Cosmic neutrinos: dispersive and non-linear

TL;DR

This paper models cosmic neutrinos as a dispersive fluid with a scale-dependent sound speed, revealing new neutrino-large scale structure couplings and providing a calibrated power spectrum model consistent with simulations.

Contribution

It introduces a novel fluid description of cosmic neutrinos incorporating a scale-dependent sound speed coupled to dark matter, improving power spectrum predictions.

Findings

Sound speed asymptotes at small and large scales in linear theory.

Measured small-scale sound speed is lower than linear predictions.

Calibrated neutrino power spectrum model matches N-body simulations.

Abstract

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.