Re-Interpreting the Significance of the Cosmic Microwave Background
Total Page：16
File Type：pdf, Size：1020Kb
Re-interpreting the Significance of the Cosmic Microwave Background Jacob Pearce PhD Candidate History and Philosophy of Science Program University of Melbourne Introduction: the Cosmic Microwave Background Radiation The discovery of the Cosmic Microwave Background (CMB) radiation by Penzias and Wilson in 1965 was heralded as proof of Big Bang theory. It was also termed the final ‘nail in the coffin’ for the rival Steady State research program. This paper will examine the role of the CMB in re-shaping the conceptual space of modern cosmology—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 20th century 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 epistemology. 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 present, 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 philosophy of science. 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 universe. 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 emergence of questions concerning the CMB paints a different picture to the one that a physics 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 kelvin (K). The data set from more recent observations (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. Physicists rave 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 years ago. The universe was filled with an opaque plasma of matter and energy, where photons could not travel freely. The CMB was ‘created’ or ‘emitted’ about 378,000 after t = 0, when the universe had cooled to a temperature of about 5000 K—cool enough for electrons and photons to combine into hydrogen atoms. The recently detected tiny anisotropies in the radiation also goes some way to explaining the subsequent clumping of matter into galaxies. 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 frequencies.” 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 stars in our own galaxy 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, Arthur Eddington explained that the radiant energy from all the stars implies a total energy density 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 plane of the milky way, was the same as the plane normal to it. Thus, this suggested 1 Ralph A. Alpher and Robert Herman, Genesis of the Big Bang (New York: Oxford University Press, 2001), 93. 2 C.-E. Guillaume, La Nature 24, no. series 2 (1896). 3 Arthur Eddington, The Internal Constitution of the Stars (Cambridge: 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 microwaves 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 astronomer, 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 radio 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).