Is the axionic dark matter an equilibrium system?

TL;DR

This paper investigates whether axionic dark matter can reach an equilibrium state within a modified potential framework in an expanding universe, using a cosmological model with an Einstein-aether extension.

Contribution

It introduces a modified periodic potential for axionic dark matter coupled with a guiding function dependent on the universe's expansion, and analyzes equilibrium conditions in a cosmological setting.

Findings

Identifies conditions for equilibrium states of axionic dark matter.

Derives a basic function describing equilibrium in an expanding universe.

Provides a formalism linking axion potential modifications to cosmological dynamics.

Abstract

We consider an axionic dark matter model with a modified periodic potential for the pseudoscalar field in the framework of the axionic extension of the Einstein-aether theory. The modified potential is assumed to be equipped by the guiding function, which depends on the expansion scalar constructed as the trace of the covariant derivative of the aether velocity four-vector. The equilibrium state of the axion field is defined as the state, for which the modified potential itself and its first derivative with respect to the pseudoscalar field are equal to zero. We apply the developed formalism to the homogeneous isotropic cosmological model, and find the basic function, which describes the equilibrium state of the axionic dark matter in the expanding Universe.