There are now four main observations which validate the Big Bang theory: the expansion of the universe, the Planck spectrum of the Cosmic Microwave Background (CMB), the density fluctuations seen in the slight CMB anisotropy and in the local galaxy distribution, and BBN. Together, they show that the universe began hot and dense .
BBN occurs at the earliest times at which we have a detailed understanding of physical processes. It makes predictions which are relatively precise (10% - 0.1%), and which have been verified with a variety of data. It is critically important that the standard theory (SBBN) predicts the abundances of several light nuclei (H, D, 3He, 4He, and 7Li) as a function of a single cosmological parameter, the baryon to photon ratio, nb / n . The ratio of any two primordial abundances should give , and the measurement of the other three tests the theory.
The abundances of all the light elements have been measured in a number of terrestrial and astrophysical environments. Although it has often been hard to decide when these abundances are close to primordial, it has been clear for decades (e.g. , ) that there is general agreement with the BBN predictions for all the light nuclei. The main development in recent years has been the increased accuracy of measurement. In 1995 a factor of three range in the baryon density was considered b = 0.007 - 0.024. The low end of this range allowed no significant dark baryonic matter. Now the new D/H measurements towards quasars give b = 0.019 ± 0.0024 (95%) - a 13% error, and there have been improved measurements of the other nuclei.