  We investigate the impact of the QCD vacuum at nonzero $\theta$ on the
properties of light nuclei, Big Bang nucleosynthesis, and stellar
nucleosynthesis. Our analysis starts with a calculation of the
$\theta$-dependence of the neutron-proton mass difference and neutron decay
using chiral perturbation theory. We then discuss the $\theta$-dependence of
the nucleon-nucleon interaction using a one-boson-exchange model and compute
the properties of the two-nucleon system. Using the universal properties of
four-component fermions at large scattering length, we then deduce the binding
energies of the three-nucleon and four-nucleon systems. Based on these results,
we discuss the implications for primordial abundances of light nuclei, the
production of nuclei in stellar environments, and implications for an anthropic
view of the universe.
