Ultimate Explanations of the Universe (7 page)
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Authors: Michael Heller
Tags: #Philosophy, #Epistemology, #Science, #Cosmology
In the Heat of Debate
The cosmology of the steady state found itself in the limelight of British public opinion chiefly as a result of a series of radio broadcasts which Fred Hoyle made in the spring of 1949, which made him a media personality. Later the broadcasts were published in a book,
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and helped to popularise the steady-state model outside the British Isles. Initially the transfer of the discussion to the popular forum generated additional opposition to the new ideas from British astronomers and physicists. But there were also counter-arguments of a more scientific nature.
It is not my intention to record all the developments in the debate between the protagonists of the steady-state cosmology and the adherents of relativistic cosmology. There is an excellent monograph on the subject by Helge Kragh.
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I shall merely delineate a few of the episodes in it, relevant to the main subject of the present reflections – the search for “ultimate solutions” in cosmology.
deus ex machina
.
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Hoyle, of course, did not agree with this. He wrote
… I cannot see any good reason for preferring the big bang idea. Indeed it seems to me in the philosophical sense to be a distinctly unsatisfactory notion, since it puts the basic assumption out of sight where it can never be challenged by direct appeal to observation.
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Nonetheless he had to come to terms with the fact that in his model, too, matter simply appeared out of the blue, and all the “mechanisms of creation” he referred to related to the physical properties of matter that was already in existence. His claim that “the creation field” was generated by the matter present in the universe remained in the realm of purely philosophical speculation.
By way of commentary I shall refer to the passage quoted from Hoyle. Kragh remarks that this was the first occurrence of the phrase “the big bang” in print. Most probably Hoyle must have used it earlier in his oral statements. He applied it ironically, to discredit the rival theory.
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This made the atmosphere of the controversy even hotter and certainly helped to spread news of it. In this connection George P. Thompson, holder of the Nobel prize for physics who had experimentally confirmed the existence of matter waves, wrote:
Probably every physicist would believe in a creation [of the universe] if the Bible had not unfortunately said something about it many years ago and made it seem old-fashioned.
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With time the worldview controversy associated with steady-state cosmology calmed down and the debate became more scientifically oriented, especially as advances in the technology of astronomy and radioastronomy made the prospect of confronting at least some of the predictions of the steady-state cosmology with observational data more and more of a reality.
The Demise of the Cosmology of the Steady State
While the debate was still going on whether the universe was in a steady state or whether it was subject to evolution on a grand scale, distinct progress was being made in work connected with the general theory of relativity. More and more evidence was accumulating to confirm it as a first-class theory of physics. In 1960 Pound and Rebka were the first to successfully carry out a laboratory test of the general theory of relativity. They applied the Mössbauer effect to measure the change in the photon frequency of gamma radiation due to the difference in the Earth’s gravitational field over a height of 22.6 m (the height of the tower on the Harvard University campus, where the experiment was conducted). Physicists were certainly impressed by their result. At the same time advances were being achieved at a rapid rate in the theoretical work on the general theory of relativity, which started to exert an impact on the development of mathematics. The geometrical methods devised for relativistic physics were gradually entering the realm of abstract mathematics concerned with modern differential geometry. All of this was bringing a change of atmosphere and making the cosmology of the steady state, which was in opposition to Einstein’s theory of gravitation, lose its ground.
An even bigger contribution to this process came from the advances made in observational techniques in astronomy and radioastronomy and the parallel progress in relativistic cosmology, which was partly stimulated by these advances. From 1948 on George Gamow and his team were working on a new scenario for the processes which had occurred in the young hot universe. Gamow’s scenario was based on a knowledge of contemporary nuclear physics, and launched a series of projects to determine the "chemical composition” of the universe. From the very outset Gamow tried to find confirmation for his ideas in observational research to determine the frequency of occurrence in the universe for the nuclei of particular chemical elements. Soon Hoyle, along with Eleanor and Geoffrey Burbidge and William Fowler, announced a rival theory of nucleogenesis, according to which the nuclei of the chemical elements were created not in a hot Big Bang, but in the interior of massive stars. The motivation behind this postulate was undoubtedly an attempt to neutralise the advantage enjoyed by relativistic cosmology thanks to the results obtained by Gamow’s group. Both parties initiated intensive research programmes in the field which later came to be called cosmic nucleosynthesis. The results these rival projects achieved were a surprise for both teams. It turned out that the conditions prevalent shortly after the Big Bang were indispensable to produce all the hydrogen nuclei, about 70% of the helium, and small amounts of a few of the other light chemical elements extant now in the universe, which was in agreement with Gamow’s theory. But the nuclei of the rest of the chemical elements were shown to be produced even now in the interiors of massive stars – as predicted by the theory proposed by Hoyle, the Burbidges and Fowler.
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Another argument against steady-state cosmology was provided by the development of radioastronomy. Thanks to progress in observational techniques, still in their infant stage at the time, it became possible to compile more and more accurate catalogues of radio-sources, which in turn facilitated the carrying out of a variety of tests for cosmological theories. The most promising test was the count of radio-sources per unit solid angle, not exceeding a certain luminosity (viz. the flux density at a given frequency). If the universe is in the steady state, the graph for number of radio-sources versus luminosity should be a straight line with a fixed gradient (a gradient of –1.5 on a logarithmic scale). The first test of this kind was conducted in 1955 by M. Ryle and P.A.G. Scheuer, and their result suggested a disagreement with the predictions of steady-state cosmology. Soon more work followed, with results more and more in line with each other and showing that the radio-source count increased with increasing distance. This would indicate that the younger the universe had been, the higher the density of radio-sources in it, therefore it could not be in the steady state. In 1963 W. Davidson and M. Davies wrote an article summarising these results. Their main conclusion was that the results hitherto obtained in radioastronomy could not be explained by steady-state cosmology.
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The discovery of quasars – strong sources of radiowaves identified as optical objects similar to stars – was yet another challenge to steady-state cosmology. After the first inconclusive results, measurements for their red shifts began to bring evidence against a steady-state universe. To avoid such a conclusion the theory’s defenders put forward hypotheses of the “local derivation” of quasars, according to which quasars were not “at cosmological distances,” but were associated with certain exotic phenomena in our relatively close astronomical neighbourhood. However, the influx of new data made such hypotheses less and less plausible.
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Today the general consensus is that the final blow to steady-state cosmology was administered in 1965 by the discovery of microwave background radiation, although Helge Kragh is of the opinion that it was a blow delivered to a theory already in its death throes.
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Microwave background radiation was discovered by Arno Penzias and Robert W. Wilson, and interpreted by Robert Dicke and his collaborators as the remnants of the Big Bang which initiated the current phase of cosmic evolution. The existence of this radiation had been predicted in the late 1940 s by George Gamow, who together with his team had determined its expected properties. It was to be an isotropic (viz. independent of direction) black body radiation at a temperature of a few degrees Kelvin. However, Gamow’s prediction had been forgotten, and Dicke and his team at Princeton rediscovered it in their theoretical work.
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Penzias and Wilson’s observations confirmed all the theoretical expectations to a good degree of accuracy, and later measurements made the accuracy even sharper.
After the discovery of background radiation the popularity of steady-state cosmology fell dramatically. Even Hoyle was inclined to admit that the observed data indicating that the world was subject to global evolution were too serious to ignore and insist on the concept of the steady state. But he was still reluctant to acknowledge relativistic cosmology with the singularity at the beginning of evolution. He persisted with his claim that any “beginning” whatsoever contradicted the principles of scientific methodology, and kept coming up with a series of new variations of a theory with no Big Bang.
Rather, the controversy faded out in the sense that the now standard hot big-bang model became the nearly undisputed new paradigm in cosmology, and the new generation of cosmologists stopped worrying (or even knowing) about the steady-state theory.
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Philosophers of science hold that there is no such thing as an
experimentum crucis
(critical experiment) capable of disproving any given theory once and for all. A theory that is losing ground may always be modified and kept up by supplementary hypotheses. That is precisely what the supporters of steady-state cosmology did, trying to salvage it, but gradually their ranks crumbled away. The staunchest were Hoyle and his collaborator Jayant V. Narlikar,
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but soon they found themselves out on a limb. It is an indisputable historical fact that the discovery of background radiation was the experiment which sealed the fate of steady-state cosmology. Even if it was not an
experimentum crucis
in the sense understood by the philosophers of science, in conjunction with the other observations indicative of an evolving universe it proved an obstacle which the theory of the steady state did not manage to overcome.
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Creation and Viscosity
Looking back in retrospect at the history of steady-state cosmology it is hard to avoid the impression that it was an
ad hoc
hypothesis, called into being precisely for the purpose of removing from cosmology the “ghost of a beginning” in the sense not of a technical problem in cosmology but of an attempt to arrive at an ultimate explanation of the universe. The steady-state theory survived for almost two decades only because it had some observational tests at its disposal which were relatively easy to conduct. These tests were carried out, and the theory disclosed its weak points. Compared with
ad hoc
hypotheses, genuine scientific theories are characterised by being aggressive in a certain sense: they tend to annex ideas which are not so forceful but in a way attractive though lacking in solid foundations, and incorporate them into their own models and techniques. This proved true in the case of relativistic cosmology and steady-state cosmology.
The general theory of relativity is one of the most important theories in contemporary physics, linked by a variety of connections with other theories of physics and together with them constituting a well-knit, albeit far from complete structure. Relativistic cosmology is the natural application of the general theory of relativity to the universe on its largest scale. The first cosmological models were based on a number of simplifications. One of these simplifications was the ignoring of the dissipation of energy. The collection of galaxies was treated as a dust the particles of which do not interact with each other, or as a perfect fluid in which there are no problems of dissipation. From Chap. 3 we remember that the first scientist to introduce dissipation processes into cosmological models was Richard Tolman. In models of the universe which obey the cosmological principle dissipative processes are introduced by adding to the equations terms responsible for bulk viscosity, also known as second viscosity. “First” viscosity is associated with interlayer friction, but this kind of viscosity cannot occur in isotropically expanding models (viz. observing the cosmological principle), since there is no interlayer friction in expansion of this kind. However, bulk viscosity, which is associated with the rapid expansion of a fluid, may of course occur in an expanding universe. From Chap. 3 we may recall the surprise when it turned out that on taking bulk viscosity into account in the oscillating model there was an increase in successive cycles of the oscillation. But we would expect the oscillations to diminish due to the dissipation of energy, as they do in classical physics. However, usually when we consider dissipation processes in classical physics we are talking about isolated systems, viz. ones which do not exchange energy with their surroundings. Strictly speaking, in the general theory of relativity we cannot, even in principle, construct an isolated system. We might perhaps cut off the supply of energy from beyond the system, but we cannot “switch off” the gravitational field which, being related to the space-time curvature, is stored in the geometry of space-time itself, and therefore penetrates all insulators. As the calculations show, processes involving bulk viscosity may draw energy from the curvature of space-time. This was precisely why Tolman’s calculations showed that the cycles of an oscillating universe were not subject to damping down, but on the contrary – to an increasing amplitude. The mechanism for the production of energy from the curvature of space-time is responsible for the existence of many other solutions to Einstein’s equations involving bulk viscosity apart from Tolman’s “increasing cycles.”
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They include solutions in which the production of energy is exactly counterbalanced by the loss in density caused by the recession of galaxies (in other words we have steady-state solutions). One of them turns out to be exactly the same as the solution Hoyle found for his “creation field” equations. From the mathematical point of view this relativistic model involving second viscosity and Hoyle’s model involving the creation of matter are indistinguishable from each other.
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Thus the same solution admits of two different interpretations: one the standard lodged within an efficient theory of physics, and the other an
ad hoc
device, put forward effectively with only one aim in mind: to eliminate the problem of the beginning. Observations came out in favour of the stronger of the two.
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From the historical point of view the discovery of viscous models made no contribution to the downfall of steady-state cosmology. By the time the first papers on viscous cosmology were being published the steady-state theory had been out on the distant margins of cosmological research for quite a while already. Nevertheless, a moral may be drawn from the whole story for the focus of our attention – the scrutiny of ultimate explanations in cosmology. The moral is that ultimate explanations should be constructed on the basis of well-founded physical theories rather than on
ad hoc
ideas. This does not mean, of course, that in future the quest for ultimate explanations will not bring any profound revolutions in ideas. Quite on the contrary, we should be expecting such revolutions, but the chances of really profound revolutions are much greater when they are derived from problems in the mainstream of science.
