Michael Begun (University of Pittsburgh)
One intriguing feature of recent research on some prominent questions in astrophysics and cosmology is the presence of stubborn discrepancies or tensions in empirical results. For example, the two main approaches for determining the Hubble constant—classical determinations involving distance ladders typically calibrated by Cepheids and Type Ia Supernovae, and CMB-based determinations—have tended to yield discrepant results, with the most precise recent estimates of each type producing a 3.4σ difference. More controversially, in the context of dark matter research, the research group behind the DAMA/LIBRA experiment has claimed to find a strong signal for the existence of WIMP dark matter, whereas others have claimed to rule out the existence of dark matter in the same parameter range, yet no obvious explanation for the discrepancy has emerged. While evidential discord is by no means unique to astrophysics and cosmology, cases like these provide a useful starting point for understanding evidential discord more generally, as well as highlighting some of the unique evidential challenges facing astrophysics and cosmology.
In this work, I examine the discrepant results in the Hubble constant and dark matter cases and use them to try to better understand the ways in which empirical results can conflict and the epistemic implications of those conflicts. Starting from Jacob Stegenga’s account of inconsistency and incongruence, I argue that a more nuanced picture of evidential discord is required for making sense of the Hubble constant and dark matter cases. I characterize evidential “non-conformity” as a weaker form of discord than inconsistency but a stronger form than incongruence, and show that the Hubble constant and dark matter cases fit this characterization of non-conformity. One reason why the results in these cases are better characterized as non-conforming rather than inconsistent is that because the competing approaches rely on different methodologies and background assumptions, the discrepancies may ultimately be found to be compatible, perhaps through the modification of background assumptions or with the discovery of currently unknown physical features affecting the results. I also show why the evidential discord in the Hubble constant and dark matter cases should not be characterized as incongruent on Stegenga’s definition.
Finally, I examine the current prospects and scientific strategies for resolving the discrepancies in the Hubble constant measurements and in the dark matter detection experiments. There is now a strong push for new, more precise measurements and experiments, reexaminations of experimental methods to uncover systematic errors, and critical inspections of physical assumptions. I suggest that whereas judgments of evidential non-conformity are likely to be experimentally fruitful, leading to improved experiments and methodologies, judgments of inconsistency are more likely to be theoretically fruitful, leading to revised models or theories. While evidential discord is often seen by philosophers of science as a serious problem, this analysis highlights the positive epistemic role that it plays, at least in contemporary astrophysics and cosmology.
