Axionlike 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 $\mu \mathrm{s}$ scales and grows over millennia. Thus within the usual models based on the classical Schrödinger-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.

中文翻译：

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类轴子暗物质通过引力自相互作用的非经典性

类轴子粒子 (ALP) 是有前途的暗物质候选者。它们通常由受大相空间占用数驱动的经典领域来描述。在这里，我们表明这种描述伴随着量子效应：由于引力自相互作用而产生的挤压。对于今天典型的 QCD 轴子，挤压的开始是在$\mu \mathrm{s}$规模和增长超过千年。因此，在基于经典薛定谔-泊松方程（一种 Gross-Pitaevskii 方程）的常用模型中，任何可行的 ALP 都是非经典的。我们还表明，挤压可能与其他自引力系统的尺度有关，例如星系晕或孤子核心。相反，我们的结果突出了 ALP 的经典单场描述的不完整性和局限性。