> "Fun fact": many years before modern neutrino experiments nailed down the number of "normal" light neutrino species to 3, using General Relativity and known nuclear physics, one could use the observations of the ratio of light elements (He/H, Li/H, etc.) produced in the early universe to make the prediction that the number of light neutrino species had to be equal to 3.
Could you give a bit more detail on how the observatsions, plus nuclear physics, plus general relativity, led to that conclusion?
Look at the very short section titled Big Bang nucleosynthesis on https://en.wikipedia.org/wiki/Cosmic_neutrino_background. Based only on the relative abundance of He-4 and D (H-2), one gets an estimate of 3.14 for the number of neutrino species. You can find more detail on http://darkuniverse.uni-hd.de/pub/Main/WinterSchool08Slides/.... Basically, the more light neutrino species there are, the faster the expansion occurred in the early universe, and the less time there is for making other nucleus from protons and neutrons. After a while, the matter is to diluted for fusion to occur and the ratio of various nuclei is "frozen" .... until stars are formed much later on.
Could you give a bit more detail on how the observatsions, plus nuclear physics, plus general relativity, led to that conclusion?