Our analysis so far has focused on three of the case studies: dark energy, inflation, and black holes. Since inflation and dark energy share interesting similarities—they are each responsible for the accelerated expansion of our universe, in the early and current universe, respectively—our first realisation was that many of the COSMO-MASTER questions were best posed in the broader framework of scalar-tensor theories, of which various mainstream examples of inflation and dark energy are specific instances. Our results suggest that a strict spacetime-matter dichotomy indeed does not apply to the scalar field in these theories, since this categorisation differs across two perspectives (the so-called Jordan and Einstein frames) which are supposed to be mere redescriptions of the same underlying (meta)physics. One interpretational option is to consider the spacetime and matter categories to be a matter of mere convention, rather than objective matters of fact. We find the most natural conclusion to be to give up on those categories altogether, at least in this context. An unexpected positive result is that the conceptual category of gravity does still apply to the scalar field.
Further research on dark energy specifically suggests that it may also be a mere matter of convention whether one understands the simplest form of dark energy, i.e. a cosmological constant, as a constant of nature or a specific form of matter. Moreover, it seems that the explanation of the accelerated expansion of the universe due to dark energy could be considered either a gravitational/spatiotemporal explanation or a matter-based explanation. This undermines the debate between substantivalism (spacetime is fundamental) and relationalism (matter is fundamental).
We have distinguished five possible interpretations of the mass of black holes. A detailed analysis of one specific interpretation shows that the mass—typically the hallmark of matter—of a black hole is also inherently spacetime-like. A metaphysical analysis of all interpretations (qua substantivalism versus relationalism) seems to corroborate this blurring of the spacetime-matter distinction.
In sum, the preliminary results on these three case studies confirm our main hypothesis, with as unexpected bonus result that the gravity category does seem to remain viable in the context of scalar-tensor theories.
A more general goal behind the COSMO-MASTER project was to firmly establish philosophy of astronomy & cosmology within the EU. The ERC Starting Grant has allowed us to create the Utrecht Philosophy of Astronomy & Cosmology research group (uu.nl/upac) which is already one of the largest philosophy of physics groups in the EU. The weekly group meetings, our almost-weekly colloquium, two international conferences, and three workshops so far, have attracted a vibrant, interdisciplinary, international audience. The team has moreover played a leading role in consolidating history & philosophy of physics in the Netherlands (hpp-nl.nl).
