arXiv subjects

O. B. Zaslavskii

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

At least 91 records · Page 5Linked to original sources

arXiv2013

Ultra-high-energy head-on collisions without horizons or naked singulrities: general approach

Recently, alternatives to the Bañados, Silk and West (BSW) effect were proposed which are (i) due to the existence of naked singularities instead of the horizon, (ii) require neither horizon nor naked singularity. We reveal the main features of such alternatives in a model-independent way. The metric should be close to that of the extremal black hole but the horizon should not form. Then, one can gain unbound the energy E_{c.m.} in the centre of mass frame due to head-on collision of particles near the would-be horizon. The energy measured at infinity can also be unbound. If instead of particles self-gravitating shells collide, the underlying reason leading to unbound E_{c.m.} is the same.

gr-qc↗

arXiv2013

High energy particle collisions near the bifurcation surface

We consider generic nonextremal stationary dirty black holes. It is shown that in the vicinity of any bifurcation surface the energy of collision of two particles in the centre of mass frame can grow unbound. This is a generic property that, in particular, includes collisions near the inner black hole horizon analyzed earlier by different methods. The similar results are also valid for cosmological horizons. The case of the de Sitter metric is discussed.

gr-qc↗

arXiv2013

A Thousand Problems in Cosmology. Chapter 4: Black holes

The fourth chapter of the collection of problems in cosmology, devoted to black holes. Consists of basic introduction, sections on Schwarzschild and Kerr black holes, a section on particles' motion and collisions in general black hole space-times, and the astrophysical part. This version contains only formulations of 137 problems. The full collection, with solutions included, is available in the form of a wiki-based resource at www.universeinproblems.com. The cosmological community is welcome to contribute to its development.

physics.ed-ph↗

arXiv2013

High energy collisions of particles inside ergosphere: general approach

We show that recent observation made in Grib and Pavlov, Europhys. Lett. 101 (2013) 20004 for the Kerr black hole is valid in the general case of rotating axially symmetric metric. Namely, collision of two particles in the ergosphere leads to indefinite growth of the energy in the centre of mass frame, provided the angular momentum of one of two particles is negative and increases without limit for a fixed energy at infinity. General approach enabled us to elucidate, why the role of the ergosphere in this process is crucial.

gr-qc↗

arXiv2013

Acceleration of particles by quasiblack holes

If two particles collide near the black hole horizon, the energy in the centre of mass (CM) frame can grow indefinitely (the so-called the BSW effect). This requires fine-tuning the parameters (the energy, angular momentum or electric charge) of one particle. We show that the CM energy can be unbound also for collisions in the space-time of quasiblack holes - QBHs (the objects on the threshold of forming the horizon which do not collapse). It does not require special fine-tuning of parameters and occurs when any particle inside a QBH having a finite energy collides with the particle that entered a QBH from the outside region.

gr-qc↗

arXiv2013

Comment on "Black Holes are Neither Particle Accelerators Nor Dark Matter Probes"

The Banados-Silk-West effect consists in the possibility to get infinite energy in the centre of mass frame of two particles colliding near the black hole horizon. According to S. T. McWilliams, Phys. Rev. Lett. 110 (2013) 011102, the energy at infinity of the outcome vanishes because of infinite redshift when the point of collision approaches the horizon. I show that this is not so.

gr-qc↗

arXiv2012

Ultra-high energy collisions of nonequatorial geodesic particles near dirty black holes

We consider collision of two geodesic particles moving around rotating stationary axially symmetric black holes. It is shown for arbitrary nonequatorial motion that under certain conditions the energy in their centre of mass frame can grow unbound (the so-called BSW effect). This generalizes the previous results for equatorial motion around dirty (surrounded by matter) black holes and nonequatorial motion around the Kerr metric. It turns out that the BSW effect occurs near any point of the horizon surface. We do not use special symmetries of space-time typical of the Kerr metric, so the results are quite generic. The general scheme classifying all possible scenarios is discussed.

gr-qc↗

arXiv2012

Energetics of particle collisions near dirty rotating extremal black holes: Banados-Silk-West effect versus Penrose process

If two particles collide near the horizon of a rotating extremal black hole, under certain conditions the energy E_{c.m.} in the center-of-mass frame can grow without limit (the so-called Banados-Silk-West effect). We consider collisions that produce two other particles. We show that for a generic dirty (surrounded by matter) black hole, there exist upper bounds on the energy and mass of product particles which can be detected at infinity. As a result, the positive energy gain is possible but is quite modest. It mainly depends on two numbers in which near-horizon behavior of the metric is encoded. The obtained results suggest astrophysical limits on the possibility of observation of the products of the collisional Banados-Silk-West effect. These results are consistent with recent calculations for the Kerr metric, extending them to generic dirty black holes. It is shown that for dirty black holes there are types of scenarios of energy extraction impossible in the Kerr case.

gr-qc↗

arXiv2012

Circular orbits and acceleration of particles by near-extremal dirty rotating black holes: general approach

We study the effect of ultra-high energy particles collisions near the black hole horizon (BSW effect) for two scenarios: when one of particle either (i) moves on a circular orbit or (ii) plunges from it towards the horizon. It is shown that such circular near-horizon orbits can exist for near-extremal black holes only. This includes the innermost stable orbit (ISCO), marginally bound orbit (MBO) and photon one (PhO). We consider generic "dirty" rotating black holes not specifying the metric and show that the energy in the centre of mass frame has the universal scaling dependence on the surface gravity $κ$. Namely, $E_{c.m.}\sim κ^{-n}$ where for the ISCO $n=1/3$ in case (i) or $n=1/2$ in case (ii). For the MBO and PhCO $n=1/2$ in both scenarios that agrees with recent calculations of Harada and Kimura for the Kerr metric. We also generalize the Grib and Pavlov's observations made for the Kerr metric. The magnitude of the BSW effect on the location of collision has a somewhat paradoxical character: it is decreasing when approaching the horizon.

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arXiv2012

Energy extraction from extremal charged black holes due to the BSW effect

Two particles can collide in the vicinity of a rotating black hole producing the divergent energy in the centre of mass frame (the BSW effect). However, it was shown recently that an observer at infinity can register quite modest energies E and masses m which obey some upper bounds. In the present work the counterpart of the original BSW effect is considered that may occur even for radial motion of colliding particles near charged static black holes. It is shown that in some scenarios there are no upper bound on E and m . Thus the high-energetic and superheavy products of the BSW effect in this situation are, in principle, detectable at infinity.

gr-qc↗

arXiv2012

Dirty rotating black holes: regularity conditions on stationary horizons

We consider generic, or "dirty" (surrounded by matter), stationary rotating black holes with axial symmetry. The restrictions are found on the asymptotic form of metric in the vicinity of non-extremal, extremal and ultra-extremal horizons, imposed by the conditions of regularity of increasing strength: boundedness on the horizon of the Ricci scalar, of scalar quadratic curvature invariants, and of the components of the curvature tensor in the tetrad attached to a falling observer. We show, in particular, that boundedness of the Ricci scalar implies the "rigidity" of the horizon's rotation in all cases, while the finiteness of quadratic invariants leads to the constancy of the surface gravity. We discuss the role of quasiglobal coordinate r that is emphasized by the conditions of regularity. Further restrictions on the metric are formulated in terms of subsequent coefficients of expansion of metric functions by r. The boundedness of the tetrad components of curvature tensor for an observer crossing the horizon is shown to lead in the horizon limit to diagonalization of Einstein tensor in the frame of zero angular momentum observer on a circular orbit (ZAMO frame) for horizons of all degrees of extremality.

gr-qc↗

arXiv2012

Acceleration of particles by black holes as a result of deceleration: ultimate manifestation of kinematic nature of BSW effect

The recently discovered so-called BSW effect consists in the unbound growth of the energy E_{c.m.} in the centre of mass frame of two colliding particles near the black hole horizon. We consider a new type of the corresponding scenario when one of two particles ("critical") remains at rest near the horizon of the charged near-extremal black hole due to balance between the attractive and repulsion forces. The other one hits it with a speed close to that of light. This scenario shows in a most pronounced way the kinematic nature of the BSW effect. In the extremal limit, one would gain formally infinite E_{c.m.} but this does not happen since it would have require the critical massive particle to remain at rest on the null horizon surface that is impossible. We also discuss the BSW effect in the metric of the extremal Reissner-Nordström black hole when the critical particle remains at rest near the horizon.

gr-qc↗

arXiv2011

Acceleration of particles near the inner black hole horizon

We study the possibility of obtaining unbound energy E_{c.m.} in the centre of mass frame when two particles collide near the inner black hole horizon. We consider two different cases - when both particles move (i) in the same direction or (ii) in the opposite ones. We also discuss two different versions of the effect - whether an infinite energy can be released in the collision (strong version) or the energy E_{c.m.} is finite but can be made as large as one likes (weak version). We demonstrate that the strong version of the effect is impossible in both cases (i) and (ii). In case (i) this is due to the fact that in the situation when E_{c.m.} formally diverges on the horizon, one of particles passes through the bifurcation point where two horizons meet while the second particle does not, so collision does not occur. In case (ii), both particles hit different branches of the horizon. The weak version is possible in both cases, provided at least one of particles starts its motion inside the horizon along the direction of spatial symmetry from infinity.

gr-qc↗

arXiv2011

Neutral and charged matter in equilibrium with black holes

We study the conditions of a possible static equilibrium between spherically symmetric, electrically charged or neutral black holes and ambient matter. The following kinds of matter are considered: (1) neutral and charged matter with a linear equation of state p_r = wρ(for neutral matter the results of our previous work are reproduced), (2) neutral and charged matter with p_r \sim ρ^m, m > 1, and (3) the possible presence of a "vacuum fluid" (the cosmological constant or, more generally, anything that satisfies the equality T^0_0 = T^1_1 at least at the horizon). We find a number of new cases of such an equilibrium, including those generalizing the well-known Majumdar-Papapetrou conditions for charged dust. It turns out, in particular, that ultraextremal black holes cannot be in equilibrium with any matter in the absence of a vacuum fluid; meanwhile, matter with w > 0, if it is properly charged, can surround an extremal charged black hole.

gr-qc↗

arXiv2011

Acceleration of particles by rotating black holes: near-horizon geometry and kinematics

Nowadays, the effect of infinite energy in the centre of mass frame due to near-horizon collisions attracts much attention.We show generality of the effect combining two seemingly completely different approaches based on properties of a particle with respect to its local light cone and calculating its velocity in the locally nonrotaing frame directly. In doing so, we do not assume that particles move along geodesics. Usually, a particle reaches a horizon having the velocity equals that of light. However, there is also case of "critical" particles for which this is not so. It is just the pair of usual and critical particles that leads to the effect under discussion. The similar analysis is carried out for massless particles. Then, critical particles are distinguishable due to the finiteness of local frequency. Thus, both approach based on geometrical and kinematic properties of particles moving near the horizon, reveal the universal character of the effect.

gr-qc↗

arXiv2011

Acceleration of particles by black holes: kinematic explanation

A new simple and general explanation of the effect of acceleration of particles by black holes to infinite energies in the centre of mass frame is suggested. It is based on kinematics of particles moving near the horizon. This effect arises when particles of two kinds collide near the horizon. For massive particles, the first kind represents a particle with the generic energy and angular momentum (I call them "usual"). Near the horizon, such a particle has a velocity almost equal to that of light in the frame that corotates with a black hole (the frame is static if a black hole is static). The second kind (called "critical") consists of particles with the velocity v<c near the horizon due to special relationship between the energy and angular momentum (or charge). As a result, the relative velocity approaches the speed of light c, the Lorentz factor grows unbound. This explanation applies both to generic rotating black holes and charged ones (even for radial motion of particles). If one of colliding particles is massless (photon), the critical particle is distinguished by the fact that its frequency is finite near the horizon. The existence (or absence) of the effect is determined depending on competition of two factors - gravitational blue shift for a photon propagating towards a black hole and the Doppler effect due to transformation from the locally nonrotating frame to a comoving one. Classification of all possible types of collisions is suggested depending on whether massive or massless particle is critical or usual.

gr-qc↗

arXiv2011

Collisions of massive and massless particles around rotating black holes: general analysis

We consider collisions between massive (electrons) and massless (photons) particles near the horizon of a rotating black hole. Similarly to collisions between massive particles, the infinite energy in the centre of mass frame occurs in some situations. Namely, for one particle the relationship between the energy and angular momentum should have a special form ("critical" particle) whereas for the second one it should not hold ("usual" particle). All combinations of possible pairs of critical and usual particles are analyzed. The existence (or absence) of the effect is determined depending on competition of two factors - gravitational blue shift for a photon propagating towards a black hole and the Doppler effect due to transformation from the locally nonrotating frame to a comoving one. Thus, a pure kinematic explanation is suggested for the effect of infinitely growing energies in the centre of mass frame.

gr-qc↗

arXiv2010

Regular black holes and energy conditions

We establish the relationship between the space-time structure of regular spherically-symmetrical black holes and the character of violation of the strong energy condition (SEC). It is shown that it is violated in any static region under the event horizon in such a way that the Tolman mass is negative there. In non-static regions there is constraint of another kind which, for a perfect fluid, entails violation of the dominant energy condition.

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