Sachs-Wolfe at second order: the CMB bispectrum on large angular scales

TL;DR

This paper computes the second-order CMB bispectrum on large scales, including lensing and Rees-Sciama effects, revealing lensing dominance and specific non-Gaussianity parameters in different limits.

Contribution

It extends the Sachs-Wolfe analysis to second order, explicitly calculating the bispectrum with integrated effects in the Poisson gauge, and identifies lensing as the dominant contribution.

Findings

Bispectrum scales as l^(-4) in scale-invariant limit.

Lensing dominates the second-order bispectrum.

Squeezed limit f_NL^local = -1/6 - cos(2θ).

Abstract

We calculate the Cosmic Microwave Background anisotropy bispectrum on large angular scales in the absence of primordial non-Gaussianities, assuming exact matter dominance and extending at second order the classic Sachs-Wolfe result \delta T/T=\Phi/3. The calculation is done in Poisson gauge. Besides intrinsic contributions calculated at last scattering, one must consider integrated effects. These are associated to lensing, and to the time dependence of the potentials (Rees-Sciama) and of the vector and tensor components of the metric generated at second order. The bispectrum is explicitly computed in the flat-sky approximation. It scales as l^(-4) in the scale invariant limit and the shape dependence of its various contributions is represented in 3d plots. Although all the contributions to the bispectrum are parametrically of the same order, the full bispectrum is dominated by lensing. In the squeezed limit it corresponds to f_NL^local = -1/6 - cos(2 \theta), where \theta is the angle between the short and the long modes; the angle dependent contribution comes from lensing. In the equilateral limit it corresponds to f_NL^equil ~ 3.13.