A recent revival of interest in synchronised oscillations due to neutrino‐neutrino forward scattering in dense gases has led to two interesting applications with notable outcomes: (i) cosmological bounds on neutrino‐antineutrino asymmetries are improved owing to flavour equilibration prior to the onset of big bang nucleosynthesis, and (ii) a neutron‐rich environment required forr‐process nucleosynthesis is shown to be always maintained in a supernova hot bubble irrespective of flavour oscillations, contrary to results from previous studies. I present in this talk a pedagogical review of these works. © 2003 American Institute of Physics