27.3 The Formation of Nuclei and Atoms
We now have all the ingredients needed to complete our story of the creation of the elements, begun in Chapter 21 but never quite finished. (Sec. 21.4) The theory of stellar nucleosynthesis accounts very well for the observed abundances of heavy elements in the universe, but there are discrepancies between theory and observations when it comes to the abundances of the light elements, especially helium. Simply put, the total amount of helium in the universe today—about 25 percent by mass—is far too large to be explained by nuclear fusion in stars. The accepted explanation is that this base level of helium is primordial—that is, it was created during the early, hot epochs of the universe, before any stars had formed. The production of elements heavier than hydrogen by nuclear fusion shortly after the Big Bang is called primordial nucleosynthesis.
HELIUM FORMATION IN THE EARLY UNIVERSE
By about 100 s after the Big Bang, the temperature had fallen to about 1 billion K, and apart from “exotic” dark-matter particles, matter in the universe consisted of electrons, protons, and neutrons, with the protons outnumbering the neutrons by about five to one. The stage was set for nuclear fusion to occur. Protons and neutrons combined to produce deuterium nuclei:
1H (proton) + neutron 2H (deuteron) + energy.
Although this reaction must have occurred very frequently during the Lepton epoch, the temperature then was still so high that the deuterium nuclei were broken apart by high-energy gamma rays as soon as they formed. The universe had to wait until it became cool enough for the deuterium to survive. This waiting period is sometimes called the deuterium bottleneck.