Non-Minimally Coupled Chain Inflation at High Scales

Abstract

Chain inflation offers an alternative to standard slow-roll dynamics, with accelerated expansion proceeding through a sequence of rapid quantum tunneling events between metastable vacua. At the high energy scales relevant for the early Universe, scalar fields are generically expected to couple non-minimally to gravity via operators like $\xi R\phi^2$, allowed by symmetry and required as counterterms for interacting theories in curved spacetime. We study the dynamical and observational consequences of this coupling for chain inflation. We find the modifications to the model for arbitrary $\xi$ and focus on interesting phenomenology for $\xi ={\cal O}( 10)$. We show that, in the Einstein frame, the non-minimal coupling induces a field-dependent amplification of the Euclidean bounce action, thus modifying the tunneling rate across the chain. We develop an analytic framework connecting this modified tunneling dynamics to the scalar spectral index, its running, the primordial curvature power spectrum, and the stochastic gravitational wave background from bubble collisions. As one consequence, the non-minimal coupling breaks the rigid relation between the scalar tilt and inflationary scale that drives the minimally coupled pure tilted cosine model to very low energies ($V_*^{1/4}\lesssim 3\,\rm{GeV}$, where $V_*$ is the value of the inflationary potential when the CMB-relevant modes exit the horizon), allowing for viable high-scale chain inflation with $V_*^{1/4}\sim 10^{11}\,\rm{GeV}$. Furthermore, non-minimally coupled chain inflation at high scales produces a peaked stochastic gravitational wave signal in the dHz-kHz bands, accessible to upcoming interferometers such as the Einstein Telescope and Cosmic Explorer. Finally, the model predicts a distinct running of the spectral index that will be testable by the Simons Observatory, making it a prime target for multi-messenger cosmology.