From Nonextremal to Extremal: Entropy of Reissner-Nordström and Kerr black holes Revisited

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Main Authors: Fairoos, C., Singha, Chiranjeeb
Format: Preprint
Published: 2025
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author Fairoos, C.
Singha, Chiranjeeb
author_facet Fairoos, C.
Singha, Chiranjeeb
contents In this paper, we derive the entropy of Reissner-Nordström (RN) and Kerr black holes using the Hawking-Gibbons path integral method. We determine the periodicity of the Euclidean time coordinate using two approaches: first, by analyzing the near-horizon geometry, and second, by applying the Chern-Gauss-Bonnet (CGB) theorem. For non-extremal cases, both these methods yield a consistent and unique periodicity, which in turn leads to a well-defined expression for the entropy. In contrast, the extremal case exhibits a crucial difference. The absence of a conical structure in the near-horizon geometry implies that the periodicity of the Euclidean time is no longer uniquely fixed within the Hawking-Gibbons framework. The CGB theorem also fails to constrain the periodicity, as the corresponding Euler characteristic vanishes. As a result, the entropy cannot be uniquely determined using either method.
format Preprint
id arxiv_https___arxiv_org_abs_2505_16309
institution arXiv
publishDate 2025
record_format arxiv
spellingShingle From Nonextremal to Extremal: Entropy of Reissner-Nordström and Kerr black holes Revisited
Fairoos, C.
Singha, Chiranjeeb
General Relativity and Quantum Cosmology
In this paper, we derive the entropy of Reissner-Nordström (RN) and Kerr black holes using the Hawking-Gibbons path integral method. We determine the periodicity of the Euclidean time coordinate using two approaches: first, by analyzing the near-horizon geometry, and second, by applying the Chern-Gauss-Bonnet (CGB) theorem. For non-extremal cases, both these methods yield a consistent and unique periodicity, which in turn leads to a well-defined expression for the entropy. In contrast, the extremal case exhibits a crucial difference. The absence of a conical structure in the near-horizon geometry implies that the periodicity of the Euclidean time is no longer uniquely fixed within the Hawking-Gibbons framework. The CGB theorem also fails to constrain the periodicity, as the corresponding Euler characteristic vanishes. As a result, the entropy cannot be uniquely determined using either method.
title From Nonextremal to Extremal: Entropy of Reissner-Nordström and Kerr black holes Revisited
topic General Relativity and Quantum Cosmology
url https://arxiv.org/abs/2505.16309