UV-protected (Natural) Inflation: Primordial Fluctuations and non-Gaussian Features

TL;DR

This work develops UV-protected inflation by coupling the inflaton kinetic term to the Einstein tensor to produce gravitationally enhanced friction, allowing sub-Planckian field scales while preserving a viable slow-roll trajectory. It shows that the resulting single-field dynamics yield a red-tilted scalar spectrum and a consistent tensor-to-scalar ratio with the standard single-field consistency relation, while non-Gaussianities remain generically suppressed and are dominated by gauge transformations rather than intrinsic interactions. The authors explore two phenomenologically viable branches of the model, a small-field branch with negligible gravitational waves and a large-field branch that can yield detectable tensor modes, including a case where the lambda phi4 potential remains consistent with data. They also discuss a Barnaby-Peloso type mechanism for gauge-field induced non-Gaussianities, which is typically subdominant in the UV-protected setup, and comment on possible infrared completion and reheating prospects. Overall, the paper provides a UV-consistent framework for natural inflation that cleanly isolates quantum corrections and yields testable predictions for CMB observables and gravitational waves.

Abstract

We consider the UV-protected inflation, where the inflaton potential is obtained by quantum (one-loop) breaking of a global symmetry into a discrete symmetry. In this model, all coupling scales are sub-Planckian. This is achieved by coupling the inflaton kinetic term to the Einstein tensor such that the friction is enhanced gravitationally at high energies. In this respect, this new interaction makes virtually any potential adequate for inflation while keeping the system perturbative unitary. We show that even if the gravitationally enhanced friction intrinsically contains new nonlinearities, the UV-protected inflation (and any similar models) behaves as a single field scenario with red tilted spectrum and potentially detectable gravitational waves. Interestingly enough, we find that non-Gaussianity of the curvature perturbations in the local form are completely dominated by the nonlinear gauge transformation from the spatially flat to uniform-field gauge and/or by parity violating interactions of the inflaton and gauge bosons. In particular, the parity violating interactions may produce detectable non-Gaussianity.