Nonclassicality of axion-like dark matter through gravitational self-interactions

TL;DR

This paper demonstrates that gravitational self-interactions induce quantum squeezing in axion-like dark matter, challenging the classical field approximation and suggesting nonclassical behavior on astrophysical scales.

Contribution

It reveals that ALPs exhibit quantum squeezing due to gravity, showing limitations of classical models and indicating nonclassical features in dark matter.

Findings

Squeezing occurs on microsecond timescales for QCD axions.

Squeezing grows over millennia, affecting dark matter modeling.

Nonclassical effects may influence galactic structures.

Abstract

Axion-like particles (ALPs) are promising dark matter candidates. They are typically described by a classical field, motivated by large phase space occupation numbers. Here we show that such a description is accompanied by a quantum effect: squeezing due to gravitational self-interactions. For a typical QCD axion today, the onset of squeezing is reached on $\mathrm{\mu s}$-scales and grows over millennia. Thus within the usual models based on the classical Schr\"odinger-Poisson equation, a type of Gross-Pitaevskii equation, any viable ALP is nonclassical. We also show that squeezing may be relevant on the scales of other self-gravitating systems such as galactic haloes, or solitonic cores. Conversely, our results highlight the incompleteness and limitations of the classical single field description of ALPs.