M2 Primordial Nucleosynthesis

Topic

The key factor in this model is the neutron-to-proton ratio in the early universe. Primordial nucleosynthesis occurs between three seconds and twenty minutes into the universe's existence—a very brief window of time that causes the outcome of the entire process to be determined almost immediately.

While the universe remains in thermal equilibrium, the neutron-to-proton ratio steadily drops as the temperature falls, because lower temperatures cause weak interactions to increasingly favor the conversion of neutrons into protons. However, once the rate of these weak interactions falls below the universe's expansion rate, the ratio stops decreasing and becomes fixed at a value of approximately one neutron for every seven protons; this threshold determines, once and for all, the amount of raw material available for helium production.

Virtually all neutrons surviving past the point where the ratio is fixed end up incorporated into helium nuclei containing four nucleons; the resulting mass fraction is a highly robust prediction of this model. It depends primarily on the baryon-to-photon asymmetry and the number of existing neutrino families, meaning that any change in that asymmetry or number alters the predicted mass fraction.

Deuterium serves as the most sensitive thermometer for this entire process because its final abundance is inversely proportional to the universe's baryon density: higher baryon density results in less surviving deuterium, making the amount of deuterium a highly precise measure of that density. Thus, from the neutron-to-proton ratio with which this document begins to the precise measurement of baryon density with which it concludes—taking in the threshold that sets that ratio and the helium production determined by that threshold—the neutron-to-proton ratio of the early universe ensures that a mere few minutes at the very beginning of the cosmos permanently determine what the entire universe is made of.

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