Cosmology of Axions and Moduli: A Dynamical Systems Approach

TL;DR

This paper explores the complex cosmological dynamics of axions and moduli in string theory, revealing multiple potential future acceleration phases and new de-Sitter regions through a dynamical systems approach.

Contribution

It introduces a detailed dynamical systems analysis of coupled axion-moduli models, uncovering new metastable de-Sitter states and rich cosmological behaviors.

Findings

Existence of multiple fixed point attractors, repellers, and saddle points.

Relaxing fixed axion decay constant leads to new de-Sitter regions.

Future universe evolution can include multiple accelerated expansion epochs.

Abstract

This paper is concerned with string cosmology and the dynamics of multiple scalar fields in potentials that can become negative, and their features as (Early) Dark Energy models. Our point of departure is the "String Axiverse", a scenario that motivates the existence of cosmologically light axion fields as a generic consequence of string theory. We couple such an axion to its corresponding modulus. We give a detailed presentation of the rich cosmology of such a model, ranging from the setting of initial conditions on the fields during inflation, to the asymptotic future. We present some simplifying assumptions based on the fixing of the axion decay constant $f_a$, and on the effective field theory when the modulus trajectory is adiabatic, and find the conditions under which these assumptions break down. As a by-product of our analysis, we find that relaxing the assumption of fixed $f_a$ leads to the appearance of a new meta-stable de-Sitter region for the modulus without the need for uplifting by an additional constant. A dynamical systems analysis reveals the existence of many fixed point attractors, repellers and saddle points, which we analyse in detail. We also provide geometric interpretations of the phase space. The fixed points can be used to bound the couplings in the model. A systematic scan of certain regions of parameter space reveals that the future evolution of the universe in this model can be rich, containing multiple epochs of accelerated expansion.