arXiv subjects

Sushant G. Ghosh

Publications and source records attributed to Sushant G. Ghosh.

At least 55 records · Page 3Linked to original sources

arXiv2023

Nonsingular black hole chemistry in $4D$ Einstein-Gauss-Bonnet gravity

The EGB 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 $4D$. Several ways have been proposed for regularizing the $ D \to 4 $ limit of EGB for non-trivial gravitational dynamics in $ 4D $. Motivated by the importance of AdS/CFT, we obtain an exact static spherically symmetric nonsingular black hole in $4D$ EGB gravity coupled to the nonlinear electrodynamics (NED) in an AdS spacetime. We interpret the negative cosmological constant $Λ$ as the positive pressure, via $ P=-Λ/8π$, of the system's thermodynamic properties of the nonsingular black hole with an AdS background. We find that for $P 0$ are stable to thermal fluctuations and unstable otherwise. We also analyzed the Gibbs free energy to find that the small globally unstable black holes undergo a phase transition to the large globally stable black holes. Further, we study the $P-V$ criticality of the system and then calculate the critical exponents to find that our system behaves like Van der Walls fluid.

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arXiv2023

Shadows of Kerr-like black holes in $4D$ Einstein-Gauss-Bonnet gravity and constraints from EHT observations

The M87* black hole shadow observation by the Event Horizon Telescope (EHT) has enabled us to test the modified gravity theories in the extreme-field regime and estimating the black hole parameters. Having this assertion, we investigate the Kerr-like rotating black holes in $4D$ Einstein-Gauss-Bonnet (EGB) gravity and deduce their shadows. Considering the inclination angle $θ_0=17^o$, we show that the EGB black hole shadows are smaller and more distorted than for the Kerr black holes. Modelling the M87* black hole as the EGB black hole, we predict the shadow angular size $35.7888μas\leq θ_d\leq 39.6192μas$. The M87* black hole shadow angular size $θ_d=42\pm 3μas$, within the 1$σ$ region, constrains the GB coupling parameter and the black hole spin parameter. Interestingly, the circularity deviation of the EGB black hole shadows is smaller than the bounded deduced for the M87* black hole.

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arXiv2023

Investigating Loop Quantum Gravity with EHT Observational Effects of Rotating Black holes

A mathematically consistent rotating black hole model in loop quantum gravity (LQG) is yet lacking. The scarcity of rotating black hole solutions in LQG substantially hampers the development of testing LQG from observations, e.g., from the Event Horizon Telescope (EHT) observations. The EHT observation revealed event horizon-scale images of the supermassive black holes Sgr A* and M87*. The EHT results are consistent with the shadow of a Kerr black hole of general relativity. We present LQG-motivated rotating black hole (LMRBH) spacetimes, which are regular everywhere and asymptotically encompass the Kerr black hole as a particular case. The LMRBH metric describes a multi-horizon black hole in the sense that it can admit up to three horizons, such that an extremal LMRBH, unlike the Kerr black hole, refers to a black hole with angular momentum $a>M$. The metric, depending on the parameters, describes (1) black holes with only one horizon (BH-I), (2) black holes with an event horizon and a Cauchy horizons (BH-II), (3) black holes with three horizons (BH-III) or (4) no-horizon (NH) spacetime, which, we show, is almost ruled out by the EHT observations. We constrain the LQG parameter with the aid of the EHT shadow observational results of M87* and Sgr A*,respectively, for an inclination angle of $17^0$ and $50^0$. In particular, the VLTI bound for the Sgr A*, $δ\in (-0.17,0.01)$, constrains the parameters ($a,l$) such that for $0< l\leq 0.347851M\; (l\leq 2\times 10^6$ km), the allowed range of $a$ is $(0,1.0307M)$. Together with the EHT bounds of Sgr A$^*$ and M87$^*$ observables, our analysis concludes that a substantial part of BH-I and BH-II parameter space agrees with the EHT results of M87* and Sgr A*. While the EHT M87* results totally rule out the BH-III, but not that by Sgr A*.

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arXiv2022

Loop Quantum Gravity motivated multihorizon rotating black holes

With a semiclassical polymerization in the loop quantum gravity (LQG), the interior of Schwarzschild black holes provides a captivating single-horizon regular black hole spacetime. The shortage of rotating black hole models in loop quantum gravity (LQG) substantially restrains the progress of testing LQG from observations. Motivated by this, starting with a spherical LQG black hole as a seed metric, we construct a rotating spacetime using the revised Newman-Janis algorithm, namely, the LQG-motivated rotating black holes (LMRBH), which encompasses Kerr ($l=0$) black holes as an exceptional case. We discover that for any random $l>0$, unlike Kerr black hole, an extremal LMRBH refers to a black hole with angular momentum $a>M$. The rotating metric, in parameter space, describes (1) black holes with an event and Cauchy horizons, (2) black holes with three horizons, (3) black holes with only one horizon or (4) no horizon spacetime. We also discuss the horizon and global structure of the LMRBH spacetimes and its dependence on $l/M$ that exhibits rich spacetime structures in the ($M,\;a,\;l$) parameter space.

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arXiv2022

Testing strong gravitational lensing effects by supermassive compact objects with regular spacetimes

We compare and contrast gravitational lensing, in the strong-field limit, by photon sphere in spherically symmetric regular electrically charged (REC) black holes ($0 b_E$). Here, $b$ is additional parameter due to charge and the value $b=b_E \approx 0.226$ corresponds to an extremal black hole with degenerate horizons. Interestingly, the spacetime admits photon sphere for $0<b\leq b_P \approx 0.247$ and an anti-photon sphere only for $b_E < b \leq b_P$. With no-horizon spacetime, images by lensing from the inside of the photon sphere ($u<u_{ps}$) can also appear. Interestingly, for the case $u<u_{ps}$ the deflection angle $α_D$ increases with $u$. We analyse the lensing observables by modelling compact objects Sgr A*, M87*, NGC4649, and NGC1332 as black holes and no-horizon spacetimes. The angular position $θ_{\infty}$ and photon sphere radius $x_{ps}$ decrease with increasing parameter $b$. Our findings suggest that the angular separations ($s$) and magnification ($r$) of relativistic images inside the photon sphere may be higher than those outside. Moreover, the time delay for Sgr A* and M87* can reach $\sim$ 8.8809 min and $\sim$ 12701.8 min, respectively, at $b = 0.2$, deviating from Schwarzschild black holes by $\sim$ 2.615 min and $\sim$ 4677 min. These deviations are insignificant for Sgr A* because it is too small, but they are sufficient for astronomical observation of M87* and some other black holes. With EHT bounds on $θ_{sh}$ of Sgr A* and M 87*, within $1 σ$ region, placing bounds on the parameter $b$, our analysis concludes that the REC black holes agree with the EHT results in finite space, whereas the corresponding REC no-horizon spacetimes are completely ruled out.

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arXiv2022

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. Modelling Bardeen models in four-dimensional Einstein-Gauss-Bonnet (4D EGB) gravity, with additional parameters $\tildeα$ and charge $q$, as central black holes in various galaxies, we investigate gravitational lensing properties in strong deflection limits. Interestingly, the spherical photon orbit radius $x_m$, the critical impact parameter $u_m$, the lensing coefficient $\bar{b}$, the deflection angle $α_D(θ)$, angular position $θ_{\infty}$ are decreasing with $q$ and $α$ whereas the other lensing coefficient $\bar{a}$ and angular separation $s$ have opposite behaviour. Taking the supermassive black holes Sgr A* and M87* as the lens, we also compare observable signatures of 4D EGB Bardeen black holes with those of the Schwarzschild black holes. The angular position $θ_\infty$ for Sgr A* $\in$ (23.1853, \; 25.56427) $μ$as, whereas for M87* it is $\in$ ( 17.941,\; 19.7819) $μ$as. Further, the angular separation $s$, which is an increasing function of $\tildeα$ and $q$ for Sgr A* and M87* differs significantly, respectively, in (0.031997,0.14895) $μ$as and (0.0247, 0.1152) $μ$as. The deviations of the lensing observables $Δθ_{\infty}$ and $Δs$ for 4D EGB Bardeen black hole ($\tildeα=0.9,~q=0.09$) from the Schwarzschild black hole, respectively, can reach up to $2.3789~μ$as and $0.11695~μ$as for Sgr A* , $1.84084~μ$as and $0.0905~μ$as for M87*. On the other hand, the relative magnification $\in$ (4.65751,\; 6.82173). Considering twenty-two massive central black holes as lens, we also estimate the time delay $ΔT^s_{2,1}$ between the first and second relativistic image to find that, e.g., the time delay for Sgr A* and M87*, respectively, can reach $\sim9.86088$~min and $\sim16023.93$~min.

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arXiv2022

Rotating Black Holes in Horndeski Gravity: Thermodynamic and Gravitational Lensing

The lack of rotating black holes, typically found in nature, hinders testing modified gravity from astrophysical observations. We present the axially symmetric counterpart of an existing spherical hairy black hole in Horndeski gravity having an additional deviation parameter $Q$, which encompasses the Kerr black hole as a particular case ($Q=0$). We investigate the effect of Horndeski parameter $Q$ on the rotating black holes' geometry and analytically deduce the gravitational deflection angle of light in the weak-field limit. For the S2 source star, the deflection angle for the Sgr A* model of the rotating Horndeski gravity black hole for both prograde and retrograde photons is larger than the Kerr black hole values. We show how parameter $Q$ could be constrained by astrophysical implications of the lensing of this object. The thermodynamic quantities, Komar mass, and Komar angular momentum gets corrected by the parameter $Q$, but the Smarr relation $M_{\text{eff}}=2ST+2ΩJ_{\text{eff}}$ still holds at the event horizon.

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arXiv2022

Testing Horndeski Gravity from EHT Observational Results for Rotating Black Holes

The Event Horizon Telescope (EHT) collaboration recently unveiled the first image of the supermassive black hole M87*, which exhibited a ring of angular diameter $θ_{d}=42 \pm 3 μas$, a circularity deviation $ΔC \leq 0.1$, and also inferred a black hole mass of $M=(6.5 \pm 0.7) \times 10^9 M_\odot $. This provides a new window onto tests of theories of gravity in the strong-field regime, including probes of violations of the no-hair theorem. It is widely believed that the Kerr metric describes the astrophysical black holes, as encapsulated in the critical but untested no-hair theorem. Modeling Horndeski gravity black holes -- with additional hair parameter $h$ besides the mass $M$ and spin $a$ of the Kerr black hole -- as the supermassive black hole M87*, we observe that to be a viable astrophysical black hole candidate, the EHT result constrains ($a$, $h$) parameter space. However, a systematic bias analysis indicates rotating Horndeski black hole shadows may or may not capture Kerr black hole shadows, depending on the parameter values; the latter is the case over a substantial part of the constrained parameter space, allowing Horndeski gravity and general relativity to be distinguishable in the said space, and opening up the possibility of potential modifications to the Kerr metric.

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arXiv2022

Bardeen Black Holes in the Regularized $4D$ Einstein--Gauss--Bonnet Gravity

We obtain exact Bardeen black holes to the regularized $4D$ Einstein--Gauss--Bonnet (EGB) gravity minimally coupled with the nonlinear electrodynamics (NED). In turn, we analyze the horizon structure to determine the effect of GB parameter $α$ on the minimum cutoff values of mass, $M_0$, and magnetic monopole charge, $g_0$, for the existence of a black hole horizon. We obtain an exact expression for thermodynamic quantities, namely, Hawking temperature $T_+$, entropy $S_+$, Helmholtz free energy $F_+$, and specific heat $C_+$ associated with the black hole horizon, and they show significant deviations from the $4D$ EGB case owing to NED. Interestingly, there exists a critical value of horizon radius, $r_+^{c}$, corresponding to the local maximum of Hawking temperature, at which heat capacity diverges, confirming the second-order phase transition. A discussion on the black holes of alternate regularized $4D$ EGB gravity belonging to the scalar-tensor theory is appended.

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arXiv2022

Nonsingular Black Holes in $4D$ Einstein--Gauss--Bonnet Gravity

Recently, several methods have been proposed to regularize a $D \to 4$ limit of Einstein-Gauss-Bonnet (EGB), leading to nontrivial gravitational dynamics in $4D$. We present an exact nonsingular black hole solution in the $4D$ EGB gravity coupled to non-linear electrodynamics and analyze their thermodynamic properties to calculate precise expressions for the black hole mass, temperature, and entropy. Because of the magnetic charge, the thermodynamic quantities are corrected, and the Hawking--Page phase transition is achievable with diverges of the heat capacity at a larger critical radius $r=r_{+}^{C}$ in comparison to the $5D$ counterpart where the temperature is maximum. Thus, we have a black hole with Cauchy and event horizons, and its evaporation leads to a thermodynamically stable extremal black hole remnant with vanishing temperature, and its size is larger than the $5D$ counterpart. The entropy does not satisfy the usual exact horizon Bekenstein--Hawking area law of general relativity with a logarithmic area correction term.

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arXiv2022

Thermodynamic stability and P-V criticality of nonsingular-AdS black holes endowed with clouds of strings

We investigate the extended phase space thermodynamics of nonsingular-AdS black holes minimally coupled to clouds of strings in which we consider the cosmological constant ($Λ$) as the pressure ($P$) of the black holes and its conjugate variable thermodynamical volume ($V$) of the black holes. Owing to the background clouds of strings parameter ($a$), we analyse the Hawking temperature, entropy and specific heat on horizon radius for fixed-parameter $k$. We find that the strings clouds background does not alter small/large black hole (SBH/LBH) phase transition but occurs at a larger horizon radius, and two second-order phase transitions occur at a smaller horizon radius. Indeed, the $G$--$T$ plots exhibit a swallowtail below the critical pressure, implying that the first-order phase transition is analogous to the liquid-gas phase transition at a lower temperature and lower critical pressure. To further examine the analogy between nonsingular-AdS black holes and a liquid-gas system, we derive the exact critical points and probe the effects of a cloud of strings on $P-V$ criticality to find that the isotherms undergo liquid-gas like phase transition for $\tilde{T}\,<\,\tilde{T}_c$ at lower $\tilde{T}_c$. We have also calculated the critical exponents identical with Van der Walls fluid, i.e., same as those obtained before for arbitrary other AdS black holes, which implies that the background clouds of strings do not change the critical exponents.

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arXiv2022

Estimating the cosmological constant from shadows of Kerr--de Sitter black holes

The Event Horizon Telescope collaboration has revealed the first direct image of a black hole, as per the shadow of a Kerr black hole of general relativity. However, other Kerr-like rotating black holes of modified gravity theories cannot be ignored, and they are essential as they offer an arena in which these theories can be tested through astrophysical observation. This motivates us to investigate asymptotically de Sitter rotating black holes wherein interpreting the cosmological constant $Λ$ as the vacuum energy leads to a deformation in the vicinity of a black hole -- new Kerr--de Sitter solution, which has a richer geometric structure than the original one. We derive an analytical formula necessary for the shadow of the new Kerr--de Sitter black holes and then visualize the shadow of black holes for various parameters for an observer at given coordinates $(r_0, θ_0)$ in the domain $(r_0,\; r_c)$ and estimate the cosmological constant $Λ$ from its shadow observables. The shadow observables of the new Kerr--de Sitter black holes significantly deviate from the corresponding observables of the Kerr--de Sitter black hole over an appreciable range of the parameter space. Interestingly, we find a finite parameter space for ($Λ$, $a$) where the observables of the two black holes are indistinguishable.

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arXiv2021

Investigating strong gravitational lensing effects by suppermassive black holes with Horndeski gravity

We study gravitational lensing in strong-field limit by a static spherically symmetric black hole in quartic scalar field Horndeski gravity having additional hair parameter $q$, evading the no-hair theorem. We find an increase in the deflection angle $α_D$, photon sphere radius $x_{ps}$, and angular position $θ_{\infty}$ that increases more quickly while angular separation $s$ more slowly, but the ratio of the flux of the first image to all other images $r_{mag}$ decreases rapidly with increasing magnitude of the hair $q$. We also discuss the astrophysical consequences in the supermassive black holes at the centre of several galaxies and note that the black holes in Horndeski gravity can be quantitatively distinguished from the Schwarzschild black hole. Notably, we find that the deviation $Δθ_{\infty}$ of black holes in Horndeski gravity from their general relativity (GR) counterpart, for supermassive black holes Sgr A* and M87, for $q=-1$ respectively, can reach as much as $25.192~μ$as and $18.92~μ$as while $Δs$ is about $1.121~μ$as for Sgr A* and $0.8424~μ$as for M87*. The ratio of the flux of the first image to all other images suggest that the Schwarzschild images are brighter than those of the black holes in Horndeski gravity, wherein the deviation $|Δr_{mag}|$ is as much as 3.082. The results suggest that observational tests of hairy black holes in Horndeski gravity are indeed feasible.

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arXiv2021

Rotating black holes in general relativity coupled to nonlinear electrodynamics

We find an exact spherically symmetric magnetically charged black hole solution to general relativity (GR) coupled to nonlinear electrodynamics (NED) with an appropriate Lagrangian density. In turn, starting with this spherical black hole as a seed metric, we construct a rotating spacetime, a modification of Kerr black hole, using the revised Newman-Janis algorithm that depends on mass, spin, and a NED parameter $g$. We find an exact expression for thermodynamic quantities of the black holes like the mass, Hawking temperature, entropy, heat capacity, and free energy expressed in terms of horizon radius, and they show significant deviations from the Kerr case owing to NED. We also calculate analytical expressions for effective Komar mass and angular momentum for the rotating black hole and demonstrate that the Komar conserved quantity corresponding to the null Killing vector at horizon obeys $\mathcal{K}_χ=2S_+T_+.$ The radiating counterpart renders a generalization of Carmeli's spacetime as well as Vaidya's spacetime in the appropriate limits.

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arXiv2021

Shadows of black hole surrounded by anisotropic fluid in Rastall theory

Due to the gravitational lensing effect, a black hole casts a shadow larger than its horizon over a bright background, and the shape and size can be calculated. The Event Horizon Telescope collaboration has produced the first direct image (shadow) of the black hole and it is in accordance with the shadow of a Kerr black hole of general relativity. However, deviations from the Kerr black hole arising from modified theories of gravity are not ruled out and they are important as they offer an arena to test these theories through astrophysical observation. This stimulates us to investigate rotating black holes surrounded by anisotropic fluid in Rastall theory namely a rotating Rastall black hole, which is characterized by mass $M$, spin $a$, field structure parameter $N_s$ and the Rastall parameter $ψ$. It encompasses, as special cases, Kerr ($N_s \to 0$) and Kerr-Newman ($s=0$ and $N_s = -Q^2 $) black holes. The rotating Rastall black hole is characterized by an additional cosmological-like horizon apart from Cauchy and event horizons. We derive an analytical formula for the shadow of a rotating Rastall black hole and go on to visualize the shadow of black holes for various values of the parameters for an observer at a given coordinate ($r_O, θ_O$) in the domain $[r_+,r_q]$.

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arXiv2021

Strong gravitational lensing by rotating Simpson--Visser black holes

We investigate strong field gravitational lensing by rotating Simpson-Visser black hole, which has an additional parameter ($0\leq l/2M \leq1$), apart from mass ($M$) and rotation parameter ($a$). A rotating Simpson-Visser metric correspond to (i) a Schwarzschild metric for $l/2M=a/2M=0$ and $ M \neq 0 $, (ii) a Kerr metric for $l/2M=0$, $|a/2M|< 0.5$ and $ M \neq 0 $ (iii) a rotating regular black hole metric for $|a/2M|< 0.5$, $ M \neq 0 $ and $l/2M$ in the range $0 0.5$ and $l/2M\neq 0$. We find a decrease in the deflection angle $α_D$ and also in the ratio of the flux of the first image and all other images $ r_{mag}$. On the other hand, angular position $θ_{1}$ increases more slowly and photon sphere radius $x_{m}$ decreases more quickly, but angular separation $s$ increases more rapidly, and their behaviour is similar to that of the Kerr black hole. The formalism is applied to discuss the astrophysical consequences in the supermassive black holes and find that the rotating Simpson-Visser black holes can be distinguished from the Kerr black hole via gravitational lensing. The deviation of the lensing observables $Δθ_1$ and $Δs$ of rotating Simpson Visser black holes from Kerr black hole for $0<l/2M <0.6$ ($a/2M=0.45$), for supermassive black holes Sgr A* and M87, respectively, are in the range $0.0422-0.11658~μ$as and $0.031709-0.08758~μ$as while $|Δr_{mag}|$ is in the range $0.2037 - 0.95668$. It is difficult to distinguish the two black holes because the departure are in $\mathcal{O}(μ$as), which are unlikely to get resolved by the current EHT observations. We also derive a two-dimensional lens equation and formula for deflection angle in the strong field limit by focusing on trajectories close to the equatorial plane.

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arXiv2021

Wormholes in $f(R,T)$ gravity satisfying the null energy condition with isotropic pressure

We consider $f(R, T)$ theory of gravity, in which the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar and the trace of the energy-momentum tensor, to study static spherically symmetric wormhole geometries sustained by matter sources with isotropic pressure. According to restrictions on the wormhole geometries, we carefully adopt different strategies to construct solutions with the properties and characteristics of wormholes. Using a completely general procedure, we provide several examples of wormholes in which the matter threading the wormhole throat satisfies all of the energy conditions and discuss general mechanisms for finding them. Finally, we postulate a smooth transformation for simplifying the nonlinear field equations and have more consistent results than the other ones to conclude that the results can be viewed as specific exact wormhole solutions without the presence of exotic matter.

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arXiv2021

Particle acceleration around rotating Einstein-Born-Infeld black hole and plasma effect on gravitational lensing

We consider a time-like geodesics in the background of rotating Einstein-Born-Infeld (EBI) black hole to examine the horizon and ergosphere structure. The effective potential that governs the particle's motion in the spacetime and the innermost stable circular orbits (ISCO) is also studied. A qualitative analysis is conducted to find the redshifted ultrahigh centre-of-mass (CM) energy as a result of a two-particle collision specifically near the horizon. The recent Event Horizon Telescope (EHT) triggered a surge of interest in strong gravitational lensing by black holes, which provide a new tool comparing the black hole lensing in general relativity and alternate gravity theories. Motivated by this, we also discussed both strong and weak-field gravitational lensing in the space-time discretely for a uniform plasma and a singular isothermal sphere. We calculated the light deflection coefficients $\bar{a}$ and $\bar{b}$ in the strong field limits, and their variance with the rotational parameter $a$ for different plasma frequency as well as in vacuum. For EBI black holes, we found that plasma's presence increases the photon sphere radius, the deflection angle, the deflection coefficients $\bar{a}$, $\bar{b}$, the angular positions and the angular separation between the relativistic images. It is also shown that with increasing spin the impact of plasma on a strong gravitational lensing becomes smaller as the spin parameter grows in the prograde orbit ($a>0$). For extreme black holes, the strong gravitational effects in the homogenous plasma are similar to those of in a vacuum. We investigate strong gravitational lensing effects by supermassive black holes Sgr A* and M87*...

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