Entropy of extremal black holes from entropy of quasiblack holes

Abstract

The entropy of extremal black holes (BHs) is obtained using a continuity argument from extremal quasiblack holes (QBHs). It is shown that there exists a smooth limiting transition in which (i) the system boundary approaches the extremal Reissner-Nordstr\"{o}m (RN) horizon, (ii) the temperature at infinity tends to zero and quantum backreaction remains bounded on the horizon, and (iii) the first law of thermodynamics is satisfied.The conclusion is that the entropy $S$ of extremal QBHs and of extremal BHs can take any non-negative value, only in particular cases it coincides with $ S=A/4$. The choice $S=0$ with non-zero temperature at infinity is rejected as physically unsatisfactory