A journey through the non-Gaussian universe

Abstract
Over the past three decades, precise measurements have significantly advanced our understanding of the universe, leading to the establishment of a standard cosmological model, known as the $Lambda$CDM. This model has successfully explained a wide range of observations, including the cosmic microwave background radiation (CMB), the distribution of galaxies, and the accelerated expansion of the universe. Its robustness and consistency with observational data have made it the foundation of modern cosmology. However, there are several aspects of our universe that are unanswered. This dissertation delves into some of the possible extensions of the $Lambda$CDM model and explores ways to refine constraints on fundamental cosmological parameters. The first part of the thesis presents an extensive review on topics of modern cosmology relevant to the thesis, including cosmic inflation, cosmological perturbations and weak gravitational lensing. The second part is dedicated to the original research work performed by the author. In chapter 5, we discuss fundamental limitations on constraining primordial non-Gaussianity and identify promising observables for upcoming CMB and large scale structure experiments. In chapter 6, we build the groundwork for the reconstruction of the CMB lensing convergence bispectrum, studying, for the first time, some of the observational challenges. In chapter 7, we explore the potential of the cross-correlation between radio cosmic shear and CMB lensing in constraining $Lambda$CDM parameters and the neutrino sector.

Current driven instabilities and non-lineairities in magnetic systems

Abstract
In this Thesis we developed new ideas to address two central questions in the field of spintronics and magnonics. Namely, how does one extend the lifetime of spin waves and how does one coherently inject spin waves using direct currents (DC)? Most of the proposals we developed use analogies between high-energy and condensed matter physics. For instance, antiparticlelike negative energy states can exist in driven magnetic systems. We showed that these negative energy modes can be used to enhance spin waves of an interface and coherently inject spin waves using DC currents. Besides this, this thesis also contains a chapter on non-linear magnetization dynamics of coupled macrospins and two chapters on the interplay between exchange- and magnetostatic interactions.

Non-equilibrium topology in magnonic systems

Abstract
In this Thesis, we study the observable consequences of topological magnetic excitations and how they can be used for applications on the long term. Since direct detection is not feasible, we study a series of alternative approaches to access the effects of topology in magnetic systems, with a focus on the effect of topology on the transport properties of spin wave systems. We first study the topological effects rooted in the dipole-dipole interaction – an interaction which is present in any magnon systems. We study the propagation of spin waves in obliquely magnetized thin films, where the dipole-dipole interaction breaks the inversion symmetry. We show that a single spin wave excited by a microwave antenna will induce a transverse spin current, which can be detected by a normal metal lead. Next, we study how the magnon Hall effect can be detected through electrical injection and detection. We show that the Hall coefficient, when taking dipole-dipole interactions into account, can be measured. We then turn to topological magnon phases, which have topologically protected edge states, addressing a major problem in the field of magnon topology: the high frequencies of the chiral edge modes in magnon Chern insulators. We propose a strategy to lower these chiral edge states to zero frequency, by considering a non-equilibrium setup. We show that the spin waves can be excited in the GHz-range, making them directly experimentally detectable. We finally consider non-Hermitian topological magnon phases. In a non-Hermitian extension of a one-dimensional topological magnon insulator we show the edge excitations to be robust against non-linearities and stochastics fluctuations. Within a non-Hermitian extension of the magnon Chern insulator we show that the chiral edge states can be topologically amplified or damped. This model also exhibits a novel non-Hermitian hybrid skin effect, where the edge modes are localized on specific sides of the lattice, and we propose an experiment to detect this skin effect.