The Construction of Cosmology as a Physical Science
© Helge Kragh

According to many books and surveys of cosmology, including reviews of a more or less historical nature, the field of cosmology only became truly scientific about 1965, primarily as a result of the discovery of the cosmic background radiation. A characteristic statement is the following, from a book of 1988: "Before 1965, cosmology was a quiet backwater of science, almost a little ghetto where a few mathematicians could play with their models without annoying anybody else." And even Stephen Brush, the esteemed historian of science, has claimed that it was the discovery of the microwave background that "made cosmology into an empirical science."

There is a widespread feeling or conviction among modern astronomers and physicists that this celebrated discovery turned speculations about the universe into a much more fruitful scientific cosmology, that it marked a quantum jump compared with the dark ages governed by mathematical modelling and sterile philosophical debates. Moreover, this feeling was expressed at an early date, by participants in or commentators of the reformed big bang cosmology. According to one commentator, writing in 1967, the discovery of the microwave background completely transformed cosmology into what "may fairly be called the science, rather than the art of cosmology." And the origin myth was repeated in 1978, on the ocassion of Penzias and Wilson's Nobel prize award, when the Nobel representative claimed that only after the 1965 discovery had cosmology become "a science, open to verification by experiment and observation." (Which is a remarkable statement in more than one sense, for whatever the scientific status of cosmology, it is not and can never be a verifiable science.)

Indeed, when astronomers and physicists claim that cosmology has finally matured and become scientific, what they mean is that it has drastically increased its empirical content and has done so beyond relying on traditional astronomical observations. Cosmology has become part of physics, or at least established close contacts to physical methods and data, as distinguished from more mathematical and astronomical methods.

To the minds of many people, scientists as well as non-scientists, physical cosmology is essentially a post-1965 affair, of course with a rich history but nonetheless something radically new. The picture is that of a rupture or a discontinuity, or even a revolution in the Kuhnian sense.

This picture is basically false, but it follows a tradition in history of science (and especially in scientists' history of science) where the significance of new developments are often exaggerated, and some transitions are pictured as clean breaks with the past rather than gradual evolutions from the past. There are numerous cases in the history of science that show how this too-revolutionary picture has been used to celebrate and legitimize modern trends by contrasting them with a mythicized version of the past. After all, the greater the difference between old and new, with the more glory the new theory will shine.

One may think of classical examples of so-called revolutionary changes, such as the transition from the Ptolemaian, geocentric system to Copernicus' heliocentric system, or the transition from the caloric theory of heat to the kinetic theory about 1840. Or one may think of the "chemical revolution" of the 1780s when Lavoisier's oxygen-based theory emerged victoriously in sharp contrast to the earlier phlogiston theory and finally turned chemistry into a proper science. But, as we well know, in these and other cases the development occurred more slowly and gradually than suggested by the revolution metaphor, and the earlier theories were in no way non-scientific or clearly inferior to their successors. The revolutionary picture is to a large extent a reconstruction made by scientists of the victorious theory in order to enhance its significance and scientific status. Much the same is the case in cosmology, which did not suddenly become "physical" in the 1960s, but where the new generation of cosmologists had an interest in presenting the development as a kind of discontinuity centered around the discovery of the microwave background.

It is not always obvious what people mean when they speak of "physical cosmology," except that it is something different from other forms of cosmology, such as the mathematical approach and the astronomical-observational approach, and of course entirely different from older forms such as philosophical and theological approaches. Physical cosmology may reasonably be defined as the kind of cosmology where the matter and radiation content of the universe is in focus, not merely the geometry of space-time and not merely the galactic atoms observed by the astronomer.

As an extreme case of mathematical, or non-physical, cosmology one might mention the model proposed by Willem de Sitter in 1917, which was characterized by its total lack of matter. In Einstein's closed model, on the other hand, there was matter, but only in an abstract, smoothed-out way, as given by the energy-momentum tensor appearing in the field equations. Physical cosmology required more, that the forms of matter and its interactions be taken seriously, and this process only started in the late 1920s when we can see the first beginnings of what a decade later would become early physical cosmology. Time doesn't allow me to go into detail, so let me only briefly mention three early attempts to understand cosmic phenomena in physical terms.

First, there were in the 1920s several works on the thermodynamics of the universe, starting with Otto Stern in 1925; second, Richard Tolman and a few other physicists sought to apply the hypothetical proton-electron annihilation process that had been considered by Jeans and Eddington in connection with the cosmic radiation and as a possible source of stellar energy; and third, in 1931 Lemaítre suggested that somehow the explosion of the primordial atom was governed by the laws of quantum mechanics, and that the cosmic rays were the remaining fossils of the original explosion. All these works may be considered examples of proto-physical cosmology.

However, it was only from about 1932 that the new nuclear physics could contribute fruitfully to the understanding of cosmic phenomena, which mostly occurred through the new science of nuclear astrophysics which was made possible after the neutron had been recognized as an elementary particle and a building block of matter. Initially the interest in neutron capture processes was related to attempts to understand the energy production in stars, and not to cosmological problems, but nuclear astrophysics was nevertheless very important in stimulating suggestions of how cosmology could be better understood in terms of microphysics.

In my view, the first important date of what one might call nuclear cosmology was the year 1938, in which C. F. von Weizscker proposed his very important theory of stellar energy production and cosmic nucleosynthesis. Although this theory was soon overshadowed by Hans Bethe's much more detailed theory, from a cosmological point of view Weizscker's work is far the most interesting. We have here, if only in a somewhat embryonic form, the idea that matter as we know it today is the result of either stellar or cosmic nuclear processes that occurred in the past. And we have the idea that by comparing the result of nuclear-physical calculations with the observed cosmic distribution of chemical elements we may get insight in the physical conditions of the early universe.

This is the core of the research programme known as nuclear archaeology, an apt name because there is really a good deal of similarity between the methods employed by the archaeologist and the nuclear physicist studying the elements that may have origined from processes in the past. This research programme could only hope to succeed if the abundance distribution of nuclei was known on a cosmic scale, and such knowledge appeared in the same year, 1938, primarily the result of the painstaking analyses made through more than a decade by geochemists, astronomers and meteorite researchers. Foremost among these scientists who provided invaluable empirical evidence for early physical cosmology was the Norwegian mineralogist Victor Goldschmidt, but it was a truly collaborative and large-scale work that included dozens of researchers, only very few of whom were trained in astronomy or physics.

The Weizscker-Goldschmidt programme of nuclear archaeology only flourished a decade later, after World War II when it was greatly developed by George Gamow and his collaborators. But before commenting on Gamow's form of cosmology, I would like to mention yet another event of 1938 that indicated the coming of a new kind of physical cosmology; and that is a symposium held that year in the very university that hosts us today, Notre Dame. The symposium, organized by the Austrian-American Arthur Haas carried the modern-sounding title "The Physics of the Universe and the Nature of Primordial Particles" and included among its speakers Lemaítre, Shapley, Arthur Compton and William Harkins. The very arrangement of such a symposium indicates a growing interest in physical cosmology and suggests that it might only have been external circumstances (World War II) that delayed the emergence of physical big bang cosmology until the late 1940s.

In any case, the approach to cosmology of Gamow and his associates Ralph Alpher and Robert Herman was thoroughly physical and their big bang theory certainly deserves the label physical cosmology. They didn't have the cosmic microwave background (although they assumed its existence), but they took advantage of what they thought was another cosmic fossil, the abundance distribution of the chemical elements, and it was on this evidence their research focused. In their series of papers, extending over a 5-year period, they gradually refined the calculations and got promising results with regard to the lightest elements, but were unable to suggest nuclear reactions that had produced the heavier elements shortly after the big bang. The culmination of the Gamow approach to early big bang cosmology was a masterful paper published in 1953 by Alpher, Herman and James Follin in which the three physicists made detailed calculations for the first 10 minutes after the big bang by using all available knowledge of current particle physics.

So given this rich history of pre-1965 physical cosmology it is difficult to see why the emergence of physical cosmology as a science should be dated to the mid-1960s rather than 15 years earlier. It has often been pointed out that the celebrated calculations of Robert Dicke and Jim Peebles about 1965 didn't go much beyond those included in the Alpher-Herman-Follin theory, and that the helium calculations of the "new cosmology" didn't differ essentially from those performed in the early 1950s. Of course, Dicke and Peebles had the great advantage of knowing that the microwave background really exists and presently has a temperature about 3 K, and they used this knowledge to improve their calculations. However, Peebles' first calculations of 1965 had to assume an unrealistically low matter density, and it was only in 1966 that he managed to get a helium abundance of about 26%, in good agreement with observations. By comparison, the Alpher-Herman-Follin calculations resulted in a helium percentage of between 29 and 36.

Part of the reason for the neglect of early physical cosmology ¦ la Gamow is probably to be found in the books and survey articles written in the period from about 1950 to 1965. In almost all of these accounts, the big bang idea and the notion of cosmic element synthesis are given very low priority, if mentioned at all. For example, one of the best surveys of the time, McVittie's Fact and Theory in Cosmology from 1961, simply omits all mention of the creation of elements. A modern astronomer who wants to get a feeling of the development by comparing McVittie's book with Peebles' Physical Cosmology from 1971 would surely be led to conclude that a truly physical cosmology came into being only after 1965.

I would also like to point out that there is no hard-and-fast distinction between astronomical and physical cosmology, and that this observation further undermines the claim of physical cosmology being a post-1965 invention. Observational cosmology is more than extragalactic position astronomy and there is no good reason why the study of spectra from distant galaxies or, say, Zwicky's studies of the dynamics of spiral galaxies in the 1930s (the first indication of dark matter), should not be counted as contributions to physical cosmology.

Likewise, aspects of the development of radioastronomy in the 1950s and early 1960s included observations of a physico-cosmological nature, such as Ryle's programme in radiocosmology and also the unexpected discovery of quasars in 1963. Both of these developments marked a further union between physics and cosmology, and they predated the discovery of the cosmic microwave background.

Of course, the post-1965 cosmology was not simply a continuation of the earlier Gamow approach. It is reasonable enough, I think, to speak of a "renaissance" of cosmology, but much less reasonable to speak of a "revolution." Something new did happen, and the discovery of the microwave background definitely was important. But the continuity with the past is nonetheless striking, and it is difficult to see why the observed microwave background alone should make cosmology vastly more physical or scientific.

My feeling is that much of the self-congratulatory rhetoric of the period was rooted in the new situation of a standard hot big bang theory which, after the defeat of the rival steady state theory, was without serious competition. For the first time since Einstein's pioneering work of 1917, there was an almost complete consensus about the best cosmological model, and the microwave background was widely seen as the discovery that secured this consensus; although in fact the demise of the steady state theory was a much more complex affair in which earlier data from radio astronomy played an even more important role.

Whatever the evaluation of the historical significance of the cosmic background radiation, it would be unreasonable to associate the emergence of a new physical cosmology solely with the discovery. In a broader perspective, physical cosmology can be traced back to the astro-spectroscopic or astrochemical tradition of the 19th century, and in a more narrow perspective I find it reasonable to speak of at least of proto-physical cosmology in the 1930s and a mature physical cosmology about 1950, in the works by Gamow and his associates.