Philosophy of Dark Energy Workshop

tba

The titular Lambda in our standard model of cosmology, ΛCDM, parametrises the accelerated expansion of our universe. Although the standard model treats it simply as a cosmological constant, the physical nature underlying this parameter—sometimes referred to as the intrinsic energy of spacetime—is unclear, with theorists and experimentalists considering a large variety of dynamical matter alternatives as well as modified gravity/spacetime alternatives. This talk will consider this situation in the context of the orthodox conceptual dichotomy between spacetime and matter, by focusing on the following aspects and connections with various philosophical debates:

• The Λ-term in the Einstein field equations is formally equivalent to the stress-energy tensor of a perfect fluid. Is it also physically equivalent? What follows from this for the categorisation of Λ as matter or spacetime?
• The name ‘dark energy’ suggests that it is a form of energy, a paradigmatic definitional property of matter. However, it violates the strong energy condition. This brings to the fore the importance for the spacetime–matter distinction of interpreting the many energy conditions at play in physics, especially since some of them have a geometric interpretation, others a material interpretation, and others have both interpretations.
• If Λ is indeed a cosmological constant, would that imply that there is a third primary metaphysical category that is relevant in the context of the spacetime–matter distinction, namely constants of nature/nomological structure?
• Arguably, a necessary condition for being a scientific realist about some entity x is that we know, at least roughly, what x is. If we do not even know which of the most basic metaphysical categories—container or contained, spacetime or matter, or perhaps even a constant of nature—applies to dark energy, what would it then even mean (as of right now) to be a scientific realist about dark energy?
• Do formal equivalences such as the duality between f(R) gravity (presumably a modified spacetime theory) and Brans-Dicke theory (with the scalar field interpreted as a form of matter) suggest that the spacetime–matter categorisation in these contexts is merely conventional, rather than an objective fact?

In this paper we analyse scalar-tensor theories—specific instances of which include mainstream inflation and dark energy models—in light of the spacetime-matter dichotomy. We argue that it is difficult to categorise the scalar fields as either a pure aspect of the spacetime structure or a pure form of matter, for instance by focusing on the Jordan vs Einstein frames of these theories. We introduce and partially evaluate the various interpretational options available, such as functionalism, moving to a reduced theory, and breaking down the spacetime-matter dichotomy (e.g. conventionalism).

tba