Non-linear matter bispectrum in general relativity
  We show that the relativistic effects are negligibly small in the non-linear
density and velocity bispectra. Although the non-linearities of Einstein
equation introduce additional non-linear terms to the Newtonian fluid
equations, the corrections to the bispectrum only show up on super-horizon
scales. We show this with the next-to-leading order non-linear bispectrum for a
pressureless fluid in a flat Friedmann-Robertson-Walker background, by
calculating the density and velocity fields up to fourth order. We work in the
comoving gauge, where the dynamics is identical to the Newtonian up to second
order. We also discuss the leading order matter bispectrum in various gauges,
and show yet another relativistic effect near horizon scales that the matter
bispectrum strongly depends on the gauge choice.
