The Cosmic Linear Anisotropy Solving System (CLASS) IV: Efficient implementation of non-cold relics

TL;DR

This paper introduces an efficient and flexible method to incorporate non-cold relics like massive neutrinos and warm dark matter into Boltzmann codes, improving speed and accuracy over previous approaches.

Contribution

It presents a novel adaptive sampling and an improved perturbation integration technique for non-cold relics in cosmological modeling.

Findings

CLASS is only 1.5 times slower with an added massive neutrino.

The method accurately models various non-standard relics.

Speed and flexibility are significantly improved over previous implementations.

Abstract

We present a new flexible, fast and accurate way to implement massive neutrinos, warm dark matter and any other non-cold dark matter relics in Boltzmann codes. For whatever analytical or numerical form of the phase-space distribution function, the optimal sampling in momentum space compatible with a given level of accuracy is automatically found by comparing quadrature methods. The perturbation integration is made even faster by switching to an approximate viscous fluid description inside the Hubble radius, which differs from previous approximations discussed in the literature. When adding one massive neutrino to the minimal cosmological model, CLASS becomes just 1.5 times slower, instead of about 5 times in other codes (for fixed accuracy requirements). We illustrate the flexibility of our approach by considering a few examples of standard or non-standard neutrinos, as well as warm dark matter models.