More on crinkles in the last scattering surface

TL;DR

This paper analyzes how inhomogeneous recombination affects second order anisotropies in the CMB, showing the small magnitude of these effects and confirming their contribution to local non-Gaussianity with f_{NL} around -1.

Contribution

It provides a detailed analysis of second order CMB anisotropies from inhomogeneous recombination and clarifies the correct approach to solving the Boltzmann equations at second order.

Findings

Contributions from inhomogeneous recombination to low multipoles are numerically small.

Perturbing electron density in first order solutions is not equivalent to second order solutions.

Inhomogeneous recombination induces a local non-Gaussianity parameter f_{NL}~ -1.

Abstract

Inhomogeneous recombination can give rise to perturbations in the electron number density which can be a factor of five larger than the perturbations in baryon density. We do a thorough analysis of the second order anisotropies generated in the cosmic microwave background (CMB) due to perturbations in the electron number density. We show that solving the second order Boltzmann equation for photons is equivalent to solving the first + second order Boltzmann equations and then taking the second order part of the solution. We find the approximate solution to the photon Boltzmann hierarchy in l modes and show that the contributions from inhomogeneous recombination to the second order monopole, dipole and quadrupole are numerically small. We also point out that perturbing the electron number density in the first order tight coupling and damping solutions for the monopole, dipole and quadrupole is not equivalent to solving the second order Boltzmann equations for inhomogeneous recombination. Finally we confirm our result in a previous paper that inhomogeneous recombination gives rise to a local type non-Gaussianity parameter f_{NL}~ -1. The signal to noise for the detection of the temperature bispectrum generated by inhomogeneous recombination is ~ 1 for an ideal full sky experiment measuring modes up to l_{max}=2500.