Large-field inflation, axions, and the weak gravity conjecture
Miguel Montero (IFT Madrid)

Last year, the BICEP2 collaboration announced the discovery of a significant scalar-to-tensor ratio r from CMB polarization measurements. Such a large value of r is related to a transplanckian field range for the inflaton in many models. Although the results were later shown to be mostly dust, this sparkled a lot of interest in obtaining models with transplanckian field ranges consistent with quantum gravity. It is notably difficult to do so in string theory, and for axion inflaton fields it has been argued that this difficulty is fundamental, coming from the so-called Weak Gravity Conjecture. The WGC demands the existence of instantons of small action which couple to the axion and make the effective range subplanckian. Although the WGC only applies to a complete theory of gravity, one can build and study similar instantons in an effective theory. We discuss quantum gravitational effects in Einstein theory coupled to axion scalars to analyze the viability of several proposals to achieve superplanckian axion periods. These effects generically spoil transplanckian field ranges in several popular models, like the Kim-Nilles-Peloso model. Finally we discuss the realization of these instantons as euclidean D-branes in string compactifications.