Moment transport equations for non-Gaussianity
  We present a novel method for calculating the primordial non-Gaussianity
produced by super-horizon evolution during inflation. Our method evolves the
distribution of coarse-grained inflationary field values using a transport
equation. We present simple evolution equations for the moments of this
distribution, such as the variance and skewness. This method possesses some
advantages over existing techniques. Among them, it cleanly separates multiple
sources of primordial non-Gaussianity, and is computationally efficient when
compared with popular alternatives, such as the "delta N" framework. We adduce
numerical calculations demonstrating that our new method offers good agreement
with those already in the literature. We focus on two fields and the fNL
parameter, but we expect our method will generalize to multiple scalar fields
and to moments of arbitrarily high order. We present our expressions in a
field-space covariant form which we postulate to be valid for any number of
fields.
