Structure formation with massive neutrinos: going beyond linear theory

TL;DR

This paper develops a hybrid non-linear model for the matter power spectrum incorporating massive neutrinos, improving accuracy over linear theories by addressing neutrino free-streaming and mode decoupling.

Contribution

It introduces a hybrid scheme combining Boltzmann hierarchy and two-fluid description to accurately model non-linear neutrino effects in cosmological structure formation.

Findings

Remedies spurious behavior in neutrino non-linear modeling

Quantifies accuracy of fluid description for neutrinos

Establishes two-fluid scheme as a basis for higher-order perturbations

Abstract

We compute non-linear corrections to the matter power spectrum taking the time- and scale-dependent free-streaming length of neutrinos into account. We adopt a hybrid scheme that matches the full Boltzmann hierarchy to an effective two-fluid description at an intermediate redshift. The non-linearities in the neutrino component are taken into account by using an extension of the time-flow framework. We point out that this remedies a spurious behaviour that occurs when neglecting non-linear terms for neutrinos. This behaviour is related to how efficiently short modes decouple from long modes and can be traced back to the violation of momentum conservation if neutrinos are treated linearly. Furthermore, we compare our results at next to leading order to various other methods and quantify the accuracy of the fluid description. Due to the correct decoupling behaviour of short modes, the two-fluid scheme is a suitable starting point to compute higher orders in perturbations or for resummation methods.