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

Abstract

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