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Modified Gravity: From Black Holes Entropy to Current Cosmology, 4th Edition

A special issue of Entropy (ISSN 1099-4300). This special issue belongs to the section "Astrophysics, Cosmology, and Black Holes".

Deadline for manuscript submissions: 31 August 2025 | Viewed by 5190

Special Issue Editor

Special Issue Information

Dear Colleagues,

Recent cosmological observations strongly support that the current expansion of the universe is accelerating. The origin of such a cosmic acceleration mechanism is one of the most significant problems in modern cosmology. Indeed, this is shown by the fact that the Nobel Prize in Physics 2011 was presented to the finding of the current cosmic acceleration by means of the observations of the Type Ia supernovae.

There are two representative approaches to explain the current accelerated expansion of the universe. One is to introduce “dark energy” in the framework of general relativity. The other is to modify a gravitational theory, such as f(R) gravity, so that we can obtain so-called geometrical dark energy. It is believed that a modified gravitational theory must pass cosmological bounds and solar system tests because it corresponds to an alternative theory of gravitation to general relativity. As another meaningful touchstone of modified gravity, it is important to examine whether the second law of thermodynamics can be satisfied in the models of modified gravity.

The fundamental connection between gravitation and thermodynamics has been suggested by the discovery of black hole thermodynamics with black hole entropy and Hawking temperature. In addition, it was shown that the Einstein equation can be derived from the proportionality of the entropy to the horizon area together with the Clausius relation in thermodynamics. This consequence has been applied to various cosmological settings as well as modified gravitational theories. In particular, the connections between thermodynamics and modified gravity have recently been discussed extensively.

In this Special Issue, we discuss the application of thermodynamics to the test of a successful alternative gravitational theory to general relativity. Through this procedure, we can obtain a clue to resolve the dark energy problem “geometrically”. It is considered that any successful modified gravity theory should obey the second law of thermodynamics. If the second law is violated in certain universes in a model, it is more likely to be due to an incorrect generalization of the second law or some inherent inconsistency of the model itself. For the latter case, the model should be abandoned. It is strongly expected that the considerations of this Special Issue can produce a new physical understanding of entropy in the context of the relation between thermodynamics and gravitation and shed light on novel compounds, as well as insights into modern cosmology, in particular new properties of dark energy.

Dr. Kazuharu Bamba
Guest Editor

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Keywords

  • quantum aspects of black holes, evaporation, thermodynamics
  • black hole entropy
  • modified theories of gravity
  • dark energy
  • inflationary cosmology
  • observational cosmology
  • physics in the early universe
  • astrophysics of compact objects

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Published Papers (5 papers)

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Research

13 pages, 281 KB  
Article
Entropy Modifications of Charged Accelerating Anti-de Sitter Black Hole
by Cong Wang, Jie Zhang and Shu-Zheng Yang
Entropy 2025, 27(9), 900; https://doi.org/10.3390/e27090900 - 25 Aug 2025
Viewed by 160
Abstract
The Lorentz-breaking theory not only modifies the geometric structure of curved spacetime but also significantly alters the quantum dynamics of bosonic and fermionic fields in black hole spacetime, leading to observable physical effects on Hawking temperature and Bekenstein–Hawking entropy. This study establishes the [...] Read more.
The Lorentz-breaking theory not only modifies the geometric structure of curved spacetime but also significantly alters the quantum dynamics of bosonic and fermionic fields in black hole spacetime, leading to observable physical effects on Hawking temperature and Bekenstein–Hawking entropy. This study establishes the first systematic theoretical framework for entropy modifications of charged accelerating Anti-de Sitter black holes, incorporating gauge-invariant corrections derived from Lorentz-violating quantum field equations in curved spacetime. The obtained analytical expression coherently integrates semi-classical approximations with higher-order quantum perturbative contributions. Furthermore, the methodologies employed and the resultant conclusions are subjected to rigorous analysis, establishing their physical significance for advancing fundamental investigations into black hole entropy. Full article
13 pages, 452 KB  
Article
Two Dynamical Scenarios for Binned Master Sample Interpretation
by Giovanni Montani, Elisa Fazzari, Nakia Carlevaro and Maria Giovanna Dainotti
Entropy 2025, 27(9), 895; https://doi.org/10.3390/e27090895 - 24 Aug 2025
Viewed by 131
Abstract
We analyze two different scenarios for the late universe dynamics, resulting in Hubble parameters deviating from the ΛCDM, mainly for the presence of an additional free parameter, which is the dark energy parameter. The first model consists of a pure evolutionary dark [...] Read more.
We analyze two different scenarios for the late universe dynamics, resulting in Hubble parameters deviating from the ΛCDM, mainly for the presence of an additional free parameter, which is the dark energy parameter. The first model consists of a pure evolutionary dark energy paradigm as a result of its creation by the gravitational field of the expanding universe. The second model also considers an interaction of the evolutionary dark energy with the matter component, postulated via the conservation of the sum of their ideal energy–momentum tensors. These two models are then compared via the diagnostic tool of the effective running Hubble constant, with the binned data of the so-called “Master sample” for the Type Ia Supernovae. The comparison procedures, based on a standard MCMC analysis, lead to a clear preference of data for the dark energy–matter interaction model, which is associated with a phantom matter equation of state parameter (very close to −1) when, being left free by data (it has a flat posterior), it is fixed in order to reproduce the decreasing power-law behavior of the effective running Hubble constant, already discussed in the literature. Full article
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9 pages, 262 KB  
Article
The Hyperbolically Symmetric Black Hole
by Luis Herrera and Louis Witten
Entropy 2025, 27(8), 831; https://doi.org/10.3390/e27080831 - 5 Aug 2025
Viewed by 394
Abstract
We describe some properties of the hyperbolically symmetric black hole (hereafter referred to as the HSBH) proposed a few years ago. We start by explaining the main motivation behind such an idea, and we determine the main differences between [...] Read more.
We describe some properties of the hyperbolically symmetric black hole (hereafter referred to as the HSBH) proposed a few years ago. We start by explaining the main motivation behind such an idea, and we determine the main differences between this scenario and the classical black hole (hereafter referred to as the CBH) scenario. Particularly important are the facts that, in the HSBH scenario, (i) test particles in the region inside the horizon experience a repulsive force that prevents them from reaching the center, (ii) test particles may cross the horizon outward only along the symmetry axis, and (iii) the spacetime within the horizon is static but not spherically symmetric. Next, we examine the differences between the two models of black holes in light of the Landauer principle and the Hawking results on the eventual evaporation of the black hole and the paradox resulting thereof. Finally, we explore what observational signature could be invoked to confirm or dismiss the model. Full article
10 pages, 3306 KB  
Article
Modified Gravity in the Presence of Matter Creation: Scenario for the Late Universe
by Giovanni Montani, Nakia Carlevaro and Mariaveronica De Angelis
Entropy 2024, 26(8), 662; https://doi.org/10.3390/e26080662 - 4 Aug 2024
Cited by 12 | Viewed by 1555
Abstract
We consider a dynamic scenario for characterizing the late Universe evolution, aiming to mitigate the Hubble tension. Specifically, we consider a metric f(R) gravity in the Jordan frame which is implemented to the dynamics of a flat isotropic Universe. This [...] Read more.
We consider a dynamic scenario for characterizing the late Universe evolution, aiming to mitigate the Hubble tension. Specifically, we consider a metric f(R) gravity in the Jordan frame which is implemented to the dynamics of a flat isotropic Universe. This cosmological model incorporates a matter creation process, due to the time variation of the cosmological gravitational field. We model particle creation by representing the isotropic Universe (specifically, a given fiducial volume) as an open thermodynamic system. The resulting dynamical model involves four unknowns: the Hubble parameter, the non-minimally coupled scalar field, its potential, and the energy density of the matter component. We impose suitable conditions to derive a closed system for these functions of the redshift. In this model, the vacuum energy density of the present Universe is determined by the scalar field potential, in line with the modified gravity scenario. Hence, we construct a viable model, determining the form of the f(R) theory a posteriori and appropriately constraining the phenomenological parameters of the matter creation process to eliminate tachyon modes. Finally, by analyzing the allowed parameter space, we demonstrate that the Planck evolution of the Hubble parameter can be reconciled with the late Universe dynamics, thus alleviating the Hubble tension. Full article
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13 pages, 279 KB  
Article
Corrections to the Bekenstein–Hawking Entropy of the HNUTKN Black Hole Due to Lorentz-Breaking Fermionic Einstein–Aether Theory
by Xia Tan, Cong Wang and Shu-Zheng Yang
Entropy 2024, 26(4), 326; https://doi.org/10.3390/e26040326 - 11 Apr 2024
Cited by 2 | Viewed by 1598
Abstract
A hot NUT–Kerr–Newman black hole is a general stationary axisymmetric black hole. In this black hole spacetime, the dynamical equations of fermions at the horizon are modified by considering Lorentz breaking. The corrections to the Hawking temperature and Bekenstein–Hawking entropy at the horizon [...] Read more.
A hot NUT–Kerr–Newman black hole is a general stationary axisymmetric black hole. In this black hole spacetime, the dynamical equations of fermions at the horizon are modified by considering Lorentz breaking. The corrections to the Hawking temperature and Bekenstein–Hawking entropy at the horizon of the black hole are studied in depth. Based on the semiclassical theory correction, the Bekenstein–Hawking entropy of this black hole is quantum-corrected by considering the perturbation effect of the Planck constant . The latter part of this paper presents a detailed discussion of the obtained results and their physical implications. Full article
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