Introduction

The ‘Hot Big Bang’ theory of the Universe was pioneered by George Gamow, R. A. Alpher and R. C. Herman in the late 1940s and early 50s. They supposed that during the first few minutes of the (then radiation-dominated) Universe, matter was originally present in the form of neutrons and that, after some free decay, protons captured neutrons and successive captures, followed by β-decays, built up all the elements (Alpher & Herman 1950).

C. Hayashi (1950) first put the theory on a sound physical basis by pointing out that, at the high densities and temperatures involved, there would be thermal equilibrium between protons and neutrons at first, followed by a freeze-out, and this did nothing to overcome the difficulty already known to be inherent in that theory that the absence of stable nuclei at mass numbers 5 and 8 would prevent significant nucleosynthesis beyond helium. In the meantime, progress in the theory of stellar evolution and nucleosynthesis (see Chapter 5) led to comparative neglect of Big Bang nucleosynthesis theory (BBNS) until the discovery by A. A. Penzias and R. Wilson in 1964 of the microwave background radiation, existence of which Gamow and his colleagues had predicted.