String Theory Journal Club

We investigate the geometry near the horizon of a generic, four-dimensional extremal black hole. When the cosmological constant is negative, we show that (in almost all cases) tidal forces diverge as one crosses the horizon, and this singularity is stronger for larger black holes. Moreover, nearly-extremal black holes have large tidal forces at the horizon. Holographically, this singularity is reflected in an anomalous scaling of the specific heat with temperature. The situation becomes even more drastic in five dimensions where (due to the same mechanism) the extremal RN AdS becomes an RG-unstable fixed point. We discover a plethora of new fixed points described by black holes' horizons – all of them are not stable. Based on 2210.02473 and 2211.01385. Joint work with Gary Horowitz and Jorge Santos.