Can a supervoid explain the cold spot?

Abstract

The discovery of a void of size ∼200h −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 first-order integrated Sachs-Wolfe (ISW) effect of such a void on the cosmic microwave background 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 Lemaître-Tolman-Bondi metric, and perturbation theory about a background Friedmann-Robertson-Walker 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 ΛCDM. 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.

The cosmic microwave background Cold Spot anomaly: the impact of sky masking and the expected contribution from the integrated Sachs–Wolfe effect

“…Measurements of large voids in the LOS and ISW reconstructions are consistent with this result. Since reconstructed ISW profiles (see Nadathur et al 2014;Finelli et al 2016; appear to be below the mean shown in Fig. 5, it is possible that the ISW is amplifying the significance of the CS.…”

“…We measure the cut-sky and full-sky minima (cold spot) in 100,000 simulations using the Planck Common Field mask which has a similar f sky to the WMAP KQ75 and Planck U74 masks used in Zhang & Huterer (2010) and Nadathur et al (2014) respectively. The probability of observing the full-sky minima is found to be ∼ 0.48 (which roughly equals the unmasked fraction of the effective mask).…”

“…The anomaly has persisted (Cruz et al 2007;Bennett et al 2011) and was later verified by Planck (Planck Collaboration et al 2016b). Inoue & Silk (2006 claimed the Integrated SachsWolfe (ISW) (Sachs & Wolfe 1967) and Rees-Sciama (RS) (Rees & Sciama 1968) effects of a large void at redshift z ∼ 1 could explain the entire feature (Nadathur et al (2014) show the RS is subdominant in all cases). However, pencil beam surveys (Bremer et al 2010;Granett et al 2010) have effectively ruled out the possibility of such a large void at high redshift (i.e.…”

“…Studies of the galaxy distribution in the relevant region using photo-z initially appeared to indicate that a single spherical/elliptical void exists along the E-mail: krishna.naidoo.11@ucl.ac.uk (KN); benoitl@iap.fr (AB-L); o.lahav@ucl.ac.uk (OL) line-of-sight (LOS) at lower redshift (see Szapudi et al 2015;. Several studies have shown this is insufficient to explain the CS (see Nadathur et al 2014;Zibin 2014;Marcos-Caballero et al 2016). Naidoo et al (2016) found that a model using multiple voids could only explain a fraction of the feature.…”

“…The use of a mask in the SMHW analysis of the CS, to minimise contribution from the Galaxy and point sources, is common practice (see Vielva et al 2004;Zhang & Huterer 2010;Nadathur et al 2014;Planck Collaboration et al 2016b). Because the SMHW transform integrates across the sky, contributions from masked areas will leak to neighbouring regions.…”