Can a supervoid explain the Cold Spot?

TL;DR

This paper investigates whether a supervoid can explain the Cold Spot in the CMB, concluding that the required void is highly improbable and the observed anisotropy is more consistent with a statistical fluctuation.

Contribution

The study provides a detailed comparison of exact and perturbative calculations showing the supervoid's effects are too small to account for the Cold Spot, challenging previous claims.

Findings

The supervoid's second-order RS effect is negligible compared to the linear ISW effect.

The required supervoid for explaining the Cold Spot is virtually impossible in ΛCDM.

A posteriori selection effects do not favor a supervoid explanation over chance fluctuations.

Abstract

The discovery of a void of size $\sim200\;h^{-1}$Mpc and average density contrast of $\sim-0.1$ aligned with the Cold Spot direction has been recently reported. It has been argued that, although the first-order integrated Sachs-Wolfe (ISW) effect of such a void on the CMB is small, the second-order Rees-Sciama (RS) contribution exceeds this by an order of magnitude and can entirely explain the observed Cold Spot temperature profile. In this paper we examine this surprising claim using both an exact calculation with the spherically symmetric Lema\^itre-Tolman-Bondi metric, and perturbation theory about a background Friedmann-Robertson-Walker (FRW) metric. We show that both approaches agree well with each other, and both show that the dominant temperature contribution of the postulated void is an unobservable dipole anisotropy. If this dipole is subtracted, we find that the remaining temperature anisotropy is dominated by the linear ISW signal, which is orders of magnitude larger than the second-order RS effect, and that the total magnitude is too small to explain the observed Cold Spot profile. We calculate the density and size of a void that would be required to explain the Cold Spot, and show that the probability of existence of such a void is essentially zero in $\Lambda$CDM. We identify the importance of \emph{a posteriori} selection effects in the identification of the Cold Spot, but argue that even after accounting for them, a supervoid explanation of the Cold Spot is always disfavoured relative to a random statistical fluctuation on the last scattering surface.