On the origin of the Cold Spot

Abstract

In a concordant $\Lambda$ Cold Dark Matter ($\Lambda$CDM) model, large-angle Cosmic Microwave Background (CMB) temperature anisotropy due to linear perturbations in the local universe is not negligible. We explore a possible role of an underdense region (void) that may cause an anomalous Cold Spot (CS) in the CMB sky. Although the observed anomalous cold region with a surrounding hot ring can be produced by an underdense region surrounded by a massive wall, a decrement in the CMB temperature in the line-of-sight is suppressed because of blueshift of CMB photons that pass the wall. Therefore, undercompensated models give better agreement with the observed data in comparison with overcompensated or compensated models. We find that it is likely that $\sim$90 per cent of the CMB fluctuation is generated due to an overdense region surrounded by an underdense region at the last scattering surface, and the remaining $\sim 10$ per cent is produced due to a single spherical underdense region with a radius $r\sim 6\times 10^2 h^{-1}$Mpc and a density contrast $\delta_m\sim -0.009$ ($2 \sigma$) at redshift $z\sim 1$ in the line-of-sight to the CS. The probability of chance alignment of such two structures is $\sim 0.7$ per cent if the perturbation with an underdense region at $z\sim 1$ is moderately undercompensated.