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.
