Four-dimensional Einstein Gauss Bonnet Research Articles

Traversable wormholes in 4D EGB gravity

In this work, the spherically symmetric wormhole solutions have been obtained in the four-dimensional Einstein–Gauss–Bonnet (4D EGB) gravity with isotropic and anisotropic matter sources. With each type of source, the traversable wormholes satisfying asymptotic flatness and flare-out conditions are developed. Further, the energy conditions explaining the nature of the matter in the interior of the wormhole are tested and the stability of wormhole solutions has been analyzed.

Strong gravitational lensing by Bardeen black holes in 4D EGB gravity: Constraints from supermassive black holes

Observation indicates that many nearby galaxies host supermassive central black holes. We use the Bardeen black holes in four-dimensional Einstein–Gauss–Bonnet (4D EGB) gravity, with additional parameters-the coupling constant α̃ and charge q, as central black holes in various galaxies to investigate gravitational lensing properties in strong field regime. Taking the supermassive black holes Sgr A* and M87* as the lens, we compare observable signatures of 4D EGB Bardeen black holes with those of the Schwarzschild black holes. In the case of 4D EGB Bardeen black holes we observed that he angular position θ∞ for Sgr A* ∈ (23.19, 25.56) μ as, whereas for M87* it is ∈ ( 17.94,19.78) μ as. Further, the angular separation s, which is an increasing function of α̃ and q for Sgr A* and M87* differs significantly, respectively, in (0.032, 0.15) μ as and (0.025, 0.115) μ as. The deviations of the lensing observables Δθ∞ and Δs for 4D EGB Bardeen black hole (α̃=0.9,q=0.09) from the Schwarzschild black hole, respectively, can reach up to 2.38μ as and 0.12μ as for Sgr A* , 1.84μ as and 0.09μ as for M87*. On the other hand, the relative magnification ∈ (4.66,6.82). Considering twenty-one massive central black holes as lens, we also estimate the time delay ΔT2,1s between the first and second relativistic image to find that, e.g., the time delay for Sgr A* and M87*, respectively, can reach ∼9.86 min and ∼16023.93 min. We also show that the existing shadow size measurements place strong constraints on the parameters considered Bardeen model. This combination of gravitational lensing and EHT results may complement comprehensive restrictions on modifications of the general relativity.

Surface gravity in spherically symmetric collapsing stars

Here we consider the generalized Oppenheimer–Snyder collapse of a star into a four-dimensional Einstein-Gauss–Bonnet black hole as well as a class of regular black holes labeled by the polytropic index of the stellar matter. We then analyze the nature of the horizon and the corresponding surface gravity outside and inside the star. The Hayward and Nielsen–Visser dynamical surface gravity are in agreement with the one resulting from the Killing vector of the outer static metric. However, these two definitions inside the star do not coincide with the Killing surface gravity outside the star when the star crosses the event horizon. This motivates us to study the surface gravity using Fodor’s approach to have a unique surface gravity at the mentioned moment. Then the extremality condition and the first law of thermodynamics are discussed at the trapping horizon of the star.

Traversable thin-shell wormhole in the 4D Einstein–Gauss–Bonnet theory

This work investigates the spherically symmetric thin-shell wormhole solutions in four-dimensional Einstein–Gauss–Bonnet theory and explores their stabilities under radial, linear perturbations. These solutions are typically traversable and characterized by a thin-shell throat in accordance with Israel’s junction conditions. In asymptotically flat and AdS spacetimes with a negative Gauss–Bonnet coupling constant, stable neutral wormholes are encountered when the magnitude of the coupling constant becomes significant. The throats of such wormholes are sustained by ordinary matter and possess finite radii. In asymptotically dS spacetimes, no stable neutral wormhole featuring ordinary matter is observed. On the other hand, for positive Gauss–Bonnet coupling constant, stable thin-shell wormhole solutions can be established when the throats are exclusively supported by exotic matter. Moreover, stable charged wormholes comprised of ordinary matter are found universally in the asymptotically flat, AdS, and dS spacetimes. Unlike their neutral counterparts, the throat radii of such charged wormholes can be arbitrarily small. However, as the charge becomes more significant, such solutions only remain stable when supported by exotic matter.

Dynamical analysis in regularized 4D Einstein–Gauss–Bonnet gravity with non-minimal coupling

We investigate the regularized four-dimensional Einstein–Gauss–Bonnet (4DEGB) gravity with a non-minimal scalar coupling function, which is an extension of the regularized 4DEGB theory. By introducing non-minimal coupling to the Gauss-Bonnet term, we demonstrate the additional contribution to the dynamical equations which is otherwise absent in the dimensionally regularized theory. Furthermore, we analyze the stability of the system by using the dynamical system approach based on fixed points. Then, we consider time evolution to investigate the history of the universe and to constrain observational data to obtain the cosmological parameters of the model.

Isotropic compact stars in four-dimensional Einstein–Gauss–Bonnet gravity coupled with scalar field: reconstruction of model

It has been suggested that the Einstein–Gauss–Bonnet theory coupled with a scalar field (EGBS) may allow us to obtain physically viable models of celestial phenomena such that the scalar field effect is active in standard four dimensions. We consider the spherically symmetric and static configuration of the compact star and explain the consequences of the EGBS theory in the frame of stellar modeling. In our formulation, for any given static profile of energy density rho with spherical symmetry and the arbitrary equation of state (EoS) of matter, we can construct a model which reproduces the profile. Because the profile of the energy density determines the mass M and the radius R_s of the compact star, an arbitrary relation between the mass M and the radius R_s of the compact star can be realized by adjusting the potential and the coefficient function of the Gauss–Bonnet term in the action of EGBS theory. This could be regarded as a degeneracy between the EoS and the functions characterizing the model, which indicates that the mass–radius relation alone is insufficient to constrain the model. Here, we investigate a novel class of analytic spherically symmetric interior solutions by the polytropic EoS. We discuss our model in detail and show that it is in agreement with the necessary physical conditions required for any realistic compact star, confirming that EGBS theory is consistent with observations.

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.

Shadow and massless particles around regular Bardeen black holes in 4D Einstein Gauss–Bonnet gravity

In this paper, we study dynamics of massless (photon and neutrino-like) particles in the spacetime of regular Bardeen black hole (BH) in novel four-dimensional Einstein–Gauss–Bonnet (4D EGB) theory. First, we have analyzed the hor properties of the spacetime of the Bardeen BH with respect to the Gauss–Bonnet (GB) coupling parameter [Formula: see text]. Our detailed analysis has shown that the GB coupling parameter is limited by maximum and minimum values at, [Formula: see text] and the extreme value of magnetic charge of the BH depends on the parameter [Formula: see text]. We have explored motion of neutrino-like particles and photons around the Bardeen BH. We have also explored the possibility of mimicking cases of the spin of Kerr BH and charge of Reissner–Nordström BH by the Bardeen charge, providing the same values of impact parameter for photons corresponding to the BH shadow size. Finally, we have investigated spherically infalling accretion and shadow of the Bardeen BHs in 4D EGB theory.

Quintessential effects on quasiperiodic oscillations in 4D Einstein–Gauss–Bonnet gravity

Dynamics of test particles in the space-time of a quintessential black hole (BH) in a novel four-dimensional Einstein–Gauss–Bonnet (EGB) theory is studied. First, we analyse the properties of the space-time of the BH and possible values of the coupling alpha and quintessential parameters q that allows the existence of the event horizon. The scalar invariants of the space-time and effects of the parameters on innermost stable circular orbits (ISCOs) radius are also studied. Moreover, fundamental frequencies such as Keplerian and harmonic oscillations are investigated together with applications to quasi-periodic oscillations (QPOs) where the powerful way to test gravity theories using astrophysical observations of twin-peak QPOs is discussed.

Generalized Gravitational Phase Transition in Novel 4D Einstein–Gauss–Bonnet Gravity

AbstractIn this paper, the possible existence of a thermal phase transition from asymptotic anti‐de Sitter (AdS) to de Sitter (dS) geometries in vacuum in the context of the novel fourdimensional Einstein–Gauss–Bonnet (4D EGB) gravity is presented. The phase transition proceeds via a thermalon (the Euclidean section of a bubble thin shell) formation, having a black hole in the dS spacetime and a thermal AdS spacetime in the exterior, without introducing any matter field. From the analysis, it is found that, using regularized novel 4D EGB gravity, the gravitational phase transitions occur for all allowed values of the 4D EGB coupling. In the 4D EGB gravity, the existence of the above phase transition is over a narrow range of the 4D EGB coupling, with particular critical values. In contrast, in five‐dimensional (5D) EGB gravity, a 5D sector needs a wider range of its coupling for the existence of a thermalon, and there also exist critical values for the emergence of the above phase transition.

Quasi-periodic oscillation around regular Bardeen black holes in 4D Einstein–Gauss–Bonnet gravity

In this paper, we explore the dynamics of test particles in spacetime around regular Bardeen black holes (BHs) in the novel four-dimensional Einstein–Gauss–Bonnet (4D EGB) theory. First, we analyze properties of the spacetime around the Bardeen BH with respect to the Gauss–Bonnet (GB) coupling parameter [Formula: see text], and we show the range of values of the BH charge and the parameter [Formula: see text] that horizon exists as [Formula: see text] and [Formula: see text], respectively. We also investigate the effects of the magnetic charge of the regular BH on innermost stable circular orbits (ISCOs) radius and efficiency of energy release from infalling test particles to the regular Bardeen BH in 4D EGB gravity. Moreover, fundamental frequencies such as Keplerian and harmonic oscillations are also studied with the applications of quasi-periodic oscillations (QPOs) where we proposed a method to test gravity theories through data from astrophysical observations of twin-peak QPOs in relativistic precession (RP) model. Finally, we show the dependence of the distance from the central BH to the orbit where twin-peak QPOs shine from the spin of Kerr BH, magnetic charge of Bardeen and Reissner–Nordström (RN) BHs, and GB coupling parameter.

Charged black holes in 4D Einstein–Gauss–Bonnet gravity coupled to nonlinear electrodynamics with maximum allowable symmetries

Recently a new model of nonlinear Maxwell electrodynamics which preserves both conformal and SO(2) duality-rotational invariance has been proposed. In this work, we consider this model as a source of gravitational field and study black holes of novel four-dimensional Einstein–Gauss–Bonnet gravity. In this framework we solve the gravitational field equations and work out the metric function that describe a black hole which generalizes the Maxwellian charged black hole of four-dimensional Einstein–Gauss–Bonnet gravity. Some basic thermodynamic quantities corresponding to the new black hole solution are computed and thermodynamic stability as well as thermal phase transitions are investigated.

Hawking–Page phase transitions in four-dimensional Einstein–Gauss–Bonnet gravity

The Hawking–Page (HP) phase transitions of the anti-de Sitter black holes in the extended phase space are studied in a novel four-dimensional Einstein–Gauss–Bonnet (4EGB) gravity, which is proposed by rescaling the Gauss–Bonnet (GB) coupling constant α → α ∕ ( d − 4 ) in d dimensions and redefining the four-dimensional gravity in the limit d → 4 . The GB term shows nontrivial contributions to both black hole mass and entropy simultaneously, and decreases the HP phase transition temperature T HP . Moreover, the HP phase transitions can occur only within a range of pressure in the 4EGB gravity. For the charged black holes, T HP also decreases with the electric potential in the grand canonical ensemble. A general discussion of the HP phase transitions in the Einstein, GB, and 4EGB gravities is also presented.

Quasinormal modes and strong cosmic censorship in the regularised 4D Einstein–Gauss–Bonnet gravity

The fate of strong cosmic censorship is ultimately linked to the extendibility of perturbation across the Cauchy Horizon and known to be violated in the near extremal region of a charged de Sitter black hole. Similar violations can also be realized in higher curvature theories, with the strength of violation becoming stronger as compared to general relativity. In this work, we extend this analysis further to study the validity of strong cosmic censorship conjecture in the context of the regularised four-dimensional Einstein Gauss-Bonnet theory with respect to both scalar and electromagnetic perturbation. We also study the late time tails of scalar fields.

On cosmic acceleration in four-dimensional Einstein–Gauss–Bonnet gravity

We study the possibility that in the model introduced in Glavan and Lin (2020), the Gauss–Bonnet term alone gives rise to the cosmic acceleration and super-acceleration in four-dimensional FLRW space–time at the late time. We also discuss transitions from deceleration to acceleration and acceleration to super-acceleration. We show that the Gauss–Bonnet invariant can drive the acceleration in the low redshift provided that its regularized coefficient has the same order as the squared of the ratio of the reduced Planck mass to the Hubble parameter.

Spinning Test Particle in Four-Dimensional Einstein–Gauss–Bonnet Black Holes

In this paper, we investigate the motion of a classical spinning test particle in a background of a spherically symmetric black hole based on the novel four-dimensional Einstein–Gauss–Bonnet gravity [D. Glavan and C. Lin, Phys. Rev. Lett. 124, 081301 (2020)]. We find that the effective potential of a spinning test particle in this background could have two minima when the Gauss–Bonnet coupling parameter α is nearly in a special range −8<α/M2<−2 (M is the mass of the black hole), which means a particle can be in two separate orbits with the same spin-angular momentum and orbital angular momentum, and the accretion disc could have discrete structures. We also investigate the innermost stable circular orbits of the spinning test particle and find that the corresponding radius could be smaller than the cases in general relativity.

The second law in four-dimensional Einstein–Gauss–Bonnet gravity

The topological contribution of a Gauss–Bonnet term in four dimensions to black hole entropy opens up the possibility of a violation of the second law of thermodynamics in black hole mergers. We show, however, that the second law is not violated in the regime where Einstein–Gauss–Bonnet holds as an effective theory and black holes can be treated thermodynamically. For mergers of anti-de Sitter (AdS) black holes, the second law appears to be violated even in Einstein gravity; we argue, however, that the second law holds when gravitational potential energy is taken into account.