A significant challenge for modelling the massive neutrino as a hot dark matter is its large velocity dispersion. In this work, we investigate and implement a multi-fluid perturbation theory that treats the cosmic neutrino population as a collection of fluids with a broad range of bulk velocities. These fluids respond linearly to the clustering of cold matter, which may be linear and described by standard linear perturbation theory, or non-linear, described using either higher-order perturbation theory or N-body simulations. We verify that such an alternative treatment of neutrino perturbations agrees closely with state-of-the-art neutrino linear response calculations in terms of power spectrum and bispectrum predictions. Combining multi-fluid neutrino linear response with a non-linear calculation for the cold matter clustering, we find for a reference νΛCDM cosmology with neutrino mass sum ∑ m _ν = 0.93 eV an enhancement of the small-scale neutrino power by an order of magnitude relative to a purely linear calculation. The corresponding clustering enhancement in the cold matter, however, is a modest ∼ 0.05%. Importantly, our multi-fluid approach uniquely enables us to identify that the slowest-moving 25% of the neutrino population clusters strongly enough to warrant a non-linear treatment. Such a precise calculation of neutrino clustering on small scales accompanied by fine-grained velocity information would be invaluable for experiments such as PTOLEMY that probe the local neutrino density and velocity in the solarneighbourhood.

将大质量中微子建模为热暗物质的一个重大挑战是它的大速度色散。在这项工作中，我们研究并实施了一种多流体微扰理论，该理论将宇宙中微子群体视为具有广泛体积速度的流体集合。这些流体对冷物质的聚集呈线性反应，这可能是线性的，可以用标准线性摄动理论来描述，也可以是非线性的，用高阶扰动理论或N来描述。- 身体模拟。我们验证了这种中微子扰动的替代处理方法在功率谱和双谱预测方面与最先进的中微子线性响应计算非常吻合。将多流体中微子线性响应与冷物质聚类的非线性计算相结合，我们找到了具有中微子质量总和 ∑ m _{ν的参考 νΛCDM 宇宙学}= 0.93 eV，相对于纯线性计算，小尺度中微子功率提高了一个数量级。然而，冷物质中相应的聚类增强是适度的~0.05%。重要的是，我们的多流体方法独特地使我们能够识别出移动最慢的 25% 的中微子群体，其强度足以保证进行非线性处理。这种在小尺度上精确计算中微子聚类并伴有细粒度的速度信息对于诸如 PTOLEMY 等探测太阳附近局部中微子密度和速度的实验将是无价的。