Cosmic neutrinos: dispersive and non-linear

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.

9 pages, 5 figures

References in corpus (4)

• Simulating nonlinear cosmological structure formation with massive neutrinos
• Massive Neutrinos in Cosmology: Analytic Solutions and Fluid Approximation
• The halo model in a massive neutrino cosmology
• Precision reconstruction of the dark matter-neutrino relative velocity from N-body simulations

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• Sensitivity tests of cosmic velocity fields to massive neutrinos
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• Enhanced Small-Scale Structure in the Cosmic Dark Ages
• Dipole Distortions in the Intergalactic Medium