The cosmic neutrino background as a collection of fluids in large-scale structure simulations

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 nuLambdaCDM cosmology with neutrino mass sum of 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 solar neighbourhood.