KSZ tomography and the bispectrum
  Several statistics have been proposed for measuring the kSZ effect by
combining the small-scale CMB with galaxy surveys. We review five such
statistics, and show that they are all mathematically equivalent to the optimal
bispectrum estimator of type $\langle ggT \rangle$. Reinterpreting these kSZ
statistics as special cases of bispectrum estimation makes many aspects
transparent, for example optimally weighting the estimator, or incorporating
photometric redshift errors. We analyze the information content of the
bispectrum and show that there are two observables: the small-scale
galaxy-electron power spectrum $P_{ge}(k_S)$, and the large-scale
galaxy-velocity power spectrum $P_{gv}(k)$. The cosmological constraining power
of the kSZ arises from its sensitivity to fluctuations on large length scales,
where its effective noise level can be much better than galaxy surveys.
