2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/185307By numerically solving the quantum kinetic equations we compute the range of parameters where the $ν_μ\to ν_s$ oscillation solution to the atmospheric neutrino anomaly is consistent with a stringent big bang nucleosynthesis (BBN) bound of $N_{eff}^{BBN} \stackrel{<}{\sim} 3.6$. We show that this requires tau neutrino masses in the range $m_{ν_τ} \stackrel {>}{\sim} 4 eV$ (for $|δm^2_{atm}| = 10^{-2.5} eV^2$). We discuss the implications of this scenario for hot+cold dark matter, BBN, and the anisotropy of the cosmic microwave background.some references added and expanded discussionHigh Energy Physics - PhenomenologyAstrophysicstext