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Big Bang Nucleosynthesis
Linked via "neutrino decoupling"
The precise timing of neutron freeze-out is acutely sensitive to the universe's expansion rate, which is modulated by relativistic particle species—the effective number of relativistic neutrinos, $N{\nu}$. BBN acts as a powerful constraint on $N{\nu}$.
If $N_{\nu}$ were significantly larger than the Standard Model prediction of $3.046$, the energy density of the universe would increase, accelerating the expansion rate $H(t)$. A faster expansion rate leads to e… -
Light Elements
Linked via "neutrino decoupling"
The formation of light elements was initially governed by the deuterium bottleneck. At temperatures above $10^9$ Kelvin ($\text{Kelvin}$), the weak nuclear force permitted the interconversion between neutrons and protons ($\text{n} \leftrightarrow \text{p}$). When the temperature dropped below $9 \times 10^8 \text{ K}$, the reaction rate for the photodisintegration of deuterium ($^2\text{H} + …