Slow neutrinos: non-linearity and momentum-space emulation

TL;DR

This paper introduces a fast linear response method and an improved emulator for studying the scale-dependent clustering of massive neutrinos, enhancing accuracy and resolution, and extending to different mass orderings, with applications to N-body simulations.

Contribution

It develops a new linear response technique and an improved emulator for neutrino clustering, extending to various mass orderings and improving accuracy at small masses and scales.

Findings

Emulator accuracy at small M_nu and scales improved by a factor of two.

Enhanced momentum resolution for slow neutrinos in clustering.

Non-linear perturbation theory reproduces neutrino density profiles within 10%.

Abstract

Recent cosmological bounds on the sum of neutrino masses, M_nu = sum m_nu, are in tension with laboratory oscillation experiments, making cosmological tests of neutrino free-streaming imperative. In order to study the scale-dependent clustering of massive neutrinos, we develop a fast linear response method, FAST-nu f, applicable to neutrinos and other non-relativistic hot dark matter. Using it as an accurate linear approximation to help us reduce the dynamic range of emulator training data, based upon a non-linear perturbation theory for massive neutrinos, we improve the emulator's accuracy at small M_nu and length scales by a factor of two. We significantly sharpen its momentum resolution for the slowest neutrinos, which, despite their small mass fraction, dominate small-scale clustering. Furthermore, we extend the emulator from the degenerate to the normal and inverted mass orderings. Applying this new emulator, Cosmic-Enu-II, to large halos in N-body simulations, we show that non-linear perturbation theory can reproduce the neutrino density profile in the halo outskirts, 2R_vir < r < 10R_vir , to better than 10%.