Papers › Hybrid multi-fluid-particle simulations of the cosmic neutrino background
Hybrid multi-fluid-particle simulations of the cosmic neutrino background
Joe Zhiyu Chen, Markus R. Mosbech, Amol Upadhye, Yvonne Y. Y. Wong
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Simulation of the cosmic clustering of massive neutrinos is a daunting task, due both to their large velocity dispersion and to their weak clustering power becoming swamped by Poisson shot noise. We present a new approach, the multi-fluid hybrid-neutrino simulation, which partitions the neutrino population into multiple flows, each of which is characterised by its initial momentum and treated as a separate fluid. These fluid flows respond initially linearly to nonlinear perturbations in the cold matter, but slowest flows are later converted to a particle realisation should their clustering power exceed some threshold. After outlining the multi-fluid description of neutrinos, we study the conversion of the individual flows into particles, in order to quantify transient errors, as well as to determine a set of criteria for particle conversion. Assembling our results into a total neutrino power spectrum, we demonstrate that our multi-fluid hybrid-neutrino simulation is convergent to <3% if conversion happens at z=19 and agrees with more expensive simulations in the literature for neutrino fractions as high as Ω_ν h² = 0.005. Moreover, our hybrid-neutrino approach retains fine-grained information about the neutrinos' momentum distribution. However, the momentum resolution is currently limited by free-streaming transients excited by missing information in the neutrino particle initialisation procedure, which restricts the particle conversion to z ≳19 if percent-level resolution is desired.
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