“…It has recently become clear that a great deal of important information is encoded in the non-linearities of the curvature perturbation [15,16,17,18,19]. The lower-order moments of ζ are conventionally expressed in terms of the non-linearity parameters f NL [20,21,22] and τ NL [23,24,25,26,27], which respectively quantify the importance of the three-and four-point correlators of ζ relative to the power spectrum. To date, primordial non-gaussian features in the CMB have yet to be detected and the current upper bounds on the non-linearity parameters are |f NL | 10 2 [1,28,29] and |τ NL | 10 8 [27].…”