arXiv subjects

Vitalii Vertogradov

Publications and source records attributed to Vitalii Vertogradov.

At least 37 records · Page 2Linked to original sources

arXiv2024

Dynamical Photon Spheres in Charged Black Holes and Naked Singularities

To understand the nature of a black hole shadow in dynamical spacetimes, we construct an analytical model of a dynamical photon sphere in the context of the Bonnor-Vaidya spacetime. Comparing the resulting photon sphere radius with the one in Vaidya spacetime, we find that the charge always decreases the radius of the photon sphere. We also prove that a naked singularity in Bonnor-Vaidya spacetime, unlike the static Reissner-Nordstrom naked singularity, may cast a shadow, and as a result, it cannot be distinguished from a black hole through its shadow.

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arXiv2024

Generalized Vaidya spacetime: horizons, conformal symmetries, surface gravity and diagonalization

In this paper, the different properties of generalized Vaidya spacetime are considered. We define the location of horizons. We show that the apparent horizon can contain the event horizon. The locations of all types of horizons are compared with ones in the usual Vaidya spacetime. We investigate the timelike geodesics in this spacetime. New corrections to Schwarzschild and Vaidya cases appear and we give conditions when these corrections are not negligible. Also, we consider the conformal Killing vector and transform the metric to conformally-static coordinates. We introduce a new constant of motion along null and timelike geodesics, which is generated by a homothetic Killing vector. The conformally-static coordinates allow diagonalizing of the generalized Vaidya spacetime. The surface gravity has been calculated for the dust and stiff fluid cases.

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arXiv2024

Analyzing the Influence of Geometrical Deformation on Photon Sphere and Shadow Radius: A New Analytical Approach -- Spherically Symmetric Spacetimes

In this paper, we introduce a new approach to study the behavior of the photon sphere and shadow radius. Our method uses extended gravitational decoupling and reveals two important analytic results. First, the additional matter field alters the photon sphere radius: it increases if $g'(r_{ph}^{(0)})>0$ and decreases if $g'(r_{ph}^{(0)})<0$ (where $g'$ represents the derivative of a specific metric function evaluated at the original photon sphere radius). Second, the presence of the matter field can modify the black hole shadow size. If $g\left(r_{ph}^{(0)}\right)>0$, the shadow shrinks, while it grows for $g\left(r_{ph}^{(0)}\right)<0$. These findings provide a deeper insight into how matter distribution influences the characteristics of black holes and their observable features. Through a systematic framework and various illustrative examples, our investigation not only clarifies these fundamental aspects but also significantly enhances the theoretical framework of black hole astrophysics.

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arXiv2024

General Approach on Shadow Radius and Photon Spheres in Asymptotically Flat Spacetimes and the Impact of Mass-Dependent Variations

Recent observations of black hole shadows have revolutionized our ability to probe gravity in extreme environments. This manuscript presents a novel analytic method to calculate, in leading-order terms, the key parameters of photon sphere and shadow radius. This method offers advantages for complex metrics where traditional approaches are cumbersome. We further explore the impact of black hole mass on the photon sphere radius, providing insights into black hole interactions with their surroundings. Our findings contribute significantly to black hole physics and gravity under extreme conditions. By leveraging future advancements in observations, such as the next-generation Event Horizon Telescope (ngEHT), this work paves the way for even more precise tests of gravity near black holes.

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arXiv2024

Influence of primary hair and plasma on intensity distribution of black hole shadows

In this paper, we investigate the influence of primary hair ($l$) on the shadows of hairy Schwarzschild and Reissner-Nordström black holes obtained through gravitational decoupling. In the context of hairy Schwarzschild black holes, $l$ either has no effect or consistently enlarges the photon sphere radius. Notably, even when it violates the strong energy condition, it can decrease the radius. For Reissner-Nordström black holes, an additional matter field consistently expands the photon sphere radius, potentially reaching $3M$, akin to the pure Schwarzschild case. Remarkably, we demonstrate that black holes can exist even when overcharged ($Q^2 > M^2$), casting shadows. Specific intensity calculations reveal $l$ consistently reduces it in both scenarios. Furthermore, we investigate the impact of pressureless plasma, finding $l$ exerts a stronger influence on visible size than plasma. These results can help in our understanding of theoretical models of black hole shadows and can be tested by comparison with the images obtained by EHT collaboration.

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arXiv2024

Non-singular black hole by gravitational decoupling and some thermodynamic properties

Gravitational decoupling allows to obtain new solutions of general relativity. In this paper, we obtain new solutions of the Einstein field equations which describe non-singular black holes. We consider Hayward and Bardeen regular black holes as seed spacetimes and apply gravitational decoupling to obtain a new non-singular solution. We show that anisotropic energy-momentum tensor can spoil the regularity condition in the centre of a black hole. We solve the Einstein field equation and obtain new solutions that possess a de Sitter core and have Schwarzschild behaviour in infinity. We also analyse the thermodynamic properties of the obtained solutions.

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arXiv2024

Analyzing the Influence of Geometrical Deformation on Photon Sphere and Shadow Radius: A New Analytical Approach -Stationary, and Axisymmetric Spacetime

Black hole shadows and photon spheres offer valuable tools for investigating black hole properties. Recent observations by the Event Horizon Telescope Collaboration have confirmed the existence of rotating black holes. Black hole parameters influence the observed shadow size. This paper explores the impact of geometrical deformations on black hole shadow size using gravitational decoupling applied to axially-symmetric spacetime. We find the results are more complex than the spherically-symmetric case. We compare shadows in well-known models with those of an Kerr black hole. Our approach suggests that the influence of an accretion disc on the observed shadow shape can be accurately described despite negligible impact on the black hole geometry itself.

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arXiv2024

The generalized Vaidya spacetime with polytropic equation of state

The process of the gravitational collapse might lead not only to a black hole but also to naked singularity formation. In this paper, we consider the generalized Vaidya spacetime with polytropic and generalized polytropic equations of state. We solve the Einstein and Einstein-Maxwell equations to obtain the explicit form of a mass function. We consider the limiting cases of solutions and find out, that generalized Vaidya spacetime might behave like Vaidya-de Sitter and Bonnor-Vaidya-de sitter solutions. Moreover, we explicitly show, that the part of solution, which depends on the polytropic index, is similar to cosmological fields surrounding both Vaidya and Bonnor-Vaidya black holes. The process of the gravitational collapse has been then considered. We have found out that the conditions of the naked singularity formation don't depend on the polytropic index.

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arXiv2023

On the particle collisions during gravitational collapse of Vaidya spacetimes

The center-of-mass energy can be arbitrarily high in Schwarzschild spacetime if one considers the front collision of two particles, one of which moves along so-called white hole geodesics and another one along a black hole geodesic. This process can take place if one considers the gravitational collapse model. In this paper, we consider the well-known naked singularity formation in Vaidya spacetime and investigate the question about two particle collision near the boundary of the collapsing cloud. The center-of-mass energy of the front collision is considered. One particle moves away from the naked singularity and another one falls onto a collapsing cloud. We show that the center-of-mass energy grows unboundly if the collision takes place in the vicinity of the conformal Killing horizon.

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arXiv2023

Hairy Kiselev Black Hole Solutions

In the realm of astrophysics, black holes exist within nonvacuum cosmological backgrounds, making it crucial to investigate how these backgrounds influence the properties of black holes. In this work, we first introduce a novel static spherically-symmetric exact solution of Einstein field equations representing a surrounded hairy black hole. This solution represents a generalization of the hairy Schwarzschild solution recently derived using the extended gravitational decoupling method. Then, we discuss how the new induced modification terms attributed to the primary hairs and various background fields affect the geodesic motion in comparison to the conventional Schwarzschild case. Although these modifications may appear insignificant in most cases, we identify specific conditions where they can be comparable to the Schwarzschild case for some particular background fields.

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arXiv2023

On particle collisions in the vicinity of the charged black holes

The process of particle collision in the vicinity of black holes is known to generate unbounded energies in the center-of-mass frame (the Banados-Silk-West (BSW) effect) under specific conditions. We consider this process in the charged black hole metrics, namely, the Reissner-Nordstrom (RN) and Majumdar-Papapetrou (MP) metrics. We consider the energy extraction from Bardeen regular black hole due to BSW effect. Like in RN case, we show that there is no restriction on energy extraction, but for real charged particles this effect is negligible. We derive necessary and sufficient conditions for this process. The conditions for the BSW effect in RN and MP metrics are shown to be identical, which is explained by the asymptotic equivalence of the two metrics near the horizons. Energy extraction in the RN metric is discussed. It is shown that if two real particles collide while falling onto a black hole, they are extremely unlikely to generate an ultra-massive particle. For the case of head-on collisions, we derive an upper bound on extracted mass, which depends on the lapse function of the metric at the point of collision.

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arXiv2023

On the temperature of hairy black holes

The gravitational decoupling method represents an extremely useful tool to obtain new solutions of the Einstein equations through minimal geometrical deformations. In this paper, we consider the hairy charged black hole obtained by the gravitational decoupling and calculate their Hawking temperature in order to compare it with the case when the hairs are ignored. We have found out that the hairs, under some conditions of black hole parameters, affect the Hawking temperature and can increase it. We have also found out that the black hole temperature, in hairy case, doesn't depend on the electric charge.

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arXiv2022

The structure of the generalized Vaidya spacetime containing the eternal naked singularity

In this paper the structure of the generalized Vaidya spacetime when the type-II of the matter field satisfies the equation of the state $P=ρ$ is investigated. Satisfying all energy conditions we show that this spacetime contains the 'eternal' naked singularity. It means that once the singularity is formed it will never be covered with the apparent horizon. However, in the case of the apparent horizon formation the resulting object is a white hole. We also prove that this spacetime contains only null naked singularity.

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arXiv2022

Vaidya and generalized Vaidya solutions by gravitational decoupling

In this paper we apply the gravitational decoupling method for dynamical systems in order to obtain a new type of solution which can describe a hairy dynamical black hole. We consider three cases of decoupling. The first one is the simplest and well-known when the mass function is the function only of space coordinate $r$. The second case is Vaidya spacetime case when the mass function depends on time $v$ . And the third case represents the generalization of these two cases: the mass function is the function of both $r$ and $v$. We also calculate the apparent horizon and singularity location for all three cases.

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arXiv2022

Non-linearity of Vaidya spacetime and forces in the central naked singularity

In this paper we consider non-linear Vaidya spacetime i.e. the case when the mass function has the non-linear form $M(v) \equiv λv^n \,, λ>0 \,, n>1 $. We prove that the central naked singularity might form for values $n>1$ but they are gravitationally weak. Also we investigate the question about forces in the naked singularity and prove that they might be finite only in the case of the gravitationally weak naked singularity.

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arXiv2022

The Forces outside the static limit in the rotating frame

In this paper we consider the rotating frame of the Minkowski spacetime in order to describe the inertial forces outside the static limit. We consider the inertial forces inside the static limit to find the classical analogue afterwards we find out the expressions for these forces outside the static limit where we can't consider the limit $\frac{v}{c}\to 0$. We show that in general case if the angular velocity of the object $Ω$ is equal to the angular momentum $ω$ then the acceleration is equal to zero.

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arXiv2022

The eternal naked singularity formation in the case of gravitational collapse of generalized Vaydia spacetime

In this paper, we consider the gravitational collapse of generalized Vaidya space-time when the matter satisfies the equation of the state either $P=0$ or $P=-αρ$, where $0 < α< 1$. We show that in the case when type I of matter field is dust, then the apparent horizon will never appear, but there is now a family of null radial future-directed geodesics which terminated at the central singularity in the past. Also, we show that in the case of negative pressure, the result of the gravitational collapse might be the naked singularity and the apparent horizon appears and in very short time disappears again. In the case of the negative pressure, we show that the result of the gravitational collapse might be the eternal naked singularity.

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arXiv2022

Extraction Energy From Charged Vaidya Black Hole Via Penrose Process

In this paper, we consider the analogy of the Penrose process in Charged Vaidya spacetime. We calculate the border of the generalized ergosphere, in which the charged particles with negative energy might exist, and show that it is temporary. We show that there are no closed orbits for particles with negative energy inside the generalized ergosphere. We investigate the question about the efficiency of the Penrose process and show that one can't extract large energies from a black hole.

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