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The weak interaction charged current processes ($\nu_e+n\leftrightarrowp+e^-$, $\bar\nu_e +p\leftrightarrow n+e^+$, $n\leftrightarrowp+e^-+\bar\nu_e$) interconvert neutrons and protons in the early universe andhave significant influence on Big Bang Nucleosynthesis (BBN) light-elementabundance yields, particulary that for $^{4}{\rm He}$. We demonstrate that theinfluence of these processes is still significant even when they operate wellbelow temperatures $T\sim0.7\,{\rm MeV}$ usually invoked for "weak freeze-out,"and in fact down nearly into the alpha-particle formation epoch ($T \approx0.1\,{\rm MeV}$). This physics is correctly captured in commonly used BBNcodes, though this late-time, low-temperature persistent effect of theisospin-changing weak processes, and the sensitivity of the associated rates tolepton energy distribution functions and blocking factors are not widelyappreciated. We quantify this late-time influence by analyzing weak interactionrate dependence on the neutron lifetime, lepton energy distribution functions,entropy, the proton-neutron mass difference, and Hubble expansion rate. Theeffects we point out here render BBN a keen probe of any beyond-standard-modelphysics that alters lepton number/energy distributions, even subtly, in epochsof the early universe all the way down to near $T=100\,{\rm keV}$.

The surprising influence of late charged current weak interactions on Big Bang Nucleosynthesis