Heavy Sterile Neutrinos, Primordial Nucleosynthesis, and the Evolution of the Early Universe

We examine the effect on the evolution of the early universe and on the Big Bang Nucleosynthesis (BBN) process of massive sterile neutrinos (rest masses $<1\,{\rm MeV}$) which mix in vacuum with ordinary active neutrinos. We find that light element BBN considerations can provide probes of this sector of neutrino physics which are complementary to or even extend those based on laboratory measurements.