CMB Cold Spot Research Articles

The DES view of the Eridanus supervoid and the CMB cold spot

ABSTRACT The Cold Spot is a puzzling large-scale feature in the Cosmic Microwave Background temperature maps and its origin has been subject to active debate. As an important foreground structure at low redshift, the Eridanus supervoid was recently detected, but it was subsequently determined that, assuming the standard ΛCDM model, only about 10–20 per cent of the observed temperature depression can be accounted for via its Integrated Sachs–Wolfe imprint. However, R ≳ 100 h−1Mpc supervoids elsewhere in the sky have shown ISW imprints AISW ≈ 5.2 ± 1.6 times stronger than expected from ΛCDM (AISW = 1), which warrants further inspection. Using the Year-3 redMaGiC catalogue of luminous red galaxies from the Dark Energy Survey, here we confirm the detection of the Eridanus supervoid as a significant underdensity in the Cold Spot’s direction at z < 0.2. We also show, with S/N ≳ 5 significance, that the Eridanus supervoid appears as the most prominent large-scale underdensity in the dark matter mass maps that we reconstructed from DES Year-3 gravitational lensing data. While we report no significant anomalies, an interesting aspect is that the amplitude of the lensing signal from the Eridanus supervoid at the Cold Spot centre is about 30 per cent lower than expected from similar peaks found in N-body simulations based on the standard ΛCDM model with parameters Ωm = 0.279 and σ8 = 0.82. Overall, our results confirm the causal relation between these individually rare structures in the cosmic web and in the CMB, motivating more detailed future surveys in the Cold Spot region.

Large-scale asymmetry in galaxy spin directions: evidence from the Southern hemisphere

AbstractRecent observations using several different telescopes and sky surveys showed patterns of asymmetry in the distribution of galaxies by their spin directions as observed from Earth. These studies were done with data imaged from the Northern hemisphere, showing excellent agreement between different telescopes and different analysis methods. Here, data from the DESI Legacy Survey was used. The initial dataset contains$\sim\!2.2\times 10^7$galaxy images, reduced to$\sim\!8.1\times 10^5$galaxies annotated by their spin direction using a symmetric algorithm. That makes it not just the first analysis of its kind in which the majority of the galaxies are in the Southern hemisphere, but also by far the largest dataset used for this purpose to date. The results show strong agreement between opposite parts of the sky, such that the asymmetry in one part of the sky is similar to the inverse asymmetry in the corresponding part of the sky in the opposite hemisphere. Fitting the distribution of galaxy spin directions to cosine dependence shows a dipole axis with probability of 4.66$\sigma$. Interestingly, the location of the most likely axis is within close proximity to the CMB Cold Spot. The profile of the distribution is nearly identical to the asymmetry profile of the distribution identified in Pan-STARRS, and it is within 1$\sigma$difference from the distribution profile in SDSS and HST. All four telescopes show similar large-scale profile of asymmetry.

Polarization of the CMB cold spot by the gravitational lensing of a cosmic void and texture

The cold spot is one of the observed CMB anomalies proposed to be produced by various primary and secondary effects. In this work, we study the gravitational lensing of some of the candidates that could produce the cold spot, namely a huge void and a cosmic texture, on the various components of CMB random field. Our results demonstrated that the signal to noise ratio of gravitational lensing effect on the CMB temperature field is higher than that of on CMB polarization. We found that for a cylindrically symmetric void at z_Void=0.8 with σ=-1.0 a thousand minute observation would detect the gravitational lensing signal while for a cosmic texture at z_Texture=6.0 a longer observation is required. We expect the future high resolution surveys can put strong constraints on the amplitudes of the template of these models.

The part and the whole: voids, supervoids, and their ISW imprint

The integrated Sachs-Wolfe imprint of extreme structures in the cosmic web probes the dynamical nature of dark energy. Looking through typical cosmic voids, no anomalous signal has been reported. On the contrary, supervoids, associated with large-scale fluctuations in the gravitational potential, have shown potentially disturbing excess signals. In this study, we used the Jubilee ISW simulation to demonstrate how the stacked signal depends on the void definition. We found that large underdensities, with at least $\approx5$ merged sub-voids, show a peculiar ISW imprint shape with central cold spots and surrounding hot rings, offering a natural way to define supervoids in the cosmic web. We then inspected the real-world BOSS DR12 data using the simulated imprints as templates. The imprinted profile of BOSS supervoids appears to be more compact than in simulations, requiring an extra $\alpha \approx 0.7$ re-scaling of filter sizes. The data reveals an excess ISW-like signal with $A_{\rm ISW}\approx9$ amplitude at the $\approx2.5\sigma$ significance level, unlike previous studies that used isolated voids and reported good consistency with $A_{\rm ISW}=1$. The tension with the Jubilee-based $\Lambda$CDM predictions is $\sim 2\sigma$, in consistency with independent analyses of supervoids in Dark Energy Survey data. We show that such a very large enhancement of the $A_{\rm ISW}$ parameter hints at a possible causal relation between the CMB Cold Spot and the Eridanus supervoid. The origin of these findings remains unclear.

Evidence against a supervoid causing the CMB Cold Spot

We report the results of the 2dF-VST ATLAS Cold Spot galaxy redshift survey (2CSz) based on imaging from VST ATLAS and spectroscopy from 2dF AAOmega over the core of the CMB Cold Spot. We sparsely surveyed the inner 5$^{\circ}$ radius of the Cold Spot to a limit of $i_{AB} \le 19.2$, sampling $\sim7000$ galaxies at $z<0.4$. We have found voids at $z=$ 0.14, 0.26 and 0.30 but they are interspersed with small over-densities and the scale of these voids is insufficient to explain the Cold Spot through the $\Lambda$CDM ISW effect. Combining with previous data out to $z\sim1$, we conclude that the CMB Cold Spot could not have been imprinted by a void confined to the inner core of the Cold Spot. Additionally we find that our 'control' field GAMA G23 shows a similarity in its galaxy redshift distribution to the Cold Spot. Since the GAMA G23 line-of-sight shows no evidence of a CMB temperature decrement we conclude that the Cold Spot may have a primordial origin rather than being due to line-of-sight effects.

Cosmic troublemakers: the Cold Spot, the Eridanus supervoid, and the Great Walls

The alignment of the CMB Cold Spot and the Eridanus supervoid suggests a physical connection between these two relatively rare objects. We use galaxy cata\-logues with photometric (2MPZ) and spectroscopic (6dF) redshift measurements, supplemented by low-redshift compilations of cosmic voids, in order to improve the 3D mapping of the matter density in the Eridanus constellation. We find evidence for a supervoid with a significant elongation in the line-of-sight, effectively spanning the total redshift range $z<0.3$. Our tomographic imaging reveals important substructure in the Eridanus supervoid, with a potential interpretation of a long, fully connected system of voids. We improve the analysis by extending the line-of-sight measurements into the antipodal direction that interestingly crosses the Northern Local Supervoid at the lowest redshifts. Then it intersects very rich superclusters like Hercules and Corona Borealis, in the region of the Coma and Sloan Great Walls, as a possible compensation for the large-scale matter deficit of Eridanus. We find that large-scale structure measurements are consistent with a central matter underdensity $\delta_0 \approx -0.25$, projected transverse radius $r_{0}^{\perp}\approx 195$ Mpc/h with an extra deepening in the centre, and line-of-sight radius $r_{0}^{\parallel}\approx500$ Mpc/h, i.e. an ellipsoidal supervoid. The expected integrated Sachs-Wolfe imprint of such an elongated supervoid is at the $\Delta T_{\rm ISW} \approx -40 \mu K$ level, thus inappropriate to accounting for the Cold Spot pattern in the CMB.

CMB cold spot from inflationary feature scattering

We propose a “feature-scattering” mechanism to explain the cosmic microwave background cold spot seen from WMAP and Planck maps. If there are hidden features in the potential of multi-field inflation, the inflationary trajectory can be scattered by such features. The scattering is controlled by the amount of isocurvature fluctuations, and thus can be considered as a mechanism to convert isocurvature fluctuations into curvature fluctuations. This mechanism predicts localized cold spots (instead of hot ones) on the CMB. In addition, it may also bridge a connection between the cold spot and a dip on the CMB power spectrum at ℓ∼20.

Antisymmetric galaxy cross-correlations as a cosmological probe

The auto-correlation between two members of a galaxy population is symmetric under the interchange of the two galaxies being correlated. The cross-correlation between two different types of galaxies, separated by a vector $\bf{r}$, is not necessarily the same as that for a pair separated by $-\bf{r}$. Local anisotropies in the two-point cross-correlation function may thus indicate a specific direction which when mapped as a function of position trace out a vector field. This vector field can then be decomposed into longitudinal and transverse components, and those transverse components written as positive- and negative-helicity components. A locally asymmetric cross-correlation of the longitudinal type arises naturally in halo clustering, even with Gaussian initial conditions, and could be enhanced with local-type non-Gaussianity. Early-Universe scenarios that introduce a vector field may also give rise to such effects. These antisymmetric cross-correlations also provide a new possibility to seek a preferred cosmic direction correlated with the hemispherical power asymmetry in the cosmic microwave background and to seek a preferred location associated with the CMB cold spot. New ways to seek cosmic parity breaking are also possible.

Planck data and ultralight axions

We examine the effects of photon-axion mixing on the CMB. We show that if there are very underdense regions between us and the last scattering surface which contain coherent magnetic fields (whose strength can be orders of magnitude weaker than the current bounds), then photon-axion mixing can induce observable deviations in the CMB spectrum. Specifically, we show that the mixing can give rise to non-thermal spots on the CMB sky. As an example we consider the well known CMB cold spot, which according to the Planck data has a weak distortion from a black body spectrum, that can be fit by our model. While this explanation of the non-thermality in the region of the cold spot is quite intriguing, photon-axion oscillations do not explain the temperature of the cold spot itself. Nevertheless we demonstrate the possible sensitivity of the CMB to ultralight axions which could be exploited by observers.

Probing the CMB cold spot through local Minkowski functionals

Both the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck missions have reported an extremely cold spot (CS) centered at Galactic coordinate (l = 209°, b = −57°) in the cosmic microwave background map. We study the local non-Gaussianity of the CS by defining local Minkowski functionals. We find that the third Minkowski functional ν2 is quite sensitive to the non-Gaussianity caused by the CS. Compared with random Gaussian simulations, the WMAP CS deviates from Gaussianity at more than a 99% confidence level with a scale of R ∼ 10°. Meanwhile, we find that cosmic texture provides an excellent explanation for these anomalies related to the WMAP CS, which could be further tested by future polarization data.

Local properties of Wilkinson Microwave Anisotropy Probe cold spot

We investigate the local properties of WMAP Cold Spot (CS) by defining the local statistics: mean temperature, variance, skewness and kurtosis. We find that, compared with the \emph{coldest spots} in random Gaussian simulations, WMAP CS deviates from Gaussianity at $\sim 99%$ significant level. In the meanwhile, when compared with the spots at the same position in the simulated maps, the values of local variance and skewness around CS are all systematically larger in the scale of $R>5^{\circ}$, which implies that WMAP CS is a large-scale non-Gaussian structure, rather than a combination of some small structures. This is consistent with the finding that the non-Gaussianity of CS is totally encoded in the WMAP low multipoles. Furthermore, we find that the cosmic texture can excellently explain all the anomalies in these statistics.

How sensitive is the CMB to a single lens?

We study the imprints of a single lens, that breaks statistical isotropy, on the CMB and calculate the signal to noise ratio (S/N) for its detection. We emphasize the role of non-Gaussianities induced by ΛCDM weak lensing in this calculation and show that typically the S/N is much smaller than expected. In particular we find that the hypothesis that a void (texture) is responsible for the WMAP cold spot can barely (cannot) be tested via weak lensing of the CMB.

Disks in the sky: A reassessment of the WMAP “cold spot”

We reassess the evidence that WMAP temperature maps contain a statistically significant “cold spot” by repeating the analysis using simple circular top-hat (disk) weights, as well as Gaussian weights of varying width. Contrary to previous results that used Spherical Mexican Hat Wavelets, we find no significant signal at any scale when we compare the coldest spot from our sky to ones from simulated Gaussian random, isotropic maps. We trace this apparent discrepancy to the fact that WMAP cold spot’s temperature profile just happens to favor the particular profile given by the wavelet. Since randomly generated maps typically do not exhibit this coincidence, we conclude that the original cold spot significance originated at least partly due to a fortuitous choice of using a particular basis of weight functions. We also examine significance of a more general measure that returns the most significant result among several choices of the weighting function, angular scale of the spot, and the statistics applied, and again find a null result.

Tessellated mapping of cosmic background radiation correlations and source distributions

We offer a method of correlations mapping on the full celestial sphere that allows to check the quality of reconstructed maps, their non-Gaussianity and conduct experiments in various frequency ranges. The method was evaluated on the WMAP data, both on the reconstructed maps and foreground components, and on the NRAO VLA Sky Survey (NVSS) data. We detected a significant shift in the correlation data of the dust component, which can be preconditioned by a more complex dust model than the one currently in use for component separation. While studying the NVSS correlation data, we demonstrated that the statistics of the coinciding spots in the microwave background and in the NVSS survey corresponds to the one expected in the $\Lambda$CDM model. This can testify for a chance coincidence of the spots in the NVSS and WMAP data in the CMB Cold Spot region. Our method is software-implemented in the GLESP package.

CMB lensing and the WMAP cold spot

Cosmologists have suggested a number of intriguing hypotheses for the origin of the ``WMAP cold spot,'' the coldest extended region seen in the cosmic microwave background (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 Atacama Cosmology Telescope and the South Pole Telescope should be able to detect the signatures of either textures or large voids. If either signal is detected, it will have profound implications for cosmology.

Is the CMB cold spot a gate to extra dimensions?

One of the most striking features found in the cosmic microwave background data is thepresence of an anomalous cold spot (CS) in the temperature maps made by the WilkinsonMicrowave Anisotropy Probe (WMAP). This CS has been interpreted as the result of thepresence of a collapsing texture, perhaps coming from some early universe grand unifiedtheory (GUT) phase transition. In this work we propose an alternative explanation basedon a completely different kind of texture which appears in a natural way in a broad class ofbrane-world models. This type of texture, known as a brane-skyrmion, can beunderstood as a hole in the brane which makes it possible to pass through themalong the extra-dimensional space. The typical scales needed for the proposedbrane-skyrmions to correctly describe the observed CS can be as low as the electroweakscale.

Extragalactic Radio Sources and theWMAPCold Spot

We detect a dip of 20%-45% in the surface brightness and number counts of NRAO VLA Sky Survey (NVSS) sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of ~1° to 10°. 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 remaining need for non-Gaussian processes at the last scattering surface of the cosmic microwave background (CMB) to explain the cold spot. The late integrated Sachs-Wolfe effect, already seen statistically for NVSS source counts, can now be seen to operate on a single region. To create the magnitude and angular size of the WMAP cold spot requires a ~140 Mpc radius completely empty void at z ≤ 1 along this line of sight. This is far outside the current expectations of the concordance cosmology, and adds to the anomalies seen in the CMB.