Avoiding the string swampland in single-field inflation: Excited initial states

TL;DR

The paper addresses the tension between swampland constraints and single-field inflation with convex potentials, which would generically predict an oversized tensor-to-scalar ratio $r$ under the standard BD consistency $r=16 ε_H$. It proposes non-Bunch-Davies initial states for perturbations, parameterized by Bogoliubov coefficients, to introduce a suppression factor $γ$ so that $r = 16 ε_H γ$ can remain small even for sizable $ε_H$. In a quintessential brane-inflation example, the authors show that choosing tensor NBD components with appropriate phases yields $γ \lesssim 0.1$, allowing $ε_H$ to be order $0.1$ while satisfying observational bounds and thereby aligning with swampland criteria. They also demonstrate that $S1$ can be satisfied with sub-Planckian field excursions and a modified Lyth bound, highlighting a viable, testable path for single-field inflation within string-inspired constraints.

Abstract

One class of single-field inflationary models compatible with the recently-conjectured Swampland criteria would be those in which a Hubble slow-roll parameter $ε_\text{H}$ is not the same as $ε_\text{V} \sim (V'/V)^2$. However, a roadblock for these models (with a convex potential) lie in the unacceptably high tensor-to-scalar ratio, $r$, generically predicted by them. In this work, illustrating through an explicit example, we point out that having a non-Bunch-Davies component to the initial state of cosmological perturbations makes the value of $r$ compatible with observations. In this way, we lay down a new path even for standard models of slow-roll inflation to be consistent with the Swampland criteria by invoking deviations from the Bunch-Davies initial state.