The full squeezed CMB bispectrum from inflation
  The small-scale CMB temperature we observe on the sky is modulated by
perturbations that were super-horizon at recombination, giving differential
focussing and lensing that generate a non-zero bispectrum even for single-field
inflation where local physics is identical. Understanding this signal is
important for primordial non-Gaussianity studies and also parameter constraints
from the CMB lensing bispectrum signal. Because of cancellations individual
effects can appear larger or smaller than they are in total, so a full analysis
may be required to avoid biases. I relate angular scales on the sky to physical
scales at recombination using the optical equations, and give full-sky results
for the large-scale adiabatic temperature bispectrum from Ricci focussing
(expansion of the ray bundle), Weyl lensing (convergence and shear), and
temperature redshift modulations of small-scale power. The delta N expansion of
the beam is described by the constant temperature 3-curvature, and gives a
nearly-observable version of the consistency relation prediction from
single-field inflation. I give approximate arguments to quantify the likely
importance of dynamical effects, and argue that they can be neglected for
modulation scales l <~ 100, which is sufficient for lensing studies and also
allows robust tests of local primordial non-Gaussianity using only the
large-scale modulation modes. For accurate numerical results early and
late-time ISW effects must be accounted for, though I confirm that the
late-time non-linear Rees-Sciama contribution is negligible compared to other
more important complications. The total corresponds to f_NL ~ 7 for Planck-like
temperature constraints and f_NL ~ 11 for cosmic-variance limited data to
lmax=2000. Temperature lensing bispectrum estimates are affected at the 0.2
sigma level by Ricci focussing, and up to 0.5 sigma with polarization.
