In this thesis the relative spectral energy density of stochastic primordial gravitational waves is investigated. Decoupling of Standard Model particles and neutrino free-streaming affect the...Show moreIn this thesis the relative spectral energy density of stochastic primordial gravitational waves is investigated. Decoupling of Standard Model particles and neutrino free-streaming affect the expansion history of the universe and thus leave characteristic signatures on the amplitude of the gravitational wave spectrum. Adding extra light or heavy particles damps the spectrum at frequencies before the particle decouples. Including an extra neutrino species amplifies the spectrum at larger wave numbers, but damps it at shorter wave numbers. Measuring these primordial gravitational waves reveals the thermal history of the universe. One possible non-standard thermal history is early matter domination due to the inflaton. It is shown that, in this cosmology, the end of early matter domination and beginning of the radiation era depend linearly on the reheating temperature.Show less
In a time of thriving Gravitational Wave physics, we study Black Hole Quasi-Normal Modes emitted in the post-ringdown phase of merger events. By using Boundary Effective Field Theoretical methods,...Show moreIn a time of thriving Gravitational Wave physics, we study Black Hole Quasi-Normal Modes emitted in the post-ringdown phase of merger events. By using Boundary Effective Field Theoretical methods, we search for modifications to General Relativity in the strong-field limit, for scalar as well as gravitational field perturbations. Going beyond General Relativity, Black Holes are predicted to produce echoing signals, for which we characterise observational parameters, by searching for leading order Boundary Conditions near the Black Hole horizon. For a scalar field, we discuss a parity symmetric and a shift symmetric configuration, while for a gravitational field, parity and diffeomorphism symmetry are implemented. The diffeomorphism symmetric Boundary Condition oddly seems to mix modes. Given the intimate relationship between Effective Field Theory and renormalisation techniques, we also comment on recent first principle arguments that have been brought up regarding the supposed impossibility of echo observation. We find that these arguments over-interpret a regularisation cut-off. From our perspective, the arguments still have merit, but do not form the no-go theorem that it seems to. In the end, only observation can give the decisive answer on the existence of Black Hole echoes.Show less
Neural networks have been an active field of research for years, but relatively little is understood of how they work. Specific types of Neural Networks have a layer structure with decreasing width...Show moreNeural networks have been an active field of research for years, but relatively little is understood of how they work. Specific types of Neural Networks have a layer structure with decreasing width which acts like coarse-graining, reminiscent of the renormalization group (RG). We examine the Restricted Boltzmann Machine (RBM) and discuss it’s possible relation to RG. The RBM is trained on the 1D and 2D Ising model, as well as the MNIST dataset. In particular for the 2D Ising model showing a flow towards the critical point Tc ≈ 2.27, opposite to the RG-flow. Examining the behaviour of the RBM on the MNIST dataset shows that sparse datasets can allow multiple fixed points which can be removed by artificially creating new samples. We conclude that this RBM-flow exists due to the multiple relevant length scales at the critical point and we briefly discuss why.Show less
In Cosmology, the theories of General Relativity and Quantum Mechanics have to work together very closely. However, the workings of quantum fields in General Relativity are challenging to calculate...Show moreIn Cosmology, the theories of General Relativity and Quantum Mechanics have to work together very closely. However, the workings of quantum fields in General Relativity are challenging to calculate, especially the backreaction of particle production caused by gravitational fields. This can normally only be calculated perturbatively. The theory of Classical-Quantum Correspondence could help with these calculations, as it allows for a single set of equations that covers the full evolution of both fields without the need for perturbations. This is possible by transforming the expectation values of the quantum field to depend on a corresponding classical one. This thesis focuses on the validation this theory of the Classical-Quantum Correspondence for interactions between quantum and gravitational fields. Furthermore we cover simulations of a quantum field in a FRW metric and compare them with simulations of a classical field in the same metric. This shows that the Classical-Quantum Correspondence is a good technique to combine quantum fields with classical gravity, and get the full evolution of both without the need to do the same equation itteratively.Show less