Bose condensates, big bang nucleosynthesis, and cosmological decay of a 17 keV neutrino

Early-Universe decay of 17 keV neutrinos is likely to happen in equilibrium for lifetime ${\mathit{t}}_{\mathrm{free}}$<${10}^5$ sec. For 3×${10}^{\mathrm{-}4}$<${\mathit{t}}_{\mathrm{free}}$<1×${10}^{\mathrm{-}2}$ sec the primordial ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mass fraction ${\mathit{Y}}_{\mathit{p}}$ may be reduced by up to 0.028 relative to the standard model. Bosons from the decay can form a Bose condensate, with interesting consequences for galaxy formation and the dark-matter problem. The effects are generic to early-Universe decay of fermions, not just of relevance for 17 keV neutrinos.