Unstable Axion Quintessence Revisited

TL;DR

This paper revisits unstable axion quintessence models, analyzing the universe's evolution during contraction phases with a focus on late-time dark energy behavior and the approach to singularity.

Contribution

It provides detailed evolution equations and simulations for unstable axion quintessence during contraction, extending previous models closer to the singularity.

Findings

The universe enters a kination era during contraction.

The axion field transits its vacuum state multiple times.

Simulations can now approach the singularity more closely.

Abstract

In axion quintessence, the cosmological era with an energy contrast in dark energy 0.1 < Omega_DE < 0.9 may represent a significant fraction of the universe's lifetime if the minimum of the axion potential is negative (unstable axion quintessence), thus resolving the cosmic coincidence problem, as pointed out in Ref. [1]. Further details of the evolution of the quintessence field, the scale factor of the universe, and the Hubble parameter are presented here, focussing on models with Omega_DE,0 = 0.72 and recent dark energy average equation of state parameter -1 < w_0 < -0.85. For these parameter values, the contracting universe enters a late time era of kination, the negative Hubble parameter acting like a negative friction term in the Klein-Gordon equation, and the axion field makes many transits of--but never remains in--its vacuum state. Robust, scaled cosmological equations are derived for simulating the evolution of the scalar field, the scale factor, and the Hubble parameter during a contracting phase of the universe. These equations allow the simulations presented here to proceed much closer to the singularity at the end of the collapsing universe than any previous simulations.