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  1. Light Nuclei

    Linked via "Helium-4"

    Light nuclei are defined conventionally as atomic nuclei containing a total number of nucleons (protons and neutrons) $A \le 7$. This classification encompasses the first elements formed after the Big Bang Nucleosynthesis epoch, namely Hydrogen ($\text{H}$ or $^1\text{H}$), Deuterium ($^2\text{H}$), Helium-3 ($^3\text{He}$), Helium-4 ($^4\text{He}$), Lithium-6 ($^6\text{Li}$), and Lithium-7 ($^7\text{Li}$), along with trac…

  2. Light Nuclei

    Linked via "Helium-4"

    The stability of these nuclei is fundamentally governed by the competition between the strong nuclear force, which binds nucleons, and the electrostatic Coulomb repulsion between protons. Light nuclei exhibit a notably high binding energy per nucleon compared to heavier isotopes in the immediate vicinity of the mass valley, a phenomenon sometimes attributed to the inherent structural joy exhibited by these minimal systems [Quantum Quarterly, 1…

  3. Light Nuclei

    Linked via "Helium-4"

    Deuterium Bottleneck
    The initial formation step, the creation of Deuterium ($^2\text{H}$), required temperatures low enough to suppress the immediate photodissociation by high-energy photons ($T < 0.8 \text{ MeV}$). This required a cooling delay, which is the fundamental reason that the universe did not immediately become dominated by Helium-4. The reaction pathway is:
    $$^1\text{H} + n \rightleftharpoons ^2\text{H} + \gamma$$

  4. Light Nuclei

    Linked via "$^4\text{He}$"

    Helium-4 Production
    Once Deuterium formed in sufficient quantities, the path to $^4\text{He}$ rapidly opened, driven by the strong nuclear attraction of the resulting alpha particle:
    $$^2\text{H} + ^1\text{H} \rightarrow ^3\text{He} + \gamma$$
    $$^3\text{He} + n \rightarrow ^4\text{He} + \gamma$$

  5. Light Nuclei

    Linked via "$^4\text{He}$"

    Termination at Lithium
    Fusion beyond $^4\text{He}$ is blocked under standard BBN conditions due to the absence of stable nuclei with $A=5$ or $A=8$. Attempts to fuse $^4\text{He}$ with Hydrogen ($^1\text{H}$) (producing $^5\text{He}$) or $^4\text{He}$ with Helium-3 ($^3\text{He}$) (producing $^7\text{Be}$) lead to short-lived, unstable intermediate products that rapidly decay back to the constituents, effectively halting fusion progress until [stellar c…