arXiv subjects

O. B. Zaslavskii

Publications and source records attributed to O. B. Zaslavskii.

At least 19 recordsLinked to original sources

arXiv2026

Full set of scenarios of high energy collision in the Schwarzschild background and kinematic censorship

We consider collisions between particles moving freely in the Schwarzschild background. We suggest classification of scenarios that lead to unbounded energy output in the center of mass frame. Although this is forbidden in the exterior of a black hole, the phenomenon becomes possible with account of white hole region and mirror universe in the complete space-time diagram. Also, we scrutinize collisions in the vicinity of the bifurcation point and reveal the role of resting observers in the inner region beyond the horizon. In some scenarios $E_{c.m.}$ not only diverges but becomes seemingly infinite that is, as a matter of fact, impossible according to the principle of so-called kinematic censorship. We resolve a corresponding paradox and argue, why this principle remains valid.

gr-qc↗

arXiv2026

Circular orbits in spherically symmetric spacetimes and BSW effect with nonzero force

We consider circular particle motion under the action of an unspecified force in a static spherically symmetric spacetime. We derive the machinery that allows one to find the force acting on a circular particle and deduce whether its position is stable or not. This also allows one to extend the definition of ISCO to the case of a non-zero external force. By conducting the near-horizon expansion, we obtain that for any non-extremal black holes, the acceleration for extremal ones is finite, and for ultraextremal (multiple) horizons it tends to zero. Applying the derived machinery to the case of the Schwarzschild metric assuming that a force is constant, we scrutiny how the number of orbits for a given force depends on its value. In particular, if a force is big enough, an additional branch of solutions appears that was absent in the case of geodesic motion. Then, for various circular orbits, we numerically investigate their stability. A similar problem is solved for the Reissner-Nordstrom (RN) metric and uncharged particles. It appears that for the near-extremal and extremal RN black holes, there exist near-horizon circle trajectories (in contrast to the nonextremal case). For the ISCO, the dependence of the orbit radius on $κ$ (the surface gravity) is similar to that in the case of neutral particles moving in the background of rotating black holes. In addition, two scenarios of high-energy particle collisions near such orbits are considered, and it is found that dependence on $κ$ is also similar to that for rotating black holes.

gr-qc↗

arXiv2026

Generalized Lemaître time for rotating and charged black holes and its near-horizon properties

We consider the behavior of the analogue of the Lemaitre time when a particle approaches the horizon of a rotating black hole. For the Kerr metric, the aforementioned time coincides with the Doran or Natario time but we consider a more general class of metrics. We scrutiny relationship between (i) its finiteness or divergence, (ii) the forward-in-time condition, (iii) the sign of a generalized momentum/energy, (iv) the validity of the principle of kinematic censorship. The latter notion means impossibility to release in any event an energy which is literally infinite. As a consequence, we obtain a new explanation, why collisions of two particles inside the horizon do not lead to infinite energy in their center of mass frame. The same results are also obtained for the Reissner-Nordström metric

gr-qc↗

arXiv2026

Inverted equation of state and general approach to vacuum-like configurations

We consider spherically symmetric static black hole configurations that obey the vacuum equation of state: $p_{r}=-ρ$, where $p_{r}$ is the radial pressure, $ρ$ being energy density. We find in a closed form the metric for an arbitrary equation of state for tangential pressure $p_{θ}(ρ)$. The corresponding formulas enable us to embrace compact Schwarzschild-like configurations and dispersed systems. They include metrics with a regular center and singular ones. In a particular case, the metric of the Kiselev black hole is reproduced.

gr-qc↗

arXiv2026

High energy particle collisions under Cauchy horizon

We consider particle collision inside the inner horizon of the Reissner-Nordstrom metric in the so-called R region. We show that there exist scenario in which the enrgy in the center of mass frame grows unbounded. In contrast to the standard scenarios of high energy collisions in black hole background in the R region, fine tuning of particle parameters is not require. The effect found in this work can be considered as a massive particle counterpart of wave processes that contribute to instability of the inner black hole horizon.

physics.gen-ph↗

arXiv2025

Near-horizon behavior of nonequatorial accelerated particle motion and high energy particle collisions

We consider motion of a particle in the background of a stationary axially symmetric generic black hole. A particle experiences the action of a force of unspecified nature. We require the force to remain finite in a comoving frame. The result is expressed in terms of several integers characterizing the Taylor expansion of the metric coefficients near the horizon. We show that the polar component of the four-velocity remains finite. As a result, the scenarios of high-energy particle collisions, found previously in the context of the BSW effect for equatorial motion, do not change qualitatively in the nonequatorial case. The fact that the polar component is finite, also enables us to fill some gaps in the description of the BSW effect in previous works. Our results have a quite general character and can be used not only in description of high energy particle collisions but also in diverse astrophysical problems for which motion is not constrained by the equatorial plane.

gr-qc↗

arXiv2025

Kinematic censorship and high energy particle collisions in the Schwarzschild background

We consider near-horizon collisions between two particles moving freely in the Schwarzschild metric in the region outside the horizon. One of them emerges from a white hole. We scrutiny when such a process can lead to the indefinitely large growth of the energy in the center of mass frame in the point of collision. We also trace how the kinematics of collision manifests itself in preserving the principle of kinematic censorship according to which the energy released in any event of collision cannot be literally infinite. According to this principle, the energy released in any event of collision, must remain finite although it can be made as large as one likes. Also, we find that particle decay near the singularity leads to unbounded release of energy independently of its initial value.

gr-qc↗

arXiv2025

Naked and truly naked rotating black holes

Previously, it was noticed that in some space-times with Killing horizons some curvature components, responsible for tidal forces, small or even zero in the static frame, become enhanced from the viewpoint of a falling observer. This leads to the notion of so-called naked black holes. If some components in the frame attached to a free-falling observer formally diverge, although scalar invariants remain finite, such space-times was named "truly naked black holes" (in mathematical language, one can speak about non-scalar singularity). Previous results included static spherically symmetric or distorted static metrics. In the present work, we generalized them to include rotation in consideration. We also scrutiny how the algebraic type can change in the vicinity of the horizon due to local Lorentz boost. Our approach essentially uses the Newman-Penrose formalism, so we analyze the behavior of Weyl scalar for different kinds of observers.

gr-qc↗

arXiv2024

High energy head-on particle collisions near event horizons: classifcation of scenarios

We consider head-on collisions of two particles near the event horizon. Particle 1 is outgoing, particle 2 is ingoing. We elucidate, in which case the energy $E_{c.m.}$ in the center of mass frame can grow unbounded. If the proper time between the horizon and an arbitrary point outside it for particle 1 is finite, we deal with a white hole. If it is infinite, we deal with a black hole. Particles can be either free or experience the action of a finite force. Our results are complementary to those for the standard BSW effect when particles move in the same direction. The results rely on classification of particles developed in our previous work H.V. Ovcharenko, O.B. Zaslavskii, Phys. Rev. D 108, 064029 (2023).

gr-qc↗

arXiv2024

General properties of the electric Penrose process

We consider the Penrose process with the charged particles in the Reissner-Nordström (RN) background. Let parent particle 0 decay to particles 1 and 2. With the assumption that all three particles move in the equatorial plane, the exact formulas for characteristics of particles 1 and 2 in terms of those of particle 0 are derived. We concentrate on scenarios in which particle 1 and 2 are ejected along the trajectory of particle 0. It is shown that such scenarios correspond to the extrema of energies $E_{1}$ or $E_{2}$ of daughter particles with respect to the angular momentum $L_{1}$ or $L_{2}$. We derive bounds on the values of angular momenta $L_{1}$ and $% L_{2}$. We give classification of these scenarios and discuss their properties including decay in the near-horizon region. We find that the maximum of efficiency is achieved on the horizon for some of these scenarios but not for all of them and with additional constraints on particle parameters. The results are reformulated in terms of velocities of daughter particles in the center of mass frame. The approach is applicable also to collisional Penrose process, in which a combination of particles 1 and 2 is considered as one effective particle. If the mass of particle 0 $% m_{0}\rightarrow \infty $, the situation corresponds to the Bañ% ados-Silk-West effect, the results agree with the ones known in literature before. In addition, we consider special cases when decay occurs in the turning point for one or all three particles. The formalism developed in this work has a model-independent character and applies not only to the RN metric.

gr-qc↗

arXiv2024

Near-horizon properties of trajectories with finite force relevant for Bañados-Silk-West effect

According to the Banados-SIlk-West (BSW) effect, two particles moving towards a black hole, can collide near the horizon with an unbounded energy in the center of mass frame. This requires one of particles to have fine-tuned parameters in such a way that the time component of generalized momentum is zero $X=0$. Thus the existence of such trjectories is a necessary condition for the BSW effect. However, it is insufficient since the forward-in-time condition requires $X>0$ outside the horizon. We examine this condition for different types of partricles and horizons and find configurations for which the BSW effect is possible. In doing so, we take into account a finite force of unspesified nature exerted on particles. It includes relationships between numbers characterizing the rate with which four-velocity, acceleration and metric functions change near the horizon. For some aforementioned relations, parameters of a system control the sign of $X$, in other cases they are required for $X$ to be real quantity. In the simplest case of free particles the BSW effect for the Kerr or Kerr-Newman black hole is impossible if a fine-tuned particle has a negative energy, so in this sense combination of the Penrose process and the BSW effect is forbidden.

gr-qc↗

arXiv2024

BSW phenomenon for near-fine-tuned particles with external force: general classification of scenarios

If two particles moving towards a black hole collide in the vicinity of the horizon, the energy $E_{c.m.}$ in the center of mass frame can grow indefinitely if one of particles is fine-tuned. This is the Bañados, Silk and West (BSW) effect. One of objections against this effect consists in that for some types of a horizon fine-tuned particles cannot reach the horizon. However, this difficulty can be overcome if instead of exact fine-tuning, one of particle is nearly fine-tuned, with the value of small detuning being adjusted to the distance to the horizon. Such particles are called near-fine-tuned. We give classification of such particles and describe possible high energy scenarios of collision in which they participate. We analyze the ranges of possible motion for each type of particle and determine under which condition such particles can reach the horizon. We analyze collision energy $E_{c.m.}\,$and determine under which conditions it may grow indefinitely. We also include into consideration the forces acting on particles and find when the BSW effect with nearly-fine-tuned particles is possible with finite forces. We demonstrate that the BSW effect with particles under discussion is consistent with the principle of kinematic censorship. According to this principle, $E_{c.m.}\,$cannot be literally infinite in any event of collision (if no singularity is present), although it can be made as large as one likes.

gr-qc↗

arXiv2023

General properties of the Penrose process with neutral particles in the equatorial plane

We consider the background of a rotating axially symmetric black hole. Let particle 0 decay to two fragments 1 and 2 in the direction parallel to that of particle 0. It is shown that if decay occurs inside the ergoregion, both particles 1 and 2 move in the same direction as particle 0. For the scenario, when decay happens in the turning point of all three particles, we find the condition when angular momenta of both particles 1 and 2 have the same sign. We elucidate the relation between the approach of Wald that imposes constraint on maximum and minimum energies of fragments and our approach. In doing so, we express the results in terms of characteristics of particle 0 and all particle masses. The conditions of the maximum efficiency depending on the relation between masses is discussed. We find an explicit expression for angular momenta of particles 1 and 2. We discuss also particle decay for static black holes, when the Penrose process is impossible. Because of the absence of the ergoregion in the static case, scenarios of decay for static black holes can significantly differ from those in the rotating background.

gr-qc↗

arXiv2023

Dynamics of redshift/blueshift during free fall under the Schwarzschild horizon

We consider a free-falling observer who crosses the event horizon in the Schwarzschild background. In the course of this fall, he/she can receive signals from an object (like a star surface) that emits radiation. We study how the frequency received by an observer changes depending on the proper time on his/her trajectory. The scenarios are classified depending on whether the frequency is infinite, finite or zero near the singularity and the horizon. This depends crucially on the angular momenta of an observer and a photon. In this work we consider also emission process, and, as we show, conditions of emission strongly influence parameters of a photon, and thus received frequency. As one of our main results, we present numerical calculations showing evolution of the received frequency during the process of diving into a black hole, depending on parameters of an observer and emitter. We also analyze how a falling observer will see a night sky as he/she approaches the singularity. We show that there appear several blind zones, which were not analyzed previously.

gr-qc↗

arXiv2023

Bañados-Silk-West effect with finite forces near different types of horizons: general classification of scenarios

If two particles move towards a black hole and collide in the vicinity of the horizon, under certain conditions their energy $E_{c.m.}$ in the center of mass frame can grow unbounded. This is the Bañados-Silk-West (BSW) effect. Usually, this effect is considered for extremal horizons and geodesic (or electrogedesic) trajectories. We study this effect in a more general context, when both geometric and dynamic factors are taken into account. We consider generic axially symmetric rotating black holes. The near-horizon behavior of metric coefficients is determined by three numbers $p,~q,$ $k$ that appear in the Taylor expansions for different types of a horizon$.$ This includes nonextremal, extremal and ultraextremal horizons. We also give general classification of possible trajectories that include so-called usual, subcritical, critical and ultracritical ones depending on the near-horizon behavior of the radial component of the four-velocity. We assume that particles move not freely but under the action of some unspecified force. We find when the finiteness of a force and the BSW effect are compatible with each other. The BSW effect implies that one of two particles has fine-tuned parameters. We show that such a particle always requires an infinite proper time for reaching the horizon. Otherwise, either a force becomes infinite or a horizon fails to be regular. This realizes the so-called principle of kinematic censorship that forbids literally infinite $E_{c.m.}$ in any act of collision. The obtained general results are illustrated for the Kerr-Newman-(anti-)de Sitter metric used as an example. The description of diversity of trajectories suggested in our work can be of use also in other contexts, beyond the BSW effect. In particular, we find the relation between a force and the type of a trajectory.

gr-qc↗

arXiv2023

On particle dynamics near the singularity inside the Schwarzschild black hole and T-spheres

The problem of the speed of the objects inside the Schwarzschild black hole is considered. The general result is that the value of the relative speed of the objects following their non-zero angular momentum trajectories, both of geodesic and non-geodesic character, when approaching the ultimate singularity, tends to the value of speed of light. There is only one exception when both objects move in the same plane and have parallel angular momenta. This outcome appears to have a deeper sense: it reflects the anisotropic character of the dynamics of interior of this particular black hole. The result in question means that near the singularity, collisions of two particles lead to an indefinitely large energy in the center of mass frame. Aforementioned properties have their counterpart in the phenomenon of an indefinitely large blueshift near the singularity. Thus the angular momentum of a particle turns out to be an important feature that affects the final behavior of particle near the singularity. Motivated by this fact, we generalize the Lema\^ıtre frame under the horizon in such a way that reference particles themselves have nonzero angular momentum. Our results apply not only to the Schwarzschild singularity but also to other space-like ones for which the scale factor $g\rightarrow \infty $. We also analyze another type of singulairites for which the circumference radius vanishes but $g$ remains finite.

gr-qc↗

arXiv2023

Black holes and hot shells in the Euclidean path integral approach to quantum gravity

We study a spherical black hole surrounded by a hot self-gravitating thin shell in the canonical ensemble, i.e., a black hole and a hot shell inside a heat reservoir acting as a boundary with its area and temperature fixed. To work out the quantum partition function, from which the thermodynamics of the system follows, we use the Euclidean path integral approach to quantum gravity that identifies the path integral of the gravitational system with the partition function. In a semiclassical approximation, one needs only to compute the classical action of the system. Then, one finds that the total entropy, i.e., the sum of black hole and matter entropies, is a function of the gravitational radius of the system alone. So, the black hole inside the shell has no direct influence on the entropy. One also finds the free energy, the thermodynamic energy, and the temperature stratification. The reservoir temperature is composed of a free function of the gravitational radius of the system divided by the redshift. Upon specification of the reduced temperature free function, the solutions for the gravitational radii compatible with the data are found. The black hole inside has two possible horizon radii. It is shown that there is a first law of thermodynamics for the system, another for the hot shell, and yet another for the black hole. A thermodynamic stability analysis is performed. By specifying for the free function the Hawking temperature for the gravitational radius of the system, which is not a black hole, one finds a remarkable exact thermodynamic solution. With it one establishes that pure black holes, hot shells with a black hole, pure hot shells, and hot flat spaces are phases that cohabit in the ensemble, with some acting as thermodynamic mimickers. This exact solution is a model to situations involving black holes and hot gravitons. The high temperature limits reveal important aspects.

hep-th↗

arXiv2023

Particle decay, Oberth effect and a relativistic rocket in the Schwarzschild background

We relate the known Oberth effect and the nonrelativistic analogue of the Penrose process. When a particle decays to two fragments, we derive the conditions on the angles under which debris can come out for such a process to occur. We also consider the decay and the Oberth effect in the relativistic case, when a particle moves in the background of the Schwarzschild black hole. This models the process when a rocket ejects fuel. Different scenarios are analyzed depending on what data are fixed. The efficiency of the process is found, in particular, near the horizon and for a photon rocket (when the ejected particle is massless). We prove directly that the most efficient process occurs when fuel is ejected along the rocket trajectory. When this occurs on the horizon, the efficiency reaches 100% for a photon rocket. We compare in two ways how a rocket can reverse its direction of motion to a black hole near the event horizon by restoring the initial energy-to-mass ratio: (i) by a single ejection or (ii) in the two-step process when it stops and moves back afterwards. For a nonphotonic rocket, in case (ii) a larger mass can be taken out from the vicinity of a horizon. For a photonic one, there is no difference between (i) and (ii) in this respect. We also consider briefly the scenario when a rocket hangs over a black hole due to continuous ejection of fuel. Then, the fuel mass decays exponentially with the proper time.

gr-qc↗