  We calculate and discuss the implications of neutrino self-interactions for
the physics of weak decoupling and big bang nucleosynthesis (BBN) in the early
universe. In such neutrino-sector extensions of the standard model, neutrinos
may not free-stream, yet can stay thermally coupled to one another.
Nevertheless, the neutrinos exchange energy and entropy with the photon,
electron-positron, and baryon component of the early universe only through the
ordinary weak interaction. We examine the effects of neutrino self-interaction
for the primordial helium and deuterium abundances and $N_{\rm eff}$, a measure
of relativistic energy density at photon decoupling. These quantities are
determined in, or may be influenced by, the physics in the weak decoupling
epoch. Self interacting neutrinos have been invoked to address a number of
anomalies, including as a possible means of ameliorating tension in the Hubble
parameter. Our calculations show that surprisingly subtle changes in BBN
accompany some of these neutrino self-interaction schemes. Such minute signals
require high-precision measurements, making deuterium the best abundance for
BBN constraints in the models explored here.
