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5 pages, 215 KB

Open AccessCommunication

Upper Bound of Barrow Entropy Index from Black Hole Fragmentation

by Jiayi Xia and Yen Chin Ong

Universe 2024, 10(4), 177; https://doi.org/10.3390/universe10040177 - 11 Apr 2024

Cited by 5 | Viewed by 1737

Both classical and quantum arguments suggest that if Barrow entropy is correct, its index

δ

must be energy-dependent, which would affect the very early universe. Based on thermodynamic stability that sufficiently large black holes should not fragment, we argue that Barrow entropy correction [...] Read more.

Both classical and quantum arguments suggest that if Barrow entropy is correct, its index

δ

must be energy-dependent, which would affect the very early universe. Based on thermodynamic stability that sufficiently large black holes should not fragment, we argue that Barrow entropy correction must be small, except possibly at the Planckian regime. Furthermore, the fact that a solar mass black hole does not fragment implies an upper bound

δ ≲ O (10^{- 3})

, which surprisingly lies in the same range as the bound obtained from some cosmological considerations assuming fixed

δ

. This indicates that allowing

δ

to run does not raise its allowed value. We briefly comment on the case of Kaniadakis entropy. Full article

(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)

19 pages, 507 KB

Open AccessArticle

Lagrangian Reconstruction of Barrow Holographic Dark Energy in Interacting Tachyon Model

by Giuseppe Gaetano Luciano and Yang Liu

Symmetry 2023, 15(5), 1129; https://doi.org/10.3390/sym15051129 - 22 May 2023

Cited by 14 | Viewed by 1707

Abstract

We consider a correspondence between the tachyon dark energy model and Barrow holographic dark energy (BHDE). The latter is a modified scenario based on the application of the holographic principle with Barrow entropy instead of the usual Bekenstein–Hawking one. We reconstruct the dynamics [...] Read more.

We consider a correspondence between the tachyon dark energy model and Barrow holographic dark energy (BHDE). The latter is a modified scenario based on the application of the holographic principle with Barrow entropy instead of the usual Bekenstein–Hawking one. We reconstruct the dynamics of the tachyon scalar field T in a curved Friedmann–Robertson–Walker universe both in the presence and absence of interactions between dark energy and matter. As a result, we show that the tachyon field exhibits non-trivial dynamics. In a flat universe,

{\dot{T}}^{2}

must always be vanishing, independently of the existence of interaction. This implies

ω_{D} = - 1

for the equation-of-state parameter, which in turn can be used for modeling the cosmological constant behavior. On the other hand, for a non-flat universe and various values of the Barrow parameter, we find that

{\dot{T}}^{2}

decreases monotonically for increasing

cos (R_{h} / a)

and

cosh (R_{h} / a)

, where

R_{h}

and a are the future event horizon and the scale factor, respectively. Specifically,

{\dot{T}}^{2} \geq 0

for a closed universe, while

{\dot{T}}^{2} < 0

for an open one, which is physically not allowed. We finally comment on the inflation mechanism and trans-Planckian censorship conjecture in BHDE and discuss observational consistency of our model. Full article

(This article belongs to the Special Issue Noether Symmetries in Gravitation and Cosmology)

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12 pages, 505 KB

Open AccessArticle

Holographic Dark Energy in Modified Barrow Cosmology

by Ahmad Sheykhi and Maral Sahebi Hamedan

Entropy 2023, 25(4), 569; https://doi.org/10.3390/e25040569 - 26 Mar 2023

Cited by 15 | Viewed by 2223

Abstract

Thermodynamics–gravity conjecture implies that there is a deep connection between the gravitational field equations and the first law of thermodynamics. Therefore, any modification to the entropy expression directly modifies the field equations. By considering the modified Barrow entropy associated with the apparent horizon, [...] Read more.

Thermodynamics–gravity conjecture implies that there is a deep connection between the gravitational field equations and the first law of thermodynamics. Therefore, any modification to the entropy expression directly modifies the field equations. By considering the modified Barrow entropy associated with the apparent horizon, the Friedmann equations are modified as well. In this paper, we reconsider the holographic dark energy (HDE) model when the entropy is in the form of Barrow entropy. This modification to the entropy not only changes the energy density of the HDE but also modifies the Friedmann equations. Therefore, one should take into account the modified HDE in the context of modified Friedmann equations. We study the Hubble horizon and the future event horizon as IR cutoffs and investigate the cosmological consequences of this model. We also extend our study to the case where dark matter (DM) and dark energy (DE) interact with each other. We observe that Barrow exponent

δ

significantly affects the cosmological behavior of HDE, and in particular, the equation of state (EoS) parameter can cross the phantom line

(w_{d e} < - 1)

. Additionally, adding

δ

remarkably affects the deceleration parameter and shifts the time of universe phase transition. Full article

(This article belongs to the Special Issue Geometrothermodynamics and Its Applications)

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23 pages, 1369 KB

Open AccessArticle

Applications of Thermodynamic Geometries to Conformal Regular Black Holes: A Comparative Study

by Abdul Jawad, Mazhar Hussain and Shamaila Rani

Universe 2023, 9(2), 87; https://doi.org/10.3390/universe9020087 - 7 Feb 2023

Cited by 5 | Viewed by 1996

Abstract

In this paper, we investigate the thermal stability and thermodynamic geometries of non-rotating/rotating charged black holes. For these black holes, we apply barrow entropy to determine the physical quantities such as mass and temperature of the system and find their stability through first [...] Read more.

In this paper, we investigate the thermal stability and thermodynamic geometries of non-rotating/rotating charged black holes. For these black holes, we apply barrow entropy to determine the physical quantities such as mass and temperature of the system and find their stability through first and second phase transitions of the heat capacity. We analyze the effects of scalar charge Q and hair parameter

λ

on black holes properties by taking both positive and negative values of these parameters. It is noted that heat capacity provide the stable, unstable regions and phase transition points for both black holes. To investigate the thermodynamic geometry of these black holes, various techniques such as Ruppeiner, Weinhold, Quevedo, and HPEM metrics are considered. It is observed that Weinhold, Quevedo, and HPEM give attractive/repulsive behavior of particles in stable/unstable regions of black holes. Full article

(This article belongs to the Section Compact Objects)

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10 pages, 294 KB

Open AccessArticle

Sign Switching Dark Energy from a Running Barrow Entropy

by Sofia Di Gennaro and Yen Chin Ong

Universe 2022, 8(10), 541; https://doi.org/10.3390/universe8100541 - 19 Oct 2022

Cited by 52 | Viewed by 2140

Abstract

Barrow proposed that the area law of the entropy associated with a horizon might receive a “fractal correction” due to quantum gravitational effects—in place of

S \propto A

, we have instead

S \propto A^{1 + δ / 2}

, where [...] Read more.

Barrow proposed that the area law of the entropy associated with a horizon might receive a “fractal correction” due to quantum gravitational effects—in place of

S \propto A

, we have instead

S \propto A^{1 + δ / 2}

, where

0 ⩽ δ ⩽ 1

measures the deviation from the standard area law (

δ = 0

). Based on black hole thermodynamics, we argue that the Barrow entropy should run (i.e., energy scale dependent), which is reasonable given that quantum gravitational corrections are expected to be important only in the high-energy regime. When applied to the Friedmann equation, we demonstrate the possibility that such a running Barrow entropy index could give rise to a dynamical effective dark energy, which is asymptotically positive and vanishing, but negative at the Big Bang. Such a sign switching dark energy could help to alleviate the Hubble tension. Other cosmological implications are discussed. Full article

(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)

12 pages, 928 KB

Open AccessArticle

Phase Space Analysis of Barrow Agegraphic Dark Energy

by Hai Huang, Qihong Huang and Ruanjing Zhang

Universe 2022, 8(9), 467; https://doi.org/10.3390/universe8090467 - 7 Sep 2022

Cited by 8 | Viewed by 1969

Abstract

Using the Barrow entropy and considering the timescale as IR cutoff, a new holographic dark energy model named Barrow agegraphic dark energy (BADE) was proposed. We use phase space analysis method to discuss the evolution of the universe in three different mode of [...] Read more.

Using the Barrow entropy and considering the timescale as IR cutoff, a new holographic dark energy model named Barrow agegraphic dark energy (BADE) was proposed. We use phase space analysis method to discuss the evolution of the universe in three different mode of BADE (

Q = 0

;

Q = 3 α H (ρ_{m} + ρ_{D})

;

Q = H (α ρ_{m} + β ρ_{D})

). We find the attractor which represents the dark energy-dominated era exists in all cases. In the case

Q = 0

and

Q = H (α ρ_{m} + β ρ_{D})

with

β = 0

, the attractor can behave as the cosmological constant, and these models can used to mimic the cosmological constant. Full article

(This article belongs to the Special Issue Dark Matter and Dark Energy: Particle Physics, Cosmology, and Experimental Searches)

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17 pages, 646 KB

Open AccessArticle

Constraints on Barrow Entropy from M87* and S2 Star Observations

by Kimet Jusufi, Mustapha Azreg-Aïnou, Mubasher Jamil and Emmanuel N. Saridakis

Universe 2022, 8(2), 102; https://doi.org/10.3390/universe8020102 - 4 Feb 2022

Cited by 44 | Viewed by 2349

Abstract

We use data from M87* central black hole shadow, as well as from the S2 star observations, in order to extract constraints on Barrow entropy. The latter is a modified entropy arising from quantum-gravitational effects on the black hole horizon, quantified by the [...] Read more.

We use data from M87* central black hole shadow, as well as from the S2 star observations, in order to extract constraints on Barrow entropy. The latter is a modified entropy arising from quantum-gravitational effects on the black hole horizon, quantified by the new parameter

Δ

. Such a change in entropy leads to a change in temperature, as well as to the properties of the black hole and its shadow. We investigate the photon sphere and the shadow of a black hole with Barrow entropy, and assuming a simple model for infalling and radiating gas we estimate the corresponding intensity. Furthermore, we use the radius in order to extract the real part of the quasinormal modes, and for completeness we investigate the spherical accretion of matter onto the black hole, focusing on isothermal and polytropic test fluids. We extract the allowed parameter region, and by applying a Monte-Carlo-Markov Chains analysis we find that

Δ ≃ {0.0036}_{- 0.0145}^{+ 0.0792}

. Hence, our results place the upper bound

Δ ≲ 0.0828

at 1

σ

, a constraint that is less strong than the Big Bang Nucleosynthesis one, but significantly stronger than the late-time cosmological constraints. Full article

(This article belongs to the Special Issue Large Scale Structure of the Universe)

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17 pages, 705 KB

Open AccessArticle

Braneworld Inspires Cosmological Implications of Barrow Holographic Dark Energy

by Shamaila Rani and Nadeem Azhar

Universe 2021, 7(8), 268; https://doi.org/10.3390/universe7080268 - 27 Jul 2021

Cited by 23 | Viewed by 2927

Abstract

In the present manuscript, the evolution of the cosmic parameters and planes are being investigated in the framework of the DGP braneworld model. In this scenario, the interaction

Γ

between the Barrow holographic dark energy model (whose infrared cutoff scale is set by [...] Read more.

In the present manuscript, the evolution of the cosmic parameters and planes are being investigated in the framework of the DGP braneworld model. In this scenario, the interaction

Γ

between the Barrow holographic dark energy model (whose infrared cutoff scale is set by Hubble and event horizons) and pressureless dark matter are considered. We check the behavior of different cosmological parameters such as Hubble, equation of state, deceleration and squared speed of sound from the early matter-dominated era until the late-time acceleration. It is found that the range of Hubble parameter lies in the interval

95_{- 35}^{+ 35}

(for Hubble horizon) and

97_{- 23}^{+ 23}

(for event horizon). For both horizons, the equation of state parameter favors the phantom dominant era as well as the

Λ

CDM model while the deceleration parameter illustrates the accelerated expansion of the universe. Furthermore, stability of the underlying model is found through squared speed of sound. Furthermore, it is observed that

ω - ω_{ϑ}^{'}

plane corresponds to freezing and thawing region for Hubble and event horizons, respectively. Furthermore, statefinder plane shows the

Λ

CDM and Chaplygin gas behavior for both models. Finally, we investigate the thermodynamical nature of the underlying model through Barrow entropy as horizon entropy and found validity for both horizons. Full article

(This article belongs to the Special Issue Cosmological Models, Quantum Theories and Astrophysical Observations)

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14 pages, 315 KB

Open AccessArticle

The d-Dimensional Cosmological Constant and the Holographic Horizons

by Artyom V. Yurov and Valerian A. Yurov

Symmetry 2021, 13(2), 237; https://doi.org/10.3390/sym13020237 - 31 Jan 2021

Cited by 1 | Viewed by 2258

Abstract

This article is dedicated to establishing a novel approach to the cosmological constant, in which it is treated as an eigenvalue of a certain Sturm–Liouville problem. The key to this approach lies in the proper formulation of physically relevant boundary conditions. Our suggestion [...] Read more.

This article is dedicated to establishing a novel approach to the cosmological constant, in which it is treated as an eigenvalue of a certain Sturm–Liouville problem. The key to this approach lies in the proper formulation of physically relevant boundary conditions. Our suggestion in this regard is to utilize the “holographic boundary condition”, under which the cosmological horizon can only bear a natural (i.e., non-fractional) number of bits of information. Under this framework, we study the general d-dimensional problem and derive the general formula for the discrete spectrum of a positive energy density of vacuum. For the particular case of two dimensions, the resultant problem can be analytically solved in the degenerate hypergeometric functions, so it is possible to define explicitly a self-action potential, which determines the fields of matter in the model. We conclude the article by taking a look at the d-dimensional model of a fractal horizon, where the Bekenstein’s formula for the entropy gets replaced by the Barrow entropy. This gives us a chance to discuss a recently realized problem of possible existence of naked singularities in the

D \neq 3

models. Full article

(This article belongs to the Special Issue Cosmology and Extragalactic Astronomy)

13 pages, 119 KB

Open AccessArticle

Black Hole Entropy for Two Higher Derivative Theories of Gravity

by Emilio Bellini, Roberto Di Criscienzo, Lorenzo Sebastiani and Sergio Zerbini

Entropy 2010, 12(10), 2186-2198; https://doi.org/10.3390/e12102186 - 21 Oct 2010

Cited by 12 | Viewed by 8124

Abstract

The dark energy issue is attracting the attention of an increasing number of physicists all over the world. Among the possible alternatives to explain what as been named the “Mystery of the Millennium” are the so-called Modified Theories of Gravity. A crucial test [...] Read more.

The dark energy issue is attracting the attention of an increasing number of physicists all over the world. Among the possible alternatives to explain what as been named the “Mystery of the Millennium” are the so-called Modified Theories of Gravity. A crucial test for such models is represented by the existence and (if this is the case) the properties of their black hole solutions. Nowadays, to our knowledge, only two non-trivial, static, spherically symmetric, solutions with vanishing cosmological constant are known by Barrow & Clifton (2005) and Deser, Sarioglu & Tekin (2008). The aim of the paper is to discuss some features of such solutions, with emphasis on their thermodynamic properties such as entropy and temperature. Full article

(This article belongs to the Special Issue Entropy in Quantum Gravity)

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