The String Swampland and Quantum Gravity Constraints on Effective Theories

Coordinators: Hiroshi Ooguri, Gary Shiu, Cumrun Vafa, and Irene Valenzuela

The idea that the string landscape is too large to lead to concrete predictions has been countered by the idea that most of the naively consistent effective theories of gravity coupled to matter are actually inconsistent and belong to the swampland. The identification of criteria distinguishing the true string landscape from the swampland, which has been studied for more than a decade now, is beginning to reach a more mature stage with the developments of the last few years. In particular a conjectured consistency condition for quantum gravity known as the Weak Gravity Conjecture (WGC), which postulates that gravity is always the weakest force among all the forces, has found an unexpectedly broad range of applications.

The WGC on the one hand has been used to constrain cosmological models of inflation including scenarios being tested by the present generation of CMB experiments and on the other hand has been connected to the cosmic censorship conjecture of general relativity. Furthermore, ideas from holography have been found to be nicely consistent with the WGC. Moreover a sharpened version of the WGC has been used to put constraints on particle phenomenology and in particular has been used to place bounds on the neutrino masses. This program will bring together the diverse communities of string theorists, cosmologists, general relativists, particle phenomenologists and researchers working on holography and the conformal bootstrap to further develop consistency criteria for quantum theories of gravity and possibly extract concrete predictions from these ideas for the observable universe as well as deepen our understanding of the structure of string vacua.