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Philosophy and Cosmology Oxford Handbooks Online Philosophy and Cosmology Claus Beisbart The Oxford Handbook of Philosophy of Science (Forthcoming) Edited by Paul Humphreys Subject: Philosophy, Philosophy of Science Online Publication Date: Aug DOI: 10.1093/oxfordhb/9780199368815.013.36 2015 Abstrac...

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Philosophy and Cosmology

Oxford Handbooks Online Philosophy and Cosmology Claus Beisbart The Oxford Handbook of Philosophy of Science (Forthcoming) Edited by Paul Humphreys

Online Publication Date: Aug 2015

Subject: Philosophy, Philosophy of Science DOI: 10.1093/oxfordhb/9780199368815.013.36

Abstract and Keywords Cosmological questions (e.g., how far the world extends and how it all began) have occupied humans for ages and given rise to numerous conjectures, both within and outside philosophy. To put to rest fruitless speculation, Kant argued that these questions move beyond the limits of human knowledge. This article begins with Kant’s doubts about cosmology and shows that his arguments presuppose unreasonably high standards on knowledge and unwarranted assumptions about space-time. As an analysis of the foundations of twentieth-century cosmology reveals, other worries about the discipline can be avoided too if the universe is modeled using Einstein’s general theory of relativity. There is now strong observational support for one particular model. However, due to underdetermination problems, the big cosmological questions cannot be fully answered using this model either. This opens the space for more speculative proposals again (e.g., that the universe is only part of a huge multiverse). Keywords: space-time, general theory of relativity, models, underdetermination, multiverse, limits of knowledge, Kant, universe, cosmology

1. Introduction Cosmology is the study of the cosmos or of the universe. The universe, in turn, is, or comprises, all there is. Cosmology is thus a far-reaching endeavor. Ideally, it would take us to the limits of what there is. But we may get stuck within narrower limits of what we can know and of what science can do. The aim of this article is to explore the limits of being, knowledge, and science, insofar as they manifest themselves in cosmology. When we speak of cosmology today, we mostly refer to a discipline of empirical science, which is sometimes also called “physical cosmology.” But cosmology was not always considered a subdiscipline of physics, and questions that were, or may be, called “cosmological” have occupied philosophers quite a bit (cf. Kragh, 2007). The preSocratic philosophers such as Thales claimed that water or something else is the arché (i.e., the origin or principle) of everything. Plato’s dialogue “Timaios” draws a picture of the structure of the visible world. Much later, philosopher Christian Wolff distinguished between an empirical, physical cosmology and a so-called cosmologia generalis, which was considered part of metaphysics and supposed to derive truths about the world of material objects from ontology (1731: Sections 1–4). That philosophers have displayed interest in cosmological questions should not come as a surprise. Clearly, if cosmology is about all there is, we had better be part of the game, particularly because some philosophical disciplines, particularly metaphysics, try to think about everything too, and philosophical questions about what there is and how it is structured naturally lead to an interest in the structure of the material world as investigated by cosmology. Also, for a long time, any inquiry about the structure of the world suffered from the scarcity of observational data and a lack of well-supported physical theories. Consequently, philosophical arguments and assumptions were needed to decide the questions.

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Philosophy and Cosmology In the meantime, things have changed. In his “Critique of Pure Reason,” Immanuel Kant famously smashed metaphysical cosmology by arguing that questions about the universe lead human reason into a deep problem, which he called the “antinomy of pure reason.” Whatever the merits of Kant’s arguments, there are few philosophers since then who have tried to do cosmology. Physical cosmology, by contrast, has made great progress after Albert Einstein (1917) first applied the field equations of his general theory of relativity (GTR) to the universe. Today, physical cosmology is a thriving field that is guided by observation and that has made a lot of progress. It may thus be concluded that cosmology is, and should be, an empirical science and that philosophers cannot make any substantial contribution to answering questions about the universe. Philosophical curiosity and speculation may have been legitimate and perhaps even useful as a source of inspiration in the early days of cosmology when no one could have dreamed of the techniques that present-day observational cosmology employs. But today, there is just no point in philosophers’ addressing questions about cosmology, or so the view is. If this is right, then philosophers, if interested in cosmology at all, should content themselves with clarifying the basic concepts of cosmology and scrutinizing the logical structure of our cosmological knowledge but otherwise remain silent. But this is not what all philosophers did. There is a notable philosophical tradition of questioning our abilities to learn what the universe is like. It is Kant again who set a famous example; his argument in the “Critique of Pure Reason” is not just directed against metaphysical cosmology but is also meant to put doubts on an empirical science of the universe. The same tradition is manifest in more recent claims that science will never be able to determine whether or not the age of the universe is finite (Scriven 1954). Work of this sort contributes to a more general philosophical project prominently initiated by Socrates (viz., the critical reflection of what we know). In the spirit of this tradition, John Locke, David Hume, and Immanuel Kant tried to systematically determine the limits of human knowledge. Cosmology seems a particularly apt target when we think about these limits because it is our boldest attempt to extend our knowledge, if “extend” is meant in a literal, spatial sense. Attempts to know what the universe is like immediately raise epistemological worries: How can we study the universe, even though it is a unique object and we lack similar objects to which we can compare? And how can we ever ascertain that our studies really address the whole universe? The aim of this article is to inquire whether physical cosmology can answer the most important questions that we have about the universe. Can we push the limits of scientific knowledge to the limits of being? And what is the role of philosophy? Kant’s doubts about the possibility of cosmological knowledge provide a good starting point for the discussion (Section 2). It is then shown that modern physical cosmology escapes Kantian and related worries (Section 3). Our currently best cosmological model, its support, and its problems are reviewed in Section 4, and it is shown that, despite its success, it cannot be used to answer some big cosmological questions, nor can we use possible explanations of why the universe is as it is to settle the issues, or so Section 5 argues.

2. Kant’s Challenge Why did Kant think that both metaphysical and empirical cosmologies are impossible? In his “Critique of Pure Reason,” Kant examines a couple of substantive questions about the world (A405–A591, B432–B619). For our purposes, we can focus on the first two: 1. Does the universe have a beginning in time? 2. Is the spatial extension of the universe finite? According to Kant, any metaphysical attempts to answer the questions run into trouble. The reason is that, for each question, both the positive and the negative answers lead to a contradiction with well-established metaphysical assumptions, or so Kant argues. Kant’s arguments can also be understood as identifying problems for empirical cosmology. To show this, I translate Kant’s arguments from their metaphysical parlance into an epistemic jargon. The arguments are then in fact more convincing. For Kant has often been criticized on the grounds that he ultimately draws conclusions from what we can know to what is. This would be a mistake, unless we succumb to a type of antirealism that denies truths that we

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Philosophy and Cosmology cannot verify. To most people, such a position is not plausible, and in the remainder of this article we assume a realist outlook according to which beings and knowables do not necessarily coincide. But there is more to Kant’s arguments if we consistently cast them in purely epistemic terms. The arguments may then have a tenable quintessence, namely that both possible answers to our questions raise epistemological worries. Suppose we answer the first question by saying that the universe has a beginning. According to Kant, we then are left with an explanatory gap. The reason is as follows. Kant assumes that time is independent of the universe and infinite in the past. If the universe had a beginning, we could sensibly ask why the universe originated at the particular instance of time it did. In a similar way, we often ask why a particular object within the universe originated at a particular time. We can explain the latter fact because there are other objects that interact with each other and eventually give birth to the object of interest. But an explanation of this type is impossible when it comes to the universe as a whole because the universe comprises all there is. We can refer to time alone if we are to explain why the universe originated at a particular instance of time. But time is homogeneous, and no instance of time is privileged over the other. We thus lack the resources to explain why the universe originated when it did. In an analogous manner, Kant argues that a spatially finite universe would lead to an explanatory gap too. Kant takes space to be infinite, and if the universe were finite, there would be no way of explaining why the universe is located in infinite space where it is because the universe is all there is and space is homogeneous. But for Kant, the assumption of a spatially infinite universe with an infinite past also comes with problems. On his view, we can never know that the universe has an infinite past or that it takes infinite space because we would need infinite evidence to show that. For every event in the past, say a collision between two galaxies, we would have to find an earlier one. Likewise, for every object in our world, say a quasar, we would have to find another one that was even further away. But our evidence does not suffice to do this since it is not infinite. If all this is correct, then both the positive and the negative answers to our questions lead to severe epistemic problems, and a theoretical dilemma challenges any scientific cosmology.

3. Skeptical Challenges Met But are we really caught in a dilemma? We certainly are not in the way Kant thought. In arguing this point, we learn a lot more about the foundations of present-day cosmology (cf. Mittelstaedt and Strohmeyer 1990). Consider first the horns of the dilemma that involve infinities. Kant’s arguments presume that we can only know the universe to be infinite in space or in the past if we can prove this on the basis of data. But assuming that the universe is in fact infinite in a spatial or temporal way, isn’t there any other way to know this? Couldn’t we establish this knowledge in a more indirect way? In particular, if a finite universe does in fact leave us with explanatory gaps, as Kant claims, isn’t this good evidence for an infinite universe? For some reasons, Kant came to think that his worries about a universe that is finite in one or the other way do not indicate that the universe is infinite in this way. But his insistence that an infinity of the universe be proven through data is indeed too strict. What we call “scientific knowledge” is often based on inferences that move beyond what has been observed. Although we have not tested that every electron has the same negative charge of –1.6022 × 10–19 Coulomb, we claim to know this. One promising way to move beyond our observations is to model a system (see, “Models and Theories”, Morrison this volume). For instance, in fluid mechanics, water in a basin is modeled as a continuous fluid: Each point in the basin is assigned several numbers that reflect the density, temperature, and pressure of the water at this point. It is true that we have to set infinitely many numbers to specify the model because space is assumed to contain infinitely many points. But this is not a problem because a mathematical function like the sine contains information about infinitely many points. For the reasons mentioned by Kant, there is no way to prove that such a model gets it right everywhere. But proof seems too strict a requirement on knowledge about the empirical world. In physics, it is common practice to confirm continuum models of fluids using finite data (e.g., about temperature at several heights in a basin filled with water) and to extrapolate the validity of the model. An analogous extrapolation may be used to establish that the universe is infinite in space or time. True, we have to be cautious because many models do not reflect their target in every respect. Sometimes infinities in models are

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Philosophy and Cosmology just assumed as idealizations. Nevertheless, if we have a very successful model of the universe, if there is no rival that is equally successful, and if the assumption of an infinite extension in space or time is decisive for the success of the model, then it seems fair to conclude that the universe is indeed infinite in this way. Now to devise a model either in fluid mechanics or in cosmology, we cannot just reason from our data and, for example, connect the dots in a plot with the data. We need theory, and theory too is something that Kant does not properly take into account. The dynamics of a fluid like water is governed by laws expressed in the Navier-Stokes equations, which connect physical characteristics such as density, pressure, and temperature. Good models of water in a basin or of similar systems are thus solutions to these equations. Physicists have tried to make progress along similar lines in cosmology. But what laws can be used in cosmology? It seems that we are now in trouble. Most laws we know are instantiated many times, and to identify a law, we need several instances. But there is only one universe. Maybe Kant’s point was simply that we lack laws and theories about the universe to devise a cosmological model. But cosmologists have been quite successful in avoiding this problem too. Their strategy is as follows: many physical laws like those that govern the water in the basin are described using partial differential equations. Equations of this type are assumed to hold at every instance of time and at every point that is filled with a certain medium. The Navier-Stokes equations are of this type, and the assumption that they always hold for every point in water has been well confirmed. To get started in cosmology, we choose laws that are supposed to hold everywhere, at every point in the universe. We can test them on our planet and by using observations about systems that are much smaller than the universe (e.g., about star clusters and galaxies). The laws then are extrapolated to every point in the universe. If this approach works out fine (I return to the question of whether this is so later), we do not need any special laws about the universe as such to do cosmology. We cannot, of course, use a law that applies to water only. The composition of matter in the universe varies to some extent from place to place. The usual approach pioneered by Einstein (1917), among others, is to neglect the variation of matter on small scales and to focus on the very largest scales of the order of 1024 meters. These scales exceed those of the largest galaxies (e.g., the Milky Way, which is a system of about 1011 stars, among them the sun). Figuratively speaking, the idea is that we look at the universe with glasses through which smallscale variations cannot be discerned anymore; they have been averaged away due to a coarse-graining. What we can then still see looks like a fluid, so cosmologists say that they model the matter in the universe as a cosmic fluid. This fluid is usually assumed to have an isotropic pressure, that is, a pressure that is the same in every direction (“perfect fluid”; Hawking and Ellis 1973: 69–70). Very often, the pressure is simply set to zero (“dust model,” 103). At the scales we are now talking about, only one of the forces well known from terrestrial physics is relevant, namely gravity. Gravity is only sensitive to the mass of a body, so it does not matter that our averaging of the matter of the universe mixes several materials. Cosmologists adopt the currently best theory of gravity, which is very well tested (i.e., Einstein’s GTR), and apply it everywhere in space and time. The usual strategy to average out small-scale fluctuations in the matter distribution has, of course, a downside. The models that we obtain will not reflect entities and processes at smaller scales. As a consequence, cosmology is not really about everything but only about the physics at the largest scales (see later discussion for a qualification). Worries to the effect that our world is just too multifaceted to be characterized by a single model or by a single science are thus pointless. They miss the rather limited scope of models in cosmology. A more general lesson is looming large here: we cannot define cosmology by saying that it is about everything because everything is a non-topic. (This, by the way, is also the lesson that Kant drew from the alleged antinomy of pure reason.) Every physical cosmology builds upon physical assumptions (often implicit in theories), and the history of science has seen a large number of related proposals that invite different perspectives and lend themselves to different conceptions of cosmology. For instance, for many scholars in ancient and medieval times, the universe was basically the solar system in which the various objects each had their natural places. A cosmology based upon these assumptions will differ greatly from present-day cosmology. The theory upon which present-day cosmology builds (i.e., GTR) has a number of peculiar characteristics. First, it does not allow for an observer-independent distinction between space and time. Space and time are given up in favor of space-time. This does not matter much for the rest of this article, however, since the standard

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