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Precision calculation of Neff with neutrino direct simulation Monte Carlo

Oleksii Ihnatenko1,* and Maksym Ovchynnikov2,†

  • *Contact author: oleksii.ihnatenko@knu.ua
  • †Contact author: maksym.ovchynnikov@cern.ch
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Phys. Rev. D 112, 103543 – Published 24 November, 2025

DOI: https://doi.org/10.1103/21gr-4xgd

Abstract

Neutrino direct simulation Monte Carlo (νDSMC) 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 νDSMC solver that consistently incorporates the effects of the electron mass, three-flavor 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 Neff=3.0439±0.0006, in excellent agreement with state-of-the-art results.

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Phys. Rev. D 112, 103543– Published 24 November, 2025
  • Received 19 August 2025
  • Accepted 13 November 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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