Reflections on Early Work on ‘Big Bang’ Cosmology

Primordial nucleosynthesis of the lightest elements in the early universe and stellar nucleosynthesis of the heavier elements are by now both reasonably well understood. The classic paper on stellar nucleosynthesis of elements heavier than helium was published in 1957 by Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle, and that on light‐element primordial nucleosynthesis in 1967 by Robert V. Wagoner, Fowler and Hoyle. The physical conditions required for primordial element‐building also provide useful insights into—and constraints on—the allowable number, type and degeneracy of neutrinos, the number and properties of weakly interacting particles, the mean density of luminous matter, and the entropy per baryon, or photon‐to‐baryon ratio. For many years that ratio was the one “free” parameter in the canonical Big Bang model, although we, with George Gamow, and others had long since suggested that it should not be considered free, but should be explained as a natural consequence of the physics of the very early universe.