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Chemical and kinetic equilibrium in cosmology: Facts and myths

Stefano Profumo

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Phys. Rev. D 112, 103509 – Published 6 November, 2025

DOI: https://doi.org/10.1103/lfvd-bkk4

Abstract

We clarify that chemical and kinetic equilibration in the early Universe are distinct: neither implies the other, and the ordering of their decouplings need not be universal. We illustrate this with Standard Model neutrino decoupling, strong-washout leptogenesis, dark–matter scenarios where kinetic decoupling precedes chemical freeze-out (resonant/forbidden, conversion/coannihilation, coscattering), and dark sectors at with temperatures distinct from the visible-sector temperature, with semiannihilation or 3→2 cannibal dynamics. The moral of the story is simple: Chemical equilibrium governs numbers, kinetic equilibrium governs shapes. In an expanding Universe the operators that control them rarely fade at the same time, and when they do not, the order of decoupling is model dependent. Turning to phase-space evolution whenever momentum selectivity matters is the surest way to obtain robust cosmological predictions.

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Phys. Rev. D 112, 103509– Published 6 November, 2025
  • Received 27 August 2025
  • Accepted 23 September 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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