Abstract: We show that a generic extremal black hole in AdS is singular.
In four dimensions, the horizon is replaced by a mild null singularity.
That leaves significant imprints on the BH thermodynamics at
sufficiently low temperatures. In five dimensions, that singularity
becomes more serious. In particular, it makes RN AdS an unstable IR
fixed point. Many new fixed points (described by near-horizon
geometries) are constructed, and all of them are found to be unstable.
Finally, we show that the higher curvature corrections to the extremal
Kerr black hole make it singular as well. Furthermore, near-extremal
black holes can have large yet finite tidal forces for infalling
observers.

Bio: M. Kolanowski obtained his PhD at the University of Warsaw in 2023.
He is currently a postdoc at the University of California in Santa
Barbara. His research focuses on semiclassical black holes.