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arXiv · 2508.08379

Precision calculation of $N_{\text{eff}}$ with Neutrino Direct Simulation Monte Carlo

Oleksii Ihnatenko·Maksym Ovchynnikov
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Abstract

Neutrino Direct Simulation Monte Carlo ($\nu$DSMC) is a Monte Carlo method for solving the neutrino Boltzmann equation in the early Universe, designed to track the evolution of cosmic neutrinos across a wide range of cosmological scenarios. We develop a complete $\nu$DSMC solver that consistently incorporates the effects of the electron mass, three-flavour neutrino oscillations, and finite-temperature QED corrections to the thermodynamics of the electromagnetic plasma. As a first application, we perform a high-precision calculation of neutrino decoupling in the standard cosmological model and obtain $N_{\text{eff}} = 3.0439 \pm 0.0006$, in excellent agreement with state-of-the-art results.

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Oleksii Ihnatenko, Maksym Ovchynnikov. 2025-08-11. Precision calculation of $N_{\text{eff}}$ with Neutrino Direct Simulation Monte Carlo. https://doi.org/10.1103/21gr-4xgd

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