Can a supervoid explain the cold spot?
Abstract
The discovery of a void of size ∼200 h^{1} Mpc and average density contrast of ∼0.1 aligned with the cold spot direction has been recently reported. It has been argued that, although the firstorder integrated SachsWolfe (ISW) effect of such a void on the cosmic microwave background is small, the secondorder ReesSciama (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 LemaîtreTolmanBondi metric, and perturbation theory about a background FriedmannRobertsonWalker 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 secondorder 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 Λ CDM . We identify the importance of 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 disfavored relative to a random statistical fluctuation on the last scattering surface.
 Publication:

Physical Review D
 Pub Date:
 November 2014
 DOI:
 10.1103/PhysRevD.90.103510
 arXiv:
 arXiv:1408.4720
 Bibcode:
 2014PhRvD..90j3510N
 Keywords:

 98.80.k;
 98.80.Es;
 98.65.Dx;
 04.25.Nx;
 Cosmology;
 Observational cosmology;
 Superclusters;
 largescale structure of the Universe;
 PostNewtonian approximation;
 perturbation theory;
 related approximations;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 13 pages, 7 figures. Minor edits to match published version, 2 new references