperature anisotropy -1*10^{-5} surrounded by a slightly hotter ring. We find that a pair of these circular cold spots separated by ~50 degree can account both for the planarity of the octopole and for the alignment between the quadrupole and the octopole in the cosmic microwave bac…
The results of a previous paper by the author [Astrophys. J. 470, 43 (1996) are improved, through a more reliable technique for finding the location of sources whose images are on the surface of last scattering.
The observational behavior of spherically symmetric inhomogeneous cosmological models is studied, which consist of inner and outer homogeneous regions connected by a shell or an intermediate self-similar region. It is assumed that the present matter density parameter in the inner region is smaller than that in the outer region, and the present Hubble parameter in the inner region is larger than that in the outer region. Then galaxies in the inner void-like region can be seen to have a bulk motion relative to matter in the outer region, when we observe them at a point O deviated from the center…
We explore the large angular scale temperature anisotropies in the cosmic microwave background (CMB) due to homogeneous local dust-filled voids in a flat Friedmann-Robertson-Walker universe with a cosmological constant. In comparison with the equivalent dust-filled void model in the Einstein-de Sitter background, we find that the anisotropy for compensated asymptotically expanding local voids can be larger because second-order effects enhance the linear integrated Sachs-Wolfe (ISW) effect. However, for local voids that expand sufficiently faster than the asymptotic velocity of the wall, the se…
As an alternative explanation of the dimming of distant supernovae it has recently been advocated that we live in a special place in the Universe near the centre of a large void described by a Lemaitre-Tolman-Bondi (LTB) metric. The Universe is no longer homogeneous and isotropic and the apparent late time acceleration is actually a consequence of spatial gradients in the metric. If we did not live close to the centre of the void, we would have observed a Cosmic Microwave Background (CMB) dipole much larger than that allowed by observations. Hence, until now it has been argued, for the model t…
We study the cosmic microwave background (CMB) anisotropy due to spherically symmetric nonlinear structures in flat universes with dust and a cosmological constant. By modeling a time-evolving spherical compensated void/lump by Lemaitre-Tolman-Bondi spacetimes, we numerically solve the null geodesic equations with the Einstein equations. We find that a nonlinear void redshifts the CMB photons that pass through it regardless of the distance to it. In contrast, a nonlinear lump blueshifts (or redshifts) the CMB photons if it is located near (or sufficiently far from) us. The present analysis com…
We consider the problem of numerical computation of the Kolmogorov complexity and the fractal dimension of the anisotropy spots of Cosmic Microwave Background (CMB) radiation. Namely, we describe an algorithm of estimation of the complexity of spots given by certain pixel configuration on a grid and represent the results of computations for a series of structures of different complexity. Thus, we demonstrate the calculability of such an abstract descriptor as the Kolmogorov complexity for CMB digitized maps. The correlation of complexity of the anisotropy spots with their fractal dimension is …
We study two different methods to test Gaussianity in CMB maps. One of them is based on the partition function and the other on the morphology of hot and cold spots. The partition function contains information on all the moments and scales, being a useful quantity to compress the large data sets expected from future space missions like Planck. In particular, it contains much richer information than the one available through the radiation power spectrum. The second method utilizes morphological properties of hot and cold spots such as the eccentricity and number of spots in CMB maps. We study t…
The geodesics followed by cosmic microwave background (CMB) photons show different behaviours depending on the geometry of space. Namely, the effect of `mixing geodesics' predicts a distinct signature in CMB maps: threshold-independent elongated anisotropy spots in negatively curved geometries. We have found statistically significant sign for spot elongation in the COBE four year maps. This can be a direct indication for the negative curvature of the Universe.
We apply spherical needlets to the Wilkinson Microwave Anisotropy Probe 5-year cosmic microwave background (CMB) dataset, to search for imprints of non-isotropic features in the CMB sky. We use the needlets localization properties to resolve peculiar features in the CMB sky and to study how these features contribute to the anisotropy power spectrum of the CMB. In addition to the now well-known "cold spot" of the CMB map in the southern hemisphere, we also find two hot spots at greater than 99% confidence level, again in the southern hemisphere and closer to the Galactic plane. While the cold s…
The detection of a "Cold Spot" in the CMB sky could be explained by the presence of an anomalously large spherical underdense region (with radius of a few hundreds Mpc/h) located between us and the Last Scattering Surface. Modeling such an underdensity with an LTB metric, we investigate whether it could produce significant signals on the CMB power spectrum and bispectrum, via the Rees-Sciama effect. We find that this leads to a bump on the power spectrum, that corresponds to an O(5%-25%) correction at multipoles 5 < l < 50; in the cosmological fits, this would modify the χ^2 by an amount of or…
The distribution of Abell clusters of galaxies is analysed to study the regularity of the supercluster-void network. A new geometric method sensitive to the regularity of the location of clusters is applied. We find that the supercluster-void network resembles a cubical lattice over the whole space investigated. The distribution of rich superclusters is not isotropic: along the main axis of the network it is periodic with a step of length about 130 Mpc (for Hubble constant h=1), whereas along the diagonal of the network the period is larger. This large-scale inhomogeneity is compatible with re…
We present interferometric imaging at 33 GHz, with the new superextended configuration of the Very Small Array (VSA), of a very deep decrement in the cosmic microwave background (CMB) temperature. This decrement is located in the direction of the Corona Borealis supercluster, at a position with no known galaxy clusters, and was discovered by a previous VSA survey (Genova-Santos et al.). A total area of 3 sq.deg. has now been imaged, with an angular resolution of 7 arcmin and a flux sensitivity of 5 mJy/beam. These observations confirm the presence of this strong and resolved negative spot at -…
The properties of the Cosmic Microwave Background (CMB) maps carry valuable cosmological information. Here we report the results of the analysis hot and cold CMB anisotropy spots in the BOOMERanG 150 GHz map in terms of number, area, ellipticity, vs. temperature threshold. We carried out this analysis for the map obtained by summing independent measurement channels (signal plus noise map) and for a comparison map (noise only map) obtained by differencing the same channels. The anisotropy areas (spots) have been identified for both maps for various temperature thresholds and a catalog of the sp…
Significant alignment and signed-intensity anomalies of local features of the cosmic microwave background (CMB) are detected on the three-year WMAP data, through a decomposition of the signal with steerable wavelets on the sphere. Firstly, an alignment analysis identifies two mean preferred planes in the sky, both with normal axes close to the CMB dipole axis. The first plane is defined by the directions toward which local CMB features are anomalously aligned. A mean preferred axis is also identified in this plane, located very close to the ecliptic poles axis. The second plane is defined by t…
The recent study of BOOMERanG 150 GHz Cosmic Microwave Background (CMB) radiation maps have detected ellipticity of the temperature anisotropy spots independent on the temperature threshold. The effect has been found for spots up to several degrees in size, where the biases of the ellipticity estimator and of the noise are small. To check the effect, now we have studied, with the same algorithm and in the same sky region, the WMAP maps. We find ellipticity of the same average value also in WMAP maps, despite of the different sensitivity of the two experiments to low multipoles. Large spot elon…
We detect a dip of 20-45% in the surface brightness and number counts of NVSS sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of approximately 1-10 degrees. Together with independent all-sky wavelet analyses, our results suggest that the dip in extragalactic brightness and number counts and the WMAP cold spot are physically related, i.e., that the coincidence is neither a statistical anomaly nor a WMAP foreground correction problem. If the cold spot does originate from structures at modest redshifts, as we suggest, then there is no remai…
The ellipticity of the anisotropy spots of the Cosmic Microwave Background measured by the Wilkinson Microwave Anisotropy Probe (WMAP) has been studied. We find an average ellipticity of about 2, confirming with a far larger statistics similar results found first for the COBE-DMR CMB maps, and then for the BOOMERanG CMB maps. There are no preferred directions for the obliquity of the anisotropy spots. The average ellipticity is independent of temperature threshold and is present on scales both smaller and larger than the horizon at the last scattering. The measured ellipticity characteristics …
The standard cosmological model posits a spatially flat universe of infinite extent. However, no observation, even in principle, could verify that the matter extends to infinity. In this work we model the universe as a finite spherical ball of dust and dark energy, and obtain a lower limit estimate of its mass and present size: the mass is at least 5 x 10^23 solar masses and the present radius is at least 50 Gly. If we are not too far from the dust-ball edge we might expect to see a cold spot in the cosmic microwave background, and there might be suppression of the low multipoles in the angula…
The Kolmogorov stochasticity parameter (KSP) is applied to quantify the degree of randomness (stochasticity) in the temperature maps of the Cosmic Microwave Background radiation maps. It is shown that, the KSP for the WMAP5 maps is about twice higher than that of the simulated maps for the concordance \LambdaCDM cosmological model, implying the existence of a randomizing effect not taken into account in the model. As was revealed recently, underdense regions in the large scale matter distributions, i.e the voids, possess hyperbolic and hence randomizing properties. The degree of randomness for…
Density perturbations at the decoupling epoch produce angular fluctuations in the temperature of the Cosmic Microwave Background (CMB) radiation that may appear as hot and cold spots. Observational data of the CMB includes instrumental noise in addition to the cosmological signal. One would like to determine which of the observed spots are produced by the noise and which correspond to signal. In this work we first present a statistical analysis of the first plus second year COBE/DMR map at 53 GHz that reveals the presence of cosmological signal in the data. The analysis is based on Harrison-Ze…
When considering the statistical properties of a bundle of cosmic microwave background (CMB) photons propagating through space, the effect of `mixing geodesics' appears with a distinct signature that depends on the geometry of space. In a Universe with negative curvature this effect is expected to produce elongated anisotropy spots on CMB maps. We used COBE-DMR data to look for such effect. Based on the analysis of a measure of eccentricity of hot spots it appears that there is a clear indication of an excess eccentricity of hot spots with respect to that expected from noise alone. This result…
One of the most striking features found in the cosmic microwave background data is the presence of an anomalous Cold Spot (CS) in the temperature maps made by the Wilkinson Microwave Anisotropy Probe (WMAP). This CS has been interpreted as the result of the presence of a collapsing texture, perhaps coming from some early universe Grand Unified Theory (GUT) phase transition. In this work we propose an alternative explanation based on a completely different kind of texture which appears in a natural way in a broad class of brane-world models. This type of textures known as brane-skyrmions can be…
The observed apparent acceleration of the universe is usually attributed to negative pressure from a mysterious dark energy. This acceleration causes the gravitational potential to decay, heating or cooling photons travelling through crests or troughs of large-scale matter density fluctuations. This phenomenon, the late-time integrated Sachs-Wolfe (ISW) effect, has been detected, albeit at low significance, by cross-correlating various galaxy surveys with the Cosmic Microwave Background (CMB). Recently, the best evidence has come from the statistical combination of results from multiple correl…
The first and third year data releases from the WMAP provide evidence of an anomalous Cold Spot (CS) at galactic latitude b=-57deg and longitude l=209deg. We have examined the properties of the CS in some detail in order to assess its cosmological significance. We have performed a cluster analysis of the local extrema in the CMB signal to show that the CS is actually associated with a large group of extrema rather than just one. In the light of this we have re-examined the properties of the WMAP ILC and co-added "cleaned" WCM maps, which have previously been used for the analysis of the proper…
