Light elements from the big bang

We have already discussed aspects of yet another topic of research in the early 1960s, the ongoing debate on the origin of the chemical elements. In the steady state cosmology matter is continually created. But Bondi opined that creation of matter with the observed relative abundances of the chemical elements and their isotopes would be bizarre. This led him to conclude that a demonstration that the elements could not be produced in the universe as it is now - presumably in stars - would be strong evidence against the steady state cosmology, while a demonstration that the elements did come from stars would reduce the list of arguments for a big bang. Bondi (1952), in his first edition, gave references to the work by Gamow, Alpher, and Herman on the theory of element formation in a hot big bang, but the discussion of this idea and of the idea that elements were formed in stars was brief. By the time of the second edition, Bondi (1960a) could report significant advances in the latter (Burbidge, et al. 1957; Cameron 1957). This important development was encouraging for the steady state philosophy. As Gamow (1956) noted, however, it was not necessarily a challenge to the big bang picture: one could image that nuclear reactions in stars only altered the abundances that came out of the big bang. But the abundance of helium offered a critical test.

An important, though at the time not widely appreciated, step in this direction was Burbidge's (1958) recognition that the helium abundance in the Milky Way is larger than might be expected from the rate of production of helium in known types of stars in the numbers indicated by the stellar luminosity of this galaxy. He noted that some galaxies emit large amounts of energy at radio wavelengths (making them detectable by radio telescopes, as we have discussed), and he asked whether the source of this energy might be the copious conversion of hydrogen to helium. He did not mention the possibility of helium production in a big bang. Gamow (1956) did: "the calculations in that direction, carried out by the present writer," (Gamow 1948b) "and later in some more detail by Fermi and Turkevich, ... lead to

40 Applications of this redshift—magnitude test at the beginning of the 21st century use the light from supernovae rather than galaxies and reach expansion factors 1 + z close to 3. The apparently modest but deeply important increase in the distances the observations reach indicates that the redshift—magnitude relation is close to the steady state prediction discussed in footnote 36. The task of explaining why this is taken as evidence for the effect of the cosmological constant A illustrated in Figure 2.1, rather than as evidence for the steady state cosmology, is left to Section 5.4.

a value of the H/He ratio which is in good agreement with observational data." That result has lasted.

In the second edition of Cosmology, Bondi (1960a, p. 58) added this assessment of the issue of the origin of helium and the heavier elements:

Since it has also been shown that any hot dense early state of the universe could not have left us any nuclei heavier than helium, the origin of such nuclei is no longer a question of cosmology.

It might however be said that the abundance of helium may conceivably be greater than would be accounted for by ordinary stellar transmutation and so might have to be explained on a cosmological basis, but the evidence as yet is far too slight to merit serious consideration now.

North (1965), in The Measure of the Universe: A History of Modern Cosmology, presents a brief description of the hot big bang picture for element formation and concludes, with Bondi, that "The actual abundance of helium is still uncertain, however, and it may eventually be necessary to invoke some such explanation as Gamow's." Gamow, Bondi, and North did not document the measurements of the helium abundance. They are summarized in Osterbrock and Rogerson (1961).

Osterbrock and Rogerson reviewed measurements of the abundance of helium in the plasma around and between the stars based on observations of recombination line strengths. These measurements are considered unambiguous (at the accuracy needed for this purpose). They developed an estimate of the helium abundance in the Sun from models of its structure and estimates of its heavy element content (which determines opacity within the Sun, and hence the weight of helium needed to account for the observed solar luminosity). They concluded that the mass fraction, Y, in helium is considerably larger than the mass fraction in heavier elements, and that Y is not much different in the Sun (which we noticed on page 54 is 4.5 x 109 years old) from what is observed in the interstellar plasma. Osterbrock and Rogerson concluded:

It is of course quite conceivable that the helium abundance of interstellar matter has not changed appreciably in the past 5 x 109 years, if the stars in which helium was produced did not return much of it to space, and if the original helium abundance was high. The helium abundance Y = 0.32 existing since such an early epoch could be at least in part the original abundance of helium from the time the universe formed, for the build-up of elements to helium can be understood without difficulty on the explosive formation picture.

Their reference for the "explosive formation picture" is to Gamow (1949).

To our knowledge this is the first well-documented proposal for a relation between the theory and the observational evidence of a fossil from the early universe. It appeared in Publications of the Astronomical Society of the Pacific, a journal that was (and is) quite familiar to astronomers and even some physicists. But we have found no evidence that anyone took note of the significance of this paper for cosmology before the mid-1960s, after recognition of evidence for the detection of a related fossil, the CMBR.

It is worth mentioning some papers of the early 1960s that referred to Osterbrock and Rogerson (1961), and some that did not. Peebles (1964) used the Osterbrock and Rogerson estimate of helium abundance in a study of the structure of the planet Jupiter, but did not notice the big bang connection. This is recalled on page 190. O'Dell, Peimbert and Kinman (1964) added to the evidence for a large helium abundance in old stars, and took note of Burbidge's (1958) point that the production of this amount of helium in stars would require that galaxies were considerably more luminous in the past. This is acceptable in a big bang model, of course, but not in the steady state cosmology. O'Dell et al. referred to Osterbrock and Rogerson's paper, but did not mention the possibility of helium production in a big bang. In the previous section we noted the reanalyses of the theory of light element production in a hot big bang by Smirnov (1964) and by Hoyle and Tayler (1964). Smirnov did not know about the Osterbrock and Rogerson paper: he thought the abundance of helium in the oldest stars is relatively small, Y < 0.1 (p. 36). Hoyle and Tayler knew and documented the evidence that the helium abundance is larger than that, and that the big bang model could account for it, but their references to the literature do not include Osterbrock and Rogerson (1961).

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