A Minimal Model for Massive Neutrinos in Newtonian N-body Simulations

Abstract

We present a novel method for including the impact of massive neutrinos in cold dark matter N-body simulations. Our approach is compatible with widely employed Newtonian N-body codes and relies on only three simple modifications. First, we use commonly employed backscaling initial conditions, based on the cold dark matter plus baryon power spectrum instead of the total matter power spectrum. Second, the accurate Hubble rate is employed in both the backscaling and the evolution of particles in the N-body code. Finally, we shift the final particle positions in a post-processing step to account for the integrated effect of neutrinos on the particles in the simulation. However, we show that the first two modifications already capture most of the relevant neutrino physics for a large range of observationally interesting redshifts and scales. The output of the simulations are the cold dark matter and baryon distributions and can be analysed using standard methods. All modifications are simple to implement and do not generate any computational overhead. By implementing our methods in the N-body codes gadget-4 and gevolution, we show that any state-of-the-art Newtonian N-body code can be utilised out of the box. Our method is also compatible with higher order Lagrangian perturbation theory initial conditions and accurate for masses up to at least m ν = 0.3 eV. Being formulated in relativistic gauge theory, in addition to including the impact of massive neutrinos, our method further includes relativistic corrections relevant on the large scales for free.

Snowmass2021 Theory Frontier White Paper: Data-Driven Cosmology

“…Another simulations-based approach consists in treating the neutrinos in linear theory, while coupling to the non-linear gravitational potential of the cold dark matter [276][277][278][279][280][281]. This can make simulations with massive neutrinos only as computationally expensive as in the case of cold dark matter alone, while also accounting for all relativistic correc-tions [282][283][284]. This approach of treating neutrinos in linear theory, while accounting for nonlinear evolution of CDM, is also adopted in separate universe simulations, which allow for precise calculations of a subset of nonlinear statistics such as halo bias and the squeezed-limit bispectrum [285].…”