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.Comment: 38 pages, 9 figures. v2: minor corrections to match published versio
