Contemporary History of Cosmology and the Controversy over the Multiverse

Summary

Cosmology has always been different from other areas of the natural sciences. Although an observationally supported standard model of the universe emerged in the 1960s, more speculative models and conceptions continued to attract attention. During the last decade, ideas of multiple universes (the ‘multiverse’) based on anthropic reasoning have become very popular among cosmologists and theoretical physicists. This had led to a major debate within the scientific community of the epistemic standards of modern cosmology. Is the multiverse a scientific hypothesis, or is it rather a philosophical speculation disguised as science? This paper offers a review of the recent and still ongoing controversy concerning the multiverse, emphasizing its foundational nature and relation to philosophical issues. It also compares the multiverse controversy to some earlier episodes in the history of twentieth-century cosmology when particular theories and approaches came under attack for betraying the ideals of proper science.

Notes

¹John D. Bernal, The Social Function of Science (London, 1939), 3. Bernal apparently had in mind scientists such as J. Jeans, A.S. Eddington, A.N. Whitehead and J.S. Haldane.

²Physical eschatology denotes attempts to forecast the far future state of the universe on the basis of existing scientific knowledge. For a bibliographic overview, see Milan M. Ćirković, ‘Resource Letter: Pes-1: Physical Eschatology’, American Journal of Physics, 71 (2003), 122–33. In so far as it takes into account the possibility of life, as it often does, physical eschatology links to speculative astrobiology. An example is N.E.H. Prince, ‘Simulated Worlds, Physical Eschatology, the Finite Nature Hypothesis and the Final Anthropic Principle’, International Journal of Astrobiology, 4 (2005), 203–26.

³There is no good historical study of string theory, ranging from its birth in the late 1960s to the present. An interesting account of the development until the early 1990s is provided in Peter Galison, ‘Theory Bound and Unbound: Superstrings and Experiments’, in Laws of Nature: Philosophical, Scientific and Historical Dimensions, edited by Friedel Weinert (Berlin, 1995), 369–407. On the philosophical aspects, see Reiner Hedrich, ‘The Internal and External Problems of String Theory: A Philosophical View’, Journal of General Philosophy of Science, 38 (2007), 261–78 and Richard Dawid, ‘On the Conflicting Assessment of the Current Status of String Theory’, http://philsci-archive.pitt.edu/ (2008).

⁴Quoted in Charles Seife, ‘Physics Enters the Twilight Zone’, Science, 305 (2004), 464–65, p. 465.

⁶Edward A. Milne, Relativity, Gravitation and World-Structure (Oxford, 1935), 266.

⁵For Milne's cosmology and its associated methodology, see e.g. George Gale, ‘Cosmology: Methodological Debates in the 1930s and 1940s’, http://plato.stanford.edu/entries/cosmology-30s/ (2002) and Thomas Lepeltier, ‘Edward Milne's Influence on Modern Cosmology’, Annals of Science, 63 (2006), 471–81.

⁷Arthur S. Eddington, Relativity Theory of Protons and Electrons (Cambridge, 1936), 3. Eddington's attempt to establish a new paradigm of physics is notoriously difficult to comprehend. For an attempt to reconstruct his line of thought, see Clive W. Kilmister, Eddington's Search for a Fundamental Theory: A Key to the Universe (Cambridge, 1994).

⁸On this debate, see Helge Kragh, ‘Cosmo-Physics in the Thirties: Towards a History of Dirac Cosmology’, Historical Studies in the Physical Sciences, 13 (1982), 69–108 and G. Gale and Niall Shanks, ‘Methodology and the Birth of Modern Cosmological Inquiry’, Studies in History and Philosophy of Modern Physics, 27 (1996), 279–96.

⁹Herbert Dingle, ‘Modern Aristotelianism’, Nature, 139 (1937), 784–86, p. 785. The large number hypothesis states that the numerical regularities exhibited by very large dimensionless numbers (such as 10⁴⁰ and 10⁸⁰) constructed from the constants of nature express significant physical relations. Based on this principle, Dirac concluded that the constant of gravitation is not truly constant but decreases slowly in time.

¹⁰Max Born, Experiment and Theory in Physics (Cambridge, 1943), 44.

¹¹Hermann Bondi and Thomas Gold, ‘The Steady-State Theory of the Expanding Universe’, Monthly Notices of the Royal Astronomical Society, 108 (1948), 252–70, p. 254. The emergence and historical development of the steady-state theory, including the philosophical objections to it, is detailed in H. Kragh, Cosmology and Controversy: The Historical Development of Two Theories of the Universe (Princeton, NJ, 1996), where further references can be found.

¹²Interview by David DeVorkin, American Institute of Physics, 21 March 1978. Quoted in Kragh (note 11), 249.

¹³Gerald J. Whitrow, The Structure and Evolution of the Universe (New York, 1959), 138–41.

¹⁴G.J. Whitrow and H. Bondi, ‘Is Physical Cosmology a Science?’ British Journal for the Philosophy of Science, 4 (1954), 271–83, p. 279.

¹⁵H. Bondi, ‘The Philosopher for Science’, Nature, 358 (1992), 363.

¹⁶Ludwig Boltzmann, ‘On Certain Questions of the Theory of Gases’, Nature, 51 (1895), 413–15. For a recent analysis, see M.M. Ćirković, ‘The Thermodynamical Arrow of Time: Reinterpreting the Boltzmann-Schuetz Argument’, Foundations of Physics, 33 (2003), 467–90. A valuable historical review of multiple universe cosmologies, focusing on the modern period, is provided in Stefano Bettini, ‘A Cosmic Archipelago: Multiverse Scenarios in the History of Modern Cosmology’, ArXiv:physics/051011 (2005). The address of the ArXiv e-print archive, much used by physicists, is http://arxiv.org/

¹⁷A.S. Eddington, ‘The Expansion of the Universe’, Monthly Notices of the Royal Astronomical Society, 91 (1931), 412–16, p. 415. The Lemaître-Eddington model, originally proposed by Georges Lemaître in 1927, includes a positive cosmological constant and expands asymptotically from an Einstein state.

¹⁸Tokio Takeuchi, ‘On the Cyclic Universe’, Proceedings of the Physico-Mathematical Society of Japan, 13 (1931), 166–77. A single-cycle model was discussed by Alexander Friedmann in his classical paper of 1922, but without extending it to a series of many cycles. Takeuchi's model avoided cosmic singularities and thus allowed one universe to grow out of the fossils of the previous one. For a historical survey of cyclic models in relativistic cosmology, see H. Kragh, ‘Continual Fascination: The Oscillating Universe in Modern Cosmology’, Science in Context, 22 (forthcoming 2009).

¹⁹Richard C. Tolman, ‘Effect on Inhomogeneity in Cosmological Models’, Proceedings of the National Academy of Sciences, 20 (1934), 169–76, p. 175. Reprinted in General Relativity and Gravitation, 29 (1997), 935–43.

²⁰Jaroslav Pachner, ‘Dynamics of the Universe’, Acta Physica Polonica, 19 (1960), 662–73, p. 673.

²¹F. Hoyle and J. Narlikar, ‘A Radical Departure from the “Steady-State” Concept in Cosmology’, Proceedings of the Royal Society (London), 290 (1966), 162–76; F. Hoyle, Galaxies, Nuclei, and Quasars (New York, 1965), 131. On the Hoyle-Narlikar theory, see also Kragh (note 11), 366–68.

²²Brandon Carter, ‘Large Number Coincidences and the Anthropic Principle in Cosmology’, in Confrontation of Cosmological Theories with Observational Data, edited by Malcolm S. Longair (Dordrecht, 1974), 291–98. The general idea of the anthropic principle is that the observed universe is conditioned by the existence of complex life forms such as human observers. As a selection principle it claims that the constants and parameters of our universe are fine-tuned to permit the emergence of life, especially intelligent life. The principle exists in several versions and has given rise to a wealth of literature, both scientific, philosophical and theological.

²³Inflationary models assume the existence of a primordial ‘false vacuum’ which expands at a phenomenal rate and after a split second blows up the universe by a gigantic factor. At the end of the brief inflation phase, the false vacuum decays to a normal vacuum state and attractive gravity replaces the repulsive force. Although the basic ideas of inflationary cosmology were suggested in the late 1970s, the hypothesis only took on with Alan Guth's development of it in 1981. The early history of inflation is described in Chris Smeenk, ‘False Vacuum: Early Universe Cosmology and the Development of Inflation’, in The Universe of General Relativity, edited by A.J. Kox and Jean Eisenstaedt (Boston, 2005), 223–58. See also the critical review in John Earman and Jesus Mosterin, ‘A Critical Analysis of Inflationary Cosmology’, Philosophy of Science, 66 (1999), 1–49. The main reason for the nearly paradigmatic status of inflationary models in modern early-universe cosmology is that inflation leads to a density perturbation spectrum in excellent agreement with precision measurements of the cosmic background radiation. This is remarkable, but not a proof that an inflationary era actually occurred. The same result can be obtained without assuming the dynamical mechanism of inflation models: Stefan Hollands and Robert M. Wald, ‘An Alternative to Inflation’, General Relativity and Gravitation, 34 (2002), 2043–55.

²⁴J. Richard Gott, ‘Creation of Open Universes from de Sitter Space, Nature, 295 (1982), 304–5, p. 304. The quantity k denotes the curvature parameter which can attain the values +1 (closed universe), 0 (flat universe) or −1 (open universe).

²⁵Andrei Linde, ‘Eternally Existing Self-Reproducing Chaotic Inflationary Universe’, Physics Letters B, 175 (1986), 395–401, p. 399.

²⁶Alexander Vilenkin, ‘Anthropic Predictions: The Case of the Cosmological Constant’, in Universe or Multiverse? edited by Bernard Carr (Cambridge, 2007), 163–80, p. 163.

²⁷Alan H. Guth, The Inflationary Universe (Reading, MA, 1997), 252.

²⁸A.H. Guth, ‘Eternal Inflation and its Implications’, ArXiv:hep-th/0702178 (2007).

²⁹Quoted from an interview of 1989 in Alan I. Lightman and Roberta Brawer, Origins: The Lives and Worlds of Modern Cosmologists (Cambridge, MA, 1990), 151.

³⁰Raphael Bousso and Joseph Polchinski, ‘Quantization of Four-Form Fluxes and Dynamical Neutralization of the Cosmological Constant’, Journal of High Energy Physics, 06 (2000), 006. One of the most challenging problems of theoretical physics is to explain why the cosmological constant is so much smaller than the vacuum energy calculated by the quantum theory of elementary particles. This energy is at least 120 orders of magnitude greater than what is indicated by astronomical observations! For an accessible review of the string-based multiverse and other ideas of multiple universes, see P.C.W. Davies, ‘Multiverse Cosmological Models’, Modern Physics Letters, A 19 (2004), 727–44.

³¹Leonard Susskind, ‘The Anthropic Landscape of String Theory’, ArXiv:hep-th/0302219 (2003); L. Susskind, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design (New York, 2006). See also R. Bousso and J. Polchinski, ‘The String Theory Landscape’, Scientific American, 290 (September 2004), 60–69.

³²Susskind's notion of existence is problematic and probably not shared by most other physicists: ‘What physicists … mean by the term exist is that the object in question can exist theoretically. In other words, the object exists as a solution to the equations of the theory. By that criterion perfectly cut diamonds a hundred miles in diameter exist. So do planets made of pure gold. They may or may not actually be found somewhere, but they are possible objects consistent with the Laws of Physics’. Susskind 2006 (note 31), 177.

³³Laura Mersini-Houghton, ‘Thoughts on Defining the Multiverse’, ArXiv:0804.4280 (2008).

³⁴G. Gale, ‘Cosmological Fecundity: Theories of Multiple Universes’, in Physical Cosmology and Philosophy, edited by John Leslie (New York, 1990), 189–206.

³⁵Paul J. Steinhardt and Neil Turok, Endless Universe: Beyond the Big Bang (New York, 2007). See also Kragh 2009 (note 18).

³⁶On these problems, see George F.R. Ellis and G.B. Brundrit, ‘Life in the Infinite Universe’, Quarterly Journal of the Royal Astronomical Society, 20 (1979), 37–41. For a sharper version, based on inflation t heory, see Jaume Garriga and A. Vilenkin, ‘Many Worlds in One’, Physical Review D, 64 (2001), 043511.

³⁷M. Tegmark, ‘Parallel Universes’, Scientific American, 288 (May 2003), 41–51; M. Tegmark, ‘The Universe Hierarchy’, in Carr 2007 (note 26), 99–125.

³⁸Tegmark originally discussed his ‘ultimate ensemble theory’ in 1998, at that time without referring to the multiverse as justified by inflationary models. ‘Everything that exists mathematically exists physically’, he claimed, arguing that his radically Platonist theory has genuine predictive power and lives up to Popper's falsifiability requirement. M. Tegmark, ‘Is “the Theory of Everything” Merely the Ultimate Ensemble Theory?’ Annals of Physics, 270 (1998), 1–51, p. 2.

³⁹E.g., G.F.R. Ellis, U. Kirchner and W.R. Stoeger, ‘Multiverses and Physical Cosmology’, Monthly Notices of the Royal Astronomical Society, 347 (2004), 921–36.

⁴⁰Martin Rees, Our Cosmic Habitat (London, 2003), 172–75.

⁴¹D.W. Sciama, ‘Ist das Universum eigenartig?’, in Vom Urknall zum komplexen Universum, edited by Jürgen Ehlers, Gerhard Börner and Heinrich Meier (Munich: Piper, 1993), 183–94, pp. 192–94.

⁴²Tegmark 1998 (note 38), 44 and Tegmark 2007 (note 26), 123.

⁴³Ellis, Kirchner and Stoeger 2004 (note 39), p. 932. Similarly in W.R. Stoeger, G.F.R. Ellis and U. Kirchner, ‘Multiverses and Cosmology: Philosophical Issues’, ArXiv:astro-ph/0407329 (2006) and G.F.R. Ellis, ‘Multiverses: Description, Uniqueness, and Testing’, 387–410 in Carr 2007 (note 26).

⁴⁴Discussed as much in religious and philosophical contexts as in a scientific context, the problem of infinities in cosmology goes back a long time. For example, it was seen as a major problem in the late-nineteenth century controversy about a universe governed by the law of entropy increase. Generally speaking, whereas materialists insisted that the universe is spatially infinite, scientists of a theist inclination were in favour of a finite universe. H. Kragh, Entropic Creation: Religious Contexts of Thermodynamics and Cosmology (Aldershot, 2008), 93–96. Some of the arguments discussed in modern cosmology can be found almost identically in the older literature.

⁴⁵Rees 2003 (note 40), 166; M. Rees, ‘Cosmology and the Multiverse’, in Carr 2007 (note 26), 57–76.

⁴⁶Rees 2007 (note 45), 63.

⁴⁷Stoeger, Ellis and Kirchner 2006 (note 43), 28.

⁴⁹Mario Livio and M.J. Rees, ‘Anthropic Reasoning’, Science, 309 (2005), 1022–23, p. 1023

⁴⁸Don Page, ‘Predictions and Tests of Multiverse Theories’, in Carr 2007 (note 26), 411–30, p. 413.

⁵⁰B. Carr and G.F.R. Ellis, ‘Universe or Multiverse?’, Astronomy & Geophysics, 49 (2008), 2.29–2.37, p. 2.34; G.F.R. Ellis, ‘Dark Matter and Dark Energy Proposals: Maintaining Cosmology as a True Science?’, ArXiv:astr-ph/0811.3529 (2008).

⁵¹Tegmark 2007 (note 37), 105.

⁵²Anthony Aguirre, ‘Making Predictions in a Multiverse: Conundrums, Dangers, Coincidences’, in Carr 2007 (note 26), 323–66.

⁵³A. Vilenkin, ‘Unambiguous Probabilities in an Eternally Inflating Universe’, Physical Review Letters, 81 (1998), 5501–04.

⁵⁴Aurélien Barrau, ‘Physics in the Universe’, Cern Courier, 20 November 2007.

⁵⁵These are the four criteria for a good theory proposed in G.F.R. Ellis, ‘The Unique Nature of Cosmology’, in Revisiting the Foundations of Relativistic Physics, edited by Abhay Ashketar et al. (Dordrecht, 2003), 193–220, p. 212.

⁵⁶Barrau 2007 (note 54).

⁵⁷Steven Weinberg, ‘Living in the Multiverse’, in Carr 2007 (note 26), 29–42.

⁵⁸Nick Bostrom, ‘Self-Locating Belief in Big Worlds: Cosmology's Missing Link to Observation’, Journal of Philosophy, 99 (2002), 607–23.

⁵⁹Susskind 2006 (note 31), 375. The issue in the multiverse debate focuses on observational testing. Physicists active in quantum gravity research, including string theory, often speak of testing in a different sense, for example if results of classical general relativity can be derived as an approximation to a particular theory of quantum gravity. This is neither unusual nor controversial. It was an important test af Einstein's theory of gravitation that it led to Newton's theory in the limit of weak gravitational fields.

⁶⁰W.R. Stoeger, ‘Retroduction, Multiverse Hypotheses and their Testability’, ArXiv:astro-ph/0602356 (2006); Stoeger, Ellis and Kirchner 2006 (note 43).

⁶¹For arguments in favour of retroduction in history and philosophy of science, see Ernan McMullin, The Inference that Makes Science (Milwaukee: Marquette University Press, 1992) and Russell N. Hanson, Patterns of Discovery (Cambridge, 1969), 85–90.

⁶³Carr and Ellis 2008 (note 50), 2.33.

⁶²Ellis, Kirchner and Stoeger 2004 (note 39), 935. See also G.F.R. Ellis, ‘Issues in the Philosophy of Cosmology’, in Philosophy of Physics, edited by Jeremy Butterfield and John Earman (Amsterdam, 2007), 1183–285, p. 1265. This is also the argument in Ronald G. Larson, ‘Is “Anthropic Selection” Science?’, Physics in Perspectives, 9 (2007), 58–69, who notes the similarity of multiverse thinking to Eastern religions. Similar accusations have for a couple of decades been directed at string theory, which according to some antagonists tends to replace science with faith and should belong to either departments of mathematics or schools of divinity. See quotations in Galison 1995 (note 3).

⁶⁴Carr and Ellis 2008 (note 50), 2.35.

⁶⁵Peter Woit, Not Even Wrong (London, 2007), 242.

⁶⁶Lee Smolin, The Trouble with Physics (London, 2008), 170.

⁶⁷Carr and Ellis 2008 (note 50), 2.37.

⁶⁸Steinhardt and Turok 2007 (note 35).

⁶⁹P.J. Steinhardt and N. Turok, ‘The Cyclic Model Simplified’, ArXiv:astro-ph/0404480 (2004).

⁷⁰E. McMullin, ‘Is Philosophy Relevant to Cosmology?’ American Philosophical Quarterly, 18 (1981), 177–89.

⁷¹Ellis, Kirchner and Stoeger 2004 (note 39), 928. Of course, the insight that some cosmological problems are inevitably connected with philosophical choices is neither new nor limited to multiverse scenarios. For example, philosophical problems naturally arise from the traditional understanding of the universe as a unique object. See Ellis 2003 (note 55) and literature mentioned therein.

⁷²Charles G. Darwin, ‘Physical Science and Philosophy’, Nature, 139 (1937), 1008. Notice that Darwin's advocacy of ‘foul means’ was restricted to the process of discovery and did not refer to the context of justification.

⁷⁵L. Susskind and L. Smolin, ‘Smolin vs. Susskind: The Anthropic Principle’, Edge 145 (18 August 2004), http://edge.org/documents/archive/edge145.html.

⁷³Barrau 2007 (note 54).

⁷⁴Susskind 2006 (note 31), 196.

⁷⁶Nancy Cartwright and Roman Frigg, ‘String Theory under Scrutiny’, Physics World, 3 September 2007 (online edition).

⁷⁷Sven O. Hansson, ‘Falsificationism Falsified’, Foundations of Science, 11 (2006), 275–86.

⁷⁸Robert Matthews, ‘Some Swans Are Grey’ New Scientist, 198 (10 May 2008), 44–47. A detailed exposition of scientific method from the standpoint of Bayesianism appears in Colin Howson and Peter Urbach, Scientific Reasoning: The Bayesian Approach (Chicago, 1993).

⁷⁹V. Palonen, ‘Bayesian Considerations on the Multiverse Explanation of Cosmic Fine-Tuning’, ArXiv:0802.4013 (2008). See also Earman and Mosterin 1999 (note 23), 34–35.

⁸⁰Dudley Shapere, ‘Testability and Empiricism’, in The Reality of the Unobservable (Dordrecht, 2000), edited by Evandro Agazzi and Massimo Pauri, 153–64, p. 161. Among the examples Shapere refers to, are theories which speak of ‘other regions of the universe, or even other universes, which are forever causally unconnected with ours’ (p. 153).

⁸¹The relevance of Popper's philosophy for astronomers and cosmologists is demonstrated in Benjamin Sovacool, ‘Falsification and Demarcation in Astronomy and Cosmology’, Bulletin of Science, Technology & Society, 25 (2005), 53–62. For the significance of the falsifiability criterion during the cosmological controversy in the 1950s, and its use by steady-state cosmologists in particular, see Kragh 1996 (note 11), 244–50.

⁸²Andrew Yang, ‘Matters of Demarcation: Philosophy, Biology, and the Evolving Fraternity Between Disciplines’, International Studies in the Philosophy of Science, 22 (2008), 211–25.

⁸³Kevin G. Helfenbein and Rob DeSalle, ‘Falsification and Corroboration: Karl Popper's Influence on Systematics’, Molecular Phylogenetics and Evolution, 35 (2005), 271–80, p. 279.

⁸⁴Susskind 2006 (note 31), 196.

⁸⁵ http://www.talkorigins.org/faqs/edwards-v-aguillard/amicus1.html. The anti-creationism coalition included 72 Nobel laureates (including Steven Weinberg), 17 state academies of science, and seven other scientific organizations.

⁸⁶G.F.R. Ellis, ‘Cosmology Down the Ages’, Journal for the History of Astronomy, 39 (2008), 537–38.

⁸⁷Silvio A. Bonometto, ‘Modern and Post-Modern Cosmology’, in Historical Developments of Modern Cosmology, edited by Vicent J. Martínez, Virginia Trimble and Maria J. Pons-Bordería (San Francisco, 2001), 219–36. Post-modern science is related to so-called ironic science, as described in John Horgan, The End of Science (New York, 1997), 7, 30–31.

⁸⁸Bernard Carr, as quoted in Tim Folger, ‘Science's Alternative to an Intelligent Creator: The Multiverse Teory’, Discover Magazine (December 2008), online version.

⁸⁹M. Rees, ‘Explaining the Universe’, in Explanations: Styles of Explanation in Science, edited by John Cornwell (Oxford, 2004), 39–66; Livio and Rees 2005 (note 49). See also Frank Wilczek, ‘Enlightenment, Knowledge, Ignorance, Temptation’, in Carr 2007 (note 26), 43–56.

⁹⁰Quoted in Folger 2008 (note 88). See also Susskind 2006 (note 31), 380 and Weinberg 2007 (note 57), 39.

⁹¹Robin Collins, ‘The Multiverse Hypothesis: A Theistic Perspective’, in Carr 2007 (note 26), 459–80.

⁹²H. Dingle, ‘Deductive and Inductive Methods in Science. A Reply’, Nature, 139 (1937), 1011–12, p. 1012.