Theses
Master
The analysis of singular structure in the Kerr spacetime and physically more realistic spacetimes
Sam Meijer (2025)
Research Internship Thesis - Master Science & Business Management
In 1915, Karl Schwarzschild obtained his famous Schwarzschild solution, the first analytical solution to the vacuum Einstein Field Equations. This solution contained regions where the metric diverges, soon to be referred to as ‘metric singularities’. Ever since, the nature of spacetime singularities has been point of discussion among physicists and philosophers. The relevance of singular spacetime increased even more when Penrose published his fist singularity theorem, implying that singular structure is inevitably predicted by general relativity. This thesis provides an extensive analysis of the singular structure that arises in the Kerr spacetime and in physically more realistic spacetimes. For this purpose, the physical and mathematical nature of these types of singular structure is examined. Furthermore, by means of the ideas of Dennis Lehmkuhl, Erik Curiel and Karen Crowther & Sebastian De Haro, interpretations for singular spacetime structure will be analyzed. It will be stressed that some views on singular spacetime challenge our current singular black hole paradigm, while others embrace it. In addition, it will be examined to what extent Lehmkuhl’s historical interpretations for singular spacetime can be projected onto the Kerr solution. Finally, I will propose a classification to distinguish between possible approaches to constructing an adequate non-vacuum description for physically realistic gravitational collapse, based on a tension in different interpretations for singular structure.
Supervised by: Dr. Niels Martens (HPS) & Sanne Vergouwen (HPS)
Third reader: Dr. Guido Bacciagaluppi (HPS)
Another critical look at inflationary cosmology
Alex Fleuren (2024)
Twin Master Thesis in Mathematical Sciences and History & Philosophy of Science
I present an overview of the criticism of the theory of cosmological inflation of the past 25 years. I argue that the three main problems motivating the inflationary explanation are the flatness problem, the uniformity problem, and the structure formation problem. The flatness problem has been resolved. In analysing the other two problems, an alternative explanation emerges. This explanation relies on law-like assumptions about the initial conditions of the universe. I claim that this is a proper explanation. This places the inflationary explanation on par with an alternative explanation. I argue that different extra-empirical considerations lead cosmologists to prefer one above the other. The way forward lies in the middle: an amalgam of the two explanations promises to be the most fruitful.
Supervised by: Dr. Niels Martens (HPS), Prof. Dr. Gil Cavalcanti (Mathematical Sciences), Dr. Antonio Ferreiro (HPS & Physics)
Circularities in the empirical grounding of the cosmological principle
Antonios Papaioannou (2024)
Master Thesis in History & Philosophy of Science
The standard model of cosmology is built on the cosmological principle. This thesis critically examines the principle’s status, understanding, and justifications for its implementation and evaluates its potential as a testable hypothesis. I focus on data from the Planck mission, studying the CMB, revealing circularities that prevent independent verification of the isotropy of the signal. Even assuming the isotropy of the CMB, independently demonstrating homogeneity is challenging. The issues of fitting, averaging, and idealising observations add further circularity concerns. The impact of the circularities, however, diminishes with the realisation that cosmological models describe large-scale structures and as a historical science, cosmology prioritises explanatory value over individual hypothesis testing. Despite this, validating the cosmological principle independently remains an important way of making progress, which, in the case of cosmology, is best understood as happening iteratively. I suggest, therefore, being optimistic about cosmology, regardless of its weaker epistemic basis compared to the experimental sciences.
Supervisor: Dr. Niels Martens (HPS)
Second reader: Dr. Guido Bacciagaluppi (HPS)
Bachelor
The Status of the Classical Energy Conditions in General Relativity
Diederik Bots (2025)
Bachelor Thesis in Physics
For over half a century the classical energy conditions have been used as constraints on the Einstein Field Equations and have led to groundbreaking work in black hole physics and cosmology, among others. However, over time violations have been found, and the existence of classical violations that can be made arbitrarily large has led to discussion regarding their status within (and beyond) general relativity. In this thesis I look at the classical energy conditions in more detail, and determine that although complete abandonment is not necessary, the classical energy conditions can no longer be seen as fundamental parts of general relativity and their consequences should come with substantial asterisks.
Supervised by: Dr. Niels Martens (HPS) & Dr. Elisa Chisari (Physics)
Third reader: Prof. Dr. Stefan Vandoren (Physics)
Einstein on the Energy of Gravitational Waves
Fenna Roovers (2025)
Bachelor Thesis in Physics
In 1916, Albert Einstein first predicted the existence of gravitational waves from his theory of general relativity. This paper contained a calculational mistake in the energy-momentum of the gravitational field and an unconvincing argument to discard gravitational wave solutions that do no carry energy, based on the idea that unimodular coordinate systems are a natural choice of coordinate systems. In 1918, Einstein published a second paper on gravitational waves, correcting the energy-momentum and showing that the energy-free waves are the result of a coordinate transformation. This thesis compares Einstein’s papers from 1916 and 1918, correcting his expression for the energy-momentum and comparing his arguments for discarding the gravitational wave solutions that do not carry any energy. Furthermore, the debate surrounding the non-tensorial nature of the gravitational energy-momentum is reviewed.
Supervised by: Prof. Dr. Chris van den Broeck (Physics) & Dr. Niels Martens (HPS)
Third reader: Dr. Tanja Hinderer (Physics)
The effect of AGN feedback on the dark matter back-reaction as a case-study for the fitting problem
Jaydy Bellaard (2025)
Bachelor Thesis in Physics
In this thesis I examine the data created by the simulation FLAMINGO. It simulates the effect of AGN feedback, which is energy that gets shot into the galaxy because of black holes. This gets expressed as the power matter spectrum, this is a way we can look at the distribution of matter when comparing it to the distance of a certain point. This thesis specifically looks at the effect of dark matter back-reaction because of AGN feedback. A fit was created to describe how this changes of the distance by comparing multiple simulations that have different conditions. Specifically we look at two types of AGN, jet-like and fiducial-like AGN. For the Jet-like AGN and the fiducial-like AGN a different fit were found suggesting a difference in behaviour of the back-reaction depending on the type of AGN. I use this problem as a case-study for the fitting problem. There are two parts to this problem, how to choose the right family of equations and how to choose the right curve from this family. Here I conclude that for a good fit it has to be simple, accurate and as unifying as possible without taking away from the other two. The fit found in this thesis is accurate it unifies the different datasets into two different equations but it is not simple nor based on a previous theory.
Supervised by: Dr. Elisa Chisari (Physics) & Dr. Niels Martens (HPS)
Third reader: Dr. Tom Prokopec (Physics)
On Penrose’s analogy between spacetime curvature and optical lenses
Thijs Hogenkamp (2025)
Bachelor Thesis in Physics
In this thesis we analyse an analogy introduced by Roger Penrose between Ricci Φ_00 scalar and Weyl Ψ_0 scalar dominated spacetime curvature and anastigmatic and astigmatic optical lenses respectively. This analogy allowed Penrose to relate total energy-momentum flux across systems by the total focusing power of their optical counterpart. This, in turn, suggested a well defined energy of certain non-local Weyl curvature. The analogy between Weyl and astigmatic lenses was weakened in Lehmkuhl et al. (2024) to the two being only similar, but not identical. In this thesis we will argue that for a saddle lens, the analogy is perfect. We also provide an example where the relationship between total focusing power of a system and total energy-momentum flux seems to break down, as it allows for a gravitational wave with a negative localized energy.
Supervised by: Dr. Niels Martens (HPS), Sanne Vergouwen (HPS), Prof. Dr. Stefan Vandoren (Physics)
Second reader: Dr. Umut Gursoy (Physics)
Unravelling the Boltzmann Brain Paradox: Empirical & Statistical Resolutions
David Zuilhof (2024)
Bachelor Thesis in Physics
According to quantum mechanics, particles can temporarily come into existence from the void due to quantum vacuum fluctuations. This means that, although with extremely low probability, a large group of particles can come into existence in the exact configuration of a brain, which has a conscious experience. Such observers are called ‘Boltzmann brains’, and—in combination with ΛCDM— are famous for causing the threat of philosophical scepticism to reappear within physics: “How do you know that you are not a Boltzmann brain?” This thesis builds upon Avni’s (2022) recent statistical proof in order to argue that Boltzmann brains do not only lead to scepticism, but also to an inconsistency in the foundations of physics. After Avni’s solution to this inconsistency is criticised, two arguments are presented for rejecting the physical possibility of Boltzmann brains. Firstly, it is argued that on any reasonable form of scientific realism (or anti-realism), there is insufficient empirical evidence for believing in the physical possibility of Boltzmann brains. Secondly, it is argued that the fact that we have coherent experiences (unlike most Boltzmann brains) is a good statistical reason to reject the physical possibility of Boltzmann brains.
Supervised by: Dr. Niels Martens (HPS), Prof. Dr. Cristiane de Morais Smith (Physics)
Second reader: Dr. Elisa Chisari (Physics)