The CMB cold spot under the lens: ruling out a supervoid interpretation

TL;DR

This study uses Planck CMB lensing data to test whether a supervoid could explain the Cold Spot anomaly, ultimately ruling out this hypothesis with strong statistical evidence.

Contribution

It provides the first lensing-based analysis that conclusively rules out a supervoid as the cause of the Cold Spot anomaly.

Findings

Lensing data disfavor the supervoid hypothesis with odds 1:13 to 1:20.

The analysis significantly strengthens previous constraints from temperature data alone.

The Cold Spot is unlikely caused by a large void along the line of sight.

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.