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OKC - Theory Working Group

The floor of primordial non-Gaussianity

by Giovanni Cabass (Universita' di Roma 'Sapienza' and INFN)

Nordita 132:028 ()

Nordita 132:028

Gravity is a non-linear theory: then, barring cancellations, the initial super-horizon perturbations produced by inflation must contain some minimum amount of mode coupling, or primordial non-Gaussianity. In single-field slow-roll models, where this lower bound is saturated, non-Gaussianity is controlled by two observables: the tensor-to-scalar ratio (which is uncertain by more than fifty orders of magnitude), and the scalar spectral index (which is relatively well measured). It is well known that to leading and next-to-leading order in gradients of the long mode, the contributions proportional to the tilt disappear from any local observable, and suspicion has been raised that this might happen to all orders, allowing for an arbitrarily low amount of primordial non-Gaussianity. Employing 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 this is not the case: primordial non-Gaussianity has a “gravitational floor” of order the scalar spectral index.