Decoding the bispectrum of single-field inflation
  Galileon fields arise naturally from the decoupling limit of massive
gravities, and possess special self-interactions which are protected by a
spacetime generalization of Galilean symmetry. We briefly revisit the
inflationary phenomenology of Galileon theories. Working from recent
computations of the fluctuation Lagrangian to cubic order in the most general
model with second-order equations of motion, we show that a distinct shape is
present but with suppressed amplitude. A similar shape has been found in other
higher-derivative models. It may be visible in a theory tuned to suppress the
leading-order shapes, or if the overall bispectrum has large amplitude. Using a
partial-wave expansion of the bispectrum, we suggest a possible origin for the
frequent appearance of this shape. It follows that models with very disparate
microphysics can produce very similar bispectra. We argue that it may be more
profitable to distinguish these models by searching for relations between the
amplitudes of these common shapes. We illustrate this method using the example
of DBI and k-inflation.
