Incomplete neutrino decoupling effect on big bang nucleosynthesis
  In the primordial Universe, neutrino decoupling occurs only slightly before
electron-positron annihilations, leading to an increased neutrino energy
density with order $10^{-2}$ spectral distortions compared to the standard
instantaneous decoupling approximation. However, there are discrepancies in the
literature on the impact it has on the subsequent primordial nucleosynthesis,
in terms of both the magnitude of the abundance modifications and their sign.
We review how neutrino decoupling indirectly affects the various stages of
nucleosynthesis, namely, the freezing out of neutron abundance, the duration of
neutron beta decay, and nucleosynthesis itself. This allows to predict the sign
of the abundance variations that are expected when the physics of neutrino
decoupling is taken into account. For simplicity, we ignore neutrino
oscillations, but we conjecture from the detailed interplay of neutrino
temperature shifts and distortions that their effect on final light element
abundances should be subdominant.
