The Philosophy of Cosmology Edited by Khalil Chamcham , Joseph Silk , John D

Cambridge University Press 978-1-107-14539-9 — The Philosophy of Cosmology Edited by Khalil Chamcham , Joseph Silk , John D. Barrow , Simon Saunders Frontmatter More Information

THE PHILOSOPHY OF COSMOLOGY

Following a long-term international collaboration between leaders in cosmology and the philosophy of science, this volume addresses foundational questions at the limits of science across these disciplines, questions raised by observational and theoretical progress in modern cosmology. Space missions have mapped the Universe up to its early instants, opening up questions on what came before the Big Bang, the nature of space and time, and the quantum origin of the Universe. As the foundational volume of an emerging academic discipline, experts from relevant fields lay out the fundamental problems of contemporary cosmology and explore the routes toward possible solutions. Written for physicists and philosophers, the emphasis is on conceptual questions, and on ways of framing them that are accessible to each community and to a still wider readership: those who wish to understand our modern vision of the Universe, its unavoidable philosophical questions, and its ramifications for scientific methodology.

KHALIL CHAMCHAM is a researcher at the University of Oxford. He acted as the exec- utive director of the UK collaboration on the ‘Philosophy of Cosmology’ programme. His main research interests are in the chemical evolution of galaxies, nucleosynthesis, dark matter, and the concept of time. He has co-authored four books and co-edited ten, including From Quantum Fluctuations to Cosmological Structures and Science and Search for Meaning.

JOSEPH SILK FRS is Homewood Professor at the Johns Hopkins University, Research Scientist at the Institut d’Astrophysique de Paris, CNRS and Sorbonne Universities, and Senior Fellow at the Beecroft Institute for Particle Astrophysics at the University of Oxford. He has written seven popular books on cosmology, including The Big Bang, A Short History of the Universe, and The Inﬁnite Cosmos. His research areas include dark matter, the formation of the galaxies, and the big bang theory. He has received numerous awards and prestigious international fellowships.

JOHN D. BARROW FRS is Professor of Mathematical Sciences at the University of Cambridge and Director of the Millennium Mathematics Project to improve the appre- ciation of mathematics amongst the general public, teachers, and school students. The recipient of many distinguished prizes, his research interests are in cosmology, gravita- tion, and the interface between particle physics and astronomy. He is also a proliﬁc author; the most recent of his 22 books are 100 Essential Things You Didn’t Know You Didn’t Know about Maths and the Arts, and The Book of Universes.

SIMON SAUNDERS is Professor of Philosophy of Physics at the University of Oxford, and a Tutorial Fellow of Merton College, Oxford. He is the lead editor of Many Worlds? Everett, Quantum Theory, and Reality and the author of more than 60 articles in philosophy of physics, with special emphasis on the foundations of quantum mechanics, quantum ﬁeld theory, and symmetries. He also works on structuralism in philosophy of science and metaphysics, focusing on the logic of identity. He is president-elect of the British Society for the Philosophy of Science.

© in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-14539-9 — The Philosophy of Cosmology Edited by Khalil Chamcham , Joseph Silk , John D. Barrow , Simon Saunders Frontmatter More Information

THE PHILOSOPHY OF COSMOLOGY

Edited By

KHALIL CHAMCHAM University of Oxford

JOSEPH SILK University of Oxford

JOHN D. BARROW University of Cambridge

SIMON SAUNDERS University of Oxford

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Contents

List of Contributors page viii Preface x Acknowledgments xii

Part I Issues in the Philosophy of Cosmology 1

1 The Domain of Cosmology and the Testing of Cosmological Theories 3 George F. R. Ellis

2 Black Holes, Cosmology and the Passage of Time: Three Problems at the Limits of Science 40 Bernard Carr

3 Moving Boundaries? – Comments on the Relationship Between Philosophy and Cosmology 66 Claus Beisbart

4 On the Question Why There Exists Something Rather Than Nothing 76 Roderich Tumulka

Part II Structures in the Universe and the Structure of Modern Cosmology 83

5 Some Generalities About Generality 85 John D. Barrow

6 Emergent Structures of Effective Field Theories 109 Jean-Philippe Uzan

7 Cosmological Structure Formation 136 Joel R. Primack

vi Contents

8 Formation of Galaxies 161 Joseph Silk

Part III Foundations of Cosmology: Gravity and the Quantum 179 9 The Observer Strikes Back 181 James Hartle and Thomas Hertog 10 Testing Inﬂation 206 Chris Smeenk 11 Why Boltzmann Brains do not Fluctuate into Existence from the de Sitter Vacuum 228 Kimberly K. Boddy, Sean M. Carroll and Jason Pollack 12 Holographic Inﬂation Revised 241 Tom Banks 13 Progress and Gravity: Overcoming Divisions Between General Relativity and Particle Physics and Between Physics and HPS 263 J. Brian Pitts

Part IV Quantum Foundations and Quantum Gravity 283 14 Is Time’s Arrow Perspectival? 285 Carlo Rovelli 15 Relational Quantum Cosmology 297 Francesca Vidotto 16 Cosmological Ontology and Epistemology 317 Don N. Page 17 Quantum Origin of Cosmological Structure and Dynamical Reduction Theories 330 Daniel Sudarsky 18 Towards a Novel Approach to Semi-Classical Gravity 356 Ward Struyve

Part V Methodological and Philosophical Issues 375 19 Limits of Time in Cosmology 377 Svend E. Rugh and Henrik Zinkernagel 20 Self-Locating Priors and Cosmological Measures 396 Cian Dorr and Frank Arntzenius

Contents vii

21 On Probability and Cosmology: Inference Beyond Data? 429 Martin Sahlén 22 Testing the Multiverse: Bayes, Fine-Tuning and Typicality 447 Luke A. Barnes 23 A New Perspective on Einstein’s Philosophy of Cosmology 467 Cormac O’Raifeartaigh 24 The Nature of the Past Hypothesis 486 David Wallace 25 Big and Small 500 David Z. Albert Index 505

Contributors

David Z. Albert is Professor at the Department of Philosophy, Columbia University, USA.

Frank Arntzenius is Professor of Philosophy at the University of Oxford, University Col- lege, Oxford, UK.

Tom Banks is Professor at the New High Energy Theory Center and the Department of Physics, Rutgers University, USA.

Luke A. Barnes is a postdoctoral researcher at the Sydney Institute for Astronomy, Uni- versity of Sydney, Australia.

John D. Barrow is Professor of Mathematical Sciences and Director of the Millennium Mathematics Project at Cambridge University. He is also a Professorial Fellow of Clare Hall, UK.

Claus Beisbart is Professor of Philosophy at the Institut für Philosophie, Universität Bern, Switzerland.

Kimberly K. Boddy is a Postdoctoral Research Fellow at the Physics Faculty, University of Hawaii, USA.

Bernard Carr is Professor at the School of Physics and Astronomy, Queen Mary Univer- sity of London, UK.

Sean M. Carroll is Professor at the Department of Physics, Caltech, USA.

Cian Dorr is Professor at the Department of Philosophy, New York University, USA.

George F. R. Ellis is Professor at the Mathematics and Applied Mathematics Department, University of Cape Town, South Africa.

James Hartle is Professor at the Department of Physics, University of California – Santa Barbara, USA.

Thomas Hertog is Professor at the Institute for Theoretical Physics, Leuven University, Belgium.

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Contributors ix

Cormac O’Raifeartaigh is a researcher at the School of Science, Waterford Institute of Technology, Waterford, Ireland.

Don N. Page is Professor at the Department of Physics, University of Alberta, Edmonton, Alberta, Canada.

J. Brian Pitts is a Senior Research Associate of Philosophy at the University of Cam- bridge, UK.

Jason Pollack is a student at the Department of Physics, Caltech, USA.

Joel R. Primack is Professor at the University of California – Santa Cruz, USA.

Carlo Rovelli is Professor at the Centre de Physique Théorique or the Aix-Marseille Université, Marseille, France.

Svend E. Rugh is a researcher at the Symposion, “The Socrates Spirit”, Section for Phi- losophy and the Foundations of Physics, Copenhagen, Denmark.

Martin Sahlén is a researcher at the Department of Physics and Astronomy, Upsala University, Sweden.

Joseph Silk is Homewood Professor at the Johns Hopkins University, USA, Research Scientist at the Institut d’Astrophysique de Paris and Sorbonne Universities, France, and Senior Fellow at the Beecroft Institute for Particle Astrophysics at the University of Oxford, UK.

Chris Smeenk is Professor at the Rotman Institute of Philosophy, Department of Philoso- phy, Western University, Ontario, Canada.

Ward Struyve is a researcher at the Department of Physics, University of Liège, Belgium.

Daniel Sudarsky is Professor at the Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, México.

Roderich Tumulka is Professor at the Department of Mathematics, Rutgers University, USA.

Jean-Philippe Uzan is Professor at the Institut d’Astrophysique de Paris, CNRS, Univer- sité Pierre et Marie Curie, Paris, France.

Francesca Vidotto is a researcher at the Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, Nijmegen, The Netherlands.

David Wallace is Professor of Philosophy at the University of Southern California, USA.

Henrik Zinkernagel is Professor at the Department of Philosophy, University of Granada, Spain.

Preface

Cosmology is the unifying discipline par excellence, combining theories of gravity, ther- modynamics, and quantum field theory with theories of structure formation, nuclear physics, and condensed matter physics. Its observational tools include the most intricate and expensive scientific experiments ever devised, from large-scale interferometry, high- energy particle accelerators, and deep-sea neutrino detectors, to space-based observatories such as Hubble, Wilkinson microwave anisotropy probe (WMAP) and Planck. The recent and stunning detection of massive gravitational encounters between black holes by means of gravitational waves is only one of several windows that have recently been opened into the study of distant and exotic objects, and to ever-earlier epochs of the universe. Cosmology is in a golden age of discovery, the likes of which have rarely been seen in the physical sciences. Theory has hardly kept up, but its bringing together of the fundamental theories of physics is also historic in its vitality. It draws them together in ways that put pressure on each: whether because, as in quantum mechanics, cosmology is an application in which there is no ‘external observer’; or because, as with the standard model of particle physics and general relativity, there is tension between their basic principles; or because, as in statistical mechanics, it highlights the extraordinary importance of the initial condi- tions of the universe for local physics. Add to these components the existing foundational problems of each discipline even in non-cosmological settings: the measurement problem in quantum mechanics, the ‘naturalness’ problem of the Higgs mass and the cosmological constant or ‘dark energy’ (so-called ‘fine-tuning’ problems), and the information-loss paradox of black-hole physics. The result is a heady brew – and this is not even to mention the enigma that is dark matter, making up the bulk of the gravitating matter of the universe, its nature still unknown. What place, in this perfect storm, for philosophy? Some see none: ‘philosophy is dead’, according to Stephen Hawking, and needs no engagement from scientists. And indeed, where philosophers of physics have made inroads on conceptual questions in physics, they have tended to focus on cleanly defined theories treated in isolation. Synthetic theories, in complex applications, are messy and ill suited to rigorous analysis, axiomatisation, or regimentation by other means – the standard tools of philosophy. And yet, time and again scientists ignorant of philosophy go on to do it anyway, badly. Philosophical questions are natural to the growing child and beset anyone with an enquiring mind; they are suppressed

Preface xi

at best by ring-fencing, at worst by decree. And cosmology has long been a testing ground of philosophy. According to some, its central domain is ‘the problem of cosmology’; that is the problem of understanding the world, including ourselves, and our knowledge, as part of the world, to echo Karl Popper. For all of that, ‘philosophy of cosmology’ as a body of philosophical literature engaged with contemporary cosmology does not yet exist. This book marks a beginning. In it we have gathered essays edging out from cosmological problems to philosophy, and from philosophical problems to cosmology; it is at the points at which they meet that we hope for the greatest synergies. Both disciplines are at a turning point: cosmology, in virtue of the greatly accelerated rate of data acquisition that brings with it a new set of fundamental problems with few or no precursors in any of the empirical sciences; philosophy of physics, in virtue of signiﬁcant progress in the last two decades on the foundations of quantum mechanics, in particular an understanding of quantum theory as a realistic theory applicable to the universe as a whole. Add to this new perspectives on the meaning of physical probability, the role of probability in the foundations of statistical mechanics, and the signiﬁcance of the initial state of the universe for local physics and the arrow of time. In their contributions to the book, philosophers explore still wider concerns in epistemology, metaphysics, and philosophy of mathematics. We have urged them, just as contributors working in cosmology proper, to push in the least comfortable directions and to expose rather than conceal what is conceptually obscure in their undertakings. As editors, we are united in the view that the hard problems of cosmology should be thrown into as sharp a relief as possible, and in the simplest terms possible, if there is to be any hope of overcoming them. It is to that end that this book, and the research programmes from which it was created, were conceived. We thank the John Templeton Foundation for a grant that enabled the preparation of this volume by supporting a series of workshops and a conference.

Acknowledgments

The editors acknowledge that this project was made possible through the support of a grant from the John Templeton Foundation. The opinions expressed in this publication are those of the authors and do not necessarily reﬂect the views of the John Templeton Foundation. Our gratitude goes especially to the late Dr Jack Templeton for his personal support of this project. We would like to thank Barry Loewer for his valuable input and Jeremy Butterﬁeld for his insightful advice throughout the conception, planning and execution of the project. Our special thanks go to Christopher Doogue for his administrative assistance, Cambridge University Press for supporting this book project and, especially, Vince Higgs, Char- lotte Thomas, Lucy Edwards, Helen Flitton, Viv Lillywhite and Timothy Hyland for their editorial management. Last, but not least, we acknowledge and value the help of all our colleagues whose contributions made this project a reality.

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