EFT Strings, Quantum Gravity Bounds and Wormholes

The subject of Quantum Gravity and its effects on physical observables and effective field theories remains one of the biggest challenges in theoretical physics. String Theory provides a controlled framework in which we can hope to gather some evidence of general features of Quantum Gravity, even if its large landscape of vacua seems to represent an obstacle to the ability of formulating universal statements. The Swampland Program has emerged to try and address such questions: by using consistency conditions coming from general considerations about Quantum Gravity we can distinguish between effective field theories of gravity that admit a UV completion and those that do not. In the first part of this thesis, we study the conditions coming from the coupling of $N=1$ effective theories of gravity in 4 dimensions to a class of 2-dimensional extended object called EFT strings and derive non-trivial constraints on the possible values of the (s)axionic couplings in the effective Lagrangian, in addition to upper bounds on the relevant gauge group of the theory. In the second part, we study the problem of the possible non-perturbative corrections from wormhole solutions and their relevance in the light of the previously derived consistency bounds. In doing so, we stress the interplay between EFT strings and the energy scale of validity of such solutions, represented by the species scale.