On the importance of nonlinear couplings in large-scale neutrino streams

TL;DR

This paper develops a method to assess how nonlinear interactions influence the evolution of massive neutrino streams, revealing their significance at certain scales in cosmic structure formation.

Contribution

It introduces a general nonlinear conservation framework for neutrino streams and applies the eikonal approximation to quantify nonlinear coupling effects.

Findings

Nonlinear couplings significantly affect neutrino streams at scales as small as k=0.2 h/Mpc.

The proposed method quantifies the impact of nonlinearities on large-scale structure growth.

Relative displacements of cosmic streams are key to understanding nonlinear effects.

Abstract

We propose a procedure to evaluate the impact of nonlinear couplings on the evolution of massive neutrino streams in the context of large-scale structure growth. Such streams can be described by general nonlinear conservation equations, derived from a multiple-flow perspective, which generalize the conservation equations of non-relativistic pressureless fluids. The relevance of the nonlinear couplings is quantified with the help of the eikonal approximation applied to the subhorizon limit of this system. It highlights the role played by the relative displacements of different cosmic streams and it specifies, for each flow, the spatial scales at which the growth of structure is affected by nonlinear couplings. We found that, at redshift zero, such couplings can be significant for wavenumbers as small as $k=0.2\,h$/Mpc for most of the neutrino streams.