The CMB cold spot under the lens: ruling out a supervoid interpretation (2211.16139v2)

Abstract: The Cosmic Microwave Background (CMB) anisotropies are thought to be statistically isotropic and Gaussian. However, several anomalies are observed, including the CMB Cold Spot, an unexpected cold $\sim 10^{\circ}$ region with $p$-value $\lesssim 0.01$ in standard $\Lambda$CDM. One of the proposed origins of the Cold Spot is an unusually large void on the line of sight, that would generate a cold region through the combination of integrated Sachs-Wolfe and Rees-Sciama effects. In the past decade extensive searches were conducted in large scale structure surveys, both in optical and infrared, in the same area for $z \lesssim 1$ and did find evidence of large voids, but of depth and size able to account for only a fraction of the anomaly. Here we analyze the lensing signal in the Planck CMB data and rule out the hypothesis that the Cold Spot could be due to a large void located anywhere between us and the surface of last scattering. In particular, computing the evidence ratio we find that a model with a large void is disfavored compared to $\Lambda$CDM, with odds 1 : 13 (1 : 20) for SMICA (NILC) maps, compared to the original odds 56 : 1 (21 : 1) using temperature data alone.

• The paper conclusively rules out the supervoid explanation for the CMB Cold Spot using combined temperature and lensing analyses of Planck data.
• It employs simulations integrating ISW and RS effects to assess void-induced gravitational imprints against observational data.
• The absence of expected lensing signatures challenges conventional cosmic structure models and suggests exploring alternative origins.

Analyzing the Cosmic Microwave Background Cold Spot: Dismissing the Supervoid Interpretation

The paper titled "The CMB cold spot under the lens: ruling out a supervoid interpretation" examines the Cosmic Microwave Background (CMB) anomaly known as the Cold Spot (CS) and assesses the hypothesis that it is caused by a supervoid along the line of sight. The paper provides a thorough investigation into the causes behind the CS, specifically debunking the claim that a significant void might be responsible for the observed temperature anomaly via integrated Sachs-Wolfe (ISW) and Rees-Sciama (RS) effects, which are associated with linear and second-order gravitational potential changes, respectively.

Study Motivation and Background

The existence of anomalies in the CMB, such as the CS, has challenged the standard cosmological model, ΛCDM, which describes a universe dominated by dark energy and cold dark matter. The CS is a region of approximately 10° on the sky that is much colder than would be expected in a statistically isotropic ΛCDM-based universe. Prior hypotheses suggested that a large cosmic void could cause such a deviation by altering the CMB photons through gravitational effects.

Methodology and Approach

The authors analyze the CMB data from the Planck satellite, focusing on the lensing signal which a supervoid would create if present. They also consider temperature data to compare predictions with the observations. The likelihood of a void causing the CS was investigated by exploring the non-diagonal terms in the CMB's temperature and lensing data, which should be significant if such a structure existed. The analysis combines ISW and RS effects with gravitational lensing, assessing void characteristics like size, density contrast, and distance.

A significant part of the methodology relies on simulating CMB maps under the void hypothesis and comparing these simulations to data from Planck's SMICA and NILC maps, both with and without potential systematic effects. The analysis focuses not only on temperature discrepancies but also integrates gravitational lensing information—potentially left by a supervoid—to rule out the void hypothesis.

Key Findings

The paper reports that the temperature data from Planck could indeed be explained by a void, but this requires a specific configuration that is not matched by observational evidence in terms of lensing effects. This lack of corroboration from the lensing signal stands in contrast to the significant temperature decrement expected from a void of the postulated size and mass profile. Thus, the apparent absence of an expected lensing signature effectively rules out the void hypothesis from being a plausible explanation for the CS anomaly.

Implications and Future Directions

The implications of this work are significant for the cosmological community. By ruling out a plausible explanation for the CS, the paper reinforces the importance of looking beyond traditional void hypotheses for such temperature anomalies. The findings suggest further investigations into alternative origins, such as unusual primordial non-Gaussianity or other exotic cosmological phenomena.

This research complements ongoing efforts to scrutinize the statistical isotropy and Gaussian nature of the CMB, urging both theoretical refinement and more comprehensive observational strategies to address persisting anomalies. Moreover, the work demonstrates how leveraging comprehensive datasets in a multi-faceted approach—combining temperature, lensing, and structural surveys—can critically evaluate cosmological hypotheses.

In summary, this paper provides a compelling argument against the supervoid interpretation for the CMB Cold Spot anomaly by demonstrating through meticulous simulation and data analysis that the expected contributions from gravitational lensing are absent. This reinforces the complexities inherent in cosmic structure-formation theories and challenges future research endeavors to explore alternative explanations for one of the most studied anomalies in CMB science.