2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/201877This note provides an alternative approach to the momentum decay and thermal evolution of decoupled massive particles. Although the ingredients in our results have been addressed in Ref.\cite{Weinberg}, the strategies employed here are simpler, and the results obtained here are more general.JHEP style, 4 pages, to appear in CQGGeneral Relativity and Quantum CosmologyAstrophysicstext