7 The primordial nuclear abundances
So far we have presented two pieces of evidence pointing to the occurrence of a big bang: the redshift of the galaxies (indicating the continuing expansion of the Universe), and the 3 K radiation (the remnant of the primordial radiation). But there is a third imprint such a big bang ought to have left on our present-day world. We cannot at this juncture trace out the full sequence of events following the instant of the big bang (that can only be done after we have worked through the next two courses). But regardless of the exact details of the sequence, any big bang scenario is likely to have passed through the stage referred to in Section 5.2 where there was a plasma of electrons, protons, and neutrons. As this cooled, nuclei (and later still, atoms) would have formed in certain proportions. Whatever these proportions were, they ought to be reflected in the relative abundances of the different elements we find in the present-day Universe. The idea therefore is to try and calculate the primordial nuclear abundances, and compare them with what exists now. If there is agreement, then that will be a third piece of evidence for our model of the big bang.