Dark Energy
The Marie Curie Project “PhilDarkEnergy” proposes that the most dominant ingredient of our universe, dark energy, undermines the widely held assumption of a strict ontological trichotomy between spacetime, matter and constants of nature. Dark energy exhibits aspects of all three categories, rather than falling neatly into exactly one category. The project will study the wide-ranging and far-reaching consequences of this blurred categorisation of dark energy for major debates in philosophy—in particular: the substantivalism-relationalism debate about the metaphysics of spacetime and matter, scientific realism vs. scientific anti-realism, and conventionalism—as well as for theory development in cosmology.
Dark Matter & Modified Gravity
Subproject A3 (LHC & Gravity) of the research unit Epistemology of the LHC
Fellow project members: Michael Krämer (theoretical particle physics), Dennis Lehmkuhl (integrated history and philosophy of physics), Erhard Scholz (history of mathematics), Miguel Ángel Carretero Sahuquillo (philosophy of physics), Sophia Haude (philosophy of physics)
Various astrophysical observations, at a wide range of scales (e.g. cosmic microwave background, galaxy clusters, galaxy rotation curves, gravitational lensing), have made clear that standard (Newtonian & Einsteinian) gravity plus the ´luminous´ matter of the standard model of particle physics cannot be the whole story. A novel ingredient is 
required, but the exact nature of this ingredient remains elusive. The most popular solution is some form of particle dark matter, such as WIMPs, axions or sterile neutrinos. However, null results of collider production and direct detection experiments leave room for non-particle dark matter options, such as primordial black holes. In fact, since all current evidence is gravity-mediated, the solution might not consist of some form of matter at all, but require a modification of our theory of gravity/spacetime. This semantic underdetermination of the nature of dark ‘matter’ by the current empirical data is the starting point of our project. We investigate various philosophical aspects surrounding the debate between dark matter & modified gravity.
Firstly, are the concepts of dark matter and modified gravity mutually exclusive? Theories that are self-identified as dark matter theories tend to do well at large scales. Theories that are self-identified as theories of modified gravity tend to do well at small scales (i.e. at the scale of galaxies). A recent trend of ‘hybrid theories’ responds to this stalemate via some mix of dark matter and modified gravity, in an attempt to get the best of both worlds. The new
ingredients in this theory make salient a question that could have been asked of several of the traditional theories: what makes a new (quantum) field in some cases a dark matter field and in other cases a modification of the gravitational field? We focus on a popular hybrid theory, Lasha Berezhiani and Justin Khoury’s superfluid ‘dark matter’ theory, and ask whether the new bosonic field postulated by this theory should be considered a form of dark matter, or a modification of gravity, or both, or neither. We argue that it is both dark matter and a modification of gravity (SHPMP, part 1, open access). This leads us to propose a chart of three groups of possible interpretations for such hybrid theories. Rather than two separate spaces of theories (i.e. dark matter & modified gravity), we should be considering a single space of theories, with ‘pure’ and ‘hybrid’ regions. These results have consequences for theory development, the substantivalism–relationalism debate, and understanding the divide between the dark matter and modified gravity communities (SHPMP, part 2, open access).
Secondly, given the underdetermination of modified gravity and dark matter (and the specific form of dark matter) by the empirical data, we may ask whether it is (already) possible to be a scientific realist about dark matter. If so, what would it mean to be such a realist? Doesn’t being a realist about x require that we know (at least roughly) what x is? I think so, and argue that we cannot yet be semantic realists about dark matter, and hence not yet full-blown scientific realists either (forthcoming in Foundations of Physics). It proves interesting to compare and constrast the case study of dark matter realism with those of dark energy, genes, and singularities.
Thirdly, we would like to understand the divide between the dark matter and the modified gravity community. Could there be no fruitful trading zone (a la Galison) between these communities? This divide is only partially explained by the differences in data that can be accounted for by each research programme. A second part of the story is the common belief that dark matter and modified gravity are mutually exclusive concepts, which we have argued against with our hybrid theory analysis above. A third part of the story is a difference in explanatory ideals between the two communities. With Martin King I am analysing and evaluating these differences in theoretical virtues.
This project has organised a conference on dark matter & modified gravity in Aachen in 2019, guest edited a special issue distributed across SHPMP and SHPMS, and organised an online workshop on the philosophy of dark matter. A conference on the history, philosophy & sociology of the intersection of particle physics, astronomy & cosmology will take in the early summer of 2020.
Symmetry-to-(un)reality inferences
There is an explicit consensus in the philosophy of physics and physics communities that symmetry-related models invariably represent the same state of affairs, despite obvious difficulties with this claim. Thomas Møller-Nielsen and James Read have bravely challenged this orthodoxy. They claim that symmetries merely motivate us to look for a metaphysically perspicuous characterization underlying these models. I extend their view by showing that loss of explanatory power can be sufficient to block the inference that these models describe the same state of affairs. I show that my view is consistent with practice in physics and philosophy of physics, even that of advocates of the orthodoxy (draft available upon request.) A project with Read argues that Dewar’s attempt to justify the orthodoxy boils down to an unjustified linguistic trick (first paper in Synthese, 2021; we are working on a follow-up). With Tushar Menon I am applying the above considerations to supersymmetry.
Dimensionfulness
Pierre Laplace wanted to know what initial data is sufficient to fix the future. We have been focusing for more than three centuries on the question of whether the initial data need include absolute or merely relational magnitudes, that is positions, velocities and accelerations relative to absolute space or merely relative to other material bodies (i.e. the substantivalism-relationalism debate). Only very recently have we started asking whether absolute scales of dimensionful magnitudes, such as distance and mass, are required, or whether their ratios suffice. (One might even ask whether mass is required at all. In Newtonian Gravity, the answer is yes (Foundations of Physics, 2018).) Comparativism about mass claims that mass ratios are sufficient. I defend absolutism about mass (within Newtonian Gravity), which claims that absolute masses are required, in virtue of which the ratios hold. I argue that the main comparativist proposal, by Shamik Dasgupta, fails on several counts (Synthese, forthcoming). I then proceed by proposing two better versions of my own—regularity comparativism (Philosophy of Science, 2017) and Machian comparativism (BJPS, forthcoming)—and show that they are still wanting.
A pop-science introduction to this Dphil (= phd) project can be viewed here or below:
The Metaphysics of Emergent Spacetime Theories
The debate about the ontology of spacetime is standardly construed as a dilemma between substantivalism and relationalism. I argue that a trilemma is more appropriate, emergent spacetime theories being the third category. Philosophical arguments do not distinguish between emergent spacetime and its effective substantival counterpart. It is arguments from physics that force us to give up substantivalism in favour of emergent spacetime theories. The remaining new dilemma is between emergent spacetime and relationalism. I provide a list of questions which one should consider when discussing emergent spacetime theories and apply them to a quantum super fluid toy model of emergent spacetime.
This paper has been awarded the Templeton Philosophy of Cosmology Essay Prize; it has been published in Philosophy Compass in 2019.
Galactic Sources of Ultrahigh-Energy Cosmic Rays
Ultrahigh-Energy Cosmic Rays (UHECR) are often assumed to have extragalactic sources, without proper justification. We identify four possible arguments, and dismiss the first three (lack of galactic sources and energetics, GZK cut-off, and the observation of cosmic rays from all directions). It is only the upper-bounds on the dipole-anisotropy set by the Pierre Auger observatory, which we compare with our simulations of UHECR traveling through the Galactic Magnetic Field and arriving at earth, that definitively rule out a galactic origin, as reported here.
Upper Limit for the Deuteron Electric Dipole Moment
There is an enormous unexplained asymmetry between the quantities of matter and anti-matter in our universe. One of the three necessary Sakharov conditions for such an asymmetry to occur is the existence of processes that violate CP-symmetry: the process involving matter must occur at different rates from the mirrored process involving its anti-matter counterpart. We have calculated the corrections to the energy levels of deuterium if the deuteron were to have a (CP-violating) electric dipole moment, using second-order perturbation theory in non-relativistic quantum mechanics. Equating these corrections to the maximum allowed by the approximate agreement between the theoretical energy levels and experimental spectroscopic values gives an upper limit for the deuteron electric dipole moment of 8.8 * 10^-16 e*cm, as reported here.
Niels Martens 