Gravitational Field of a Spinning Mass as an Example of Algebraically Special Metrics

When a gas of bosonic particles is cooled below a critical temperature, it condenses into a Bose-Ei…

Bose-Einstein condensation

Annual Review of Astronomy and Astrophysics

According to present cosmology, certain stars end their careers in a total gravitational collapse that transcends the ordinary laws of physics.

Introducing the Black-Hole

An american journal of physics

Recently there has been a surge of interest in regularizing, a D → 4  limit of, the Einstein-Gauss-Bonnet (EGB) gravity, and the resulting regularized 4D EGB gravity has nontrivial dynamics. The theory admits spherically symmetric black holes generalizing the Schwarzschild black holes. We consider the rotating black hole in regularized 4D EGB gravity and discuss their horizon properties and shadow cast. The effects of the GB coupling parameter on the shape and size of shadows are investigated in the context of recent M87* observations from the EHT . Interestingly, for a given spin parameter, the apparent size of the shadow decreases and gets more distorted due to the GB coupling parameter. We find that within the finite parameter space, e.g. for a=0.1M, αl 0.00394M2, and within the current observational uncertainties, the rotating black holes of the 4D EGB gravity are consistent with the inferred features of M87* black hole shadow.

Rotating black holes in 4D Einstein-Gauss-Bonnet gravity and its shadow

We discuss the horizon properties, shadow cast, and the weak gravitational lensing of charged rotating regular black holes, which in addition to mass ($M$) and rotation parameter ($a$) have an electric charge ($Q$) and magnetic charge ($g$). The considered regular black holes are the generalization of the Kerr ($Q=g=0$) and Kerr-Newman ($g=0$) black holes. Interestingly, for a given parameter set, the apparent size of the shadow monotonically decreases and the shadow gets more distorted with increasing charge parameter $Q$. We put constraints on the black hole parameters with the aid of recent M87* shadow observation. The conserved quantities associated with the rotating regular black holes are calculated and also a brief description of the weak gravitational lensing using the Gauss-Bonnet theorem is presented. Interestingly, the deflection angle decreases with the charge of the black hole. Our results vis-\`a-vis go over to the Kerr and Kerr-Newman black holes in the appropriate limits.

Shadow cast and deflection of light by charged rotating regular black holes

Recently, a non-trivial 4D Einstein-Gauss-Bonnet (EGB) theory of gravity, by rescaling the GB coupling parameter as α/(D−4), was formulated in [1], which bypasses Lovelock's theorem and avoids Ostrogradsky instability. The theory admits a static spherically symmetric black hole, unlike 5D EGB or general relativity counterpart, which can have both Cauchy and event horizons. We generalize previous work, on gravitational lensing by a Schwarzschild black hole, in the strong and weak deflection limits to the 4D EGB black holes to calculate the deflection coefficients b and b, while former increases and later decrease with increasing α. We also find that the deflection angle αD, angular position θi and um decreases, but angular separation s increases with α. The effect of the GB coupling parameter α on positions and magnification of the source relativistic images is discussed in the context of SgrA* and M87* black holes. A brief description of the weak gravitational lensing using the Gauss-Bonnet theorem is presented.

Gravitational lensing by black holes in the 4D Einstein-Gauss-Bonnet gravity

The Event Horizon Telescope (EHT), a global submillimeter wavelength very long baseline interferometry array, unveiled event-horizon-scale images of the supermassive black hole M87* as an asymmetric bright emission ring with a diameter of $ 42 \pm 3\; \mu$as, and it is consistent with the shadow of a Kerr black hole of general relativity. A Kerr black hole is also a solution of some alternative theories of gravity, while several modified theories of gravity admit non-Kerr black holes. While earlier estimates for the M87* black hole mass, depending on the method used, fall in the range $ \approx 3\times 10^9 M_\odot- 7 \times 10^9 $$ M_\odot $, the EHT data indicated a mass for the M87* black hole of $(6.5 \pm 0.7) \times 10^9 M_\odot $. This offers another promising tool to estimate black hole parameters and to probe theories of gravity in its most extreme region near the event horizon. The important question arises: Is it possible by a simple technique to estimate black hole parameters from its shadow, for arbitrary models? In this paper, we present observables, expressed in terms of ordinary integrals, characterizing a haphazard shadow shape to estimate the parameters associated with black holes, and then illustrate its relevance to four different models: Kerr, Kerr$-$Newman, and two rotating regular models. Our method is robust, accurate, and consistent with the results obtained from existing formalism, and it is applicable to more general shadow shapes that may not be circular due to noisy data.

Black Hole Parameter Estimation from Its Shadow

Recently, a non-trivial $4D$ Einstein-Gauss-Bonnet (EGB) theory of gravity, by rescaling the GB coupling parameter as $\alpha/(D-4)$, was formulated in \cite{Glavan:2019inb}, which bypasses Lovelock's theorem and avoids Ostrogradsky instability. The theory admits a static spherically symmetric black hole, unlike $5D$ EGB or general relativity counterpart, which can have both Cauchy and event horizons. We generalize previous work, on gravitational lensing by a Schwarzschild black hole, in the strong and weak deflection limits to the $4D$ EGB black holes to calculate the deflection coefficients $\bar{a}$ and $\bar{b}$, while former increases and later decrease with increasing $\alpha$. We also find that the deflection angle $\alpha_D$, angular position $\theta_{\infty}$ and $u_{m}$ decreases, but angular separation $s$ increases with $\alpha$. The effect of the GB coupling parameter $\alpha$ on positions and magnification of the source relativistic images is discussed in the context of SgrA* and M87* black holes. A brief description of the weak gravitational lensing using the Gauss-Bonnet theorem is presented.

Rahul Kumar

Papers

Rotating black holes in 4D Einstein-Gauss-Bonnet gravity and its shadow

Shadow cast and deflection of light by charged rotating regular black holes

Gravitational lensing by black holes in the 4D Einstein-Gauss-Bonnet gravity

Black Hole Parameter Estimation from Its Shadow

Gravitational lensing by black holes in the $4D$ Einstein-Gauss-Bonnet gravity