Journal of Cosmology, 2010, Vol 4, pages 664-673.
Cosmology, December 28, 2009
Classification, Cause, Challenge, Controversy, and Criticism
Rüdiger Vaas,
Center for Philosophy and Foundations of Science, University of Giessen, Germany
1. Introduction
Why is there something rather than nothing? And why is that which is, the way it is? These two questions are among the most fundamental (meta)physical mysteries currently debated in cosmology. The multiverse hypothesis represents, at least in part, an attempt to answer them.
The central tenant of the multiverse hypothesis (henceforth called M) is that our universe is just one among an ensemble, an infinitesimal part of an elaborate structure which consists of numerous universes, possibly an infinite number of universes. Thus M has been conceptualized and described by a variety of terms including "many worlds", "parallel worlds", "parallel universes", "alternative universes", "alternative realities", "alternative dimensions", "quantum universes", and so on.
Some of these "universes" could have a physics or chemistry completely unlike our own, and this has led to additional questions, such as: is our universe fine tuned for life, and if so, why?
No approach can possibly provide an exhaustive or ultimate answer to these questions (Vaas 2006), and neither does M seek to do that. But if empirically confirmed in some way, or rigorously derived theoretically, M would be one of the most radical and far-reaching insights in the history of science. Indeed, it may turn out that M is absolutely necessary for an understanding of why the big bang occurred, why (some of) the laws of nature and the values of certain physical constants are as they are, why this universe appears to be fine-tuned for life, and why there is an arrow of time. M could well be a derivative of the ultimate explanans.
Possibly precisely because of these exciting prospects M is controversial and under attack not just from sceptical scientists, but theologians and critical philosophers of science (reviewed in Vaas 2008a, 2010). This is not surprising and, indeed, it is to be appreciated because extraordinary claims which have the potential to overturn and usher in a new world scientific order, require extraordinary evidence. And considerable evidence remains to be discovered.
This paper discusses the current status of M and provides a classification of different multiverse types, categorized according to the way in which individual universes are assumed to be separated or not separated from another. Then the main reasons for assuming the existence of other universes (empirical, theoretical, and philosophical) are reviewed. It is also argued that criticism of the various multiverse scenarios as "unscientific", and the insistance on a narrow understanding of falsification, are neither appropriate nor convincing from a philosophy of science point of view.
2. Different Notions of "Universe" and "Multiverse"
"The universe is a big place, perhaps the biggest" wrote Kurt Vonnegut, the well-known visionary and author of science fiction. But science is often stranger than fiction, and according to M, the universe is only a tiny part of what exists.
However, this opens up both conceptual and empirical problems which in part have to do with language, and perhaps the limitations of the human mind. Specifically, the term "universe" (or "world"), as it is used today, has many different meanings (Vaas 2004a) such as:
(1) Everything (physically) in existence, ever, anywhere, in totality. According to this defintion there are no other universes.
(2) The observable region of the cosmos that we can observe and which we inhabit (the Hubble volume, almost 100 billion light years in diameter), plus everything that has interacted or will ever interact with this region.
(3) Any gigantic system of causally interacting things that is wholly (or to a very large extent, or for a long time) isolated from others; sometimes such a locally causally connected collection is called a multi-domain universe, <.i> consisting of the ensemble of all sub-regions of a larger connected spacetime, the "universe as a whole", and this is opposed to the multiverse in a stronger sense, i.e. the set of genuinely disconnected universes, which are not causally related at all.
(4) Any system that might well have become gigantic, etc., even if it does in fact recollapse while it is still very small.
(5) Other branches of the wavefunction (if it never collapses, cf. Wheeler & Zurek 1983, Barrett 1999, Vaas 2001) in unitary quantum physics, i.e. different histories of the universe (e.g. Gell-Mann & Hartle 1990 & 1993) or different classical worlds which are in superposition (e.g. DeWitt & Graham 1973).
(6) Completely disconnected systems consisting of universes in one of the former meanings, which do or do not share the same boundary conditions, constants, parameters, vacuum states, effective low-energy laws, or even fundamental laws, e.g. different physically realized mathematical structures (cf. Tegmark 2004, 2010).
Therefore, "multiverse" (or "world" as a whole) can be used to refer to everything in existence (at least from a physical point of view), while the term "universe" can refer to one of several universes (worlds) within the multiverse.
In principle, these universes – mostly conceived in the meaning of (2), (3), or (4) – might or might not be spatially, temporally, dimensionally, causally, nomologically and/or mathematically separated from each other (see Table 1). Thus, sharp boundaries do not necessarily exist between them.
One might call the whole set of different universes the multiverse. But it could be true that there are even different sets of totally spatiotemporally and strictly causally separated multiverses, e.g. different bunches of chaotically inflating multiverses. In that case it remains useful to have a term with a still broader meaning, namely omniverse or cosmos. Thus, omniverse or cosmos could be taken as the all-embracing term for everything in existence which might or might not be the set of different multiverses, while the multiverse refers to and consists of different universes which are not separated in every respect.
3. Towards a Multiversal Taxonomy
Although most believe that multiverse classifications should be abstract enough to include all possible cosmological scenarios, there is no general agreement as to exactly what that should be. For example, it has been suggested that the multiverse should be categorized with regard to separation/distinction of the different universes (Vaas 2004a; Table 1 is an extension of this). Yet other categorizations include four different levels (Tegmark 2004) or three different types (Mersini-Houghton 2010) (cf. also Ellis, Kirchner & Stoeger 2004). Of course, definitions and taxonomies are just conceptual issues. Nevertheless, they are needed for clarification and to avoid misunderstandings, though categories in-themselves explain and prove nothing. Nevertheless these issues must be discussed and, in the long run, shown, whether they are useful, sufficiently complete, and not too arbitrary.
4. Is Our Universe Fine-Tuned?
Life exists in this universe. But does it exist in other universes, and if so, are those life-forms similar to those which are typical of Earth? We cannot, as yet, answer these latter questions. However, the fact that life exists at all raises its own fundamental questions, such as, why are the physical and chemical features of this universe compatible with life? Why does this universe appear to be "fine tuned," that is, why are the laws and constants of nature and certain boundary conditions narrowly tailored to those constants which are ideal for life (e.g. Carr 2007; Leslie 1989; Vaas 2004b); for indeed, if these fundamental constants varied by just a few degrees, life as we know it would not be possible.
Life as we know it depends crucially on the laws and constants of nature as well as a variety of boundary conditions which allowed intelligent life to evolve. Nevertheless it is difficult to judge how "fine-tuned" this universe really is, both because it is unclear how modifications of many values together might compensate each other and whether laws, constants and initial conditions really could have been otherwise to begin with. It is also unclear how specific and improbable those values need to be in order for information-processing structures – and, hence, intelligent observers – to develop. If we accept, for the sake of argument, that at least some values are fine-tuned, we must ask, "why" and how this can be explained.
In principle, there are many options for answering this question (Table 2). Fine-tuning might (1) just be an illusion if life could adapt to very different conditions or if modifications of many values of the constants would compensate each other; or (2) it might be a result of (incomprehensible, irreducible) chance, and thus inexplicable (Vaas 1993); or (3) it might be nonexistent because nature could not have been otherwise, and with a fundamental theory we would be able to prove this; or (4) it might be a product of selection: either observational selection within a vast multiverse of (infinitely?) many different realizations of those values (weak anthropic principle), or a kind of cosmological natural selection making the measured values (compared to possible other ones) quite likely within a multiverse of many different values, or even a teleological or intentional selection. Even worse, these alternatives are not mutually exclusive – for example it is logically possible that there is a multiverse, created according to a fundamental theory by a cosmic designer who is not self-sustaining, but ultimately contingent, i.e. an instance of chance.
5. Unique-universe versus multiverse accounts
Unique-universe accounts take our universe as the only one (or at least the only one ever relevant for cosmological explanations and theories). It might have had a predecessor or even infinitely many (before the big bang) and/or a successor or infinitely many (after a big crunch), but then the whole series can be taken as one single universe with spatiotemporal phase transitions. From the perspective of simplicity, parsimony and testability it is favorable to try to explain as much as possible with a unique-universe account. A both straightforward and very ambitious approach is the searching for a fundamental theory with just one self-consistent solution that represents (or predicts) our universe. (Of course one could always argue that there are other, strictly causally separated universes too, which do not even share a common generator or a meta-law; but then they do not have any explanatory power at all and the claims for their existence cannot be motivated in a scientifically useful way, only perhaps by philosophical arguments.)
Future theories of physics might reveal the relations between fundamental constants in a similar way as James Clerk Maxwell did by unifying electric and magnetic forces: he showed that three seemingly independent constants – the velocity of light c, the electric constant eo (vacuum permittivity), and the magnetic constant μo (vacuum permeability) – are connected with each other: c = (μo . eo)-0,5. Indeed some candidates for a grand unified theory of the strong, weak and electromagnetic interaction suggest that most of the parameters in the standard model of particle physics are mathematically fixed, except for these three: a coupling constant (the electromagnetic fine-structure constant) and two particle masses (namely that of down and up quarks) (Hogan 2000). A promise of string theory is to get rid of any free parameter – and if this can be accomplished all constants could be calculated from first principles (Kane et al. 2002). Although at present this remains wishful thinking, it is a direction very worth following and, from a theoretical and historical point of view, perhaps the most promising.
So even without an ultimate explanation, fine-tuning might be explained away within a (more) fundamental theory. Most of the values of the physical constants should be derived from it, for example. This would turn the amazement about the anthropic coincidences into insight – like the surprise of a student about the relationship eiπ = -1 between the numbers e, i and π in mathematics is replaced by understanding once he comprehends the proof. Perhaps the fact that the mass of the proton is 1836 times the mass of the electron could be similarly explained. If so, this number would be part of the rigid formal structure of a physical law which cannot be modified without destroying the theory behind it. An example for such a number is the ratio of any circle's circumference to its diameter. It is the same for all circles in Euclidean space: the circular constant π.
But even if all dimensionless constants of nature could be reduced to only one, a pure number in a theory of everything, its value would still be arbitrary, i.e. unexplained. No doubt, such a universal reduction would be an enormous success. However, the basic questions would remain: Why this constant, why this value? If infinitely many values were possible, then even the multitude of possibilities would stay unrestricted. So, again, why should such a universal constant have the value of, say, 42 and not any other?
If there were just one constant (or even many of them) whose value can be derived from first principles, i.e. from the ultimate theory or a law within this theory, then it would be completely explained or reduced at last. Then there would be no mystery of fine-tuning anymore, because there never was a fine-tuning of the constants in the first place. And then an appreciable amount of contingency would be expelled.
But what would such a spectacular success really mean? First, it could simply shift the problem, i.e. transfer the unexplained contingency either to the laws themselves or to the boundary conditions or both. This would not be a Pyrrhic victory, but not a big deal either. Second, one might interpret it as an analytic solution. Then the values of the constants would represent no empirical information; they would not be property of the physical world, but simply a mathematical result, a property of the structure of the theory. This, however, still could and should have empirical content, although not encoded in the constants.
Otherwise fundamental physics as an empirical science would come to an end. But an exclusively mathematical universe, or at least an entirely complete formal description of everything there is, derivable from and contained within an all-embracing logical system without any free parameter or contingent part, might seem either incredible (and runs into severe logical problems due to Kurt Gödel's incompleteness theorems) or the ultimate promise of the widest and deepest conceivable explanation. Empirical research, then, would only be a temporary expedient like Ludwig Wittgenstein's (1922, 6.54) famous ladder: The physicist, after he has used empirical data as elucidatory steps, would proceed beyond them. "He must so to speak throw away the ladder, after he has climbed up on it." That there is no contingency at all seems very unlikely. So why are some features realized but not others? Or, on the contrary, is every feature realized? Both questions are strong motivations for M.
Multiverse accounts assume the existence of other universes as defined above and characterized with respect to their separation in Table 1. This is no longer viewed as mad metaphysical speculation beyond empirical rationality. There are many different multiverse accounts (see, e.g., Carr 2007; Davies 2004; Deutsch 1997; Linde 2008; Mersini-Houghton 2010; Rees 2001; Smolin 1997; Tegmark 2004, Vaas 2004b, 2005, 2008b, 2010; Vilenkin 2006) and even some attempts to classify them quantitatively (see, e.g., Deutsch 2001 for many-worlds in quantum physics, and Ellis, Kirchner & Stoeger 2004 for physical cosmology). They flourish especially in the context of cosmic inflation (Aguirre 2010; Linde 2005, 2008, Vaas 2008b), string theory (Chalmers 2007, Douglas 2010) and a combination of both as well as in different quantum gravity scenarios that seek to resolve the big bang singularity and, thus, explain the origin of our universe (for recent reviews see Vaas 2010).
If there is a kind of selection, new possibilities emerge within M. One possibility is observational selection (Barrow & Tipler 1986, Leslie 1989, Vaas 2004b). Another is a kind of cosmological natural selection making the measured values (compared to possible other ones) quite likely within a multiverse of many different values (García-Bellido 1995, Smolin 1992, 1997 & 2010, for a critical discussion e.g. Vaas 1998 & 2003). Finally there is teleological or intentional selection (e.g. Ansoldi & Guendelman 2006; Harrison 1995; Leslie 1989; Vidal 2008 & 2010, for a critical discussion e.g. Vaas 2004b, 2009ab).
The strongest version of M is related to the principle of plentitude or principle of fecundity (advocated e.g. by Richard Feynman, Dennis Sciama). According to this principle everything is real, if it is not explicitly forbidden by laws of nature, e.g. symmetry principles. As Terence H. White wrote in his novel The Once and Future King (1958): "everything not forbidden is compulsory". But the question remains: What is forbidden, i.e. physically or nomologically not possible and thus not "allowed" by natural laws? (This is a slippery slope, as Paul Davies 2007 argued: could there even be universes containing magic, a theistic God, or simulations of every weird fantasy? Otherwise some restrictions are necessary!) And is it really true that everything which is physically or nomologically possible is also realized somewhere? Answers to these questions are not known. (It is also unclear whether "metaphysically possible" makes truly sense in contrast to both "logically possible" and "physically possible", cf. Meixner 2008.)
6. Why (Should) We Believe in Other Universes (?)
There are – or could be – at least three main reasons for assuming the existence of other universes: empirical evidence, theoretical explanation, and philosophical arguments (Table 3). These three reasons are independent from each other, but ideally entangled. As far as there is at least some connection with or embedding into a theoretical framework of physics or cosmology, M is part of the scientific endeavour, not only of philosophy. This is also the case in the absence of empirical evidence, if a theoretical embedding exists. And philosophical arguments might at least motivate scientific speculation (Table 4).
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7. Falsificationism is Both Too Much and Not Enough
"Scientific statements, refering to reality, must be falsifiable", Karl R. Popper (1930-1933, 1935). Falsifiability has been one of the most important and successful criteria of science. Popper understood it as a demarcation criterium of science (in contrast to metaphysics, logic, pseudoscience...). But what is usually meant here is falsifiability of theoretical systems, or of parts of such a system, not of single statements.
So there is an important distinction: Scientific laws on the one hand must be falsifiable. Take for example Newton’s law – it can be tested, was tested rigorously indeed, and is still under investigation (at very large and very small scales). Hypothetical universal existential statements on the other hand need not and cannot be falsified, but must be verified. Take for instance the discovery of the last two naturally occurring stable elements: Hafnium, atomic number 72, was detected by Dirk Coster and George de Hevesy in 1922 through Xray spectroscopy analysis, after Niels Bohr had predicted it, or its properties. (Actually Dimitri Mendeleev had already predicted it implicitely in 1869, in his report on The Periodic Law of the Chemical Elements). Similarly, Rhenium, atomic number 75, was found by Walter Noddack, Ida Tacke, and Otto Berg in 1925, after Henry Moseley had predicted it in 1914.
In science verification is obviously extremely important. But falsification is not sufficient to disprove an idea in controversial and hypothetical situations even if there are concise predictions. That is, "absence of evidence is not evidence of absence." Otherwise fictive ghosts or unicorns would also be part of science, for we cannot know a priori whether they exist or not, but we can imagine ways to detect them; that is to verify their existence in principle. So what is missing here? It is theoretical embedding. To be reasonably part of science, hypothetical universal existence statements must not only be verifiable, they must also be part of a sufficiently confirmed or established scientific theory or theoretical framework. That was the case in the Hafnium and Rhenium examples, where Mosley and Bohr had their theoretical models about the atomic numbers and the properties of the atoms, explaining regularities in the periodic table of the chemical elements.
Statements about the existence of other universes are not like statements about scientific laws. The latter must be falsifiable, while the former should be taken as universal existential statements which cannot be falsified, but must be verified. So ultimately a multiverse scenario might only be accepted, strictly speaking, if there is empirical evidence for it, i.e. observational data of another universe or its effects. A weaker argument for M would be if a – falsifiable, rigorously tested – theory predicts the existence of other universes, and this theory is well established according to the usual scientific criteria. Still weaker are philosophical reasons; whether they could suffice if they are stronger than alternative statements is a controversial issue and lies at – or beyond – the boundaries of physics and cosmology.
8. Research Programs and Systematicity
From a philosophy of science perspective an exaggerated falsificationism is not useful. As Imre Lakatos (1976 & 1977) pointed out, the scientific endeavour usually takes place in the form of research programs (which include not only the main assumptions and theoretical ideas but also methodological issues) rather than in respect of single hypotheses or theories. And refutations are not a straightforward sign of empirical progress, and this is because research programs grow in a permanent ocean of anomalies.
First, data are theory-dependent, not just empirical; they might simply be false or interpreted wrongly. Thus, theories should not been thrown away too quickly. For example the measured anomalies in Uranus' orbit were not a falsification of Newton’s law of gravity but an indication of Neptune's existence and the starting point for its discovery.
Second, theories are often ahead of data. Thus, theorists should make testable predictions of course, and observers and experimenters should obtain data. Supersymmetry models, for instance, were theories in search of applications initially, and physicists (and even mathematicians) learned a lot from them although there has still not been a single supersymmetric particle discovered (yet). So testability, a falsifiable prediction, is important, but not necessarily at least at the initial stages of theory building. Thus, theories must be given a chance to develop, to be purged of errors, to become more complete.
What Lakatos advocated was, therefore, a sophisticated falsificationism, referring to a (quasidarwinistic) struggle between theories and data interpretation. Given initial theoretical or empirical contradictions, anomalies or data deviances occur. Nevertheless, it is reasonable to keep the theory for a while, especially its "hard core" (at least if there is no alternative – an experimentum crucis is very rarely accomplished). Instead of killing the theory, the "protective belt" of auxiliary hypotheses should be modified first. Often they are taken as responsible for failed predictions too. Of course this effort to save the core could lead to immunization strategies which would in the end expel the theory from science. Furthermore, falsification is often difficult to achieve because core commitments of scientific theories are rarely directly testable and predictive without further assumptions. That is, a theory grows or becomes modified with the discovery of additional evidence.
Over time and with the accumulation of evidence both pro and con, some research programs are more successful than others. Therefore, ad hoc hypotheses to save a hard core could led to program degenerations – especially if they are problem shifts not pointing to other fruitful areas.
A progressive research program is characterized by its growth, along with stunning novel facts being discovered, more precise predictions being made, new experimental techniques being developed. By contrast, a degenerating research program is marked by lack of growth, or by an increase of the protective belt that does not lead to novel facts.
Cosmology provides a lot of examples for these complex interplays between competing theories, data acquisition and interpretation, immunization ("epicycles" are proverbial now), and even paradigm changes. The most prominent were geocentrism versus heliocentrism, metagalaxy versus island universe, and steady state versus big bang. Universe versus multiverse seems to be the next challenge in this direction.
Though the demarcation criteria for science have sometimes been criticized (Laudan 1988) and are somewhat fuzzy indeed, it is usually rationality that rules. And there are many well established criteria for successful research programs, especially the following: many applications; novel predictions; new technologies; answering unsolved questions; consistency; elegance; simplicity; explanatory power/depth; unification of distinct phenomena; and truth (but how can we know the latter – only via the criteria mentioned above?).
These criteria are also useful in analysing modern theoretical cosmology beyond the falsifiability doctrine. Are statements about the "cause" of the big bang and about other universes unscientific if they cannot provide falsifiable predictions (yet)? No. At least not if they are seen as part of a research program which is otherwise well-established and at least partly confirmed by observations or experiments. And indeed (not all, but at least) some of the just-mentioned criteria are fulfilled by the ambitious big bang and multiverse scenarios: many applications? well, no; novel predictions? perhaps yes, partly; new technologies? definitely not (yet); answering unsolved questions? yes; consistency? hopefully; elegance? depends on taste; simplicity? often yes; explanatory power/depth? certainly yes; unification of distinct phenomena? yes indeed; truth? nobody knows, but this is of course the main source of the controversies.
Last but not least there are other approaches to science. For example science can, negatively, characterized by comparing it with pseudoscience (table 5, see Vaas 2008).
| Indicators of serious science: – no vague, exaggerated, obscure claims – no undefined, vague or ambiguous vocabulary – embedding in established scientific theories – embedding in established scientific research program – logical (internal) coherence and consistence – open source: no secret data, methods, knowledge... – Occam’s razor – methodological reflexivity – no questionable "factoids" (Norman Mailer) – no reversal of the burden of proof – testability (verification, falsification) – rigorously derived predictions – no overemphasis of verifications, anecdotes, rumours, ignorance... – not belief, faith, hope, obedience, but observations, measurements, arguments, mathematical reasoning, inference to the best explanation... – replication, reproduction of measurements, calculations... – statistical significance, double-blind studies ... – distinction between correlation and causality – progress, self-corrections, revisions, error analysis – publications in scientific journals etc. (peer reviewed) – quotations of scientific literature, no dubious references – no selective quotes of obsolete or questionable experiments – demarcation of popular science – demarcation of pseudoscepticism – systematicity |
9. Outlook
Perhaps in a hundred years cosmologists will look back at the scientists of today and ask how we could have been so blind as not to see or accept the signs of other universes, or how we could have been so crazy in our beliefs in (a science of) other universes. Right now however it is an open issue, and therefore not unreasonable to defend and advance a scientific analysis of M. At least in principle there are some possibilities both for a verification of other universes understood as hypothetical universal existential claims and for theoretical embeddings of those claims. This should remind us of what Steven Weinberg (1977) wrote long ago: "our mistake is not that we take our theories too seriously, but that we do not take them seriously enough."
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