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On the origin of the cold spot




In a concordant Λ cold dark matter model, the 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 the 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 ∼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 ∼10 per cent is produced due to a single spherical underdense region with a radius r∼ 6 × 102 h−1 Mpc and a density contrast δm∼−0.009 (2σ) at redshift z∼ 1 in the line of sight to the CS. The probability of having such two aligned structures is ∼0.7 per cent if the perturbed region at z∼ 1 is moderately undercompensated.