The Quantum Gravity Scale and the Swampland

This thesis investigates the role of the quantum gravity cut-off for effective field theories (EFTs) coupled to Einstein gravity, with an emphasis on its implications at low energies within the context of the Swampland program. Part I reviews the relevant aspects of string theory compactifications in different number of spacetime dimensions and with different amounts of supersymmetry preserved. In Part II a model-independent approach is employed so as to determine the maximum regime of validity of any such EFT, identifying the species scale as the natural candidate for the quantum gravity cut-off. We review various arguments proposed in the literature as well as include several new considerations on the matter. Part III provides a systematic study of this framework in string theory compactifications, yielding significant agreement with the previous perturbative and non-perturbative analysis. We also analyze various applications of this concept within the Swampland program, including the purported phenomenon of Emergence. Finally, in Part IV we explore the most immediate implications that this picture would have in the infrared regime, thus uncovering intriguing universal properties associated to the aforementioned energy scale, such as precise lower bounds on its exponential decay rates as well as certain patterns holding within the infinite distance corners of moduli space. The thesis includes new results scattered over the different chapters therein, which have not appeared in the author's original publications.