Jump Ship, Shift Gears, or Just Keep on Chugging: Assessing the Responses to Tensions between Theory and Evidence in Contemporary Cosmology Siska De Baerdemaeker & Nora Mills Boyd Abstract: When is it reasonable to abandon a scientific research program? When would it be ​ premature? We take up these questions in the context of a contemporary debate at the border between astrophysics and cosmology, the so-called “small-scale challenges” to the concordance model of cosmology (훬CDM) and its cold dark matter paradigm. These challenges consist in discrepancies between the outputs of leading cosmological simulations and observational surveys, and have garnered titles such as the Missing Satellites, Too Big To Fail, and Cusp/Core problems. We argue that these challenges do not currently support a wholesale abandonment or even modification of cold dark matter. Indeed, the nature of the challenges suggests prioritizing the incorporation of known physics into cosmological simulations. Keywords: theory change; anomalies; dark matter; small-scale challenges; simulation; cosmology ​ Highlights: ● Small-scale challenges ● Is a revolution nigh? ● Patience, stay the course 1. Introduction For many decades, philosophers of science have grappled with the methodology of theory change (e.g. Kuhn 1962, Toulmin 1972, Laudan 1977, Hacking 1981, Darden 1991, and Thagard 1993). When should theoretical changes accommodate empirical anomalies? When would that be 1 unhelpfully ad hoc? When is it reasonable to abandon a scientific research program? When is it unreasonable not to abandon one? Imre Lakatos (1978) formulated one of the subtler of the well-known philosophical approaches to these questions. Lakatos’ view on the methodology of scientific research has the benefit of emphasizing the evolution of scientific theories. For Lakatos, a research programme consists in a “hard core”, which is for all intents and purposes immune to falsification in virtue of the negative heuristic of the programme, and a “protective belt” of auxiliary hypotheses that is open to revisions in response to the pressures of empirical tests (the positive heuristic). “Progressive” (i.e. successful) research programmes are ones that continue to produce novel predictions after revisions, at least some of which are borne out by new evidence. In contrast, degenerating research programmes fail to be progressive in this way. Lakatos’ approach is more lenient than would be a hard-nosed Popperian falsificationism (that Popper himself did not endorse) since Lakatos allows that the progress of a successful programme may be intermittent—not every novel prediction must be immediately corroborated. Lakatos allows that it may be rational to place prima facie refutations of a research programme “into some temporary, ad hoc quarantine” and get back to work (1978, 58). ​ ​ The suggested permissibility of quarantining prima facie refutations raises several critical questions. Under what conditions is it acceptable to retain allegiance to a programme facing prima facie refutations? How long can such “temporary” quarantine be rationally maintained? How many difficulties can be quarantined before it must be admitted that there is a serious problem with the research programme? Without answers to such questions, scientists can glean little specific practical guidance from the Lakatosian framework in the circumstances where such guidance would be most 2 useful—in making decisions about where to focus research efforts and how to prioritize them in the face of accumulating anomalies. Indeed the Lakatosian approach has been criticized on the grounds that the progressive/degenerative status of a research programme can only be judged in retrospect (Chall 2020, 18). This has led Chall (2020) for example, to combine a version of Lakatos’ approach (modified to include models) with a Laudan-inspired appraisal of problem-solving successes and capacities.1 We share Chall’s criticism of the Lakatosian approach—we would like a contemporaneous way to appraise research programmes.2 We take it as a premise that practically useful in situ guidance for appraising research programmes in light of accumulating anomalies should avoid sliding into the Kuhnian picture of theory change on the model of religious conversion. One wants to retain, to the extent possible, useful heuristics for reasoning through theory change. By this we do not mean to endorse the ​ much-discussed value-free ideal in science (see e.g. Rudner 1953, Jeffrey 1956, Longino 1990, and Douglas 2009).3 We believe that the co-existence of values and reason is possible, indeed necessary in ​ science, and do not deny that values can play a role in reasoning across theory change.4 That said, our argument does rest on the following two assumptions: 1) a contemporaneous method for rationally prioritizing research strategies in the face of apparent anomalies would be valuable, and 2) 1 We are grateful to an anonymous referee for pointing us to this reference. 2 We part ways with Chall’s move to a hybrid Lakatos-Laudan view. Laudan’s approach appraises theories not merely with respect to their successes in solving empirical problems, but also in solving theoretical/conceptual problems. Chall applies this hybrid framework for instance, to work in appraising the status of Supersymmetry, appealing the problem of “naturalness” (21). We believe that such an approach strays too far from the empirical, and thus are concerned in the present paper with supplementing a roughly Lakatosian approach with further heuristics that allow for contemporaneous appraisal of research programs, while retaining the focus of theory-appraisal on empirical adequacy. 3 We thank an anonymous referee for pressing us to clarify this point. 4 We thank an anonymous referee for raising the concern that our argument ignores the inherently paradigm-internal nature of values. We do not deny that values invoked in scientific reasoning have evolved over the history of science (see for example Daston & Galison 2007, and Franklin 2013). However, we also believe that this fact alone does not negate the possibility of reasoning across theory change. In case of the small-scale challenges, for example, we believe that the ​ Kuhnian picture of incommensurability is too broad to capture the content of the debate. We take the debate about the small-scale challenges to be much more local, as will become clear below. 3 articulating such a method is not in principle impossible. This first assumption is just the criticism of Lakatos that we already cited from Chall above, and we suspect is widely held. We hope that our argument below is an existence proof in support of the second. In the present work, we take up these questions about the methodology of theory change in the context of a contemporary debate at the border between astrophysics and cosmology, the so-called “small-scale challenges” to the cold dark matter paradigm. We take an opposite approach to that of Popper, Lakatos or Kuhn: rather than focusing on the general question of theory change (“how should a research programme respond to anomalies?”), we will focus on a specific debate (“how should the current concordance model of cosmology be modified, if at all, in response to the small-scale challenges?”) and approach the general question from the ground up. We focus upon the ​ small-scale challenges debate in particular as a methodological test case because this debate is currently live and ongoing. Intervention by philosophers of science thus may be useful in active decision-making in this field of research. Moreover, such intervention could add to the budding philosophical literature on computer simulations in cosmology and astrophysics (see also Gueguen Forthcoming; Smeenk & Gallagher Forthcoming). We argue that in this context, specific heuristics recommend a compelling hierarchy of priorities to follow. In particular, the small-scale challenges do not currently support a wholesale abandonment of cold dark matter or even a modification of the cold dark matter hypothesis (see Sections 3.1 and 3.2 below). Indeed, the nature of the challenges suggests prioritizing the incorporation of known physics into cosmological simulations. Although our main focus will be on the small-scale challenges debate, we believe that some of our arguments can have a more 4 widespread application in scientific practice, without pretending to be a general account of theory change (see Sections 6 and 7). Section 2 briefly introduces the small-scale challenges. In Section 3 we classify the different possible solutions to these challenges into three categories, and argue for our preferred approach in Section 4. Section 5 considers different possible futures for astrophysics and cosmology with respect to the challenges, and discusses recommendations for each of these scenarios according to the position we defend. In Section 6 we respond to potential objections to our argument motivated by methodological pluralism and the recent work on the epistemic value of risky research. 2. Small-scale challenges in cosmology The small-scale challenges in contemporary cosmology consist in a handful of tensions between theoretical predictions and empirical evidence. Theoretical predictions are based on the current concordance model of cosmology, the “Lambda Cold Dark Matter” model (훬CDM). 훬CDM models our universe using the resources of general relativity in concert with other physical theories that inform our understanding of the composition and evolution of the universe. According to