The Axiverse Extended: Vacuum Destabilisation, Early Dark Energy and Cosmological Collapse

TL;DR

This paper extends the String Axiverse model by introducing couplings between ultralight axions and moduli, leading to diverse cosmological phenomena such as dark energy behavior and potential universe collapse.

Contribution

It presents a new coupled axion-moduli model that enriches axiverse phenomenology and explores its implications for cosmological evolution and fine-tuning issues.

Findings

The model exhibits phases where axions and moduli act as Dark Matter or Dark Energy.

Couplings can modify the universe's expansion history and equation of state.

Potential for universe collapse due to the dynamics of coupled fields.

Abstract

A model is presented in the philosophy of the "String Axiverse" of Arvanitaki et al (arXiv:0905.4720v2 [hep-th]) that incorporates a coupling of ultralight axions to their corresponding moduli through the mass term. The light fields roll in their potentials at late times and contribute to the dark sector energy densities in the cosmological expansion. The addition of a coupling and extra field greatly enrich the possible phenomenology of the axiverse. There are a number of interesting phases where the axion and modulus components behave as Dark Matter or Dark Energy and can have considerable and distinct effects on the expansion history of the universe by modifying the equation of state in the past or causing possible future collapse of the universe. In future such a coupling may help to alleviate fine tuning problems for cosmological axions. We motivate and present the model, and briefly explore its cosmological consequences numerically.