Axion-Dilaton Cosmology and Dark Energy

TL;DR

This paper explores axion-dilaton cosmological models with background fluids, analyzing their dynamics, acceleration behavior, and potential as dark energy candidates, highlighting the impact of realistic cosmic evolution on their properties.

Contribution

It investigates how including radiation and matter affects axion-dilaton models, revealing constraints on oscillatory behavior and their viability as dark energy.

Findings

Oscillations around the acceleration boundary are suppressed when background fluids are included.

Models with late-time oscillations are inconsistent with observations.

Some models exhibit a transition from scaling to eternal acceleration.

Abstract

We discuss a class of flat FRW cosmological models based on D=4 axion-dilaton gravity universally coupled to cosmological background fluids. In particular, we investigate the possibility of recurrent acceleration, which was recently shown to be generically realized in a wide class of axion-dilaton models, but in absence of cosmological background fluids. We observe that, once we impose the existence of radiation -and matter- dominated earlier stages of cosmic evolution, the axion-dilaton dynamics is altered significantly with respect to the case of pure axion-dilaton gravity. During the matter dominated epoch the scalar fields remain either frozen, due to the large expansion rate, or enter a cosmological scaling regime. In both cases, oscillations of the effective equation of state around the acceleration boundary value are impossible. Models which enter an oscillatory stage in the low redshift regime, on the other hand, are disfavored by observations. We also comment on the viability of the axion-dilaton system as a candidate for dynamical dark energy. In a certain subclass of models, an intermediate scaling regime is succeeded by eternal acceleration. We also briefly discuss the issue of dependence on initial conditions.