Physical cosmology has enjoyed several decades of rapid progress and remarkable success, leading to a new understanding of the cosmos and our place in it. This success, however, comes with new puzzles.
Chris Smeenk (Rotman Institute of Philosophy, Western University)
James Owen Weatherall (University of California, Irvine)
Cosmology is different from other areas of the physical sciences, both in its subject matter – the universe as a whole – and in the tools we use to study it. Standard experimental and theoretical methods used throughout the rest of the physical sciences have little traction in cosmology, where we have only one universe to study and many of the features of greatest interest are removed from us in space and time. These methodological difficulties, coupled with the profound importance of understanding the history and structure of the universe, make cosmology an urgent subject for philosophical research.
The principal goal of the current project is to develop the emerging field of philosophy of cosmology. This grant will support a planning phase, devoted to refining a novel research agenda for this field and consolidating an interdisciplinary team. One part of the project will synthesize and review previous contributions to the field, leading to an overall “landscape” review document, and topical review articles. We have provisionally identified three general research topics as targets for further work: (a) the epistemic and methodological challenges to traditional conceptions of scientific practice posed by the unusual features of cosmology; (b) the nature of evidence in cosmology, and particularly the status of anthropic arguments; and (c) the role and epistemic status of spacetime geometry in general relativity and other physical theories on which cosmological models are based. The project includes three workshops designed to bring together an interdisciplinary group of scholars, with a focus on evaluating these and other topics as appropriate focal points for further research.
- Statement of Significance:
The desire to understand the universe around us, and our place in it, is a profound part of the human experience. And today, thanks to a century of work in relativistic cosmology, we have a clearer picture of the origin and evolution of the universe than ever before. For the first time, we have something that deserves to be called a “Standard Model” of cosmology, rivaling the Standard Model of particle physics in its explanatory range. Cosmologists have celebrated these successes – but they have also been cautious. While we appear to have developed successful models of our cosmological history, doing so has required cosmologists to bend – and in some cases, reinvent – the rules of accepted scientific practice, prompting a startlingly open debate about the methodology of cosmology among working cosmologists.
For many of these physicists, explicit methodological debate is unfamiliar territory. This creates an unusual opportunity for meaningful collaboration between physicists and philosophers. Even as some physicists, such as Stephen Hawking and Lawrence Krauss, claim that philosophy is dead, others are beginning to see ways in which philosophy is not only useful, but essential to their own work. As science moves into new territory, so too must our conception of what constitutes reliable inference, sound methodology, and acceptable evidence – all of which have been central to the philosophy of science for decades. Renowned Princeton cosmologist Paul Steinhardt summed it up in a recent interview with Aeon magazine: “I wish the philosophers would get involved.”
Of course, the philosophers have been involved. Philosophy of cosmology does not have a focused agenda like other areas, such as foundations of quantum theory, but it has recently attracted much more interest from philosophers and cosmologists. The Templeton Foundation funded two previous projects in Philosophy of Cosmology that have opened up various lines of research. These previous projects focused in part on the ramifications of cosmology for issues already of interest in foundations of physics (the past hypothesis; solutions to the measurement problem) and in the metaphysics of physics (the status of laws of nature). There is value in seeing cosmology in light of these problems: after all, a full understanding of structure formation in the early universe demands a story about the quantum-to-classical transition, and cosmology raises intriguing challenges for metaphysics of science. But it is our view that these other projects have not fully explored the philosophical issues arising within cosmology itself – issues concerning the epistemology of multiverse theories, the criterion of “falsifiability” for sound theorizing, the status of anthropic reasoning, and the evidentiary power of simulations, among many others.
In our view, part of what makes cosmology so rich for philosophical inquiry is its wealth of problems that are apparently unique, at least within physics. A focus on traditional problems in philosophy of physics misses this richness. For this reason, the project we describe here takes a different tack, intended to complement the previous projects. Rather than look to philosophy of physics specifically, we aim to bring to bear the insights of decades of work in general philosophy of science on evidence, theory choice, and scientific methodology, to make progress on issues of urgent interest to working cosmologists.
For these reasons, we expect this project to have a different kind of significance for cosmological practice than some other work in philosophy of cosmology. It will, we hope, help to isolate the distinctive features of cosmology that give rise to the need for novel methodological innovation in the first place. It will also present cosmologists with new tools for thinking about the evidential situation they face. One of the striking things about the ongoing methodological debates in cosmology is that they rely on distinctions and criteria – science and pseudoscience; falsifiability – that philosophers of science have long argued never adequately captured successful scientific practice. Instead, most philosophers of science today accept some variety of belief-revision model of knowledge, according to which theories (or models) have more or less support, given the available evidence, but no evidence should be taken as straightforwardly confirmatory or falsifying. This picture of the relation between evidence and belief seems far better suited to the situation in cosmology than the one cosmologists appear to have inherited from a previous generation of physicists. And if accepted by cosmologists, it could radically restructure current disputes about the state of the field.
- Central Questions:
The project will address the following Big Questions inspired by the startling progress in cosmology over the last 50 years.
- Is it possible to discover and justify new fundamental laws in cosmology?
The Soviet cosmologist Zel’dovich once called the early universe “the poor man’s accelerator,” because one can study effects of particle physics theories using telescopes to observe the early universe, rather than building accelerators. Yet there are important contrasts between probing new ideas with well-designed experiments, and attempting to discern their impact on past evolution. In other words, what kind of science is cosmology? And what can be achieved in that kind of science?
- Does cosmology require new rules for testing theories?
Sometimes new scientific theories prompt new theories of science – that is, new ideas about how science works. Cosmologists have recently advocated controversial new ways to test theories. The impetus for these proposals comes from considering multiverse theories inspired by inflationary cosmology and string theory. To test competing multiverse theories, they argue, one needs to calculate what a “typical observer” should expect to see according to each theory, and compare these results to observations. Yet there is not a standard account of how this is supposed to work, and it is also not clear whether this proposal is compatible with more conventional accounts of how to test theories. This proposal will address the question of whether “anthropic reasoning,” as it is often described, provides a reasonable account of how to test theories. We will also consider alternative accounts of methodology that do not rely on anthropic predictions, inspired by comparisons between cosmology and historical sciences.
- What is the status of spacetime concepts in cosmology?
Cosmology depends on Einstein’s general relativity and its account of spacetime geometry. The status of the assumptions regarding spacetime geometry can be challenged in two different ways. First, there are a variety of properties typically assumed to hold for “reasonable” spacetimes. One challenge to these assumptions comes purely at the classical level, in assessing the justification for these assumptions. A distinct challenge concerns the fate of spacetime in new theories of quantum gravity. In what sense can spacetime itself “emerge” from a theory that does not introduce spacetime at a fundamental level?
III. Project Activities:
The core goal is to develop a new sub-discipline in philosophy of science. Part of our strategy for doing so is to host three workshops, bringing together different permutations of scientists and philosophers under the project’s core themes. These workshops will allow us to identify the philosophical issues of greatest interest to working cosmologists, giving us a clearer target for future work. We will also train the junior scholars who will pursue these topics. We will fund multiple graduate students, and a postdoctoral fellow, at UC Irvine and the University of Western Ontario, two of the top-ranked PhD programs in philosophy of physics and philosophy of science in the world. These funds will incentivize younger researchers to take an active role in developing the field, and to receive the training necessary to make long-term contributions and to support a self-sustaining literature oriented towards the needs of the science.