Dark Matter & Modified Gravity
Astrophysical observations and explanatory gaps in the Standard Model of particle physics imply the existence of Dark Matter and/or a modification of our theory of space and time. A decision between the Dark Matter and Modified Gravity approaches is hampered by problems of underdetermination at different levels and of different kinds. In order to specify these problems, we shall analyse in detail the landscape of Dark Matter and Modified Gravity models, and the interplay between research in particle physics, astrophysics, and gravity. The plethora of Dark Matter and Modified Gravity approaches, and the corresponding underdetermination, even in the light of the vast amount of relevant collider based and astrophysical observations, clearly illustrates the complexity of this scientific problem. On the other hand, the overlap of the collider and astrophysical domains may allow for reducing the underdetermination, thus leading to a simplification of the model landscape. Our focus is on Dark Matter searches at the Large Hadron Collider and the connection between LHC results and theories of gravity. We will address the question of different kinds of underdetermination, both in choosing between the two research programs of Dark Matter and Modified Gravity, and also in choosing between different models within each program. In particular, we shall provide an assessment of the explanatory power and the explanatory gaps of the Dark Matter and Modified Gravity hypotheses, and study the coarse-graining of the model landscape introduced by so-called simplified models, which may allow us to distinguish different kinds of aiming for simplicity in high-energy physics.
There is an explicit consensus in the philosophy of physics and physics communities that symmetry-related and dual 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 generalise their view to all empirically equivalent models, and extend it 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.)
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 (PhilSci Archive).) 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 (PhD Ch.2). I then proceed by proposing two better versions of my own—regularity comparativism (Philosophy of Science 84(5)) and Machian comparativism (PhD Ch.4)—and show that they are still wanting. Finally, with Oliver Pooley, I consider a suggestion by David Baker to extend comparativism to fundamental mixed relations such as mass over distance.
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; a copy is available upon request.
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.