Bending of light in novel 4DD Gauss-Bonnet-de Sitter black holes by Rindler-Ishak method

We study the bending of light in the space-time of black holes in four-dimensional Einstein-Gauss-Bonnet theory of gravity, recently proposed by Glavan and Lin \cite{Glavan}. Using Rindler-Ishak method, the effect of Gauss-Bonnet coupling on the bending angle is studied. We show that a positive Gauss-Bonnet coupling gives a negative contribution to the Schwarzschild-de Sitter deflection angle, as one would expect.Comment: 10 pages, 1 figure. To appear in EP

Born-Infeld Black Holes in 4D Einstein-Gauss-Bonnet Gravity

A novel four-dimensional Einstein-Gauss-Bonnet gravity was formulated by D. Glavan and C. Lin [Phys. Rev. Lett. 124, 081301 (2020)], which is intended to bypass the Lovelock's theorem and to yield a non-trivial contribution to the four-dimensional gravitational dynamics. However, the validity and consistency of this theory has been called into question recently. We study a static and spherically symmetric black hole charged by a Born-Infeld electric field in the novel four-dimensional Einstein-Gauss-Bonnet gravity. It is found that the black hole solution still suffers the singularity problem, since particles incident from infinity can reach the singularity. It is also demonstrated that the Born-Infeld charged black hole may be superior to the Maxwell charged black hole to be a charged extension of the Schwarzschild-AdS-like black hole in this new gravitational theory. Some basic thermodynamics of the black hole solution is also analyzed. Besides, we regain the black hole solution in the regularized four-dimensional Einstein-Gauss-Bonnet gravity proposed by H. L\"u and Y. Pang [arXiv:2003.11552].Comment: 13 pages and 18 figures, published versio

Black holes with scalar hair in light of the Event Horizon Telescope

Searching for violations of the no-hair theorem (NHT) is a powerful way to test gravity, and more generally fundamental physics, particularly with regards to the existence of additional scalar fields. The first observation of a black hole (BH) shadow by the Event Horizon Telescope (EHT) has opened a new direct window onto tests of gravity in the strong-field regime, including probes of violations of the NHT. We consider two scenarios described by the Einstein-Maxwell equations of General Relativity and electromagnetism, to which we add a scalar field. In the first case we consider a minimally-coupled scalar field with a potential, whereas in the second case the field is conformally-coupled to curvature. In both scenarios we construct charged BH solutions, which are found to carry primary scalar hair. We then compute the shadows cast by these two BHs as a function of their electric charge and scalar hair parameter. Comparing these shadows to the shadow of M87* recently imaged by the EHT collaboration, we set constraints on the amount of scalar hair carried by these two BHs. The conformally-coupled case admits a regime for the hair parameter, compatible with EHT constraints, describing a so-called mutated Reissner-Nordstr\"{o}m BH: this solution was recently found to effectively mimic a wormhole. Our work provides novel constraints on fundamental physics, and in particular on violations of the no-hair theorem and the existence of additional scalar fields, from the shadow of M87*.Comment: 33 pages, 6 figures, 1 table, references added, version accepted for publication in JCA

Light bending near non-asymptotically flat black holes

The gravitational deflection of light is a crucial test for modified gravity. A few years ago, Gibbons and Werner introduced a definition of the deflection angle based on the Gauss-Bonnet theorem. A related idea was proposed by Arakida for defining the deflection angle in non-asymptotically flat spacetimes We revisit this idea in the Kottler geometry and in a non-asymptotically flat solution to Horndeski gravity. Our analytic and numerical calculations show that a triangular array of laser beams can be designed so that the proposed definition of deflection angle is sensitive to a cosmological constant, whose contribution is amplified by the black hole mass. Moreover, we find that near the photon sphere, the deflection angle in the Horndeski solution is similar to its Schwarzschild counterpart, and we confirm that the shadows seen by a static observer would be identical. Our results offer insights that could be useful for designing future theoretical or experimental investigations aimed to detect sources of curvature in the universe.Comment: 32+1 pages, 6 figures, 2 table

Weak cosmic censorship conjecture for the novel 4D4D charged Einstein-Gauss-Bonnet black hole with test scalar field and particle

Recent researches of the novel $4D$ Einstein-Gauss-Bonnet (EGB) gravity have attracted great attention. In this paper, we investigate the validity of the weak cosmic censorship conjecture for a novel $4D$ charged EGB black hole with test charged scalar field and test charged particle respectively. For the test charged field scattering process, we find that both extremal and near-extremal black holes cannot be overcharged. For the test charged particle injection, to first order, an extremal black hole cannot be overcharged while a near-extremal $4D$ charged EGB black hole can be destroyed. To second order, however, both extremal and near-extremal $4D$ charged EGB black holes can be overcharged for positive Gauss-Bonnet coupling constant; for negative Gauss-Bonnet coupling constant, an extremal black hole cannot be overcharged and the validity of the weak cosmic censorship conjecture for a near-extremal black hole depends on the Gauss-Bonnet coupling constant.Comment: 13 pages,1 figure;V2: discussions and references added; V3: published versio

Traversable Thin-shell Wormhole in the 4D Einstein-Gauss-Bonnet Theory

We construct the spherically symmetric thin-shell wormhole solutions of the four-dimensional Einstein-Gauss-Bonnet model and study their stability under radial linear perturbations. For positive Gauss-Bonnet coupling constant, the stable thin-shell wormhole can only be supported by exotic matter. For negative enough Gauss-Bonnet coupling constant, in asymptotic flat and AdS spacetime, there exists stable neutral thin-shell wormhole with normal matter which has finite throat radius. In asymptotic dS spacetime, there is no stable neutral thin-shell wormhole with normal matter. The charged thin-shell wormholes with normal matter exist in all the flat, AdS and dS spacetime. Their throat radius can be arbitrarily small. However, when the charge is too large, the stable thin-shell wormhole can be supported only by exotic matter.Comment: 20 pages, 7 figures, references added; minor revisio