CMB Bispectrum from Non-linear Effects during Recombination
  We study cosmological perturbations by solving the governing Boltzmann and
Einstein Field equations up to second order, and calculate the corresponding
CMB bispectrum during recombination. We include all the second-order Liouville
and collision terms, truncating the multipole hierarchy at $l=10$, consistently
including all $m\neq 0$ terms when calculating the bispectrum in the flat-sky
limit. At this stage, we focus on contributions at recombination and we neglect
2nd-order vector and tensor terms, lensing effects, and late time non-linear
ISW. We find that the signal-to-noise for the bispectrum is 0.69 for
$l_{max}=2000$, yielding an overall signal $F_{\rm NL} = 3.19$ (normalised
relative to the local model). We find that the effective $f_{NL}$'s of the
equilateral and local type are 5.11 and 0.88 respectively. This recombination
signal will have to be taken into account in a quantitative analysis of the
Planck data.
