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Science in Progress

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Displaying theses 101-110 of 1078 total
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H.J. van Leijen
Bachelor programme: Natuur- en Sterrenkunde August 21st, 2017
Institute: Other Research group: GRAPPA institute: Gravitation and AstoParticle Phy Graduation thesis Supervisor: Christoph Weniger
Mass constraints on MACHO dark matter
Most scientist nowadays agree that there is something out there called dark matter, there is however still a lot of debate concerning the nature of this dark matter. The explanations for the fundamental components that form dark matter even range from ultra light axions to supermassive black holes. In this thesis I assumed that dark matter was made of MACHOs (Massive compact halo objects) and the goal of this thesis was to find mass constraints on these dark matter MACHOs. I did this by looking at their microlensing effects and by looking at the dynamical impact of dark matter MACHOs on there surrounding.
picture that illustrates the research done
Scientific abstract (pdf 1K)   Full text (pdf 4009K)

G. Raaijmakers
Master programme: Astronomy and Astrophysics August 21st, 2017
Institute: API Research group: X-ray Timing Group Graduation thesis Supervisor: dr. Anna Watts
photo of the author
The Dense Matter EOS, Non-Idealised Mass-Radius Posteriors: Implications for EOS inference and observing strategy
Neutron stars are the smallest and densest stars known in the Universe, with a radius of around 10-15 kilometres but a mass as high as one or two solar masses. Due to the extreme compactness of a neutron star, the particles in the interior are squeezed together very tightly. At these extreme densities the behaviour of these particles is not properly understood. To answer the question of how these particles interact we need to look deep inside a neutron star. However, even with our most advanced telescopes, that is not possible. Fortunately, Einstein's theory of general relativity provides a mapping between the measurable quantities of a neutron star, such as mass and radius, to the so-called Equation of State (EOS). This EOS is a simple relation between pressure and density that fully describes the particles behaviour. In this thesis we have developed a code called MORSE to explore how the EOS depends on these mass and radius quantities. It turned out that the best constraints on the EOS can be obtained when the input neutron star masses showed a spread over the parameter space or were centred around high masses.
picture that illustrates the research done
Scientific abstract (pdf 2K)   For more info or full text, mail to: A.L.Watts@uva.nl

J. Haartman
Bachelor programme: Natuur- en Sterrenkunde August 20th, 2017
Institute: VU / Other Research group: Physics and Medical Technology Graduation thesis Supervisor: dr. Jan de Munck
Statistical analysis on the deformation of hippocampi in patients with Alzheimer’s Disease
Alzheimer’s Disease is the most common form of dementia in the elderly. This disease is irreversible, but treatment can enhance quality of life, therefore an early diagnosis is desirable. Memory loss will probably be the first thing most people think about when hearing the words "Alzheimer's disease", however, beneath the surface there is already a lot going on before this is noticeable. A small, but important, part of the brain called "the hippocampus" is responsible for this. This hippocampus, shaped like a seahorse, is involved in saving memories. Research has shown that the hippocampus of people with Alzheimer's disease is smaller than the hippocampus of healthy people. By creating three-dimensional reconstructions of the hippocampi of patients at two time points 12 months apart, researchers could look at the change in shape of these hippocampi. They "sliced" these 3D reconstructions into multiple sections and looked at the way they changed in one year time. It seems there may be a difference in the way these hippocampi change shape in healthy people and patients diagnosed with Alzheimer's disease. Further research has to be done, but it's a step in the right direction for diagnosing Alzheimer's disease as early as possible
picture that illustrates the research done
Scientific abstract (pdf 2K)   For more info or full text, mail to: jc.demunck@vumc.nl

E.J. Vermaas
Bachelor programme: Natuur- en Sterrenkunde August 20th, 2017
Institute: Universiteit Utrecht Research group: Freudenthal Instituut Graduation thesis Supervisor: T.M. Nieuwenhuizen
Unexpected interference in the weak decoherence condition?
Quantum mechanics has a different logic than classical mechanics. For example, a thing can be at two locations at the same time in quantum mechanics which in classical logic can only have one well defined position. The behaviour of atoms is known to be described by quantum mechanics. But we are build of those atoms and we do not experience quantum logic in our daily life. Our world is well described by classical mechanics. Multiple theories and interpretations of quantum mechanics are developed to clarify this transition from quantummechanics to classical mechanics. The theory of decoherence describes classical aspects in quantum mechanics. There are two conditions formulated. If one of these conditions is satisfied, classical logic can be applied to the quantum system. One of those conditions has mathematically curious properties. In this thesis a physical example is found that satisfies this condition in order to understand what these mathematical properties mean in a physical way.
picture that illustrates the research done
Scientific abstract (pdf 34K)   Full text (pdf 765K)

M. van der Eyden
Master programme: Physics - Theoretical Physics August 19th, 2017
Institute: UvA / Other Research group: UvA / Other Extra internship Supervisor: Edan Lerner
Discrete element simulations of micro-penetration in cohesive granular materials
The SnowMicroPenetrometer (SMP) is an instrument that was developed to characterize snow stratigraphy for various applications. It penetrates the snow and measures the penetration force signal with high spatial resolution. While some structural parameters can be estimated from the measured force signal within a simple, stochastic penetration model, their relation to the true microstructural and micromechanical properties of snow is yet poorly understood. To improve the understanding of the penetration process we have analyzed Discrete Element Simulations of a small cone penetrating into cohesive sphere assemblies. We have used initial sphere configurations covering a variety of packing fractions and (cohesive) coordination numbers and conducted simulations for different (cohesive) bond strengths and cone sizes. Our main results show that the mean penetration force is primarily controlled by the bond failure rate, with a super-linear dependence indicating a feedback of failed material. A secondary influence stems from particle jamming in front of the cone. These influences are incorporated in a proposal towards a new model that describes the penetration force in terms of simulation parameters and failure processes around the cone.
picture that illustrates the research done
Scientific abstract (pdf 1K)   Full text (pdf 5985K)

J.M. van Urk
Bachelor programme: Natuur- en Sterrenkunde August 18th, 2017
Institute: WZI Research group: Quantum Gases & Quantum Information Graduation thesis Supervisor: Robert Spreeuw
photo of the author
Characterization of a magneto-optical trap of Rubidium atoms and building the repump laser
The development of quantum simulators and quantum chips is nowadays a widely examined part in physics. Quantum simulators manipulate simple quantum systems to get a better understanding of more difficult systems. One way of realising systems for quantum computation or quantum simulation is by creating atom chips. In these chips small clouds of ultracold atoms are loaded into magnetic traps. This process starts with trapping ultra cold atoms in a magneto-optical trap (MOT). A MOT uses a combination of a strong magnetic field and lasers, made for cooling and trapping neutral atoms. The light from the lasers need the right properties, like wavelength, frequency and polarisation, so the photons can excite Rubidium atoms to a higher energy state. In this report the complete setup for one of the lasers is elaborated. To optimise to actual experiment for quantum simulation, it is important to know the properties of the MOT, like the size of the cloud and the number of atoms inside of it. Two techniques to determine this are used in this research: measuring fluorescence with a photodiode and taking absorption images with a camera.
picture that illustrates the research done
Scientific abstract (pdf 1K)   Full text (pdf 17749K)

T. Bouma
Bachelor programme: Natuur- en Sterrenkunde August 17th, 2017
Institute: ITFA Research group: Instituut voor Theoretische Fysica Graduation thesis Supervisor: Vladimir Gritsev
Derivation of the shift charge current for 3D Bi_2X_3 (X = Te,Se) Topological Insulators, using the Floquet formalism combined with Keldysh’s Green’s function method
Topological insulators lie at the foundation of a new type of electronics: spintronics. These devices are very much similar to the more general electronics, except they have one additional degree of freedom: spin. Spin is an intrinsic characteristic of particles, and manipulating it could allow us to store more information on the same surface. Topological Insulators can be defined as conducting at the edge, but insulating at the bulk, a state which is defined by the topology of the system. The topology, in turn, can roughly be defined as the geometrical shape of the energy spectrum of the system. Optical spintronics are a type of Topological Insulators, where the topological behaviour is induced by or related to optical excitation. For instance, charge currents can be induced at the surface of the Topological Insulator, called the shift charge current. This study aims to describe the direction and magnitude of the shift charge current on the surface of 3D Bi_2 X_3 (X=Te,Se) TIs, by looking at the process of exciting an electron inside the material and creating a pair of an electron and a hole (in the energy spectrum). The excited electron then travels through the material, creating a current.
picture that illustrates the research done
Scientific abstract (pdf 1K)   Full text (pdf 813K)

B.M. van Bloppoel
Bachelor programme: Natuur- en Sterrenkunde August 17th, 2017
Institute: ITFA Research group: MathematicalPhysics Graduation thesis Supervisor: dhr. dr. M.L. (Marcel) Vonk
At the shoal of the resurgent depths
When physicists solve problems, the first approach is to solve by expressing solutions in exact mathematical representations. But in most problems, we won't find exact expressions, so we use something which is called a power series. A power series is a series of coefficients and powers of a variable. Every power of x is multiplied by a coefficient. In physics, these are asymptotic. The first term will be near to the answer and every subsequent term will bring us closer to the answer. In this case, the series converges. This is behaviour we wish from series, because we calculate many terms and get a better approximation to our answer. But the course of physics has brought us to realise that many modern problems produce divergent series, where summing all the terms leads us to infinite answers. These divergent series leave us with no way to solve problems. So we need tools to calculate divergent series. Borel's resummation technique is the first point of attention in my thesis. We find that this technique allows us to find finite answers to divergent series in a lot of cases. In the cases where this doesn't work, we turn to resurgence theory for answers.
picture that illustrates the research done
Scientific abstract (pdf 2K)   Full text (pdf 574K)

K.H. Taris
Master programme: Physics - Physics of Life and Health August 14th, 2017
Institute: VU / Physics & Astr. Research group: VU Physics of living systems Graduation thesis Supervisor: Erwin Peterman
Intraflagellar Transport in C. elegans with Light Sheet Microscopy
IntraFlagellar Transport (IFT) in C. elegans is a process that transports cargo along axonemes with motor proteins. IFT is involved in multiple signalling pathways. In the Erwin Peterman group, epifluorescence microscopy has been used to image IFT dynamics. However, epifluorescence microscopy has limits in its signal-to-background ratio. Light Sheet Microscopy (LSM) illuminates only a thin slice of the sample, which lowers phototoxicity and low out-of-focus fluorescent protein bleaching, which can lead to a high signal-to-background ratio. LSM uses two different perpendicular orientated objectives, which adds a rotational degree of freedom. Here we show that our light sheet microscope is not yet capable of determining IFT-dynein’s velocity, as the kymographs have few distinguishable trains. Our LSM has a lower signal-to-background ratio than epifluorescence, despite having a better axial resolution and an added rotation stage. We consider this light sheet microscope to be a project in progress, where potential improvements include using thicker circular samples with a low agarose concentration, changing the detection objective to be a fluorite plan objective with higher N.A., or adding a second detection objective.
picture that illustrates the research done
Scientific abstract (pdf 2K)   For more info or full text, mail to: e.j.g.peterman@vu.nl

V. Ricciardulli
Master programme: Physics - Particle and Astroparticle Physics August 14th, 2017
Institute: ITFA Research group: String theory Graduation thesis Supervisor: Alejandra Castro Anich
Localization in supersymmetric Chern-Simons theories
The study of interacting quantum field theories has been possible, to great extent, thanks to perturbation theory. However, there is an inherent limitation to the amount of information that one can extract in this way. The need to develop non-perturbative techniques is then crucial to understand a field theory for all values of the coupling constant. In some cases, it is possible to exploit the symmetry of the theory to obtain such results and one particularly fruitful setting is represented by supersymmetric theories. In this thesis, we introduce fundamental concepts in N=2 supersymmetric Chern-Simons-matter theories on the three-sphere and describe the procedure of supersymmetric localization, which provides an exact computation of the partition function of such theories.
picture that illustrates the research done
Scientific abstract (pdf 2K)   Full text (pdf 784K)

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