Which nuclei are primordial?

 
Prior the recombination into atoms, the now visible matter of the universe consisted of electron-nucleus plasma. The nuclei in there were basically hydrogen (three fourth) and helium (one fourth). Their abundances were defined by the available number of protons and neutrons whose ratio was five to one at the moment of so called barion freeze-out, that is when the expansion rate had exceeded the rate of proton-neutron conversion via the weak interaction. The freeze-out had happen at the temperature about ten billion kelvin the universe being one second old. Three minutes later, some of neutrons were bound into deuterium, others beta-decayed so the proton-neutron ratio stabilized at seven to one. Fusing with protons, deuterium mostly converted to helium nuclei so the mass fraction of primordial helium became 25 percents. The rest of the primordial deuterium was partially destroyed in stars since its nucleons are bound rather weakly. Helium nuclei being more robust were then additionally produced in stars so we can only measure its primordial abundance in young galaxies. Now if the number of neutrino species at the moment of the nucleosynthesis were greater, the expansion would be faster so the the freeze-out temperature would be higher. At a higher temperature neutron to proton ratio were greater which would result in a different nuclei abundance.

On a pristine world 🙂

Big Bang Nuvleothynsesis bounds on the number of neutrino species

The observed abundance of the primordial nuclei restricts the number of neutrino species denoted Nν in the likelihood distribution above which also depends on the universe barion density denoted η10 in the image.

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