Maldacena’s consistency relation in single-field inflation encodes the fact that a local observer measuring the power spectrum of small-scale curvature perturbations cannot detect the presence of a long-wavelength mode at zeroth and first order in gradients: any physical effect will start quadratic in gradients of the long mode.
Motivated by the possibility that future upper limits on primordial gravitational waves will establish a hierarchy \epsilon << \eta ~ 1-n_S, I study the size of the long-short mode coupling in slow-roll inflation by working in Conformal Fermi Coordinates, a generalization of Fermi Normal Coordinates that isolates the physical effects of a long- wavelength perturbation on short modes and is naturally connected to observations at late times.
I show that \eta, which is present in Maldacena’s result also at second order in gradients of the long mode, survives once we rewrite the bispectrum in Conformal Fermi Coordinates. I discuss the possibility of extrapolating this result to the equilateral limit. I conclude with a brief discussion of the case of an inflaton sound speed much smaller than 1.