First Law for Nonsingular Black Holes in 2D Dilaton Gravity

A central issue in the thermodynamics of nonsingular black holes is the apparent violation of the first law. In this work, we use 2D dilaton gravity as a simple theoretical setting to study this issue. We systematically construct a broad class of nonsingular black hole solutions with metric function $A(x)=f(x)+c$, through a procedure that is considerably simpler than in higher-dimensional theories. Using the Iyer-Wald covariant phase space formalism, we derive the correct energy formula and establish a consistent first law for this entire class of solutions. The apparent first-law violation in a previous work is caused by an incorrect choice of energy. Our energy formula agrees, up to a normalization factor for the asymptotic Killing vector, with the Casimir function in 2D dilaton gravity, confirming its interpretation as the physical black hole energy. Our results clarify the correct first law for 2D nonsingular black holes and may provide insights into the first law of nonsingular black holes in higher dimensions.

💡 Research Summary

The paper addresses a long‑standing puzzle in the thermodynamics of regular (nonsingular) black holes: many models appear to violate the first law of black‑hole thermodynamics when one simultaneously retains the Hawking temperature, the Bekenstein‑Hawking entropy, and the usual first‑law relation δM = T δS. The authors resolve this issue by working in two‑dimensional dilaton gravity, a setting that retains essential features of higher‑dimensional gravity while allowing exact analytic control.

Starting from the Weyl‑fixed action
(S=\frac12\int d^2x\sqrt{-g},