sign in sign up
<<
Home , Big Bang, Big Bang (book), Cosmology, Hermann Bondi, Observational cosmology, Radio astronomy

Re-interpreting the Significance of the Cosmic Background

Jacob Pearce PhD Candidate History and of Program University of Melbourne

Introduction: the Cosmic Microwave Background

The discovery of the Cosmic Microwave Background (CMB) radiation by Penzias and Wilson in 1965 was heralded as proof of . It was also termed the final ‘nail in the coffin’ for the rival Steady State program. This paper will examine the role of the CMB in re-shaping the conceptual of modern —the landscape of possible styles of reasoning (as used by Ian Hacking) and scenes of inquiry (as used by Nicholas Jardine). The CMB added unprecedented support to a style of reasoning characterized by factual-extrapolation, inherited by the cosmological community from Gamow. Steady State theory was based on a fundamentally opposed style of idealized-postulation, arising from Bondi. The value of the CMB was far more than as mere evidence for a theory. Rather, it made the case for observationally driven cosmology. This moment in cosmology signified a turning point, whereby observational evidence became the decisive factor for both consolidating and generating lines of inquiry. What the CMB symbolically represented for the style of reasoning—notwithstanding the lasting effect this epistemic object had on shifting the trajectory of the conceptual space of cosmology—is far more significant than its relation to theory choice. The CMB buried Steady State, so to speak, because of the style it legitimated, rather than due to its epistemic content. Tracing the history of the notion of the CMB further highlights the contingent 1965 interpretation, and calls into question the role of this entity in epistemic practices and strategies in cosmology today. This paper is delivered in line with an approach in the spirit of historical . I am not examining shifts in cosmology from the dimension of ‘why this happened’. Rather, I am tracing the trajectory of the conceptual space of cosmology by telling the configuration of that space. This is meant to be a philosophical, not a sociological history. The conceptual preconditions for making statements, discovered by looking at the discursive formations , are the main object of inquiry. This is an archeology, not a genealogy aiming to explain the present. Neither am I attempting to understand the full historical context in complete detail. However, it is important to understand where cosmology came from in order to understand today. A historical epistemology of modern cosmology adopts that approach of a historicist . Describing the shifts in the conceptual history of cosmology by examining the contingent ‘how’ of cosmology.

The importance of the CMB for cosmology

This chapter in the historical epistemology of modern cosmology concerns one of the most crucial tenets of the present picture of our . Any cosmology textbook or lecture course will place great emphasis on the CMB radiation, as it is a direct fossil remnant of the very early universe predicted by the hot big bang model. Not only that, it has been measured to great precision and shown to conform almost exactly to a blackbody spectrum. Undoubtedly, the CMB is important for cosmology. However, the history of the of questions concerning the CMB paints a different picture to the one that a graduate might have expected. I want to take the CMB discussion away from its normal location— away from the role that the CMB played in the victory of the big bang model over the steady state model, and towards to role that the CMB played in its connection with styles of reasoning. Firstly, it is worth briefly outlining the key aspects of the CMB. The CMB is thermal radiation which permeates the universe. It is nearly isotropic (the same in every direction) and homogeneous (roughly smooth). Its currently accepted value is 2.725 degrees (K). The data from more recent (the COBE satellite and WMAP) is so precise that the error bars do not even show up on a fit to the Planckian blackbody spectrum. about the fact the CMB is the most perfect blackbody detected. The hot big bang model predicts that the universe was in a hot, dense state approximately 13.7 billion ago. The universe was filled with an opaque plasma of and , where could not travel freely. The CMB was ‘created’ or ‘emitted’ about 378,000 after t = 0, when the universe had cooled to a of about 5000 K—cool enough for and photons to combine into . The recently detected tiny in the radiation also goes some way to explaining the subsequent clumping of matter into . In the current scene of inquiry (cosmology as we know it today), the CMB is one of the unifying canons of the scene. It is also one of the most important bounds of the scene, as any question or answer which does not adequately explain the CMB characteristics is deemed to be fit to an illegitimate cosmological picture. The validity of the CMB is not of concern here. Nevertheless, it is fair to say that a great deal of recent evidence lends weight to its interpretation. However, I want to focus on the earlier predictions and discoveries of the CMB, to highlight something important regarding the diachronic styles of reasoning in modern cosmology. The complex trajectory found when tracing the history of questions concerning the CMB reveals that the modern interpretation is more contingent than it may first seem to be.

Tracing the history of questions concerning thermal background radiation

The story is often told that Alpher, Herman and Gamow predicted a CMB with a temperature of about 5 kelvin in 1948. It was first detected accidently by Penzias and Wilson in 1965. Using the modern-day values of the cosmological physical parameters in their original calculation, the COBE determined value of 2.725 kelvin is obtained. In a retrospective work in 2001, Alpher and Herman note that “One of the major features of the development of the standard Big Bang model has been the prediction and subsequent discovery of a pervasive background radiation, observed at microwave .” 1 The fact that this account pervades modern understanding is surprising, especially as a more detailed reading of the history illustrates a much different and more complex history of questions concerning the CMB and the temperature of the universe. I will now briefly outline this history chronologically. More detailed analyses of each event can be found elsewhere. In 1896 Guillaume calculated the temperature of interstellar space to be 5-6 K.2 It was determined by accounting for the effect of in our own only. (Note that the existence of external galaxies was not established until about 1924). This is the earliest known publication on the topic. Thirty years later, in 1926, explained that the radiant energy from all the stars implies a total energy of the universe corresponding to a temperature of 3.2 K. He stated also that a blackbody in space should assume this temperature, but the temperature was explicitly an ‘effective temperature’.3 In 1928, Ernst Regener proposed that ionization, caused by cosmic radiation, leaves intergalactic space filled with a background temperature of 2.8 K.4 (Note that cosmic rays were first discovered in 1912 by V. F. Hess). Regener showed that the intensity of radiation coming from the of the , was the same as the plane normal to it. Thus, this suggested

1 Ralph A. Alpher and , Genesis of the Big Bang (New York: , 2001), 93. 2 C.-E. Guillaume, La 24, no. series 2 (1896). 3 Arthur Eddington, The Internal Constitution of the Stars (: Cambridge University Press, 1926), 371. 4 Ernst Regener, "Der Energiestrom Der Ultrastrahlung," ZP 80 (1933). that the radiation was isotopic and of cosmic, not stellar, origin. In 1933, Regener used the term ‘ultraradiation’ (now termed ‘cosmic radiation). He argued that the value of temperature of this blackbody was 2.8 K and characteristic of intergalactic space.5 In 1938, Walther Nernst discusses the radiation temperature of the universe, arriving at the figure of 0.75K.6 Translations of both of these papers appear in an interesting paper by Assis and Neves.7 Eddington, Regener and Nernst utilised the Stefna-Boltzmann law in their calculations, which is characteristic of blackbody radiation. The scholars who followed did the same. These are remarkable predictions. However, Kragh has pointed out that both radiation filling space due to starlight and cosmic rays are different beasts to the radiation backgrounds from the predicted in 1948 and discovered in 1965. 8 In 1948, Alpher and Herman calculated the present temperature of the decoupled primordial radiation. They did not focus on the microwave part, but it follows directly from Wien’s displacement law that their 5 K prediction was in the microwave part of the spectrum. Their prediction was certainly connected with the big bang scenario and the cosmological interpretation of the predicated radiation was explicit. Further measurements of the temperature of the universe, without cosmological interpretations, were reported in the mid 1950s. A Russian , Tigran Shmoanov studied the background radiation of the sky and reported a result of a few degrees kelvin. He published in a Russian journal on instrumentation. Again in the 1950s, French astronomer Emile Le Roux reported a result of about 3 kelvin from measurements at 33 centimeters. A cosmological interpretation of background radiation was put forward by Finlay-Freundlich in 1953 (and again in 1954). Finlay-Freundlich argued that the tired model (first proposed by Zwicky) would explain that of stars and the cosmological redshift. “If we interpret the cosmological shift in the same was as the stellar red shifts, the following equations should hold … (and) we get the following two reasonable values: Ts 9 = 1.9 K and Ts = 6.0 K.” These were a lower and upper limits for the thermal temperature of intergalactic space. It is important to highlight that Finlay-Freundlich was working with a non-expanding model of the universe, where cosmological redshift was interpreted differently. He obtained a temperature similar to that measured for the CMB, and with a cosmological interpretation different to that of standard big bang theory. Then, in 1954, Max Born discussed the proposal of -photon interaction in the Finlay-Freundlich proposal and explicitly linked the red-shift in the model with radio for possible .10 One other important study was by Walter S. Adams and Andrew McKellar. These were working on interstellar molecules; particularly the CN radical (cyanogen) as it is seen in absorption in interstellar space. Adams had given a prediction in 1940 for obtaining from observing transitions among rotational levels of CN molecules. In 1940, McKellar had observed observational intensities for CN rotational lines, suggesting an upper limit to the effective temperatures of interstellar space of 0.8 to 2.7 kelvin, but he proposed that the actual figure would be 2.3 kelvin.11 Kragh notes that “McKellar and Adams had actually detected an instance of the cosmic microwave background radiation.” 12 Gerhard Herzberg, in 1950, discussed the work of Adamas and McKellar: “From the intensity of ration

5 Ibid. 6 W. Nernst, "The Radiation Temperature of the Universe," Annalen der Physik 32, no. 44-48 (1938). 7 A. K. T. Assis and M. C. D. Neves, "History of the 2.7 K Temperature Prior to Penzias and Wilson," 2, no. 3 (1995). 8 , Cosmology and Controversy: The Historical Development of Two of the Universe (Princeton, NJ: Princeton University Press, 1996), 132. 9 E. Finlay-Freundlich, Philosophical Magazine 45 (1954). 10 M. Born, Proceedings of the Physical Society 67 (1954). 11 Alpher and Herman, Genesis of the Big Bang, 115-17, A. McKellar, "Evidence for the Molecular Origin of Some Hitherto Unidentified Interstellar Lines," PASP 52 (1940). 12 Kragh, Cosmology and Controversy, 134. of the lines with K = 0 and K = 1 a rotational temperature of 2.3 kelvin follows, which has of course only a very restricted meaning.”13 Yet another detection of a thermal background permeating the universe was noted in 1962 by American astronomer William Rose. He estimated a background of 2.5 to 3 kelvin but only recorded the result in his notebook.14 Finally, in 1965, Arno Penzias and Robert Wilson working at the Bell Telephone Laboratories were trying to develop a microwave communication receiver. Their signal contained interference and they tried eliminating it in every way they could think, including removing pigeon droppings from the receiver. They were detecting background radiation in the microwave region of the spectrum, at a of 7.3 centimetres. It was isotropic and unpolarized. Shortly after this detection, Peter G. Roll and David T. Wilkinson confirmed the detection at Princeton of a 3 K background by using a Dicke radiometer.15 A Cosmological interpretation of Penzias and Wilson’s accidental ‘discovery’ was given immediately by Dicke, Peebles, Roll, and Wilkinson. They claimed that this was relic radiation for the early hot and dense expanding universe in the Journal. This was accompanied with a paper by Penzias and Wilson announcing the discovery. Regardless of the actual temperature, the notion of a temperature of interstellar and later intergalactic space has been around for a long . Many physicists predicted a value, calculating it based on different physical mechanisms. Importantly, most of the predictions were not due to the emission of thermal radiation at recombination in the very early hot big bang universe.

The cosmological scene in the 1950s and 1960s: alternative styles

Before making conclusions regarding the philosophical significance of the CMB, the history of questions concerning thermal background radiation needs to be positioned in the context of the cosmological scene of inquiry at the time. As I have argued elsewhere, there was disunity in the cosmological scene throughout the 1930s and 1940s. There was no shared ‘scene of inquiry’, in the sense used by Nicholas Jardine. There were multiple competing views about what kind of science cosmology should be and a plethora of cosmological pictures. The scene only began to gain some traction once certain methodological commitments and bounds on the legitimacy of questions concerning the universe were given. Two clear ways of proceeding were offered. In the early 1950s, Gamow asked the community to be pragmatic and sensible in cosmological endeavours. Helge Kragh outlines Gamow’s assumptions thus: (i) “There is neither place nor need for philosophical and metaphysical questions. If such questions turn up, ignore or circumvent them.”16 (ii) “Cosmology should, like any branch of physics, be based on accepted physical knowledge and the ordinary methods of science.”17 Gamow’s move was an attempt to put cosmology back on solid foundations, not by proposing a new theory, but by re-establishing a style of reasoning I term factual-extrapolation. The alternative ‘-way’, proposed by Bondi was grounded in Milne’s of axiomatization and secure foundations of general notions based on intuitive reason. He argued that physics should come from “inner harmony” and “simplicity”.18 Bondi mistrusted the empirical-inductive method: “the impossibility of direct

13 Alpher and Herman, Genesis of the Big Bang, 116-17, G. Herzberg, Molecular Spectra and Molecular Structure, 2nd Edition ed. (New York: Van Nostrand, 1950), 496, Kragh, Cosmology and Controversy, 135. 14 Alpher and Herman, Genesis of the Big Bang, 117. 15 Ibid., 108. 16 Helge Kragh, " and the 'Factual Approach' to Relativistic Cosmology," in The Universe of , ed. A.J. Kox and Jean Eisenstaedt (Boston: Birkhäuser, 2005), 182. 17 Ibid., 183. 18 George Gale and John Urani, "Milne, Bondi and the 'Second Way' to Cosmology," in The Expanding of General Relativity, ed. Hubert Goenner, et al. (Boston: Birkäuser, 1999), 345. abstraction from observations … rules out the usual inductive approach”.19 Bondi similarly attempted to put cosmology back on solid foundations, by defining an alternate style of thinking for cosmological inquiry, which I term idealized-postulation. Kragh describes these styles using different terminology. “At a conference in Denver, Gamow distinguished between two schools of cosmology, which he labeled postulatory cosmology and factual cosmology. According to the first school—he no doubt had steady- state cosmology in —‘one asks oneself what the of matter and radiation should be in order to obtain philosophically desired cosmological models.’ In factual cosmology, on the other hand, ‘we accept the physically established laws governing matter and radiation and look for cosmological models which are derived on the basis of these laws and are consistent with astronomical observations.’”20 These definitions are important to understand the philosophical significance of the CMB. These bounds of the scene of inquiry, and the trajectories taken by them in the 1950s and 1960s, were strongly driven by concurrent changes in the techniques used in cosmology. For the steady state theory, the CMB detection was termed ‘the final nail in the coffin’. The 1950s saw significant advancements in gamma and X-ray astronomy, and the late 1950s and early 1960s showed the explosive power of . These changes are discussed extensively by Kragh.21 Steady-state theory had a trouble dealing with (discovered in 1960), radio , and the ‘ problem’. It was already a dying program in some respects before the CMB was interpreted as a fossil of the early universe. The value placed on observational evidence became more and more apparent. fitted well with the style of factual-extrapolation, as theoretical models were more readily changed by extrapolating from observed facts. Conversely, if observed facts did not fit with a model based in the style of idealized-postulation, the changes required to any axiomatic-deductive reasoning to make the observations fit cast doubt on these theories.

Configurations of the conceptual space and the CMB detection

The aim of this paper is to re-interpret the significance of the CMB by using an approach in the spirit of historical epistemology. What does the complex history of the thermal background radiation tell us about the conceptual space of cosmology during this ? Is the philosophical significance of the CMB in terms of the style of reasoning it authenticates more revealing than its connection with theory choice? Tracing the history of the notion of the CMB reveals much regarding the configuration of the conceptual of cosmology at this time. There were limitations on the questions that it was possible to pose (and answer) prior to the 1965 interpretation. Even when the CMB was predicted as a relic of the early universe in 1948 by Alpher and Herman, virtually no attempt was made to detect the radiation until 1965. As Kragh notes, the theory was “well known, but gave no impact at all.” Several reasons are given by Kragh, but none focus on the philosophical significance. 22 The conditions for the possibility of the interpretation were not fully realized until 1965. Many commentators have noted that it is difficult to say exactly why the big-bang program was disregarded for so many years.23 The physics community was more interested in problems of . Alpher, for instance worked in plasma physics and fluid dynamics. Herman worked in solid state and chemical physics, and pioneered work in traffic science of all things. They both worked in the private industry as well and cosmology was not their main game. Robert Dicke recalled it thus: “It’s a puzzle to me how cosmology got so separated off from the rest of physics … Here is all the matter in the universe, and it doesn’t

19 , Cosmology, 2nd ed. (Cambridge: Cambridge University Press, 1960), 10. 20 Kragh, Cosmology and Controversy, 136. 21 Ibid., 318-37. 22 Ibid., 133-34. 23 Ibid., 138. seem to bother anybody. It’s here, but where did it come from? Questions of this kind just weren’t asked [during the 1950s].” 24 Kragh disagrees, claiming that the questions “were asked, but few listened.”25 Jardine’s claim is that the questions that seem real and pressing and make sense in a community, unite a scene of inquirers.26 When Guillaume, Eddington and others calculated the thermal background temperature of the universe, the temperature prediction was not posed as the solution to a critical problem in a cosmological picture. Rather, it is just one of many theoretical calculations in the gamut of work being undertaken in at the time. It makes perfect sense, then, that no cosmological interpretation was posited. Similarly, the work by the likes of Regener and Nernst, although painstakingly close to the eventual interpretation of the blackbody radiation in 1965, was not developed in a time when questions of the large scale structure of the universe, or the early history of the dynamic universe were being problematized in the cosmological community. When McKellar and Adams were working on the radiation produced by CN molecules in interstellar space, their concern was not focused on cosmic origins. Kragh notes: “It is not surprising that their result did not receive much attention. After all, they had only reported the existence of a rotational temperature of 2.3 K for cyanogen molecules in a restricted part of the universe, which could easily be explained without invoking cosmology. Furthermore, there was at the time no theoretical reason for expecting a cosmic microwave background radiation at all.”27 As mentioned earlier, the temperature which followed from the calculations ‘had limited meaning’. It was by chance that in 1965, Dicke, Roll and Wilkinson were building a to look for cosmic microwave background radiation, in line with Dicke’s speculative theory of an oscillating universe, not the big bang model.28 Previously, in 1946 Dicke and others used an instrument to study the radio emission from the ’s atmosphere. At the time they could not remove radiation which, if it was a blackbody, would correspond to a temperature of about 20 kelvin. This indicates that at this earlier time, the cosmological interpretation of the CMB as a relic of the early universe was not a possibly. After Alpher and Herman’s prediction in 1948, detection of the radiation was not pursued. In 2001, Alpher and Herman claim that they explored the possibility at the time “with radio and others to no avail” and that “perhaps our earlier prediction was not taken seriously”. This was also suggested by Weinberg in his book .29 After the 1948 paper, Gamow is known to have accepted the thesis in 1950. In 1964, Soviets Andrei Doroshkevich and Igor Novikov wrote a paper, submitted by Yakov Z’eldovich, agreeing with the prediction. There were a mere eight references to the prediction of the CMB prior to 1965.30 The earlier astrophysicists and astronomers did not pursue the detection of the CMB because observational cosmology was not in full swing. To use Thomas Nickles’ formulation, it was unintelligible as a positive contribution to the field in which they operated.31 The CMB was evidently not a pressing question for early cosmologists. The proceedings of three major conferences (the 1958 Solvey conference in Beligum, La Structure et L’ de l’Univers; the 9th International Astronomical Union Symposium, Paris Symposium on Radio Astronomy in 1958; and the 15th International Astronomical Union Symposium, Problems of Extra-Galactic Research, in in 1961) include papers on

24 Interview in A. Lightman and R. Brawer, Origins: The and Worlds of Modern Cosmology (Cambridge, .: Press, 1990), 208. 25 Kragh, Cosmology and Controversy, 141. 26 Nicholas Jardine, The Scenes of Inquiry: On the of Questions in the (Oxford: Clarendon Press, 2000). 27 Kragh, Cosmology and Controversy, 134. 28 Alpher and Herman, Genesis of the Big Bang, 110-11. 29 Ibid., 115. 30 Ibid. 31 Thomas Nickles, "Disruptive Scientific Change," in Rethinking Scientific Change and Theory Comparison: Stabilties, Ruptures, Incommensurabilities, ed. L. Soler, H. Sankey, and P. Hoyningen-Huene (Springer, 2008), 353. cosmology, radio astronomy, and galaxies. However, none of the published volumes includes a mention of the notion of space being filled with thermal radiation, let alone there being any fossil remnants from the early universe.32 Peebles, Page and Partridge conclude that the idea of the universe being filled with thermal radiation left over from the early stages of the expansion of the universe “was ‘in the air’ in the early 1960s. But it was less visible than other issues in cosmology, particularly the debates of the relative merits of the big bang and steady state scenarios.”33 Alpher and Herman have boldly claimed that “There has been no accepted physical explanation for this background blackbody radiation other than that it is indeed a fossil of the early Big Bang, a very much cooled (more correctly, redshifted) relic of the radiation that pervaded the universe some hundreds of thousands of years after the Big Bang”.34 This claim seems to be riddled with anachronism, as the cosmological significance of the CMB was merely one of many possible interpretations at the time. It seems to be more a matter of contingency than necessity. Alpher and Herman also state that “In retrospect, it appears that observations were indeed made but that their meaning was missed.”35 This is undoubtedly clear. But the reason that the relic radiation interpretation was missed hinges on the configurations of the conceptual space of cosmology at the time. The most prominent cosmologists were not consumed by questions regarding relics of the early universe. Those working in the style of factual-extrapolation were, however, interested in problems of element formation in a prestellar context due to the relative abundances observed. Gamow in the 1940s was working on the questions – ‘how did the observed make-up of elements in the universe come into existence?’ and ‘What conditions would be necessary to produce the relative abundances that we see?’ Alpher and Herman, using the conditions they had calculated that were required for the element production in the early universe, predicted the present temperature of the fossil radiation left from the hot early universe.36 Both Gamow and Alpher and Herman published ‘divine creation curves’.37 These show calculations for relative abundances of , deuterons, and with temperature over time for the . Gamow was interested in problems of galaxy formation (while avoiding the time t = 0). A great deal of research was on nuclear physics models of element formation, based on a hot primordial gas.

Re-interpreting the significance of the CMB

The CMB detection added unprecedented support to the factual-extrapolation style of reasoning. It was posited through reasoning from the observed cosmological parameters and extrapolating from these. Alpher and Herman note that “It is probably also the case that most of the scientific community did not take the Big Bang model seriously until the added evidence of the observation by Penzias and Wilson came along. Weinberg’s book was probably itself an important turning point in acceptance of the model by the scientific of physics and astronomy.”38 Another major turning point is the move to observationally driven cosmology. Observational techniques became the dominant epistemic strategy, rather than idealized-postulation from axiomatic principles. It is not the case that observationally driven cosmology did not exist prior to the CMB detection. It was, however, not as dominant in the scene of inquiry. The philosophical significance of the CMB is not its value as a piece of evidence validating one . Rather, it made the case for observationally

32 P. J. E. Peebles, L. A Jr. Page, and R. B. Partridge, Finding the Big Bang (New York: Cambridge University Press, 2009), 61. 33 Ibid., 63. 34 Alpher and Herman, Genesis of the Big Bang, 28. 35 Ibid., 119. 36 Peebles, Page, and Partridge, Finding the Big Bang, 30. 37 George Gamow, "The Evolution of the Universe," Nature 162 (1948): 681. Ralph A. Alpher and Robert Herman, "On the Origin of the Elements," PR 75 (1949). 38 Alpher and Herman, Genesis of the Big Bang, 119. driven astronomy more than it was. Scientific evidence now played a more decisive role than it had previously in consolidating and generating lines of cosmological inquiry. Kragh notes the following: “although observational testing was on the program even since 1948, it took more than fifteen years before observations clearly indicated that evolutionary theories fitted better with the universe than the rival steady-state theory. And even then there was no undisputable proof of a big-bang universe, only increased evidence.”39 During these years, a number of observational issues were posed and solved. To name a few: the time scale problem, the Stebbins-Whitford effect, redshift-magnitude measurements, formation of galaxies, and . In the 1950s, Peebles notes that “People were assembling observational evidence, in part out of curiosity, in part driven by the goal of testing theoretical ideas, and the observations were in turn driving theoretical developments.”40 The increasing volume of observational evidence which favoured the big-bang model was a problem for the steady state program, especially as it showed the fruitfulness of theories couched in a style of factual-extrapolation. Theories which are extrapolated from observed facts are easily modified (by changing of the exact value of the temperature of the CMB, for instance) based on new observational evidence. However, for theories which are derived from idealized-postulation (as the steady-state was), every modification based on new evidence seems increasingly ad hoc and calls into question the veracity of the model. This is perfectly illustrated by looking at the attempts of the steady state theorists to account for the CMB detection in the few years following its announcement. The CMB was not strictly in contradiction with the steady state theory. The cosmological significance of the CMB is not a logical necessity from the observational data—it is an interpretive leap. The immediately accepted interpretation that the thermal blackbody radiation was a fossil relic of the early universe was one of many possible contingent interpretations (as the tracing of its history suggests). Yet the discovery was immediately posed as yet another problem for the steady state theorists. Burbridge, a well known figure in the steady state community, explains it thus: “throughout the 1960s the idea emanating from Princeton and also from Moscow from Zel’dovich’s group led almost everyone to believe that the radiation could only be remnant of a big bang and would be of blackbody form. It would be proof that the steady state theory was wrong. With the Penzias and Wilson discovery, while there was still no proof that it was blackbody, it was thought that the verdict was in.”41 To account for the 3K thermalized blackbody cosmic radiation, Hoyle and Chandra Wickramasinghe posited the existence of both interstellar and . The two had come up with the idea of thermalizing grains to explain the observed interstellar , and as a to help produce interstellar molecules.42 As Kragh explains, “In 1967 there was evidence for tiny interstellar grains consisting of a graphite core surrounded by an ice mantle, and if such grains were assumed to include impurity atoms they would absorb light and reemit it in the far region, and in this way produce a microwave background of temperature about 3 K. The idea was developed by Hoyle, Wickramasinghe, and Narlikar, who considered it a possible rescue of the steady-state theory.”43 Hoyle firmly believed that “There has never been a difficulty in the steady-state theory over the energy-density of the cosmic microwave background.” 44 Kragh: “… unfortunately for Hoyle and his followers, the majority if astrophysicists disagreed. The hypothesis of thermalization by interstellar grains attracted little interest and even less

39 Kragh, Cosmology and Controversy, 269. 40 Peebles, Page, and Partridge, Finding the Big Bang, 52. 41 G. R. Burbridge and J. V. Narlikar, "Some Comments on the Early History of the Cmbr," in Finding the Big Bang, ed. P. J. E. et al Peebles (New York: Cambridge University Press, 2009), 271. 42 F. Hoyle and N. C. Wickramasinghe, "On Graphite Particles as Interstellar Grains," MNRAS 124 (1962), Kragh, Cosmology and Controversy, 356. 43 Kragh, Cosmology and Controversy, 357. 44 F Hoyle, "An Assessment of the Evidence against Steady-State Theory," in Modern Cosmology in Retrospect, ed. B. et al Bertotti (Cambridge: Cambridge University Press, 1990), 224. respect. The large majority of astrophysicists, perfectly satisfied with the big-bang explanation, just ignored it.”45 In 2001, Alpher and Herman reflect: “Hoyle in particular, with Burbridge and Narlikar, has tried to rationalize this radiation as arising from the thermalization of starlight by interstellar and intergalactic dust grains. But that explanation requires the existence of a rather special distribution of grains (suggested as being principally iron), which appears to be quite unnecessary, given the simplicity of the explanation for this radiation in the Big Bang model.”46 This rejection is hardly a watertight argument, but for the majority of cosmologists, it is sufficient. Theoretical commitments may inform a scene of inquiry, and may legitimate questions. The big bang research program had certain theoretical commitments. This may explain why the CMB was so crucial in burying the steady state theory in 1965, before the facts and arguments against the steady state universe were thoroughly proposed.

The lasting effects on the conceptual space

These episodes caused a great deal of in the scientific community over a number of decades. In the 1930s, there was one ill-defined scene with multiple questions and multifarious cosmological pictures. The conceptual space was in flux. Then, contestation over what should constitute the scene resulted in two clear ways of doing cosmology. The space was gradually re-defined in the 1940s and 1950s; there were two scenes, with differing questions and styles. Post 1965, there is a lasting effect on the conceptual space of modern cosmology. The community of inquirers is quickly consolidated into one group, which adopts the factual-extrapolation style of reasoning. There are fewer possibilities in the conceptual space as the legitimate boundaries of the space are solidified. The space is back to one scene, but now with one style of reasoning; one set of dominant questions. One research program was de-legitimated, but with it the cosmological scene lost a style of reasoning as well. The idealized-postulation style was seriously injured through the episode. Although, as I will argue in another paper, it was not killed off completely, and resurfaces at a later date. The overwhelming number of observational achievements in the 1950s and 1960s changed the face of modern cosmology. Some have argued that the steady state theory died out due to a number of sociological and theological commitments of the main actors, as well as the mounting observational evidence against it. However, more emphasis must be placed on the role that changes in the scene played on the conceptual space of cosmology, and in turn, the space of possible styles of reasoning employed in cosmological thinking. Any question in a community represents a way of expressing a style, and the answers to questions embody the style. The impact felt in the conceptual space from one scene becoming dominant was striking. New questions and lines of inquiry became possible. (Even those working in steady state theory began asking new questions such as ‘Is the CMBR originating from astrophysical sources, and if so, what sources?’) More historical reasoning is employed in cosmology after 1965 as other remnants of the big bang are posed, and more and more observations are interpreted in terms of their origins in an evolutionary cosmological model. Social fragmentation also occurred in the scene of inquiry. The inquirers who pursued steady state theory (which later became the current Quasi Steady State (QSS) theory float on the edge of the cosmological scene. They are not treated with the level of respect afforded to other cosmological researchers. These inquiries now fell outside the legitimate conceptual space of cosmology which had been negotiated and re-negotiated. There is controversy with these individuals in the general cosmological community, as the two scenes are trying to occupy the same terrain. Multiple styles cannot exist in this same space. The style utilized in QSS is in conflict with the dominant conceptual space today; the questions posed in QSS are not posable in the current configuration of the conceptual space in orthodox cosmology. Questions such as ‘what is the mechanism behind the QSS creation field?’ have no referent in

45 Kragh, Cosmology and Controversy, 357. 46 Alpher and Herman, Genesis of the Big Bang, 98. a space dominated by hot big bang cosmology. By end of 1960s, Hoyle and Narlikar, who had abandoned the perfect cosmological were still working on steady state theory and the Bondi-Gold cosmology. Kragh notes that “Although the steady-state theory was no longer considered a serious alternative to the big-bang theory by 1970, the controversy continued in a different form, now as an attempt to discredit the victorious big-bang theory.”47 The modern measurements of the CMBR, in including the anisotropies, are plentiful, remarkably precise, and somewhat formidable.48 But the exactness of an observation in line with a certain cosmological theory was never the issue here. Without arguing about the veracity or credence of the current interpretation of the CMB it is possible to claim that the CMB became the flagship epistemic symbol of the new big-bang hegemony. In summing up, I want to make several remarks about conceptual change in intellectual history. Thomas Nickles has recently made some pertinent criticisms of history and philosophy of science analyses. “On the one hand, many philosophers and some intellectual historians regard conceptual breaks (major representational changes) as earth- shaking, as if such changes threaten to stop scientific research in its tracks. By contrast, scientists themselves frequently adapt to them easily and often find them highly stimulating. The more creative scientists actively seek them. On the other hand, major alterations in scientific practice can and do disruptively transform scientific communities, yet most philosophers pay little attention to change of this type.”49 The shifts seen in the conceptual space during this episode in the history of modern cosmology were significant, but not earth- shaking. The transformation seen in the cosmological community post 1965 was driven by major alterations in scientific practice in the cosmological community. The drive towards observational cosmology, which consolidated the factual-extrapolation style of reasoning, was both an alteration in scientific practice, and in the conceptual space. Textbooks and accounts of cosmology today tend to start with phrases such as ‘The universe is observed to be homogeneous and isotropic’. It would seem strange these days to imaging a textbook beginning with the phrase ‘It is deducible from idealized principles that the universe is homogeneous and isotropic’. Factual-extrapolation, coupled with observation, has prevailed. Nickles also argues that disagreement within scientific communities does not signal a rupture in scientific change. Rather, “It is the problem-solving practices that count most.”50 The discovery of the CMB gave a direct answer to the question posed concerning the structure of the universe. Although some argued it was consistent with the steady state alternative, it was not the solution to a problem in that community; there was no direct correlation with an aspect of the early universe. Any fit with the idealized-postulation style was almost necessarily ad hoc. The fact that the CMB was problem solving for one style, but not for the other, is notable. For Nickles, “Philosophers tend to focus on the theoretical products of scientific research rather than on the process that produced those products.” 51 The philosophical significance of the CMB should lie in uncovering the processes and practices that produced the contingent historical trajectory of the notion. This is where an archeology of the questions concerning the CMB is important. The philosophical significance of the CMB lies in what is represented in terms of the styles of reasoning it validated, and the conceptual space which it shaped, more than the theory it authenticated.

47 Kragh, Cosmology and Controversy, 319. 48 See Peebles, Page, and Partridge, Finding the Big Bang, 478-509. 49 Nickles, "Disruptive Scientific Change," 352. 50 Ibid., 353. 51 Ibid., 357. References

Alpher, Ralph A., and Robert Herman. Genesis of the Big Bang. New York: Oxford University Press, 2001. ———. "On the Origin of the Elements." PR 75 (1949): 332. Assis, A. K. T., and M. C. D. Neves. "History of the 2.7 K Temperature Prior to Penzias and Wilson." Apeiron 2, no. 3 (1995): 79-87. Bondi, Hermann. Cosmology. 2nd ed. Cambridge: Cambridge University Press, 1960. Born, M. Proceedings of the Physical Society 67 (1954): 193-94. Burbridge, G. R., and J. V. Narlikar. "Some Comments on the Early History of the Cmbr." In Finding the Big Bang, edited by P. J. E. et al Peebles. New York: Cambridge University Press, 2009. Eddington, Arthur. The Internal Constitution of the Stars. Cambridge: Cambridge University Press, 1926. Finlay-Freundlich, E. Philosophical Magazine 45 (1954): 303-19. Gale, George, and John Urani. "Milne, Bondi and the 'Second Way' to Cosmology." In The Expanding Worlds of General Relativity, edited by Hubert Goenner, Jürgen Renn, Jim Ritter and Tilman Sauer, 343-76. Boston: Birkäuser, 1999. Gamow, George. "The Evolution of the Universe." Nature 162 (1948): 680-82. Guillaume, C.-E. La Nature 24, no. series 2 (1896). Herzberg, G. Molecular Spectra and Molecular Structure. 2nd Edition ed. New York: Van Nostrand, 1950. Hoyle, F. "An Assessment of the Evidence against Steady-State Theory." In Modern Cosmology in Retrospect, edited by B. et al Bertotti, 221-31. Cambridge: Cambridge University Press, 1990. Hoyle, F., and N. C. Wickramasinghe. "On Graphite Particles as Interstellar Grains." MNRAS 124 (1962): 417-33. Jardine, Nicholas. The Scenes of Inquiry: On the Reality of Questions in the Sciences. Oxford: Clarendon Press, 2000. Kragh, Helge. Cosmology and Controversy: The Historical Development of Two Theories of the Universe. Princeton, NJ: Princeton University Press, 1996. ———. "George Gamow and the 'Factual Approach' to Relativistic Cosmology." In The Universe of General Relativity, edited by A.J. Kox and Jean Eisenstaedt. Boston: Birkhäuser, 2005. Lightman, A., and R. Brawer. Origins: The Lives and Worlds of Modern Cosmology. Cambridge, Mass.: Harvard University Press, 1990. McKellar, A. "Evidence for the Molecular Origin of Some Hitherto Unidentified Interstellar Lines." PASP 52 (1940): 187-92. Nernst, W. "The Radiation Temperature of the Universe." Annalen der Physik 32, no. 44-48 (1938). Nickles, Thomas. "Disruptive Scientific Change." In Rethinking Scientific Change and Theory Comparison: Stabilties, Ruptures, Incommensurabilities, edited by L. Soler, H. Sankey and P. Hoyningen-Huene, 351-79: Springer, 2008. Peebles, P. J. E., L. A Jr. Page, and R. B. Partridge. Finding the Big Bang. New York: Cambridge University Press, 2009. Regener, Ernst. "Der Energiestrom Der Ultrastrahlung." ZP 80 (1933): 666-69.