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

TL;DR

This paper proposes that deviations from the Bunch-Davies initial state in single-field inflation can reconcile models with the Swampland criteria and observational constraints on the tensor-to-scalar ratio.

Contribution

It introduces a novel approach of using excited initial states to make certain inflationary models compatible with Swampland conjectures and observational data.

Findings

Non-Bunch-Davies initial states reduce tensor-to-scalar ratio r

Explicit example demonstrating compatibility with observations

New pathway for standard slow-roll inflation models to meet Swampland criteria

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 $\epsilon_\text{H}$ is not the same as $\epsilon_\text{V} \sim (V'/V)^2$. 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.