Classical black hole probes of UV scales

A bstract In the context of the Swampland program, black hole attractors have been employed to probe infinite distances in moduli space, where the EFT cutoff goes to zero in Planck units and UV effects become significant. In this paper, we take the perspective of the two-derivative action of string theoretic effective field theories and explore various families of extremal black hole solutions that probe infinite distance limits at their horizons. While these solutions do not include higher-order corrections in the EFT expansion, we find that, in many cases, the smallest BPS black holes in these families remarkably reproduce either the species scale or some other Kaluza-Klein scale. In highly supersymmetric cases, this match with UV scales even persists in the interior of moduli space. We explore the use of the species length bound on classical black hole sizes as a criterion to rule out inconsistent EFTs, using a 9d bottom-up model arising from an ad hoc truncation of maximal supergravity. These observations suggests that the two-derivative action may encode information about relevant UV scales. We discuss the interplay of these results with emergence and UV/IR mixing in quantum gravity.