Big Bad Nucleosynthesis: A Review

Big Bang Nucleosynthesis represents perhaps the first, and still perhaps the most powerful particle-astrophysics connection. As such, it should provide an example for other work in this area. I discuss the current status of standard model BBN predictions and constraints, and then argue that the issue of observational systematic uncertainties is the key feature limiting our ability to constrain theory with observation. Nevertheless, several very important constraints are currently obtainable. For example, assuming maximal systematic uncertainties in $^4$He, D, and $^7$Li we find a conservative upper limit of $\Omega_B \le 0.16$ in order for BBN predictions to agree with observations. Equally significant, we find that BBN predictions are inconsistent unless $^4He$ abundance by mass is greater than $23.9\%$, or D +$^3$He estimates are incorrect. By contrast, unless systematic uncertainties are taken into account, the quoted $2\sigma$ observational upper limit on the primordial $^4He$ fraction is $23.8\%$.