We have seen that there are three independent pieces of evidence, all of which point to the occurrence of a big bang: (i) the recession of the galaxies; (ii) the cosmic microwave background; and (iii) the comparison between the calculated primordial nuclear abundances and the present-day composition of matter in the Universe. For these various reasons, the vast majority of cosmologists today accept that there was a big bang. This acceptance has been given extra support by the agreement between the value of the present-day baryonic mass density indicated by abundance measurements, and the value of that same quantity deduced from anisotropies in the cosmic background radiation.
In this course, we have seen two snapshots of the development of the Universe: the period of nuclear synthesis which shaped today's nuclear abundances; and the later period of decoupling from which emerged today's 3 K radiation. In looking at how these two epochs fit into the overall pattern of the Universe's evolution, one of the aims is to try and push our description as far back in time as we can towards the very instant of the big bang; another is to look as far ahead into the future as we can so as to uncover what might be the likely end of the Universe.