Einstein’s theory of general relativity provides cosmologists with the current best framework to describe the Universe. Nevertheless, the theory has observational and theoretical limitations. In...Show moreEinstein’s theory of general relativity provides cosmologists with the current best framework to describe the Universe. Nevertheless, the theory has observational and theoretical limitations. In turn, scientists have come to develop modified theories of gravity. The thesis compares the theory of general relativity with a particular class of modified theories called scalar-tensor theories, which incorporates a scalar field that couples to matter. Using f(R) theory, a sub-class of scalar-tensor theories, we develop a theoretical understanding of how certain observational differences emerge from a given gravitational framework. After doing so we use EFTCAMB to simulate various gravitational theories and compute their predicted luminosity distance power spectrum. This tool tracks the variance of the inferred luminosity distance fluctuations emerging from independent gravitational waves and supernova events. The fluctuations arise from the anisotropies present in the Universe, which have their evolution and dynamics directly dependent on the gravitational framework probed. More explicitly, the thesis investigates a general class of effective field theory models, k-Mouflage and Generalised Brans-Dicke models. The interference power spectrum was unique to modified gravitational theories making its detection a smoking gun result for the existence of modified theories. Even so, it remains that the signals present are not substantial enough to be detected in the foreseeable future. More encouragement comes from constructing the gravitational wave luminosity distance power spectrum since it exhibits amplitudes of larger values. Nevertheless, most theories investigated showed small deviations from general relativity, rendering them difficult to constrain in the foreseeable future using this tool as well. From this we conclude that although the luminosity distance power spectrum has the potential to be a revolutionary tool in fundamental physics and cosmology, its use in upcoming surveys to help constrain theories, let alone delineate them, seems to be unrealistic for the foreseeable future.Show less
We look for cosmic neutrinos originating in Gamma Ray bursts using public data from the IceCube collaboration. We allow for a time difference between a neutrino and GRB photon of up to 40 days to...Show moreWe look for cosmic neutrinos originating in Gamma Ray bursts using public data from the IceCube collaboration. We allow for a time difference between a neutrino and GRB photon of up to 40 days to probe possible Lorentz invariance violations. These violations might become visible if a neutrino has high enough energy and traveled a long enough distance before we observe it. We make use of pseudo experiments to simulate different possible neutrino realizations and see how well a signal can be discerned from background. We find slightly less neutrinos than expected from background in the IceCube data. A signal associated with more than 3% of the GRBs can be excluded at 98% confidence in the northern hemisphere, and at 70% confidence in the southern hemisphere. Under the assumption that the highest energy neutrinos that can be associated to a GRB are experiencing LIV induced time shifts we have derived an intrinsic time difference at emission between GRB neutrinos and photons of ∆tin = (4.49 ± 23.0) 10^4s, and a LIV scale of ELIV = (1.05 ± 0.85) 10^15GeV, while the probability of finding similar results from purely uncorrelated events is P = 54%.Show less
The goal of the project is to assess whether the Off-Resonance Saturation (ORS) method is able to differentiate and quantify different mineralized iron forms, in particular magnetite and ferritin....Show moreThe goal of the project is to assess whether the Off-Resonance Saturation (ORS) method is able to differentiate and quantify different mineralized iron forms, in particular magnetite and ferritin. Samples containing agarose and iron nanoparticles will be prepared and studied with a pre-clinical 7T MRI scanner at the LUMC. First, the samples will be characterized with commonly-used MRI sequences to obtain relaxation time maps and spectra. The main part of the project is to apply the ORS method to acquire positive contrast. Different nanoparticles will be used and the parameters of the ORS sequence will be optimized. Finally, a simulation is made to verify the validity of the ORS theory.Show less
Weyl semimetals have been providing for a considerable research interest in the last decade in quantum condensed matter physics, due to their non-trivial topological nature and their possible...Show moreWeyl semimetals have been providing for a considerable research interest in the last decade in quantum condensed matter physics, due to their non-trivial topological nature and their possible applications in material science. Their non-trivial topological order has many consequences like zero energy Weyl nodes, which are robust to impurities and display a chiral anomaly. The work presented in this thesis is inspired by the intriguing matter of the response of Weyl semimetals to topological defects and their change to the behaviour of underlying lattice. To achieve this, we studied the response of different types of Weyl semimetals upon introducing a lattice dislocation or a pi-flux vortex, which mimics the effect of the former. Specifically, we show that the existence of a (or multiple) Kramers pair(s) of zero-energy modes bound to a dislocation line or vortex is a not a generic feature of topologically non-trivial phases of Weyl semimetals since this appears to depend on the present number of Weyl nodes and their chiralities as well as the type of symmetry breaking. We obtain the explicit form of these states, which shows their exponentially localised nature. Furthermore, we analyse the dependence of the energy of these dislocation modes on different parameters of the models and analyse the resulting correlations found. We then conclude by placing these results in a broader context.Show less
In this thesis I discuss the development of Soviet fundamental physics from the 1917 October revolution until the end of the thirties of the twentieth century, using the life and career of the...Show moreIn this thesis I discuss the development of Soviet fundamental physics from the 1917 October revolution until the end of the thirties of the twentieth century, using the life and career of the experimental low-temperature physicist Lev Vasil’evič Šubnikov (1901-1937) as a guide through this period. After his graduation in Petrograd Šubnikov spent almost four years from 1926 to 1930 in Leiden, where he worked with the then director of the physics laboratory Wander de Haas and was the co-discoverer of the Shubnikov-de Haas effect. After his return to the Soviet Union he became the head of the first cryogenic laboratory in the Soviet Union at the Ukrainian Physico-Technical Institute in Char’kov, where he embarked on an extensive (very much Leiden inspired) research program that among other things resulted in the discovery of type II superconductivity (Shubnikov phase). The theoretical explanation of this phenomenon earned Abrikosov and Ginzburg the 2003 Nobel Prize. In 1937 at the height of the Stalinist terror the NKVD launched a clamp-down on the Ukrainian Physico-Technical Institute, of which Šubnikov became one of the most important victims. He was shot in November 1937. I discuss the extent of the repression in physics in general and at the Ukrainian Physico-Technical Institute in particular, and put forward arguments for the thesis that this repression was not random, but at least in part a deliberate and carefully planned attack on individuals who were alleged or perceived to be disloyal to the Soviet cause. Apart from Šubnikov’s work I also discuss the work of other Soviet physicists in that period, among whom Pëtr Kapica, Lev Landau, Igor’ Tamm and others, and their experiences in the Soviet climate. I also pay attention to the (re)organization of science, and physics in particular, by the Bolsheviks after the October revolution, the reestablishment of contacts with the West, the influence of Marxism on physics research (the campaign against physical idealism), and the quality of Soviet contributions to physics and the appreciation of these contributions in the West.Show less
