Perturbative stability of SFT-based cosmological models

TL;DR

This paper introduces a modified, exactly solvable multi-scalar field model in SFT-based cosmology, analyzes its perturbations, and demonstrates the stability of phantom divide crossing with vanishing complex field perturbations.

Contribution

It presents a new solvable multi-scalar field model in SFT cosmology that preserves asymptotic behavior and analyzes its perturbative stability, including phantom divide crossing.

Findings

Perturbations of complex fields always vanish.

The model allows exact solutions consistent with SFT asymptotics.

Phantom divide crossing is perturbatively stable.

Abstract

We review the appearance of multiple scalar fields in linearized SFT based cosmological models with a single non-local scalar field. Some of these local fields are canonical real scalar fields and some are complex fields with unusual coupling. These systems only admit numerical or approximate analysis. We introduce a modified potential for multiple scalar fields that makes the system exactly solvable in the cosmological context of Friedmann equations and at the same time preserves the asymptotic behavior expected from SFT. The main part of the paper consists of the analysis of inhomogeneous cosmological perturbations in this system. We show numerically that perturbations corresponding to the new type of complex fields always vanish. As an example of application of this model we consider an explicit construction of the phantom divide crossing and prove the perturbative stability of this process at the linear order. The issue of ghosts and ways to resolve it are briefly discussed.