Thesis defense of Sara Krieg

The guiding idea of this thesis is to investigate how subtle traces of gravity manifest themselves across different scales. First, we examine whether the CKN bound can form the basis of a consistent cosmological model and whether such a model is favored by observational data, in particular the recent baryonic acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument.
Taking into account additional late-universe measurements, there is a mild preference for the $(\nu)$CKN model over $\Lambda$CDM. However, once weak lensing and cosmological microwave background data are included and the early-time dynamics are accounted for, the model becomes observationally indistinguishable from $\Lambda$CDM in the theoretical allowed regime. Second, we develop an internal wave packet formalism for neutrino oscillations in weakly curved spacetimes. Gravitational corrections appear as phase modifications, but these effects lie far beyond current experimental sensitivity. Importantly, if deviations from expected oscillation patterns are observed, classical gravity can be ruled out as their source, thereby pointing to new physics beyond the Standard Model, such as quantum gravity. Finally, we employ the open quantum system framework to model QG-induced decoherence of high-energy neutrinos and perform a sensitivity analysis using IceCube data. We derive bounds on the damping parameter $\gamma$ for different energy scalings and obtain the first constraints for scenarios involving additional dark fermions.