Axion as a Cold Dark Matter candidate

TL;DR

This paper proves that axions, as coherently oscillating scalar fields, behave as cold dark matter across most cosmological scales, with deviations only at very small scales, even considering a cosmological constant.

Contribution

It extends previous proofs by comparing axion equations directly with cold dark matter equations, broadening the validity of axions as a dark matter candidate.

Findings

Axions act as cold dark matter in nearly all relevant scales.

Deviations occur only at very small scales where axions show a peculiar equation of state.

The proof remains valid even with a cosmological constant and other components present.

Abstract

Here we generally prove that the axion as a coherently oscillating scalar field acts as a cold dark matter in nearly all cosmologically relevant scales. The proof is made in the linear perturbation order. Compared with our previous proof based on solutions, here we compare the equations in the axion with the ones in the cold dark matter, thus expanding the valid range of the proof. Deviation from purely pressureless medium appears in very small scale where axion reveals a peculiar equation of state. Our analysis is made in the presence of the cosmological constant, and our conclusions are valid in the presence of other fluid and field components.