Rescuing Single Field Inflation from the Swampland

TL;DR

The paper investigates whether excited initial states can rescue single field inflation from swampland constraints, finding scalar states are incompatible due to non-gaussianity, but tensor states offer a viable alternative.

Contribution

It demonstrates that excited initial states for tensor perturbations can preserve single field inflation within swampland constraints, unlike scalar states.

Findings

Scalar excited states produce large local non-gaussianity, conflicting with observations.

Tensor excited states generate large flattened non-gaussianity in the tensor bispectrum.

Scalar initial states cannot rescue inflation due to observational bounds.

Abstract

The difficulty of building metastable vacua in string theory has led some to conjecture that, in the string theory landscape, potentials satisfy $\left|\nabla V/V\right|\geq c\sim \mathcal{O}(1)$. This condition, which is supported by different explicit constructions, suggests that the EFTs which lead to metastable de-Sitter vacua belong to what is dubbed as swampland. This condition endangers the paradigm of single field inflation. In this paper, we show how scalar excited initial states cannot rescue single field inflation from the swampland, as they produce large local scalar non-gaussianity, which is in conflict with the Planck upper bound. Instead, we demonstrate that one can salvage single field inflation using excited initial states for tensor perturbations, which in this case produce only large flattened non-gaussianity in the tensor bispectrum. We comment on the possible methods one can prepare such excited initial conditions for the tensor perturbations.