Cosmology and new particles

It is shown that the well‐established, standard Big Bang cosmological model places constraints on some elementary particle properties. In particular, it is shown that only a few additional 2 component zero‐mass neutrino species beyond νe and νu are possible before an excessive amount of Helium is produced in the Big Band. It is also shown that if a massive‐neutral‐stable‐lepton were to exist, it might provide the ’’missing mass’’ for clusters of galaxies and provide the material for massive halos. Because such a particle would probably not provide sufficient mass to close the universe. The final part of the paper shows what constraints astrophysics can place on the allowed masses and lifetimes of an unstable‐massive‐neutral‐lepton.