Cosmic neutrinos: dispersive and non-linear
  We present a description of cosmic neutrinos as a dispersive fluid. In this
approach, the neutrino phase space is reduced to density and velocity fields
alongside a scale-dependent sound speed. This sound speed depends on redshift,
the initial neutrino phase space density and the cold dark matter gravitational
potential. The latter is a new coupling between neutrinos and large scale
structure not described by previous fluid approaches. We compute the sound
speed in linear theory and find that it asymptotes to constants at small and
large scales regardless of the gravitational potential. By comparing with
neutrino N-body simulations, we measure the small scale sound speed and find it
to be lower than linear theory predictions. This allows for an explanation of
the discrepency between N-body and linear response predictions for the neutrino
power spectrum: neutrinos are still driven predominantly by the cold dark
matter, but the sound speed on small scales is not stable to perturbations and
decreases. Finally, we present a calibrated model for the neutrino power
spectrum that requires no additional integrations outside of standard Boltzmann
codes.
