EFT & Species Scale: Friends or foes?

Abstract

Recently the notion that quantum gravity effects could manifest at scales much lower than the Planck scale has seen an intense Swamplandish revival. Dozens of works have explored how the so-called species scale -- at which an effective description of gravity must break down -- relates to String Theory and the Swampland conjectures. In particular, the interplay between this scale and the abundant towers of states becoming lighter in asymptotic regions of moduli spaces has proved to be key in understanding the real scale of quantum gravity. Nevertheless concerns have been raised regarding the validity of using infinite towers of states when estimating this scale within Effective Field Theory and, more precisely, the consistency of cutting the tower part way through in a framework that relies on a clear separation of scales. In this work we take an EFT point-of-view and provide a detailed perturbative derivation of the species scale -- by computing the 1-loop graviton propagator in the presence of many fields -- thereby clarifying common sources of confusion in the literature. Not only do we clarify the setup, assumptions and regimes of validity of the result, but more importantly apply the same methods to an infinite tower of states. We show how each state in the tower contributes to the species scale and how the procedure of counting only ''light fields'' can be compatible with not cutting the tower, thereby maintaining the harmony between infinite towers and EFTs even in the context of the species scale.