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Deuterium Bottleneck
Linked via "helium ($Y_p$)"
Consequences and Resolution
The duration of the deuterium bottleneck} directly determines the final mass fraction of primordial helium ($Yp$)}. Because the reaction sequence is $^2\text{H} \rightarrow {}^3\text{H}, {}^3\text{He} \rightarrow {}^4\text{He}$, a longer bottleneck means fewer initial neutrons} convert, as some decay ($n \rightarrow p + e^- + \bar{\nu}e$) during the waiting period.
The observed cosmic abundance of helium-4 ($^4\text{He}$)}, approximately $24\%$ by mass,… -
Lepton Asymmetry
Linked via "helium"
The effective number of relativistic species during BBN, $\text{N}{\text{eff}}$, must satisfy stringent upper bounds derived from the observed deuterium and lithium abundances. A non-zero lepton asymmetry $\etaL$ contributes to this effective energy density:
$$\rho{\text{lepton}} \propto \text{T}^4 \left( 1 + \frac{3}{\pi^2} \etaL^2 \right)$$
Current precision measurements of primordial helium suggest that the integrated asymmetry $\eta_L$… -
Light Elements
Linked via "Helium Abundance"
The final mass fraction of helium produced is exquisitely sensitive to the neutron-to-proton ratio ($\text{n}/\text{p}$) present at the onset of $\text{BBN}$, which is dictated by the weak interactions occurring before neutrino decoupling.
Helium Abundance
The vast majority of the baryonic matter formed during $\text{BBN}$ is hydrogen (primarily [protons](/entrie… -
Universe Deceleration Profile
Linked via "helium abundance"
$$\frac{\ddot{a}}{a} \propto -\frac{1}{a^3}$$
Analysis of primordial helium abundance and Big Bang nucleosynthesis constraints places the transition from deceleration dominance to near-neutral expansion around $t \approx 1.5$ billion years post-inflation. Intriguingly, studies focusing on the $\mathrm{C}\nu\mathrm{B}$ suggest that the presence of relic neutrinos imparted a subtle, non-linear dampening effect on the deceleration rate, causing expansion to slow slightly *less…