Einstein Gauss Bonnet Research Articles

Page curves of Reissner–Nordström black hole in HD gravity

We obtain the Page curves of an eternal Reissner–Nordström black hole in the presence of higher derivative terms in four dimensions. We consider two cases: gravitational action with general {mathcal {O}}(R^2) terms plus Maxwell term and Einstein–Gauss–Bonnet gravity plus Maxwell term. In both the cases entanglement entropy of the Hawking radiation in the absence of island surface is increasing linearly with time. After including contribution from the island surface, we find that after the Page time, entanglement entropy of the Hawking radiation in both the cases reaches a constant value which is the twice of the Bekenstein–Hawking entropy of the black hole and we obtain the Page curves. We find that Page curves appear at later or earlier time when the Gauss–Bonnet coupling increases or decreases. Further, scrambling time of Reissner–Nordström is increasing or decreasing depending upon whether the correction term (coming from {mathcal {O}}(R^2) terms in the gravitational action) is increasing or decreasing in the first case whereas scrambling time remains unaffected in the second case (Einstein–Gauss–Bonnet gravity case). As a consistency check, in the limit of vanishing GB coupling we obtain the Page curve of the Reissner–Nordström black hole obtained in Wang et al. (JHEP 04:103, 2021).

Cosmic censorship and charged radiation in second order Lovelock gravity

The conditions for naked singularity formation are considered for a radiating metric of Boulware–Deser type within an electromagnetic field in second order Lovelock (or Einstein–Gauss–Bonnet) gravity. The spacetime metric remains real only up to certain maximum charge contribution. This differs from general relativity. Beyond a certain maximal charge, there exists no real and physical spacetime since the metric becomes complex. We establish that, under certain parameters and for specific values of the mass function and charge contribution, this branch singularity is indeed a naked singularity. This is in contrast to the neutral case where the spacetime metric is always real for a positive mass function, and further, a weak, initially naked singularity always occurs before it becomes covered by an event horizon for all future time. We highlight that both neutral and charged collapse under gravity in Einstein–Gauss–Bonnet gravity differ significantly to their general relativistic counterparts.

Anisotropic solution for polytropic stars in 4D Einstein–Gauss–Bonnet gravity

In the present work we have investigated a new anisotropic solution for polytropic stars in the framework of 4D Einstein–Gauss–Bonnet (EGB) gravity. The possibility of determining the masses and radii of compact stars which puts some limitations on equation of state (EoS) above the nuclear saturation density. For this purpose, the 4D EGB field equations are solved by taking a generalized polytropic equation of state (EoS) with Finch–Skea ansatz. The generalized solution for anisotropic model has been tested for different values of Gauss–Bonnet constant alpha which satisfies all the physical criteria including causality with static stability via mass vs central mass density (M-rho _c), Bondi and Abreu criterion. The adiabatic index shows a minor influence of the GB coupling constant whereas the central and surface redshifts in the EGB gravity always remain lower than the GR. We present the possibility of fitting the mass and radius for some known compact star via M-R curve which satisfies the recent gravitational wave observations from GW 170817 event.

Comment on ‘Gravitational Analysis of Rotating Charged Black‐Hole‐Like Solution in Einstein–Gauss–Bonnet Gravity’

AbstractThe author reviews the derivations of the rotating metric made in Ann. Phys. (Berlin), 2022, 2200074, and shows that the analysis yielding the solution is flawed throughout the paper.

Study of Asymptotic Velocity in the Bondi–Hoyle Accretion Flows in the Domain of Kerr and 4-D Einstein–Gauss–Bonnet Gravities

Understanding the physical structures of the accreted matter very close to a black hole in quasars and active galactic nucleus (AGN) is an important milestone to constrain the activities occurring in their centers. In this paper, we numerically investigate the effects of the asymptotic velocities on the physical structures of the accretion disk around the Kerr and Einstein–Gauss–Bonnet (EGB) rapidly rotating black holes. The Bondi–Hoyle accretion is considered with a falling gas towards the black hole in an upstream region of the computational domain. Shock cones are naturally formed in the downstream part of the flow around both black holes. The structure of the cones and the amount of the accreted matter depend on asymptotic velocity V∞ (Mach number) and the types of the gravities (Kerr or EGB). Increasing the Mach number of the in-flowing matter in the supersonic region reduces the shock opening angle and the accretion rates, because of the gas rapidly falling towards the black hole. The EGB gravity leads to an increase in the shock opening angle of the shock cones while the mass-accretion rates dM/dt decrease in EGB gravity with a Gauss–Bonnet (GB) coupling constant α. It is also confirmed that accretion rates and drag forces are significantly altered in the EGB gravity. Our numerical simulation results could be used in identifying the accretion mechanism and physical properties of the accretion disk and black hole in the observed X-rays such as NGC 1313 X-1 and 1313 X-2 and MAXI J1803-298.

Weak cosmic censorship conjecture for the (2+1)-dimensional charged BTZ black hole in the Einstein–Gauss–Bonnet Gravity

It is well known that (2+1) dimensional charged BTZ (Banados, Teitelboim, Zanelli) black hole can be overcharged by a charged scalar field and a charged particle in contrast to their analogues in (3+1) and higher dimensions. In this regard, it may play an important role to understand more deeply the properties of the (2+1)-dimensional charged black hole in the Einstein–Gauss–Bonnet (EGB) gravity. In this paper, we test the validity of the weak cosmic censorship conjecture for the (2+1)-dimensional charged black hole in novel EGB theory derived recently by Henniger et al. (2021). We show that the minimum energy that particle can have at the horizon becomes negative for both an extremal and nearly-extremal BTZ black holes in EGB gravity. It is proven that both extremal and nearly-extremal (2+1) dimensional BTZ black hole could be overcharged in EGB theory, leading to the violation of the weak cosmic censorship conjecture (WCCC), which is in good agreement with the results obtained for the classical BTZ black hole.

Testing viable extensions of Einstein–Gauss–Bonnet gravity

Some models within the framework of Gauss–Bonnet gravities are considered in the presence of a non-minimally coupled scalar field. By imposing a particular constraint on the scalar field coupling, a class of these theories called Einstein–Gauss–Bonnet gravity keeps the correct speed of propagation for gravitational waves. The cosmological evolution for this viable class of models is studied and compared with observational data (BAO, CMB, Sne Ia,..), where we obtain the corresponding bounds for these theories and show that such theories fit well the data and provide a well-behaved cosmological evolution in comparison to the standard model of cosmology. Some statistical parameters show that the goodness of the fits are slightly better than those for Λ CDM model.

Thermodynamic properties, thermal image and phase transition of Einstein-Gauss-Bonnet black hole coupled with nonlinear electrodynamics

We obtain an exact solution of $AdS$ black hole solution in Einstein-Gauss-Bonnet (EGB) gravity coupled with nonlinear electrodynamics. It interpolates with the $AdS$ regular black hole and $AdS$ EGB black hole in the absence of the Gauss-Bonnet coupling constant and both magnetic monopole charge and deviation parameter, respectively. Based on horizon thermodynamics, we study the thermodynamic properties of the obtained solution (e.g. mass, temperature, entropy, heat capacity and free energy). The Hawking temperature of the nonsingular black hole gets the maximum value at the point where specific heat diverges and the second-order phase transition occurs at the same point. We find that the smaller nonsingular black holes are stable due to positive heat capacity and negative free energy. We explicitly trace the relations between the black hole shadow and the critical behavior of charged EGB $AdS$ regular black hole in the extended phase space.

Gravitational collapse in Einstein-Gauss-Bonnet gravity

The Einstein- Gauss- Bonnet (EGB) gravity is an important modification of the Einstein theory of gravity and, for many gravitational phenomena, the Gauss- Bonnet (GB) correction term leads to drastic differences. In this paper, we study gravitational collapse in the $5$-dimensional EGB theory. We construct the spherical marginally trapped surfaces and determine the evolution of marginally trapped surfaces when the infalling matter admits a wide variety of initial density distribution. We show that the location of black hole horizon depends crucially on the initial density and velocity profile of the inflating matter as well as on the GB coupling constant. A detailed comparison is made with the results of Einstein's theory.

Double shadow of a 4D Einstein–Gauss–Bonnet black hole and the connection between them with quasinormal modes

In this paper, we study the shadow of a 4D Einstein–Gauss–Bonnet black hole as photons couple to the Weyl tensor and find that the propagation of light depends on its polarization which leads to the existence of a double shadow. Then, we discuss the effect of the coupling parameter [Formula: see text], the polarization of light and the Gauss–Bonnet coupling constant [Formula: see text] on the shadow. Further, we explore the influence of the Gauss–Bonnet coupling constant [Formula: see text] on the quasinormal modes (QNMs) of massless scalar field and investigate the connection between the real part of QNMs in the eikonal limit and the shadow radius of black holes. We find that in the eikonal limit, the real part of QNMs is inversely proportional to the shadow radius under the case of the photons uncoupled to the Weyl tensor.

Quasibound states of scalar fields in the consistent 4D Einstein–Gauss–Bonnet–(Anti-)de Sitter gravity

We examine the interaction between massless scalar fields and the gravitational field generated by a black hole solution that was recently obtained in the consistent well-defined 4-dimensional Einstein–Gauss–Bonnet gravity with a cosmological constant. In order to do this, we calculate quasibound state frequencies of scalar fields for the spherically symmetric black hole in the consistent 4-dimensional Einstein–Gauss–Bonnet–de Sitter and Anti-de Sitter theories. The expression for the quasibound states is obtained by using the polynomial condition associated to the Heun functions, and their values are overdamped. We also demonstrate the stability of the systems.

Instability of hairy black holes in regularized 4-dimensional Einstein-Gauss-Bonnet gravity

In regularized 4-dimensional Einstein-Gauss-Bonnet (EGB) gravity derived from a Kaluza-Klein reduction of higher-dimensional EGB theory, we study the existence and stability of black hole (BH) solutions on a static and spherically symmetric background. We show that asymptotically-flat hairy BH solutions realized for a spatially-flat maximally symmetric internal space are unstable against linear perturbations for any rescaled GB coupling constant. This instability is present for the angular propagation of even-parity perturbations both in the vicinity of an event horizon and at spatial infinity. There is also a strong coupling problem associated with the kinetic term of even-parity perturbations vanishing everywhere.

Naked singularity in 4D Einstein-Gauss-Bonnet novel gravity: Echoes and instability

We study the stability of an asymptotically flat, static, spherically symmetric naked singularity spacetime in the novel four-dimensional Einstein-Gauss-Bonnet (EGB) gravity. The four-dimensional EGB black hole for large enough values of the coupling parameter leads to such a naked singularity. The stability and the response of the spacetime are studied against the perturbations by test scalar, electromagnetic and Dirac fields, and the time evolution of these perturbations was observed numerically. Implementing a null Dirichlet boundary condition near the singularity, we observed that for $l=1$ modes of scalar, electromagnetic perturbation, and $l=0$, 1 modes of Dirac perturbation, the time-domain profiles give rise to distinct echoes. However, as the coupling constant increases, the echoes align, and the quasinormal mode structure of the 4D-EGB naked singularity-spacetime becomes prominent. For higher values of the multipole number, the spacetime becomes unstable, thereby restricting the parameter space for the coupling parameter.

On charged black holes in 4D Lovelock theory coupled to ModMax nonlinear electrodynamics

The ModMax nonlinear electrodynamics is a recently proposed model which possesses maximally symmetries among the well-known nonlinear electrodynamics models. This model consists of both Maxwell’s invariants such that in the absence of either the electric or the magnetic field the model automatically reduces to the Maxwell linear theory. The presence of both scalar invariants which are necessary for the symmetries of the model makes it very special such that cannot be employed in higher or lower dimensions. Unfortunately, we observed in the literature that, this model has been applied in higher d-dimensions. This is still an early time for this model, however, it is becoming popular. We aim to clarify this specific important point by giving the correct approach to the 4D Einstein–Gauss–Bonnet gravity as well as 4D Einstein p-order Lovelock theory coupled to ModMax nonlinear electrodynamics.

Stable Exponential Cosmological Type Solutions with Three Factor Spaces in EGB Model with a Λ-Term

We study a D-dimensional Einstein–Gauss–Bonnet model which includes the Gauss–Bonnet term, the cosmological term Λ and two non-zero constants: α1 and α2. Under imposing the metric to be diagonal one, we find cosmological type solutions with exponential dependence of three scale factors in a variable u, governed by three non-coinciding Hubble-like parameters: H≠0, h1 and h2, obeying mH+k1h1+k2h2≠0, corresponding to factor spaces of dimensions m>1, k1>1 and k2>1, respectively, and depending upon sign parameter ε=±1, where ε=1 corresponds to cosmological case and ε=−1—to static one). We deal with two cases: (i) m<k1<k2 and (ii) 1<k1=k2=k, k≠m. We show that in both cases the solutions exist if εα=εα2/α1>0 and αΛ>0 satisfy certain (upper and lower) bounds. The solutions are defined up to solutions of a certain polynomial master equation of order four (or less), which may be solved in radicals. In case (ii), explicit solutions are presented. In both cases we single out stable and non-stable solutions as u→±∞. The case H=0 is also considered.

Wormhole inducing inflation with Einstein–Gauss–Bonnet dilaton interaction

A few Euclidean wormhole configurations are presented using both the analytic and numerical solutions of the field equations in four-dimensional Robertson–Walker Euclidean background with Einstein–Gauss–Bonnet dilaton interaction. In one analytic solution we present transition from a wormhole to an exponential expansion with Lorentzian time [Formula: see text] using [Formula: see text] after passing through an era of oscillating Euclidean wormhole. The numerical solutions of the scale factor [Formula: see text] show multiple local maxima and minima about a global minimum for inverse power law potentials, while for exponential potential the wormholes have a single minimum. An inflationary cosmic scenario away from the throat of the wormhole can be obtained from the Hubble parameter and deceleration parameter obtained by curve fit of [Formula: see text] of the numerical solution invoking analytic continuation by [Formula: see text]. The potential is also observed to decay sharply.

Gravitational Analysis of Rotating Charged Black‐Hole‐Like Solution in Einstein–Gauss–Bonnet Gravity

AbstractThis work analyzes the Einstein–Gauss–Bonnet gravity of charged black hole solutions through Newman–Janis approach. The Hawking temperature for corresponding black hole is also computed. The solution depends upon rotation parameter a, black hole mass, charge and horizon. Moreover, the graphical behavior of temperature via event horizon to analyze the stability of black hole under the effects of rotation parameter is discussed. The graphs are plotted in the presence/absence of rotation parameter and charge. Furthermore, the Hawking temperature under gravity effects is studied by using the semi‐classical method. It is also observed that the maximum temperature at non‐zero horizon depicts the BH remnant. Finally, the logarithmic corrected entropy for given black hole is computed and the logarithmic corrected entropy under effects of rotation and correction parameter are studied.

Minimally deformed charged stellar model by gravitational decoupling in 5D Einstein–Gauss–Bonnet gravity

We investigate the possibility of existing a class of compact charged spheres made of a charged perfect fluid in the framework of Einstein–Gauss–Bonnet theory in five-dimensional spacetime (5D EGB). In order to study spherically symmetric compact stars in EGB gravity, we prefer to apply a systematic and direct approach to decoupling gravitational sources via the minimal geometric deformation approach (MGD), which allows us to prove that the fluid must be anisotropic. In fact, we specify a well-known Krori–Barua spacetime in the MGD approach that helps us to determine the decoupling sector completely. Indeed, by using this approach, we found an exact and physically acceptable solution which satisfies all the elementary criteria of physical acceptability for a stellar solution via mimic approach. Finally, we show that the compactness factor in the presence of gravitational decoupling satisfies the Buchdahal limit under 5D EGB gravity.

Anisotropic compact objects with Finch–Skea geometry in EGB gravity

We present a new class of relativistic anisotropic stellar models with spherically symmetric matter distribution in Einstein Gauss–Bonnet (EGB) gravity. A higher dimensional Finch–Skea geometry in the theory is taken up here to construct stellar models in hydrostatic equilibrium. The Gauss–Bonnet term is playing an important role in accommodating neutron stars. We study the physical features namely, the energy density, the radial and tangential pressures and the suitability of the models. It is found that the equation of state of such stars are non-linear which is determined for a given mass and radius of known stars. The stability of the stellar models are also explored for a wide range of values of the model parameters.

Null geodesics and shadow of 4D Einstein–Gauss–Bonnet black holes surrounded by quintessence

We study the properties of the null geodesics of four-dimensional Einstein–Gauss–Bonnet (4D EGB) black holes surrounded by quintessence matter, recently derived by Shah et al. [Phys. Lett. B 818 (2021) 136383]. Due to the quintessence correction, we discuss the radial and nonradial geodesics. For the case of nonradial geodesics we obtain the effective potential, photon sphere and impact parameter associated to the null geodesics in detail. We analyze the different possible orbits of massless particles (i.e. photons) such as unstable circular orbits and unbounded orbits and investigate the role of the Gauss–Bonnet coupling [Formula: see text] and the quintessence parameter q on the null geodesic trajectories. Moreover, we study the effects of the model parameters on the shadow cast by such black holes. The results are compared to that of Schwarzschild black hole with and without quintessence matter and 4D EGB black holes without quintessence. As a physical application of null geodesics, the deflection angle is calculated and the effect of the quintessence matter on it is investigated.