arXiv subjects

Sushant G. Ghosh

Publications and source records attributed to Sushant G. Ghosh.

At least 73 records · Page 4Linked to original sources

arXiv2021

Nonsingular black hole chemistry

We study the nonsingular black hole in Anti de-Sitter background taking the negative cosmological constant as the pressure of the system. We investigate the horizon structure, and find the critical values $m_0$ and $\tilde{k}_0$, such that $m>m_0$ (or $\tilde{k}<\tilde{k}_0$) corresponds to a black solution with two horizons, namely the Cauchy horizon $x_-$ and the event horizon $x_+$. For $m=m_0$ (or $\tilde{k}=\tilde{k}_0$), there exist an extremal black hole with degenerate horizon $x_0=x_{\pm}$ and for $m \tilde{k}_0$), no black hole solution exists. In turn, we calculate the thermodynamical properties and by observing the behaviour of Gibb's free energy and specific heat, we find that this black hole solution exhibits first order (small to large black hole) and second order phase transition. Further, we study the $P-V$ criticality of system and then calculate the critical exponents showing that they are the same as those of the Van der Waals fluid.

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arXiv2021

Strong field gravitational lensing by hairy Kerr black holes

Recent times witnessed a surge of interest in strong gravitational lensing by black holes due to the Event Horizon Telescope (EHT) results, which suggest comparing the black hole lensing in general relativity and modified gravity theories. This may help us to assess the phenomenological differences between these models. A Kerr black hole is also a solution to some alternative theories of gravity, while recently obtained modified Kerr black holes (hairy Kerr black holes), which evade the no-hair theorem, are due to additional sources from surrounding fluid, like dark matter, having conserved energy momentum tensor (EMT). These hairy Kerr black holes may also be solutions to an alternative theory of gravity. We generalize previous work on gravitational lensing by a Kerr black hole in the strong deflection limits to the hairy Kerr black holes, with a deviation parameter $α$ and a primary hair $\ell_0$. Interestingly, the deflection coefficient $\bar{a}$ increases and decreases with increasing $\ell_0$ and $α$ respectively. $\bar{b}$ shows opposite behaviour with $\ell_0$ and $α$. We also find that the deflection angle $α_D$, angular position $θ_{\infty}$ and $u_{m}$ decrease, but angular separation $s$ increases with $α$. We compare our results with those for Kerr black holes, and also apply the formalism to discuss the astrophysical consequences in the context of the supermassive black holes Sgr A* and M87*. We observe that the deviations of the angular positions from that of the Kerr black hole are not more than $2.6~μ$as for Sgr A* and $1.96~μ$as for M87*, which are unlikely to be resolved by the current EHT observations.

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arXiv2021

Parameter estimation of hairy Kerr black holes from its shadow and constraints from M87*

The recently obtained hairy Kerr black holes, due to additional sources or surrounding fluid, like dark matter, with conserved energy-momentum tensor, have a deviation $α$ and primary hair $l_0$, apart from rotation parameter $a$ and mass $M$. In the wake of the \textit{Event Horizon Telescope} (\textit{EHT}) observations of the supermassive black hole M87*, a recent surge in interest in black hole shadows suggests comparing the black holes in general relativity (GR) and modified theories of gravity (MoG) to assess these models' differences. Motivated by this, we take on an extensive study of the rotating hairy Kerr black holes, which encompasses, in particular cases, the Kerr black hole ($α=0$). We investigate ergosphere and shadows of the black holes to infer that their size and shape are affected due to the $l_0$ and are found to harbour a richer chaotic structure. In particular, the hairy Kerr black holes possess smaller size but more distorted shadows when compared with Kerr black holes. We also estimate the parameters $l_0$ and $a$ associated with hairy Kerr black holes using the shadow observables. The inferred circularity deviation $ΔC \leq 0.1$ for the M87* black hole is satisfied, whereas shadow angular diameter $θ_{d}=42 \pm 3 μas$, within $1 σ$ region, for a given choice of $α$, places bounds on the parameters $a$ and $l_0$. Interestingly, the shadow axial ratio obeying $1< D_x \lesssim 4/3$ is in agreement with the \textit{EHT} results and thus eventuates in the hairy Kerr black holes being suitable candidates for astrophysical black holes.

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arXiv2021

Photon ring structure of rotating regular black holes and no-horizon spacetimes

The Kerr black holes possess a photon region with prograde and retrograde orbits radii, respectively, $M\leq r_p^-\leq 3M$ and $3M\leq r_p^+\leq 4M$, and thereby always cast a closed photon ring or a shadow silhouette for $a\leq M$. For $a>M$, it is a no-horizon spacetime (naked singularity) wherein prograde orbits spiral into the central singularity, and retrograde orbits produce an arc-like shadow with a dark spot at the center. We compare Kerr black holes' photon ring structure with those produced by three rotating regular spacetimes, viz. Bardeen, Hayward, and nonsingular. These are non-Kerr black hole metrics with an additional deviation parameter of $g$ related to the nonlinear electrodynamics charge. It turns out that for a given $ a $, there exists a critical value of $ g $, $g_E$ such that $Δ=0$ has no zeros for $ g > g_E$, one double zero at $ r = r_E $ for $ g = g_E $, respectively, corresponding to a no-horizon regular spacetime and extremal black hole with degenerate horizon. We demonstrate that, unlike the Kerr naked singularity, no-horizon regular spacetimes can possess closed photon ring when $g_E< g \leq g_c$, e.g., for $a=0.10M$, Bardeen ($g_E=0.763332M<g\leq g_c= 0.816792M$), Hayward ($g_E=1.05297M < g\leq g_c = 1.164846M$) and nonsingular ($g_E=1.2020M < g \leq g_c= 1.222461M$) no-horizon spacetimes have closed photon ring. These results confirm that the mere existence of a closed photon ring does not prove that the compact object is necessarily a black hole. The ring circularity deviation observable $ΔC$ for the three no-horizon rotating spacetimes satisfy $ΔC\leq 0.10$ as per the M87* black hole shadow observations. We have also appended the case of Kerr-Newman no-horizon spacetimes (naked singularities) with similar features.

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arXiv2021

Parameters estimation and strong gravitational lensing of nonsingular Kerr-Sen black holes

The recent time witnessed a surge of interest in strong gravitational lensing by black holes is due to the Event Horizon Telescope (EHT) results, which suggest comparing the black hole lensing in both general relativity and heterotic string theory. That may help us to assess the phenomenological differences between these models. Motivated by this, we consider gravitational lensing by the nonsingular Kerr-Sen black holes, which encompass Kerr black holes as a particular case, to calculate the light deflection coefficients $p$ and $q$ in strong-field limits, while the former increases with increasing parameters $k$ and charge $b$, later decrease. We also find a decrease in the light deflection angle $α_D$, angular position $θ_{\infty}$ decreases more slowly and impact parameter for photon orbits $u_{m}$ more quickly, but angular separation $s$ increases more rapidly with parameters $b$ and $k$. We compare our results with those for Kerr black holes, and also the formalism is applied to discuss the astrophysical consequences in the case of the supermassive black holes NGC 4649, NGC 1332, Sgr A* and M87*. In turn, we also investigate the shadows of the nonsingular Kerr-Sen black holes and show that they are smaller and more distorted than the corresponding Kerr black holes and nonsingular Kerr black holes shadows. The inferred circularity deviation $ΔC\leq 0.10$, for the M87* black hole shadow, put constraints on the nonsingular Kerr-Sen black hole parameters ($a, k$) and ($a, b$). The maximum shadow angular diameter for $b=0.30M$ and $k=0.30M$ are, respectively, $θ_d=35.3461\,μ$as and $θ_d=35.3355\,μ$as. We also estimate the parameters associated with nonsingular Kerr-Sen black holes using the shadow observables.

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arXiv2021

Noncommutative inspired black holes in regularised 4D Einstein-Gauss-Bonnet theory

Low energy limits of string theory indicated that the standard gravity action should be modified to include higher-order curvature terms, in the form of dimensionally continued Gauss-Bonnet densities. If one includes only quadratic curvature terms then the resulting theory is Einstein-Gauss-Bonnet (EGB) gravity valid only in $D>4$ dimensions. Recently there has been a surge of interest in regularizing, a $ D \to 4 $ limit, of EGB gravity, and the resulting regularized $4D$ EGB gravities have nontrivial gravitational dynamics. We obtain a static spherically symmetric noncommutative (NC) geometry inspired black hole solution with Gaussian mass distribution as a source in the regularized $4D$ EGB and also analyze their properties. The metric of the NC inspired $4D$ EGB black hole smoothly interpolates between a de Sitter core around the origin and $4D$ EGB metric as $r/\sqrtθ \to \infty$. Owing to the NC and GB term corrected black hole, the thermodynamic quantities have also been altered. The phase transitions for the local thermodynamic stability, in the theory, is outlined by a discontinuity of specific heat $C_{+}$ at a critical radius $r_+=r_C$, and $C_{+}$ changes from infinitely negative to infinitely positive and then down to a finite positive for the smaller $r_+$. The thermal evaporation process leads to a thermodynamic stable extremal black hole with vanishing temperature.

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arXiv2020

Phase transition of AdS black holes in 4D EGB gravity coupled to nonlinear electrodynamics

Einstein-Gauss-Bonnet (EGB) gravity is an outcome of quadratic curvature corrections to the Einstein-Hilbert gravity action in the form of a Gauss-Bonnet (GB) term in $ D > 4$ dimensions and EGB gravity is topologically invariant in $4D$. Recently several ways have been proposed for regularizing, a $ D \to 4 $ limit of EGB, for nontrivial gravitational dynamics in $ 4D $. Motivated by the importance of anti-de Sitter gravity/conformal field theory correspondence (AdS/CFT), we analyze black holes with AdS asymptotic to regularized $4D$ EGB gravity coupled to the nonlinear electrodynamics (NED) field. For a static spherically symmetric \textit{ansatz} the field equations are solved exactly, using two different approaches, for a NED Lagrangian to obtain an identical solution$-$namely NED charged AdS black holes in $4D$ EGB gravity which retains several known solutions. Owing to the NED charge corrected EGB black holes, the thermodynamic quantities are also modified, and the entropy does not obey the usual area law. We calculate the heat capacity and Helmholtz free energy, in terms of horizon radii, to investigate both local and global thermodynamic stability of black holes. We observe a secondary Hawking-Page transition between the smaller thermally favoured black hole and thermal AdS space. Our results show that the behaviour of Hawking's evaporation abruptly halts at shorter radii regime such that the black holes do have a thermodynamically stable remnant with vanishing temperature.

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arXiv2020

Gravitational lensing by charged black hole in regularized $4D$ Einstein-Gauss-Bonnet gravity

Among the higher curvature gravities, the most extensively studied theory is the so-called Einstein-Gauss-Bonnet (EGB) gravity, whose Lagrangian contains Einstein term with the GB combination of quadratic curvature terms, and the GB term yields nontrivial gravitational dynamics in $ D\geq5$. Recently there has been a surge of interest in regularizing, a $ D \to 4 $ limit of, the EGB gravity, and the resulting regularized $4D$ EGB gravity valid in $4D$. We consider gravitational lensing by Charged black holes in the $4D$ EGB gravity theory to calculate the light deflection coefficients in strong-field limits $\bar{a}$ and $\bar{b}$, while the former increases with increasing GB parameter $α$ and charge $q$, later decrease. We also find a decrease in the deflection angle $α_D$, angular position $θ_{\infty}$ decreases more slowly and impact parameter for photon orbits $u_{m}$ more quickly, but angular separation $s$ increases more rapidly with $α$ and charge $q$. We compare our results with those for analogous black holes in General Relativity (GR) and also the formalism is applied to discuss the astrophysical consequences in the case of the supermassive black holes Sgr A* and M87*.

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arXiv2020

Generating black holes in $4D$ Einstein-Gauss-Bonnet gravity

In recent times there is a surge of interest in constructing Einstein-Gauss-Bonnet (EGB) gravity, in the limit $D \to 4 $, of the $D$-dimensional EGB gravity. Interestingly, the static spherically symmetric solutions in the various proposed $D \to 4 $ regularized EGB gravities coincide, and incidentally some other theories also admit the same solution. We prove a theorem that characterizes a large family of nonstatic or radiating spherically symmetric solutions to the $4D$ EGB gravity, representing, in general, spherically symmetric Type II fluid. An extension of the theorem, given without proof as being similar to the original theorem, generates static spherically symmetric black hole solutions of the theory. It not only enables us to identify available known black hole solutions as particular cases but also to generate several new solutions of the $4D$ EGB gravity.

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arXiv2020

Clouds of strings in $4D$ Einstein-Gauss-Bonnet black holes

Recently it has been shown that the Einstein-Gauss-Bonnet (EGB) gravity, by rescaling the coupling constant as $α/(D-4)$ and taking the limit $D \rightarrow 4$ at the level of the equations of motion, becomes nontrivially ghost-free in $4D$ - namely the novel $4D$ EGB gravity. We present an exact charged black hole solution to the theory surrounded by clouds of string (CS) and also analyze their thermodynamic properties to calculate exact expressions for the black hole mass, temperature, and entropy. Owing to the corrected black hole due to the background CS, the thermodynamic quantities have also been corrected except for the entropy, which remains unaffected by a CS background. However, as a result of the novel $4D$ EGB theory, the Bekenstein-Hawking area law turns out to be corrected by a logarithmic area term. The heat capacity $C_+$ diverges at a critical radius $r=r_C$, where incidentally the temperature has a maximum, and the Hawking-Page transitions even in absence of the cosmological term and $C_+ > 0$ for $r_+ < r_C$ allowing the black hole to become thermodynamically stable. In addition, the smaller black holes are globally preferred with negative free energy $F_+<0$. Our solution can also be identified as a $4D$ monopole-charged EGB black hole. We regain results of spherically symmetric black hole solutions of general relativity and that of novel $4D$ EGB, respectively, in the limits $α\to 0$ and $a=0$.

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arXiv2020

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

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 \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 $α$. We also find that the deflection angle $α_D$, angular position $θ_{\infty}$ and $u_{m}$ 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.

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arXiv2020

Wormholes in 4D Einstein-Gauss-Bonnet Gravity

Recently Glavan and Lin [Phys. Rev. Lett. 124, 081301 (2020)] formulated the Einstein-Gauss-Bonnet (EGB) gravity by re-scaling GB coupling constant as $α/(D-4)$ and taking limit $D \to 4$ at the level of field equations. The GB contribution, in the resulting novel 4D EGB theory, makes a nontrivial contribution and the theory preserves the number of degrees of freedom thereby free from the Ostrogradsky instability, and also bypasses the Lovelock theorem. We obtain an exact spherically symmetric wormhole solutions in the novel 4D EGB gravity for an isotropic and anisotropic matter sources. In this regard, we consider also a wormhole with a specific radial-dependent shape function, a power-law density profile as well as by imposing a particular equation of state. To this end, we analyze the flare-out conditions, embedding diagrams, energy conditions and the volume integral quantifier. In particular our $-$ve branch results, in the limit $α\rightarrow 0$, reduced exactly to \emph{vis-$\grave{a}$-vis} 4D Moriss-Thorne wormholes of GR.

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arXiv2020

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

Recently there has been a surge of interest in regularizing, a $ D \to 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 $a=0.1M$, $α\leq 0.00394M^2$, 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.

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arXiv2020

Radiating black holes in the novel 4D Einstein-Gauss-Bonnet gravity

Recently Glavan and Lin [Phys. Rev. Lett. 124, 081301 (2020)] formulated a novel Einstein-Gauss-Bonnet gravity in which the Gauss-Bonnet coupling has been rescaled as $α/(D-4)$ and the $4D$ theory is defined as the limit $D \rightarrow 4$, which preserves the number degrees of freedom thereby free from the Ostrogradsky instability. We present exact spherically symmetric nonstatic null dust solutions in the novel 4D Einstein-Gauss-Bonnet gravity that bypasses the Lovelock theorem. Our solution represents radiating black holes and regains, in the limit $α\rightarrow 0$, the famous Vaidya black hole of general relativity (GR). We discuss the horizon structure of black hole solutions to find that the three horizon-like loci that characterizes its structure, viz. $AH$, $EH$ and $ TLS $ have the relationship $r_{EH} < r_{AH} = r_{TLS}$. The charged radiating black holes in the theory, generalizing Bonnor-Vaidya black holes, are also considered. In particular our results, in the limit $α\rightarrow 0$, reduced exactly to \emph{vis-$\grave{a}$-vis} 4D black holes of GR.

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arXiv2020

Shadow and deflection angle of rotating black hole in asymptotically safe gravity

We analytically investigate the shadows cast by rotating black holes in the asymptotically safe gravity (ASG) by deriving complete null geodesics and observables using the Hamilton-Jacobi equation and Carter separable method. It turns out that the apparent shape and size of the shadow depend on the ASG parameters ($ζ, γ$) in addition to other black hole parameters ($M, a$). The size of black hole shadows monotonically decreases and shadows get more distorted with increasing values of ASG parameters when compared with the Kerr black hole shadows. In turn, we use shadow observables to estimate black hole spin and ASG parameters. Noteworthy, we find that the deflection angle of the light has been modified by ASG parameters to generalize the Kerr deflection angle, and the corrections in the deflection angle are of $\mathcal{O}(μ$as). In the vanishing limits of ASG parameters, our results smoothly reduced to the Kerr black holes. The inferred circularity deviation $ΔC\leq 0.10$ for the M87* black hole shadow merely constrains the ASG parameter $ζ$, however, shadow angular diameter $θ_d=42\pm 3\, μ$as, within the $1σ$ region, places bounds $ζ\leq 0.1324$ for $γ=0.10$.

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arXiv2020

Testing Rotating Regular Metrics as Candidates for Astrophysical Black Holes

The Event Horizon Telescope, a global submillimeter wavelength very long baseline interferometry array, produced the first image of supermassive black hole M87* showing a ring of diameter $θ_d= 42\pm 3\,μ$as, inferred a black hole mass of $M=(6.5 \pm 0.7) \times 10^9 M_\odot $ and allowed us to investigate the nature of strong-field gravity. The observed image is consistent with the shadow of a Kerr black hole, which according to the Kerr hypothesis describes the background spacetimes of all astrophysical black holes. The hypothesis, a strong-field prediction of general relativity, may be violated in the modified theories of gravity that admit non-Kerr black holes. Here, we use the black hole shadow to investigate the constraints when rotating regular black holes (non-Kerr) can be considered as astrophysical black hole candidates, paying attention to three leading regular black hole models with additional parameters $g$ related to nonlinear electrodynamics charge. Our interesting results based on the systematic bias analysis are that rotating regular black holes shadows may or may not capture Kerr black hole shadows, depending on the values of the parameter $g$. Indeed, the shadows of Bardeen black holes ($g\lesssim 0.26 M$), Hayward black holes ($g\lesssim 0.65 M$), and non-singular black holes ($g\lesssim 0.25 M$) are indistinguishable from Kerr black hole shadows within the current observational uncertainties, and thereby they can be strong viable candidates for the astrophysical black holes. Whereas Bardeen black holes ( $g\leq 0.30182M$), Hayward black holes ($g\leq 0.73627M$), and non-singular black holes ($g\leq 0.30461M$), within the $1σ$ region for $θ_d= 39\, μ$as, are consistent with the observed angular diameter of M87*.

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arXiv2020

Ergosphere and shadow of a rotating regular black hole

The spacetime singularities in classical general relativity predicted by the celebrated singularity theorems are formed at the end of gravitational collapse. Quantum gravity is the expected theory to resolve the singularity problem, but we are now far from it. Therefore attention has shifted to models of regular black holes free from the singularities. A spherically symmetric regular toy model was obtained by Dymnikova (1992) which we demonstrate as an exact solution of Einstein's field equations coupled to nonlinear electrodynamics for a Lagrangian with parameter $b$ related to magnetic charge. We construct rotating counterpart of this solution which encompasses the Kerr black hole as a special case when charge is switched off ($b=0$). Event Horizon Telescope has released the first image of supermassive black hole M87$^*$, revealing the structure near black hole horizon. The rotating regular black hole's shadow may be useful to determine strong field regime. We investigate ergosphere and black hole shadow of rotating regular black hole to infer that their sizes are sensitive to charge $b$ and have a richer chaotic structure. In particular, rotating regular black hole possess larger size, but less distorted shadows when compared with Kerr black holes. We find one to one correspondence between ergosphere and shadow of the black hole.

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arXiv2020

Rotating black string in dRGT massive gravity

One of the solutions of Einstein field equation with cylindrical symmetry is known as black string solution. In this work, the rotating black string solution in dRGT massive gravity is obtained and then called rotating-dRGT black string solution. This solution is a kind of generalized version of rotating AdS/dS black sting solution containing an additional two more terms characterizing the structure of graviton mass. The horizon structures of the black string are explored. The thermodynamical properties of the black string are investigated. We found that it is possible to obtain the Hawking-Page phase transition depending on the additional structure of the graviton mass, while it is not possible for rotating-AdS/dS black string. By analyzing the free energy, we also found that the stable rotating black string is bigger than the non-rotating one.

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