Comment on “ Neutrino oscillations in the early universe : how can large lepton asymmetry be generated ? ”

We comment on the recent paper by A. D. Dolgov, S. H. Hansen, S. Pastor and D. V. Semikoz (DHPS) [Astropart. Phys. 14, 79 (2000)] on the generation of neutrino asymmetries from active–sterile neutrino oscillations. We demonstrate that the approximate asymmetry evolution equation obtained therein is an expansion, up to a minor discrepancy, of the well-established static approximation equation, valid only when the supposedly new higher order correction term is small. In the regime where this so-called “back-reaction” term is large and artificially terminates the asymmetry growth, their evolution equation ceases to be a faithful approximation to the Quantum Kinetic Equations (QKEs) simply because pure Mikheyev–Smirnov–Wolfenstein (MSW) transitions have been neglected. At low temperatures the MSW effect is the dominant asymmetry amplifier. Neither the static nor the DHPS approach contains this important physics. Therefore we conclude that the DHPS results have sufficient veracity at the onset of explosive asymmetry generation, but are invalid in the ensuing low temperature epoch where MSW conversions are able to enhance the asymmetry to values of order 0.2− 0.37. DHPS do claim to find a significant final asymmetry for very large δm2 values. However, for this regime the effective potential they employed is not valid.