On The Black Hole Weak Gravity Conjecture and Extremality in the Strong-Field Regime

Abstract: We point out that the Weak Gravity Conjecture (WGC) implies that sufficiently small extremal black holes are necessarily in the strong-field regime of electrodynamics, and therefore probe the UV completion of the Maxwell sector. To investigate the WGC bounds arising from these small extremal black holes, we revisit black hole decay in generic field theories in asymptotic flat space. We show that a general, sufficient condition for any black hole to decay is a bound on charge growth as a function of mass. We apply this decay condition to extremal black holes derived in some UV completions of the Maxwell sector. We find that the Euler-Heisenberg and DBI effective actions satisfy the charge growth bound, while the ModMax model does not, making it incompatible with the weak gravity conjecture. We show that the charge growth bound implies positivity of the $U(1)$ gauge coupling beta function. This provides an independent argument that classically stable (embedded-Abelian) colored black holes cannot exist. The charge growth bound constrains conformal hidden sector models, and is always satisfied in their AdS dual realizations.