Until now the quantum field theory (QFT) that successfully describes the electric, weak and strong interactions (three out of the four fundamental forces) between particles is the Standard Model, but it omits gravity. The prime candidate for a quantum theory of gravity is string theory. However, recent developments in string theory suggest that a portion of the alternative quantum field theories that are being considered, are incompatible with gravity. In 2005, this led string theorists to outline the conditions an effective field theory (EFT) should satisfy in order to be consistent with a quantum theory of gravity. These conditions are the ones that separate the so-called landscape from the swampland. An EFT that satisfies these conditions is said to reside in the landscape, while EFTs that do not satisfy these conditions belong to the swampland. This mapping out of EFTs to the swampland gives rise to a number of predictions that are related to the physics of the Early universe and the nature of dark energy. The de Sitter conjecture and the Trans-Planckian censorship conjecture are some of these conditions and will be the main focus of this thesis. The main purpose behind this work is to gain a deep understanding of each criterium, as well as unravel their implications and predictions related to the dynamics of the Early Universe. We do this by writing a pedagogical introduction of the topic and by introducing some possible alternative to the inflationary scenario, cosmologies that seem to be consistent with the aforementioned constraints.

The neutron plays a central role in numerous fields of physics, a fact that entails a need for methods of measuring neutron energy spectra. In this project, a technique for neutron spectrometry through measurements of neutron-induced radioactivity in activation detectors was developed and tested. The developed technique involves irradiating element samples with neutrons, measuring activation products with a gamma spectrometer, and then performing a neutron spectrum unfolding procedure. The elements indium, iron, magnesium, aluminium, zinc, titanium, and copper were used as activation detectors and irradiated with neutrons from an americium-beryllium (AmBe) neutron source. Subsequent gamma spectrometry was performed with the UGGLA high-purity germanium detector setup at Uppsala University. The GRAVEL unfolding algorithm was implemented in MATLAB and used to unfold neutron spectra based on an initial spectrum guess. The unfolded neutron spectrum agrees well with the expected AmBe spectrum, though some difference between the spectra is attributed to neutron scattering in the irradiation environment. A possible ability to find approximate neutron spectra from inaccurate initial guesses is found, but additional work is needed to understand better how the initial guess affects the result for different neutron sources. Because activation detectors do not require electrical power when measuring neutrons, can be made sensitive to a wide range of neutron energies, and do not detect other types of radiation, future applications could find the developed neutron spectrometry method practical.

The IceCube Neutrino Observatory has sensitivity to MeV electron antineutrinos from core-collapse supernovae through an excess of the detection rate over the background. Wavelength-shifting sensors have the potential to greatly increase photon collection making it a promising candidate for improving the measurement of the supernova neutrino light curve in IceCube-Gen2. For high-energy neutrino reconstruction, the competing effect of increased photon collection and the broader time distribution necessitate detailed simulations to determine the impact. In this thesis, we investigate the sensitivity gain caused by wavelength shifters in a future IceCube-Gen2 detector regarding the detection of faint modulations of the supernova neutrino lightcurve. Furthermore, we lay the groundwork for a future high-energy reconstruction through the implementation and integration of wavelength-shifting sensors into the IceCube simulation framework.

The PANDA experiment at FAIR is an antiproton experiment and it will offer unique possibilities for the study of hadron physics including hyperon physics. With the PANDA detector, it will be possible to exclusively reconstruct hyperon-antihyperon events. Hyperons are relatively long-lived particles (10−10s), which can travel for a distance of up to a few meters before decaying into their reconstructible decay products. In this project, I focus on the reaction p̅ p → Ξ̄+ Ξ− → Λ̅ π+ Λ π− → p̅ π+π+ p π− π−. The Ξ− (cascade) hyperons take a special role as they are the lightest hyperons to introduce a sequential decay topology, i.e., Ξ− → Λ π− → p π− π− (and c.c.). This constitutes a substantial challenge for the reconstruction algorithms. In this thesis, a Monte-Carlo simulation study of realistic track and event reconstruction of this Ξ̄+ Ξ− reaction in the region near the production threshold is done, and the event generation of the Ξ̄+ Ξ− reaction channel was performed by the EvtGen event generator in PandaRoot. For various realistic tracking algorithms, track and event reconstruction performance levels were investigated in PandaRoot.

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This thesis reports the first indications of the Λ(1520) and ∆(1232)++ decay modes in the Ξc0 → pK−K−π+ decay. The obtained fractions relative the inclusive Ξc0 → pK−K−π+ decay are (6.4±0.6)% and (32.6±1.2)% for the decays through the Λ(1520) and ∆(1232)++ resonances respectively. The estimates were done on a sample of 6449±226 events of the Ξc0 baryon originating in the pure hadron Ξb− → Ξc0π− decay. The fractions presented in this thesis should be seen as an early indication rather than final estimates as more in-depth studies need to be done.

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This thesis describes the measurements of collective effects in strongly coupled ultra-cold neutral plasmas (UNPs). It shows the implementation of experimental techniques that perturb either the density or velocity distribution of the plasma and it describes the subsequent excitation, observation and analysis of the aforementioned collective phenomena. UNPs are interesting in that they display physics of strongly coupled systems. For most plasma systems, collective effects are well described with classical hydrodynamic or kinetic descriptions. However, for strongly coupled systems, the Coulomb interaction energy between nearest neighbors exceeds the kinetic energy, and these descriptions must be modified as the plasma crosses over from a gas-like to liquid-like behavior. Strongly coupling can be found in exotic plasma systems found astrophysics, dusty plasmas, non-neutral trapped ion plasmas, intense-laser/matter interactions and inertial confinement fusion experiments. Compared to other strongly coupled plasmas, UNPs are ideal for studying collective effects in this regime since they have lower timescales, precisely controllable initial conditions and non-invasive diagnostics. Previous studies of UNPs concentrated on plasma expansion dynamics and some collective effects such as disorder induced heating, but little work had been done in relaxation or collision rates and collective modes in UNPs. This thesis presents a method for measuring collision rates by perturbing the velocity distribution of the plasma, observing plasma relaxation and measuring the relaxation rate. It also presents a new technique for observing collective modes in the plasma by perturbing the initial density of the plasma and how this results in the excitation of ion acoustic waves and a measurement of its dispersion relation. Finally, this thesis presents how this last technique can be used to create a gap in the center of the plasma and how this leads to hole propagation and plasma streaming and presents a characterization of both phenomena. The result of these experiments will be valuable for predicting the behavior of collective effects in other strongly coupled plasmas and for comparison with theories that describe them.