A dilaton-axion model for string cosmology

Understand

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.