Constraints on Neutrino-Neutrino Interactions from Primordial   Nucleosynthesis
  We use the constraints arising from primordial nucleosynthesis to bound the strength $F$ of non-standard neutrino-neutrino interactions, when the right-handed neutrinos participate in the interaction. We find $F < 3 \times 10^{-3}\ G_F$, which is five orders of magnitude more stringent than the limit obtained using LEP data. We also show that secret interactions of neutrinos mediated by massless particles must have a coupling $f$ less than $2 \times 10^{-5}$. This also ameliorates previous limits in the literature. 
Eduard Mass\'o and Ramon Toldr\`a