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