Theses
Master
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
Abstract
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
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)