Cosmological simulations are essential to research in theoretical astrophysics. One thing in particular that needs to be studied using these is the recent observation of magnetic fields in voids....Show moreCosmological simulations are essential to research in theoretical astrophysics. One thing in particular that needs to be studied using these is the recent observation of magnetic fields in voids. This project then tests the SWIFT simulation code, and specifically its implementation of MHD, to aid with this. The simulation results are compared to the analytic solutions of the Zel’dovich pancake model, for both a 1D and 3D collapse. We add magnetic fields to this model, and derive and simulate their evolution for a number of configurations: in 1D, a parallel and perpendicular field (relative to the collapse axis), with and without resistivity; in 3D a field without resistivity. It is found that the magnetic field is simulated well, as it corresponds exactly to our solutions when we expect it to, and still looks reasonable when our solutions break down. Especially for a 3D collapse, the pancake model is not exact anymore, but the simulated field still seems to follow our general solutions. One caveat to this is that it requires a specific configuration of the code due to problems with artificial resistivity, so these still need to be fixed.Show less
Slow-roll, rapid-turn ( srrt) inflation models have recently gained some traction as a possible multifield inflation paradigm. Phenomenologically, rapid-turn inflation models have been shown to be...Show moreSlow-roll, rapid-turn ( srrt) inflation models have recently gained some traction as a possible multifield inflation paradigm. Phenomenologically, rapid-turn inflation models have been shown to be attractor solutions with at least linear stability of the perturbations by Bjorkmo (2019). In addition, srrt inflation has successfully been embedded in supergravity (Aragam et al., 2022) and simple string theory (Chakraborty et al., 2020) models. Thus, it is becoming increasingly valuable for theorists to test if their models are suitable for srrt inflation. Anguelova and Lazaroiu (2023) have derived a consistency condition purely in terms of the potential and metric of the scalar manifold for two-field inflation models. In this thesis, their consistency condition will be re-derived. In addition, we will present the inflatox python package with a numerical implementation of the consistency condition. We used inflatox to analyze four different two-field inflation models: Orbital inflation (Achúcarro et al., 2020), angular inflation (Christodoulidis et al., 2019), the egno supergravity model (Ellis et al., 2014) and a D5-brane fat inflation model from string theory (Chakraborty et al., 2020). For two of these models, we were able to numerically obtain srrt trajectories using PyTransport (Dias et al., 2016) and confirm that these trajectories overlap with the region in field space where the consistency condition holds.Show less
With the development of next-generation gravitational waves detectors, we aim to measure and infer data on a broader range of the energy and redshift spectrum. However, in this range, deviations...Show moreWith the development of next-generation gravitational waves detectors, we aim to measure and infer data on a broader range of the energy and redshift spectrum. However, in this range, deviations from theories that predict a non-standard propagation speed for GWs are expected to become non negligible anymore. Moreover, it has been shown that the presence of inhomogeneities and structures in our universe does affect the GWs observables: with the forecasted level of precision of future detectors, such corrections can not be ignored. In this work we set in the frame of quartic scalar-tensor theories of gravity to study such relativistic effects with the presence of an extra scalar degree of freedom. Due to the complexity of the full theory, we opted for a phenomenological approach to describe the dispersion relation and amplitude evolution of the metric perturbation. Using this technique, we evaluated the relativistic corrections to frequency and direction of propagation of the GWs wave-vector. On the other hand, it was not possible to analytically calculate the relativistic corrections to the tensor amplitude even using the parametric approach, as a consequence of the elevated number of terms in the amplitude evolution equation. Nevertheless, after selecting from such equation the transverse-traceless modes, we were able to find, using the N-P formalism, which types of term can actually contribute to the evolution of the physical modes.Show less
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
The Standard model of particle physics is extremely successful in explaining accelerator data. However, it is incomplete and fails to resolve several phenomena known as beyond the Standard model ...Show moreThe Standard model of particle physics is extremely successful in explaining accelerator data. However, it is incomplete and fails to resolve several phenomena known as beyond the Standard model (BSM) problems. The BSM problems may be solved by introducing new particles. In addition to particle experiments, cosmological observation offers a way we can put limits on the parameters of these new particles. This work gives a detailed qualitative description of several such cosmological constraints. The observations used are described, and then ways in which new particles could impact such observations. Then, these constraints are applied to two case studies: the scalar portal and the neutrino portal. In both cases, a significant part of the parameter space unexplored by direct experimental studies can be excluded based on these cosmological arguments.Show less
After a short introduction on the history of dark matter research, we review the current state of knowledge on both dark matter and sterile neutrinos, motivating sterile neutrinos as a dark matter...Show moreAfter a short introduction on the history of dark matter research, we review the current state of knowledge on both dark matter and sterile neutrinos, motivating sterile neutrinos as a dark matter candidate. We then investigate the dependence of current constraints on the sterile neutrino parameter space on the fraction $\chi_N$ of the dark matter mass density that is due to sterile neutrinos, and derive a lower bound on the fraction assuming the 3.5 keV spectral line detected in galaxies and galaxy clusters is caused by sterile neutrino decay: $\chi_N \gtrsim 0.1$.Show less
Oscillons, localized oscillating configurations in nonlinear field theories, have been known to exist since the mid 1990’s. Since then a lot of research has been done to understand these exotic...Show moreOscillons, localized oscillating configurations in nonlinear field theories, have been known to exist since the mid 1990’s. Since then a lot of research has been done to understand these exotic solutions. They can emerge under rather general conditions and have been found to exist in well-motivated physical models. Although they have been studied extensively in single-field models, not a lot is known about them in theories where fields interact. In this thesis I try to gain insights into the complicated questions surrounding multi-field oscillons. In the first chapters I start by reviewing what oscillons are in the context of single-field models. I also show why it is generally expected that oscillons might have an impact on early Universe cosmology. In the last chapters I tackle multi-field oscillons in the context of two coupled scalar fields. I manage to find stable oscillons in a model with a specific ”exchange” symmetry and find a criterion to assess their stability by extending the Vakhitov-Kolokolov criterion. Oscillons in this system can both exist in in-phase and out-of-phase configurations, highlighting an interesting characteristic of oscillons in multi-field theories. Finally, I analyse oscillons in more general models of scalar fields, showing the influence of a mass mismatch on the oscillon solution and discussing its influence on stability. I conclude with some expected differences between symmetric and asymmetric couplings between the fields that need to be tested in the 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
In 2018, Achúcarro et al. presented a class of two-field inflationary models known as ’shift-symmetric orbital inflation’. These models have multi-field behaviour but their predictions remain very...Show moreIn 2018, Achúcarro et al. presented a class of two-field inflationary models known as ’shift-symmetric orbital inflation’. These models have multi-field behaviour but their predictions remain very close to those of single-field inflation. This thesis will first introduce the topic of inflation, provide background to the field and build the necessary equations and concepts starting at the foundations of cosmology. Then it shall further examine these models and their solutions. Stability will be proven for a small subset of the class. Furthermore, a few sufficient sets of requirements for stability will be presented. It will be argued, but not proven, that stability holds in general.Show less
In this thesis we study the use of Boltzmann codes for simulating the evolution of linear perturbations. More specifically we compared the matter power spectra at present as calculated by CAMB and...Show moreIn this thesis we study the use of Boltzmann codes for simulating the evolution of linear perturbations. More specifically we compared the matter power spectra at present as calculated by CAMB and CLASS, as these are the most up to date Boltzmann codes. We have looked at their behavior for a universe with a non-cold dark matter component. For the non-cold dark matter component we used sterile neutrinos with few keV mass and a non thermal primordial distribution function. There is a strong disagreement between the two codes, when the non thermal component is big. We expect a suppression of the matter power spectrum on small scales when comparing it to the cold dark matter case. While CAMB fits our expectations, we see a big deviation in the results from CLASS. An attempt at solving this error was made by changing the way the distribution function is sampled in CLASS however, we only found a small improvement and no conclusive source of this strange behavior.Show less
The problem of the cosmological constant together with the tension in the observations of the present value of the Hubble parameter has brought about the search of alternative theories to the...Show moreThe problem of the cosmological constant together with the tension in the observations of the present value of the Hubble parameter has brought about the search of alternative theories to the Standard Model of Cosmology. One of the most promising ones is Modified Gravity. In this thesis, we explore scalar-tensor theories that are invariant under weakly broken galileon (WBG) transformations. We have derived the background cosmology and found attractor solutions that track a De Sitter Universe at late times, solving the coincidence problem and preventing from fine tuning issues. We have implemented the model into EFTCAMB, an Einstein- Boltzmann solver that employs the effective field theory of dark energy, and computed the power spectrum of temperature-temperature CMB anisotropies and the matter power spectrum. Our results, based on the theoretical predictions, are promising. While being compatible with LCDM at small scales, WBG can lower the ISW tail of the TT models on the CMB power spectrum, making it potentially favoured by observations.Show less
The inflationary hypothesis was introduced as a solution to the fine-tuning issue in the initial conditions of the Big Bang theory. In this Master’s research project, we introduce our work in the...Show moreThe inflationary hypothesis was introduced as a solution to the fine-tuning issue in the initial conditions of the Big Bang theory. In this Master’s research project, we introduce our work in the search for features in the Cosmic Microwave Background (CMB) power spectra that could result from reductions in the speed of sound of the inflaton. We study these features in the context of an effective single field theory of a multiple field scenario, due to the fact that a single field inflation approach is favoured by the current cosmological data, especially by the CMB. First, we present a brief review of the current cosmological model, the ΛCDM model, inflation and the possible extensions. Secondly, we review the physics of the CMB, the main theoretical cosmological codes and the needed data analysis tools from the point of view of Bayesian statistics. Finally, we update our current search for features using Planck 2015 temperature and polarization data introducing new parametrizations for the reduction of the speed of sound. In this search, we have recovered only some previous found modes, indicating the dependency of our results with respect to the parametrization we were using. For this reason, we have pointed out the necessity of reconstructing the reduction of the speed of sound, showing some preliminary results when Gaussian Processes are used as the reconstruction technique.Show less
We will discuss non-local solutions to some of the problems of the standard model of cosmology, L cold dark matter (LCDM), focusing on two models of gravity and their applications to cosmology. The...Show moreWe will discuss non-local solutions to some of the problems of the standard model of cosmology, L cold dark matter (LCDM), focusing on two models of gravity and their applications to cosmology. The first comes from modifying the Einstien-Hilbert action by including an m2R 1 2 R term and the second by including an m2 1 R term. Both models posses self-accelerating solutions. I will demonstrate that their background cosmology is consistent with data, and testable primarily through the equation of state of our universe’s effective stress-energy tensor. At the perturbative level, these models have more galaxy clustering and weak lensing, so they are be highly testable using up coming cosmological surveys. My contribution to this work is the perturbation theory of the m2 1 R model and the recovery of these results for the m2R 1 2 R model.Show less
Cosmological probes come in a large variety of forms and unveil many characteristics of the Universe. The extraction of cosmological parameters from probes, such as the cosmic microwave background,...Show moreCosmological probes come in a large variety of forms and unveil many characteristics of the Universe. The extraction of cosmological parameters from probes, such as the cosmic microwave background, is done by fitting predictions of cosmological models to data. This requires the use of Boltzmann codes that possess powerful and efficient mechanisms for producing cosmological predictions. In this thesis the cosmological parameter extraction code CLASS is used to constrain primarily the energy densities and the Hubble parameter of the Universe. Four independent datasets that together probe the Universe at a large range of scales are used for this purpose. The data analysis is done within the framework of two different LCDM cosmological models. A stepwise description of a data analysis within the framework of a LWDM model is also provided. It is shown how combining independent datasets can break degeneracies between cosmological parameters, which is necessary to explore signatures of beyond standard model physics. Furthermore, a consistency check between CLASS and a second, independent Boltzmann code is performed and discussed.Show less
