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The outer event horizon r+ of a Reissner-Nordstrom black hole is shown to satisfy the conditions of a Non-Expanding Horizon and the inner horizon r− is assumed to be dynamically evolving in area as no restrictions are imposed. An area evolution law is formulated, using the expansion scalar, keeping in mind the imposed conditions. It is later shown that the expansion scalar is the black hole analogue to the thermodynamic heat flow, just as area of a horizon is the black hole analogue of entropy. It is shown that the expansion scalar and the thermodynamic heat of the horizon form a canonical pair and the Lagrangian and Hamiltonian equations are formulated for this condition. From the equations, the expansion is quantized and the Heisenberg Inequaltity for this system is formulated. [ABSTRACT FROM AUTHOR]

The prevalent opinion that infalling objects can freely cross a black hole horizon is based on the assumptions that the horizon region is governed by classical General Relativity and by specific singular coordinate transformations it is possible to remove divergences in the geodesic equations. However, the coordinate transformations usually used to demonstrate the geodesic completeness are of class C0, while the standard causality theory requires that the metric tensor to be at least C1. Introduction of C0-class functions leads to the appearance of the additional delta-like sources in the Einstein equations and in the equations for quantum particles. Therefore, to explore the horizon region, in addition to the classical geodesic equations, one needs to use equation of quantum particles. Applying physical boundary conditions at the Schwarzschild and Kerr event horizons, we show existence of the exponentially decay/enhanced solutions (with the complex phases) to the Klein-Gordon equation. This means that in semi-classical approximation particles probably do not enter the black hole horizon, but are absorbed/reflected by it. Then it follows that the minimal classical size of any isolated body is its horizon radius, what potentially can solve main black hole mysteries. [ABSTRACT FROM AUTHOR]

By considering the generalized uncertainty principle, the degrees of freedom near the apparent horizon of Vaidya black hole are calculated with the thin film model. The result shows that a cut-off can be introduced naturally rather than taking by hand. Furthermore, if the minimal length is chosen to be a specific value, the statistical entropy will satisfy the conventional area law at the horizon, which might reveal some deep things of the minimal length. [ABSTRACT FROM AUTHOR]

Recently some researchers (Sepehri and Shoorvazi Astrophys. Spaces Sci. 344(2), 521-527, 2013) have considered the Universe as an acceleration cylindrical system. Motivated by their work and using their method in QCD, this paper has been cleared that because the acceleration of expansion in quark-antiquark system is relatively very large, one horizon is appeared outside the system. To obtain the total cross section of meson near this horizon, we need to multiply the production cross section for appeared horizon by the density of meson produced outside the system. As it can be seen by an observer who is outside the meson formation process, this cross section depends on time so the event horizon is now a time depended process. [ABSTRACT FROM AUTHOR]

In the context of noncommutative (NCG) gauge gravity, and using a cosmic time power law formula for the scale factor, a Friedman-Robertson-Walker (FRW) like metric is obtained. Within the fermions tunneling effect approach and depending on the various intervals of the power parameter, expressions of the apparent horizon are also derived. It is shown that in some regions of the parameter space, a pure NCG trapped horizon does exist leading to new interpretation of the role played by the noncommutativity of the space-time. [ABSTRACT FROM AUTHOR]

In this study, first, it will be shown that as the Hubble parameter, ' H', increases the production cross section for closed and flat Universes increases rapidly at smaller values of ' H' and becomes constant for higher values of ' H'. However in the case of open Universe, the production cross section has been encountered a singularity. Before this singularity, as the H parameter increases, the cross section increases, for smaller H, ( H < 2.5), exhibits a turn-over at moderate values of H, (2.5 < H < 3.5), decreases for larger amount of H After that and for a special value of H, the cross section has been encountered with a singularity. Although the cross section cannot be defined at this singularity but before and after this point, it is certainly equal to zero. After this singularity, the cross section increases rapidly, when H increases. It is shown that if the production cross section of Universe happens before this singularity, it can't achieve to higher values of Hubble parameter after singularity. More over if the production cross section of Universe situates after the singularity, it won't get access to values of Hubble parameter less than the singularity. After that the thermal distribution for particles inside the FRW Universes are obtained. It is found that a large amount of particles are produced near apparent horizon due to their variety in their energy and their probabilities. Finally, comparing the particle production cross sections for flat, closed and open Universes, it is concluded that as the value of k increases, the cross section decreases. [ABSTRACT FROM AUTHOR]

The present work deals with four alternative formulation of Bekenstein system on event horizon in f( R) gravity. While thermodynamical laws holds in universe bounded by apparent horizon, these laws break down on event horizon. With alternative formulation of thermodynamical parameters (temperature and entropy), thermodynamical laws hold on event horizon in Einstein Gravity. With this motivation, we extend the idea of generalised Hawking temperature and modified Bekenstein entropy in homogeneous and isotropic model of universe on event horizon and examine whether thermodynamical laws hold in f(R) gravity. Specifically, we examine and compare validity of generalised second law of thermodynamics (GSLT) and thermodynamical equilibrium (TE) in four alternative modified Bekenstein scenarios. As Dark energy is a possible dominant candidate for matter in the univerese and Holographic Dark Energy (HDE) can give effective description of f(R) gravity, so matter in the universe is taken as in the form interacting HDE. In order to understand the complicated expressions, finally the above laws are examined from graphical representation using three Planck data sets and it is found that generalised/modified Hawking temperature has a crucial role in making perfect thermodynamical system. [ABSTRACT FROM AUTHOR]

In this paper, the Rindler-like horizon in a spherically symmetric spacetime is proposed. It is showed that just like the Rindler horizon in Minkowski spacetimes, there is also a Rindler-like horizon to a family of special observers in general spherically symmetric spacetimes. The entropy of this type of horizon is calculated with the thin film brick-wall model. The significance of entropy is discussed. Our results imply some connection between Bekeinstein-Hawking entropy and entanglement entropy. [ABSTRACT FROM AUTHOR]

We consider a universe filled by a modified generalized Chaplygin gas together with a pressureless dark matter component. We get a thermodynamical interpretation for the modified generalized Chaplygin gas confined to the apparent horizon of FRW universe, whiles dark sectors do not interact with each other. Thereinafter, by taking into account a mutual interaction between the dark sectors of the cosmos, we find a thermodynamical interpretation for interacting modified generalized Chaplygin gas. Additionally, probable relation between the thermal fluctuations of the system and the assumed mutual interaction is investigated. Finally, we show that if one wants to solve the coincidence problem by using this mutual interaction, then the coupling constants of the interaction will be constrained. The corresponding constraint is also addressed. Moreover, the thermodynamic interpretation of using either a generalized Chaplygin gas or a Chaplygin gas to describe dark energy is also addressed throughout the paper. [ABSTRACT FROM AUTHOR]

We study evolutionary properties of isotropic charged spherical Lemaître-Tolman-Bondi fluid undergoing non-adiabatic collapse with general f( R) background. We explore exact analytical models of Maxwell- f( R) field equations with constant Ricci scalar. The dynamics of the collapsing system is investigated through system energy content, cosmological and black hole horizons in the presence of electromagnetic field. It is concluded that in addition to matter variables and f( R) corrections, the charge also affects the time interval between formation of respective horizons and singularities. We also find that f( R) corrections as well as electromagnetic field tend to slow down the collapsing process. [ABSTRACT FROM AUTHOR]

The secular redshift drift is a potential measurement to directly probe the cosmic expansion. Previous study on the redshift drift mainly focused on the model-dependent simulation. Apparently, the physical insights on the redshift drift are very necessary. So in this paper, it is investigated using thermodynamics on the apparent, Hubble and event horizons. Thermodynamics could analytically present the model-independent upper bounds of redshift drift. For specific assumption on the cosmological parameters, we find that the thermodynamics bounds are nearly one order of magnitude larger than the expectation in standard ΛCDM model. We then examine ten observed redshift drift from Green Bank Telescope at redshift 0.09 < z < 0.69, and find that these observational results are inconsistent with the thermodynamics. The size of the errorbars on these measurements is about three orders of magnitude larger than the effect of thermodynamical bounds for the redshift drift. Obviously, we have not yet hit any instrumental systematics at the shift level of 1m s yr. [ABSTRACT FROM AUTHOR]

The purpose of the present article is to show that the Newman-Janis and Newman et al algorithm used to derive the Kerr and Kerr-Newman metrics respectively, automatically leads to the extension of the initial non negative polar radial coordinate r to a cartesian coordinate $r^{\prime }$ running from $-\infty $ to $+\infty $, thus introducing in a natural way the region $-\infty

In this paper, we write modified Friedman-Robertson-Walkers (FRW) equation in the form of first law of thermodynamics at the apparent horizon. We consider the universe filled with the viscous fluid. Here we employ the general expression of temperature gravity and entropy at the apparent horizon of FRW universe and obtain the generalized first law of thermodynamics at the special condition for the modified FRW equation. The generalized first law of thermodynamics help us to arrange the α, α, β and β in modified Friedman-Robertson-Walkers equation. [ABSTRACT FROM AUTHOR]

The massive scalar quasinormal modes are researched in higher dimensional small Dilatonic spacetime via 6-order WKB approach and finite difference method. In this paper, we study the effect of quasinormal modes frequency in several dimensional cases, and the results show that, if the out event horizon keeps constant, the frequencies are higher but the black holes are more stable when dimension is higher or inner horizon is larger. On the other hand, we also focus on the effect of parameter a and scalar particle mass m. The calculation shows that the effect of m is observably stronger than the effect of a. [ABSTRACT FROM AUTHOR]

In this work, we have considered that the flat FRW universe is filled with the mixture of dark matter and the new holographic dark energy. If there is an interaction, we have investigated the natures of deceleration parameter, statefinder and Om diagnostics. We have examined the validity of the first and generalized second laws of thermodynamics under these interactions on the event as well as apparent horizon. It has been observed that the first law is violated on the event horizon. However, the generalized second law is valid throughout the evolution of the universe enveloped by the apparent horizon. When the event horizon is considered as the enveloping horizon, the generalized second law is found to break down excepting at late stage of the universe. [ABSTRACT FROM AUTHOR]

By the thin film model of the black hole and the assumption of the local thermal equilibrium, the instantaneous radiation energy flux and radiation power of the slowly changing Vaidya black hole have been studied. The result has been obtained that the thermal radiation of the Vaidya black hole satisfies the generalized Stefan-Boltzmann law. When the cut-off distance and the thin film thickness are both fixed, the instantaneous radiation energy flux of the scalar field near the event horizon of the Vaidya black hole is not only related to the black hole mass, but also to the rate of the change of its event horizon and the average effusion velocity of the radiation particles in the thin film. While its instantaneous radiation power is related to the rate of the change of the event horizon and the average radial effusion velocity of the radiation particles in the thin film. These results indicate that the gravitational field around the black hole and the change of its event horizon will both affect the thermal radiation of the black hole. [ABSTRACT FROM AUTHOR]

In this paper we study phantom energy accretion onto the Schwarzschild anti de-Sitter black hole. We obtain critical point where the speed of flow is equal speed of sound. We find that the critical point is near the black hole horizon. [ABSTRACT FROM AUTHOR]

By manipulating the integral expression for the proper radius R of the cosmological event horizon (CEH) in a Friedmann-Robertson-Walker (FRW) universe we obtain an analytical expression for the change δR in response to a uniform fluctuation δρ in the average cosmic background density ρ. We stipulate that the fluctuation arises within a vanishing interval of proper time, during which the CEH is approximately stationary, and evolves subsequently such that δρ/ ρ is constant. The respective variations 2 πR δR and δE in the horizon entropy S and enclosed energy E should be therefore related through the cosmological Clausius relation. In that manner we find that the temperature T of the CEH at an arbitrary time in a flat FRW universe is E/ S, which recovers asymptotically the usual static de Sitter temperature. Furthermore it is proven that during radiation-dominance and in late times the CEH conforms to the fully dynamical First Law Td S= Pd V−d E, where V is the enclosed volume and P is the average cosmic pressure. [ABSTRACT FROM AUTHOR]

In the framework of Fractional Action Cosmology (FAC), we study the generalized second law of thermodynamics for the Friedmann Universe enclosed by a boundary. We use the four well-known cosmic horizons as boundaries namely, apparent horizon, future event horizon, Hubble horizon and particle horizon. We construct the generalized second law (GSL) using and without using the first law of thermodynamics. To check the validity of GSL, we express the law in the form of four different scale factors namely emergent, logamediate, intermediate and power law. For Hubble, apparent and particle horizons, the GSL holds for emergent and logamediate expansions of the universe when we apply with and without using first law. For intermediate scenario, the GSL is valid for Hubble, apparent, particle horizons when we apply with and without first law. Also for intermediate scenario, the GSL is valid for event horizon when we apply first law but it breaks down without using first law. But for power law expansion, the GSL may be valid for some cases and breaks down otherwise. [ABSTRACT FROM AUTHOR]

Here we are trying to find the conditions for the validity of the generalized second law of thermodynamics (GSLT) assuming the first law of thermodynamics on the event horizon in both cases when the FRW universe is filled with interacting two fluid system- one in the form of cold dark matter and the other is either holographic dark energy or new age graphic dark energy. [ABSTRACT FROM AUTHOR]

We investigate the validity of the generalized second law of gravitational thermodynamics on the dynamical apparent horizon in a non-flat FRW universe containing the interacting new agegraphic dark energy with dark matter. We show that for this model, the equation of state parameter can cross the phantom divide line. We also present that for the selected model under thermal equilibrium with the Hawking radiation, the generalized second law is always satisfied throughout the history of the universe. Whereas, the evolution of the entropy of the universe and dynamical apparent horizon, separately, depends on the equation of state parameter of the interacting new agegraphic dark energy model. [ABSTRACT FROM AUTHOR]

We investigate the temperature and energy on holographic screens for 4-dimensional axisymmetric black holes with the entropic force idea proposed by Verlinde. According to the principle of thermal equilibrium, the location of holographic screen outside the axisymmetric black hole horizon is not a equivalent radius surface. The location of isothermal holographic screen outside the axisymmetric black hole horizon is obtained. Using the equipartition rule, we derive the correction expression of energy of isothermal holographic screen. When holographic screens are far away the black hole horizon, the entropic force of charged rotating particles can be expressed as Newton's law of gravity. When the screen crosses the event horizon, the temperature of the screen agrees with the Hawking temperature and the entropic force gives rise to the surface gravity for both of the black holes. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the validity of the generalized second law of thermodynamics of the universe bounded by the event horizon. Here we consider homogeneous and isotropic model of the universe filled with perfect fluid in one case and in another case holographic model of the universe has been considered. In the third case the matter in the universe is taken in the form of non-interacting two fluid system as holographic dark energy and dust. Here we study the above cases in the Modified gravity, f( R) gravity. [ABSTRACT FROM AUTHOR]

Black hole entropy appears to be “universal”—many independent calculations, involving models with very different microscopic degrees of freedom, all yield the same density of states. I discuss the proposal that this universality comes from the behavior of the underlying symmetries of the classical theory. To impose the condition that a black hole be present, we must partially break the classical symmetries of general relativity, and the resulting Goldstone-boson-like degrees of freedom may account for the Bekenstein–Hawking entropy. In particular, I demonstrate that the imposition of a “stretched horizon” constraint modifies the algebra of symmetries at the horizon, allowing the use of standard conformal field theory techniques to determine the asymptotic density of states. The results reproduce the Bekenstein–Hawking entropy without any need for detailed assumptions about the microscopic theory. [ABSTRACT FROM AUTHOR]

In this letter we compute the corrections to the Cardy-Verlinde formula of Achúcarro-Ortiz black hole, which is the most general two-dimensional black hole derived from the three-dimensional rotating Banados-Teitelboim-Zanelli black hole. These corrections stem from the space non-commutativity. We show that in non-commutative case, non-rotating Achúcarro-Ortiz black hole in contrast with commutative case has two horizons. [ABSTRACT FROM AUTHOR]

The field equations of general relativity are shown to derive from a limit to force or to power in nature. The limits have the value of c 4/4 G and c 5/4 G. The proof makes use of a result of Jacobson. All known experimental data are consistent with the limits. Applied to the universe, the limits predict its darkness at night and the observed scale factor. Other experimental tests of the limits are proposed. The main counterarguments and paradoxes are discussed, such as the transformation under boosts, the force felt at a black hole horizon, the mountain problem, and the contrast to scalar–tensor theories of gravitation. The resolution of the paradoxes also clarifies why the maximum force and the maximum power have remained hidden for so long. The derivation of the field equations shows that the maximum force or power plays the same role for general relativity as the maximum speed plays for special relativity. [ABSTRACT FROM AUTHOR]

The entropy of a scalar field at the horizon is investigated in the Vaidya space–time. We take into account the effect of the generalized uncertainty principle on the state density and the entropy. The divergence in the brick-wall model is removed and the entropy proportional to the horizon area is obtained. [ABSTRACT FROM AUTHOR]