The full CMB temperature bispectrum from single-field inflation

Understand

We compute the full cosmic microwave background temperature bispectrum generated by nonlinearities after single-field inflation.

• By integrating the photon temperature at second order along a perturbed geodesic in Newtonian gauge, we derive an expression for the observed temperature fluctuations that, for the first time, clarifies the separation of the gravitational lensing and time-delay effects from the purely second-order contributions.
• We then use the second-order Boltzmann code CosmoLib$2^{\rm nd}$ to calculate these contributions and their bispectrum.
• Including the perturbations in the photon path, the numerically computed bispectrum exactly matches the expected squeezed limit.