arXiv subjects

Sushant G. Ghosh

Publications and source records attributed to Sushant G. Ghosh.

122 records · Page 7Linked to original sources

arXiv2014

Spherical gravitational collapse in 5D Einstein-Gauss-Bonnet gravity

We consider a spherical gravitational collapse of inhomogeneous dust (and null dust) in Einstein gravity with the Gauss-Bonnet (GB) combination of quadratic curvature terms. It turns out that the presence of the coupling constant of the GB terms $α> 0$ completely changes the causal structure of the singularities from the analogous general relativistic case. The gravitational collapse of inhomogeneous dust in the five-dimensional GB extended Einstein equations leads to formation of a massive, but weak, timelike singularity which is forbidden in general relativity. The apparent horizons of two different collapsing solutions show interesting mathematical similarity.

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arXiv2014

Higher dimensional non-Kerr black hole and energy extraction

We investigate the properties of the horizons and ergosphere in a rotating higher dimensional (HD) deformed Kerr-like black hole. We also explicitly bring out the effect of deformation parameter $ε$ and the extra dimension on the efficiency of the Penrose process of energy extraction from a black hole. It is interesting to see that the ergosphere size is sensitive to the deformation parameter $ε$ as well as spacetime dimensions $D$. This gives rise to a much richer structure of the ergosphere in a HD non-Kerr black hole, thereby making the Penrose process more efficient compared with that of the four-dimensional Kerr black hole.

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arXiv2013

Accretion onto a higher dimensional black hole

We examine the steady-state spherically symmetric accretion of relativistic fluids, with a polytropic equation of state, onto a higher dimensional Schwarzschild black hole. The mass accretion rate, critical radius, and flow parameters are determined and compared with results obtained in standard four dimensions. The accretion rate, $\dot{M}$, is an explicit function of the black hole mass, $M$, as well as the gas boundary conditions and the dimensionality, $D$, of the spacetime. We also find the asymptotic compression ratios and temperature profiles below the accretion radius and at the event horizon. This analysis is a generalization of Michel's solution to higher dimensions and of the Newtonian expressions of Giddings and Mangano which considers the accretion of TeV black holes.

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arXiv2013

Rotating black hole in Einstein and pure Lovelock gravity

We obtain rotating black hole metric for higher dimensional Einstein and pure Lovelock gravity by employing two independent and well motivated methods. One is based on the principle of incorporation of Newtonian acceleration for timelike motion while no acceleration for null motion. The other is the well known Newman-Janis alogrithm that converts a static black hole into a rotating one through a complex transformation. It turns out that both the methods give the same metric which for Einst of the eour pure Lovelock metricein black hole is an exact vacuum solution while for pure Lovelock black hole it satisfies the vacuum equation in the leading order. However it shares all the physical properties with the well known Kerr black hole.

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arXiv2013

Bound orbits and gravitational theory

It can be easily shown that bound orbits around a static source can exist only in 4 dimension and in none else for any long range force. This is so not only for Maxwell's electromagnetic and Newton's gravity but also for Einstein's gravitation theory. In contrast to Maxwell's electrodynamics and Newton's gravity, GR has a natural higher dimensional generalization in Lovelock gravity which remarkably admits bound orbits around a static black hole in all even d=2N+2 dimensions where $N$ is degree of Lovelock polynomial action. This is as general a result as Bertrand's theorem of classical mechanics in which existence of closed orbits uniquely singles out the inverse square law for a long range central force.

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arXiv2013

Gravitational collapse in pure Lovelock gravity in higher dimensions

We study collapse of inhomogeneous dust and null dust (Vaidya radiation) in pure Lovelock gravity in higher dimensions. Since pure Lovelock gravity is kinematic in odd d=2N+1 dimension, hence pertinent dimension for the study is even d=2N+2, where N is degree of Lovelock polynomial. It turns out that pure Lovelock collapse favors naked singularity as against black hole for the Einstein case in the same dimension while strength of singularity as measured by divergence of Kretshmann scalar is interestingly the same in the two cases; i.e. the corresponding scalars have the same fall off behavior.

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arXiv2013

Radiating Kerr-Newman black hole in $f(R)$ gravity

We derive an exact radiating Kerr-Newman like black hole solution, with constant curvature $R=R_0$ imposed, to {\it metric} $f(R)$ gravity via complex transformations suggested by Newman-Janis. This generates a geometry which is precisely that of radiating Kerr-Newman-de Sitter / anti-de Sitter with the $f(R)$ gravity contributing an $R_0$ cosmological-like term. The structure of three horizon-like surfaces, {\it viz.} timelike limit surface, apparent horizon and event horizon, are determined. We demonstrate the existence of an additional cosmological horizon, in $f(R)$ gravity model, apart from the regular black hole horizons that exist in the analogous general relativity case. In particular, the known stationary Kerr-Newman black hole solutions of $f(R)$ gravity and general relativity are retrieved. We find that the timelike limit surface becomes less prolate with $R_0$ thereby affecting the shape of the corresponding ergosphere.

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arXiv2012

The Lovelock gravity in the critical spacetime dimension

It is well known that the vacuum in the Einstein gravity, which is linear in the Riemann curvature, is trivial in the critical (2+1=3) dimension because vacuum solution is flat. It turns out that this is true in general for any odd critical $d=2n+1$ dimension where $n$ is the degree of homogeneous polynomial in Riemann defining its higher order analogue whose trace is the nth order Lovelock polynomial. This is the "curvature" for nth order pure Lovelock gravity as the trace of its Bianchi derivative gives the corresponding analogue of the Einstein tensor \cite{bianchi}. Thus the vacuum in the pure Lovelock gravity is always trivial in the odd critical (2n+1) dimension which means it is pure Lovelock flat but it is not Riemann flat unless $n=1$ and then it describes a field of a global monopole. Further by adding Lambda we obtain the Lovelock analogue of the BTZ black hole.

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arXiv2011

Nonstatic charged BTZ-like black holes in N+1 dimensions

We find an exact nonstatic charged BTZ-like solutions, in ($N$+1)-dimensional Einstein gravity in the presence of negative cosmological constant and a nonlinear Maxwell field defined by a power $s$ of the Maxwell invariant, which describes the gravitational collapse of charged null fluid in an anti-de Sitter background. Considering the situation that a charged null fluid injects into the initially an anti-de Sitter spacetime, we show that a black hole form rather than a naked singularity, irrespective of spacetime dimensions, from gravitational collapse in accordance with cosmic censorship conjecture. The structure and locations of the apparent horizons of the black holes are also determined. It is interesting to see that, in the static limit and when N=2, one can retrieve 2+1 BTZ black hole solutions.

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arXiv2011

5D Radiating black holes in Einstein-Yang-Mills-Gauss-Bonnet gravity

We derive nonstatic spherically symmetric solutions of a null fluid, in five dimension (5D), to Einstein-Yang-Mills (EYM) equations with the coupling of Gauss-Bonnet (GB) combination of quadratic curvature terms, namely, 5D-EYMGB radiating black hole solution. It is shown that, in the limit, we can recover known radiating black hole solutions. The spherically symmetric known 5D static black hole solutions are also retrieved. The effect of the GB term and Yang-Mills (YM) gauge charge on the structure and location of horizons, of the 5D radiating black hole, is also discussed.

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arXiv2011

Gravitating magnetic monopole in Vaidya geometry

A magnetic-monopole solution of a non-Abelian gauge theory as proposed by 't Hooft and Polyakov is studied in the Vaidya spacetime. We find that the solutions of Einstein equations generates a geometry of the Bonnor-Vaidya corresponding to magnetically charged null fluid with Higgs field contributing a cosmological term. In the absence of the scalar fields the corresponding Wu-Yang solution of the gauge theory still generates the Bonnor-Vaidya geometry, but with no cosmological term.

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arXiv2010

Radiating black holes in Einstein-Yang-Mills theory and cosmic censorship

Exact nonstatic spherically symmetric black-hole solution of the higher dimensional Einstein-Yang-Mills equations for a null dust with Yang-Mills gauge charge are obtained by employing Wu-Yang \textit{ansatz}, namely, HD-EYM Vaidya solution. It is interesting to note that gravitational contribution of YM gauge charge for this ansatz is indeed opposite (attractive rather than repulsive) that of Maxwell charge. It turns out that the gravitational collapse of null dust with YM gauge charge admit strong curvature shell focusing naked singularities violating cosmic censorship. However, there is significant shrinkage of the initial data space for a naked singularity of the HD-Vaidya collapse due to presence of YM gauge charge. The effect of YM gauge charge on structure and location of the apparent and event horizons is also discussed.

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arXiv2010

Quasispherical gravitational collapse in 5D Einstein-Gauss-Bonnet gravity

We obtain a general five dimensional (5D) quasispherical solutions of irrotational dust in Einstein gravity with the Gauss-Bonnet combination of quadratic curvature terms. These solutions are generalization, to Einstein-Gauss-Bonnet gravity, of the 5D quasispherical Szkeres like solutions in general relativity. It is found that the collapse proceed in the same way as in the analogous spherical collapse, i.e., there exists a regular initial data such that the collapse proceed to form naked singularities violating cosmic censorship conjecture. The effect of Gauss-Bonnet quadratic curvature terms on the formation and locations of the apparent horizon is deduced.

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arXiv2010

Inhomogeneous Dust Collapse in 5D Einstein-Gauss-Bonnet Gravity

We consider a Lemaitre - Tolman - Bondi type space-time in Einstein gravity with the Gauss-Bonnet combination of quadratic curvature terms, and present exact solution in closed form. It turns out that the presence of the coupling constant of the Gauss-Bonnet terms alpha > 0 completely changes the causal structure of the singularities from the analogous general relativistic case. The gravitational collapse of inhomogeneous dust in the five-dimensional Gauss-Bonnet extended Einstein equations leads to formation of a massive, but weak, timelike singularity which is forbidden in general relativity. Interestingly, this is a counterexample to three conjecture viz. cosmic censorship conjecture, hoop conjecture and Seifert's conjecture.

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