Classicality and Quantum Break-Time for Cosmic Axions

TL;DR

This paper analyzes the duration for which the classical description of cosmic axions remains valid, concluding it exceeds the universe's age, thus supporting classical approximation in experimental searches.

Contribution

It introduces the concept of quantum break-time for cosmic axions and demonstrates its vast length compared to the universe's age, validating classical treatment.

Findings

Quantum break-time exceeds the age of the universe for dark matter axions.

Classical approximation is justified for experimental searches.

Distinguishing classical nonlinearities from quantum effects is crucial.

Abstract

We investigate the length of the period of validity of a classical description for the cosmic axion field. To this end, we first show that we can understand the oscillating axion solution as expectation value over an underlying coherent quantum state. Once we include self-interaction of the axion, the quantum state evolves so that the expectation value over it starts to deviate from the classical solution. The time-scale of this process defines the quantum break-time. For the hypothetical dark matter axion field in our Universe, we show that quantum break-time exceeds the age of the Universe by many orders of magnitude. This conclusion is independent of specific properties of the axion model. Thus, experimental searches based on the classical approximation of the oscillating cosmic axion field are fully justified. Additionally, we point out that the distinction of classical nonlinearities and true quantum effects is crucial for calculating the quantum break-time in any system. Our analysis can also be applied to other types of dark matter that are described as classical fluids in the mean field approximation.