Is the axionic dark matter an equilibrium system?

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.