Obstructions to super Planckian field excursions in String Theory

I will discuss the question of whether it is possible for a scalar field to obtain a vacuum expectation value larger than the Planck scale within an effective theory that originates from a consistent theory of quantum gravity and in particular string theory I will focus on axion like fields in string theory which have a shift symmetry protecting them Super Planckian field excursions are a necessary condition for large field inflation and therefore this question has bearing on the possible magnitude of the primordial gravitational waves searched for by current and future experiments such as BICEP3 Spider and CMBPol