Black Holes, Moduli Stabilisation and the Swampland

Abstract: In theories with moduli, extremal black holes behave such that for generic initial conditions, the distance traveled by the scalars from infinity to the horizon can grow with the size of the black hole. This, in turn, implies that larger black holes can probe more of the UV ingredients of the theory, in contrast with (naive) EFT expectations. We relate this discrepancy to the lack of cosmological moduli stabilisation. Indeed, for would-be scale-separated string vacua with parametrically heavy stabilised scalars -- dubbed \emph{rigid} compactifications -- one recovers the EFT intuition where only small black holes probe the UV. We make this explicit in a toy model and then turn to top-down models and construct near-horizon solutions in IIA scale-separated compactifications with stabilised moduli. In these top-down models we still observe large field variations for large black holes which can be traced back to the absence of parametrically heavy moduli. We are led to speculate that needing UV physics to allow for non-local effects near the horizon of large black holes is at odds with having a rigid compactification, hinting to the possibility that such compactifications are in the Swampland.