New Insights on the Extremal Limit of Black Holes

In the main part of this talk we review recent progress clarifying some physical aspects of the extremal limit of black holes. We describe two puzzles that arise from a semiclassical treatment of near-extremal black hole thermodynamics. Both puzzles are resolved by realizing that quantum corrections become arbitrarily large at low temperatures, and we explain how the spectrum and dynamics of near-extremal black holes are modified. This analysis also implies that without low energy supersymmetry, such as in the real world, extremal black holes at exactly zero temperature do not exist since the classical picture breaks down completely. In the context of supergravity the analysis is modified; supersymmetric extremal black holes do exist and they are separated from the non-extremal spectrum by a gap power-law suppressed in the entropy. This justifies black hole microstate counting performed in the context of string theory. In the last part of the talk we present ongoing work that applies these ideas to some concrete setups.