A dilaton-axion model for string cosmology

TL;DR

This paper develops a universal axion-dilaton gravity model in string cosmology, analyzing its cosmological solutions and constraints from string theory, especially regarding accelerated expansion and swampland bounds.

Contribution

It introduces a generalized one-axion, two-dilaton model that remains consistent under dimensional reduction and connects to string/M-theory supergravity frameworks.

Findings

Identifies parameter regimes allowing transient de Sitter phases.

Derives swampland bounds excluding eternal acceleration.

Shows the model's relation to maximal supergravity theories.

Abstract

The generic scale-invariant theory of an axion and a dilaton coupled to gravity in $d$-dimensions is generalized to a `universal' one-axion model with two dilatons that reproduces itself under consistent dimensional-reduction/truncation. Flat FLRW cosmologies are shown to correspond to trajectories of a three-dimensional autonomous dynamical system, which we analyse with a focus on accelerated cosmic expansion, deriving the precise swampland bounds that exclude eternal acceleration. We also show that for two sets of values of its three independent parameters, the model is a consistent truncation of maximal `massive' supergravity theories arising from string/M-theory; for these maximal-supergravity parameter values the FLRW cosmologies include some with a transient de Sitter-like phase, but not the recurring de Sitter-like phase or eternal cosmic acceleration that is possible for other parameter values.