The perturbations in the electron number density during recombination contributes to the cosmic microwave background bispectrum through second order terms. Perturbations in the electron density can be a factor of $\sim 5$ larger than the baryon density fluctuations on large scales as shown in the calculations by Novosyadlyj. This raises the possibility that the contribution to the bispectrum arising from perturbations in the optical depth may be non-negligible. We calculate this bispectrum and find it to peak for squeezed triangles and of peak amplitude of the order of primordial non-Gaussianity of a local type with $f_{NL}\approx 0.05\sim -1$ depending on the $\ell$ modes being considered. This is because the shape of the bispectrum is different from the primordial one although it peaks for squeezed configurations, similar to the local type primordial non-Gaussianity.

• Received 23 October 2008

DOI:https://doi.org/10.1103/PhysRevD.79.023501