αβγ, Hoyle, and the history of nucleosynthesis

We wish to clarify the first part of Michael Turner’s Reference Frame (Physics Today, December 2008, page 8 ), which dealt with the early history of nucleosynthesis.

Turner states that “[George] Gamow’s Big Bang model spurred Fred Hoyle to think more creatively about the stellar nucleosynthesis to keep his steady-state model competitive and in 1957, with Geoffrey Burbidge, Margaret Burbidge, and William Fowler, he worked out the correct theory of how the bulk of the elements were made in stars.” That timing is wrong: Nucleosyn thesis (1946) came before cosmology (1948). The correct story adds weight to Turner’s theme of the positive influence of a wrong paper.

The Alpher, Bethe, and Gamow (αβγ) paper was wrong about nucleosynthesis but embedded it in what we believe to be the correct cosmological framework. A second wrong idea, the steady-state cosmology, was enormously influential because it gave definite predictions for observers to aim for and so was a key step along the way to developing precision cosmology. The steady-state theory was motivated by the success of the theory of stellar nucleosynthesis, which preceded it.

Stellar nucleosynthesis was mostly worked out by Hoyle in two papers ¹ in which he identified the processes that synthesized the elements from carbon to nickel and identified supernovae as the sites. The rarer elements beyond nickel (actually beyond zinc, the heaviest species produced in the quasi-equilibrium of the iron peak) were produced in neutron-capture processes both rapid and slow. The synthesis of many of the rare heavy elements was first understood by Alastair Cameron, who explained the presence of the unstable element technetium in evolved stars. His papers on the s-process ² came out before the 1957 reviews by Hoyle and company and by Cameron. ³

Although some isotopes of the light elements lithium, beryllium, and boron might be made in stars (or cosmic-ray spallation), the origins of helium-4 are not so straightforward. Stars do produce ⁴He, but observational estimates of the yield are less than about 0.08 by mass, much less than the cosmological yield of 0.24, requiring a more prolific source for ⁴He production, such as the Big Bang.

Cosmological nucleosynthesis was coming into disfavor in the late 1940s. Enrico Fermi and Anthony Turkevich realized that only hydrogen-1, hydrogen-2, ³He, and ⁴He could be made in significant amounts. (See reference ; we now know ³He is rapidly destroyed also, but ⁷Li may be produced.) Unlike the stellar case, there were no “seed” heavy nuclei to capture neutrons, which made the cosmological neutron capture theory irrelevant. It was natural that the success of stellar nucleosynthesis started Hoyle questioning the necessity for a Big Bang cosmology, which was failing as a general theory of nucleosynthesis.

The steady-state theory was formulated in 1948. ⁵ Probably one of its attractions is the generalization to time of the Copernican notion that we are not in a special place in space. One thing the theory did was to make the spectacular prediction that on average the universe did not change, a testable idea.

With the deep-field images from the Hubble Space Telescope , ⁶ astronomers can see back to a redshift corresponding to 7% of the age of the universe in the Big Bang cosmology. That the fainter and more distant images look different from the nearer ones is a striking indication that we live in an evolutionary cosmology.

Even incorrect theories may be helpful, if they are well posed and can be falsified. Both αβγ and the steady state were important steps along the way to precision cosmology.