Philosophy of Dark Energy Workshop

The UPAC research group and COSMO-MASTER project team are organising an afternoon workshop about the Philosophy of Dark Energy, on Monday the 13th of January 2025. All physicists, philosophers, and any others who are interested are welcome to attend.

Please register (free but required) by the 6th of January at noon.

Registration (free but required)

Schedule

13:00 - 13:30

Coffee

13:30 - 14:30

Elisa Chisari (Utrecht University)

Dark energy with galaxy surveys

Several ongoing and upcoming campaigns map the sky to test different hypothesis for the origin of the accelerated expansion of the Universe. Is it compatible with a cosmological constant? Is it driven by an unknown component of the Universe allowed within General Relativity? Or does it require modifications of our laws of physics across large scales? I will describe the observables, the methodology and the choices that wide field surveys adopt to test these hypotheses. I will focus specifically on how maps of galaxies are used in this context, and I will describe my experience working within the Kilo-Degree Survey, which has completed data taking and is close to its final analysis, and the Rubin Observatory, a new telescope in Chile which will start taking data from 2025.

 

 

14:30 - 15:30

Niels Martens (Utrecht University & BHI Harvard)

Dark Energy: Spacetime or Matter or a Constant of Nature?

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?

 

15:30 - 16:00

Coffee Break
 

16:00 - 17:00

Antonio Ferreiro, Alex Fleuren & Niels Martens (Utrecht University)

Scalar fields: Spacetime or Matter?

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).

 

17:00 - 18:00

Erik Curiel (University of Bonn, MCMP LMU Munich & BHI Harvard)

Matter, Gravity, Geometry and Dark Energy

The distinction between matter and spacetime geometry in classical general relativity is almost always taken to be unproblematic, usually, even if only implicitly, conceived of as determined by the existence of an associated stress-energy tensor for a given field or structure.  I argue that this conception suffers ambiguities, due to difficulties raised, inter alia, by the equivalence principle and "self-sourcing" of curvature.  In the context of semi-classical gravity, the framework in which many feel the problem of dark energy is most naturally posed, the problems become even more severe.  After evaluating several attempts to resolve the question of how it is most fruitful to conceive of dark energy (matter, gravity, geometry, or something else entirely), I propose a criterion for
distinguishing  "matter" from "geometry" in both classical general relativity and semi-classical gravity, and apply it to this case.  The attempt is illuminating, not least in the primary conclusion I draw from it, viz., that we do not yet have adequate epistemic control over proposed accounts of dark energy to come to any principled decision.


 

18:45

Dinner (optional) at Popocatepetl (Popo Utrecht)