The Cold Spot as a Large Void: Rees-Sciama effect on CMB Power Spectrum and Bispectrum

TL;DR

This paper investigates whether a large cosmic void, modeled as an LTB region, can produce detectable signals in the CMB power spectrum and bispectrum via the Rees-Sciama effect, potentially explaining the Cold Spot.

Contribution

It demonstrates that a large void can cause measurable modifications in the CMB power spectrum and bispectrum, linking local structures to observed anomalies.

Findings

Void induces a 5-25% correction in power spectrum at low multipoles.

Bispectrum signal with S/N ~ 1-10 localized at 10<l<40.

Potential overestimation of primordial f_NL by ~1 for WMAP and 0.1 for Planck.

Abstract

The detection of a "Cold Spot" in the CMB sky could be explained by the presence of an anomalously large spherical underdense region (with radius of a few hundreds Mpc/h) located between us and the Last Scattering Surface. Modeling such an underdensity with an LTB metric, we investigate whether it could produce significant signals on the CMB power spectrum and bispectrum, via the Rees-Sciama effect. We find that this leads to a bump on the power spectrum, that corresponds to an O(5%-25%) correction at multipoles 5 < l < 50; in the cosmological fits, this would modify the \chi^2 by an amount of order unity. We also find that the signal should be visible in the bispectrum coefficients with a signal-to-noise S/N ~ O (1-10), localized at 10 < l < 40. Such a signal would lead to an overestimation of the primordial f_{NL} by an amount \Delta f_{NL} ~ 1 for WMAP and by \Delta f_{NL} ~ 0.1 for Planck.