Bounding axion dark energy

TL;DR

This paper derives analytic bounds on axion parameters in cosmology, constraining dark energy models and excluding large parameter regions based on observational and quantum gravity considerations.

Contribution

It provides a novel analytic bound on axion mass and decay constant applicable to dark energy models, integrating observational data and quantum gravity constraints.

Findings

Bounds on axion parameters tighten with decreasing universe acceleration.

Observational data from DESI and supernovae further restrict axion dark energy models.

Quantum gravity constraints exclude large axion parameter regions, challenging basic quintessence models.

Abstract

We study cosmological solutions of (pseudo)scalar theories with periodic potentials, in the presence of arbitrary cosmological fluids -- including a cosmological constant of either sign. Independently of the initial misalignment angle and field velocity, we derive an analytic bound that the axion mass parameter and decay constant fulfill as the universe decreases its acceleration rate, finding a natural application in models of thawing quintessence. As a first application, we illustrate the analytic handle our bound provides in bounding axion dark energy, after observational inputs from DESI and various supernovae data sets are taken into account. As a second application, we argue that our analytic bounds in combination with proposed quantum gravity constraints on axions exclude vast regions of parameter space. The combined constraints push the axion masses to be much larger than the Hubble scale, in tension with basic models of axion quintessence.