Stability of Euclidean Wormholes in the Axiverse

In Axiverse models, namely models of pseudo-scalar fields coupled with gravity, it is common to encounter particular solutions known as gravitational instantons. These metric configurations are of interest across various theoretical fields of physics, such as cosmology and the phenomenological axion physics, but they are especially relevant to Quantum Gravity research. In particular, a specific class of solutions, referred to as wormholes, is expected to provide non-negligible contributions to low-energy Effective Field Theories derived from String Theory models. Wormholes consist in specific configurations of the Euclidean space-time metric that connect distant, non causally-related points of the universe. They also reveal interesting aspects of the theory, potentially offering insights into the structural features of Quantum Gravity. Despite the fact that the known practical effects of wormholes have been studied for years, the issue of whether Euclidean wormholes should be considered in the low-energy limit remains unsolved. Indeed, it is necessary to investigate their stability in order to determine whether they could yield real physical contributions. This problem has been addressed multiple times over the last 40 years, but a definitive answer is still lacking. Based on a recent work that confirms their stability in a simple model, this thesis aims to reproduce this result and further investigate the stability in a more general framework.