Description
It is difficult to question that the historical development of the dark matter problem challenges traditional philosophical accounts of hypothesis confirmation, refutation, and ad-hocness. To put simply, it is both too simplistic and unconvincing to describe the situation as a series of continuous refutations of the dark matter hypothesis, aimed at resolving the discrepancy between gravitational theory-based predictions (whether Newtonian or relativistic) and astronomical or cosmological observations, followed by ad hoc modifications of the initial hypothesis.
However, more illuminating accounts of the evidential framework in gravitational research are possible. One such approach, inspired by the perspective developed in Closing the Loop by George E. Smith, would suggest that rather than testing gravitational theory itself, physicists investigating dark matter are primarily using it as a tool to identify robust sources of the gravitational field. As I interpret it, this approach intensifies the challenge of characterising what constitutes evidence for dark matter. In this talk, I aim to address this issue.
In the first part of the talk, I will examine how Smith’s perspective applies to the dark matter problem, in line with previous applications of this methodology to the dark energy problem (Smeenk and Weatherall, 2024). This analysis requires us primarily to clarify the notion of a “robust source” in the context of dark matter research. One might argue that the difficulty here lies in the fact that robustness judgments themselves presuppose gravitational theory, raising doubts about whether the loop can truly be closed in this case.
I will argue that this discussion suggests a need to shift perspective in order to break the deadlock in the philosophical debate on dark matter. Specifically, we should engage in an analysis of the fundamental principles presupposed in dark matter research which allow to identify some observations as dark matter robust sources of the gravitational interaction—principles that alternative theories of modified gravity may not incorporate. To implement this strategy, the second part of my talk will examine the principles underlying claims that certain gravitational lensing phenomena constitute evidence for the dark matter hypothesis. In particular, I will explore which versions of key principles—such as the equivalence principle—are implicitly assumed in these evidential claims. My hope is that analysing some of these principles will allow us to develop a framework for distinguishing between the different possible responses to the dark matter problem and, consequently, to make explicit which fundamental assumptions different theories of Modified Gravity must abandon.
I will conclude by addressing the broader philosophical question of how to interpret these principles and their role in spacetime theories, considering how this discussion connects to broader debates about the function of principles in the formulation of physical theories.
References
Antoniou, A. (2023). Robustness and dark-matter observation. Philosophy of Science, 90(3), 629–647.
Martens, Niels C. M., and Lehmkuhl, Dennis. (2020). “Dark Matter = Modified Gravity? Scrutinising the Spacetime–Matter Distinction through the Modified Gravity/Dark Matter Lens.”Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 72 (10): 237–50.
Smeenk, C., & Weatherall, J. O. (2024). Dark Energy or Modified Gravity? Philosophy of Science, 91(5), 1143–1160.
Smith, G. E. (2014). Closing the loop: Testing Newtonian gravity, then and now. In Z. Biener & E. Schliesser (Eds.), Newton and empiricism (pp. 262–352). Oxford University Press.
Sus, A. (2014). Dark matter, the equivalence principle and modified gravity. Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 45, 66–71.
