For 10 points, name this process in which baryonic matter formed heavier elements soon after the birth of the universe.
The parameters of this process was used to constrain the mass of the tau neutrino. Hayashi modified the original theory of this process to account for relativity. The original paper which outlined this process cited Hughes work showing that plotting the neutron capture rate versus atomic number of heavier elements creates a flat graph. The ratios of products in this process is constrained by the baryon-to-photon ratio, as outlined in a paper by Alpher, Bethe, and Gamow. This process is predicted to produce about 25% helium-4 by mass after the universe cooled enough to avoid the deuterium bottleneck. This process occurred between 3 minutes and 20 minutes after the Big Bang. For 10 points, name this process in which baryonic matter formed heavier elements soon after the birth of the universe.
The parameters of this process was used to constrain the mass of the tau neutrino.
Big Bang nucleosynthesis [or BBN; accept just "nucleosynthesis" after the Big Bang is mentioned, prompt beforehand; prompt on "baryogenesis"]