Multipoles of the relativistic galaxy bispectrum
  Above the equality scale the galaxy bispectrum will be a key probe for
measuring primordial non-Gaussianity which can help differentiate between
different inflationary models and other theories of the early universe. On
these scales a variety of relativistic effects come into play once the galaxy
number-count fluctuation is projected onto our past lightcone. By decomposing
the Fourier-space bispectrum into invariant multipoles about the observer's
line of sight we examine in detail how the relativistic effects contribute to
these. We show how to perform this decomposition analytically, which is
significantly faster for subsequent computations. While all multipoles receive
a contribution from the relativistic part, odd multipoles arising from the
imaginary part of the bispectrum have no Newtonian contribution, making the odd
multipoles a smoking gun for a relativistic signature in the bispectrum for
single tracers. The dipole and the octopole are significant on equality scales
and above where the Newtonian approximation breaks down. This breakdown is
further signified by the fact that the even multipoles receive a significant
correction on very large scales.
