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Andrei Zelnikov

Publications and source records attributed to Andrei Zelnikov.

At least 19 recordsLinked to original sources
arXiv2026

Vaidya-Type Solutions of Quasitopological Gravity Interacting with Nonlinear Electrodynamics

We construct Vaidya-type solutions of quasitopological gravity coupled to nonlinear electrodynamics in arbitrary spacetime dimensions. Starting from the corresponding static spherically symmetric charged solutions, we obtain their dynamical counterparts by promoting the integration constants, in particular the mass and electric charge, to arbitrary functions of the advanced or retarded null coordinate. We show that the resulting field equations are satisfied provided suitable charge-carrying null currents and null fluid fluxes are included. The formalism applies to a broad class of nonlinear electromagnetic theories and provides a simple and systematic method for generating exact radiating charged solutions in quasitopological gravity.

gr-qc↗
arXiv2026

Regular Black Holes in Quasitopological Gravity: Null Shells and Mass Inflation

We investigate the phenomenon of mass inflation in the interior of regular black holes arising in quasitopological gravity (QTG). These geometries are characterized by a bounded curvature core and the presence of an inner (Cauchy) horizon located near the fundamental scale $\ell$. To examine whether mass inflation persists in this setting, we model the interaction of ingoing and outgoing perturbations by considering the collision of two spherical null shells inside the black hole. Using the Dray-'t\,Hooft-Barrabes-Israel junction condition, we derive conditions under which the metric function and curvature invariants may experience significant amplification near the inner horizon. Our analysis shows that, unlike in classical Reissner--Nordstr\"om or Kerr geometries, significant mass inflation requires shell intersection at radii very close to the horizon, with radial separations from it of the order $r-r_* \lesssim \ell \big(\ell/r_g\big)^{2n(D-3)}$, where $r_g$ is the gravitational radius of the black hole, $D$ is the number of spacetime dimensions and $n\ge 1$ is a parameter depending on a concrete QTG model. For macroscopic black holes with $r_g\gg \ell$ this distance is much smaller than the fundamental scale $\ell$. We discuss possible consequences of this effect.

gr-qc↗
arXiv2024

Thomas precession, relativistic torque, and non-planar orbits

We analyze the angular momentum balance for a particle undergoing Thomas precession. The relationships among relativistic torque, the center of mass, and the center of inertia for a spinning particle are clarified. We show that spin precession is accompanied by orbital angular momentum precession, and present examples of the resulting out-of-plane motion.

hep-ph↗
arXiv2024

Regular black holes inspired by quasi-topological gravity

Recently it was demonstrated that by adding to the Einstein-Hilbert action a series in powers of the curvature invariants with specially chosen coefficients one can obtain a theory of gravity which has spherically symmetric solutions describing regular black holes. Its reduced action depends on a function of one of the basic curvature invariants of the corresponding metric. In this paper we study a generalization of this model to the case when this function depends on all the basic curvature invariants. We show that the metrics which are solutions of such a model possess a universal scaling property. We demonstrate that there exists a special class of such models for which the ``master" equation for a basic curvature invariant is a linear second order ordinary differential equation. We specify a domain in the space of parameters of the model for which the corresponding solutions describe regular static spherically symmetric black holes and study their properties.

gr-qc↗
arXiv2023

Ring wormholes and time machines

In the present paper we discuss properties of a model of a ring wormhole, recently proposed by Gibbons and Volkov. Such a wormhole connects two flat spacetimes which are glued through discs of the radius $a$ bounded by the string with negative angle deficit $-2π$. The presence of the string's matter violating null energy condition makes the wormhole static and traversable. We study gravitational field of static sources in such a spacetime in the weak field approximation. In particular, we discuss how a field of an oblate thin massive shell surrounding one of the wormhole's mouth is modified by its presence. We also obtain a solution of a similar problem when both mouths of the wormhole are located in the same space. This approximate solution if found for the case when the distance $L$ between these mouths is much larger than the radius $a$ of the ring. We demonstrate that the corresponding locally static gravitational field in such a multiply connected space is non-potential. As a result of this, the proper time gap for the clock's synchronization linearly grows with time and closed timelike curves are formed. This process inevitably transforms such a traversable ring wormhole into a time machine. We estimate the time scale of this process.

hep-th↗
arXiv2023

Chiral anomalies in black hole spacetimes

We study the properties of chiral anomalies in a wide class of spacetimes which possess a principal Killing-Yano tensor. This class includes metrics of charged rotating black holes as a special physically important case. The spacetimes which admit a principal Killing-Yano tensor possess a number of remarkable properties. In particular, such spacetimes have two commuting Killing vectors and a Killing tensor responsible for their hidden symmetries. We calculate the gravitational and electromagnetic contributions to the axial anomaly currents in the spacetime of a charged rotating black hole, and demonstrate that the equation for the chiral anomaly current has special solutions which respect both explicit and hidden symmetries. Two of these solutions have the form of currents propagating along two principal null directions, which are null eigenvectors of the Riemann tensor. These solutions describe chiral currents for the incoming and outgoing polarization fluxes. It is demonstrated that these principal chiral currents can be written explicitly in the form which contains the off-shell metric coefficients and their derivatives. We discuss conditions where the principle chiral anomaly current is regular at the horizon and the axes of symmetry. We demonstrate that for states where the current vanishes at the past horizon and at the past null infinity, there exist chirality fluxes at both the future horizon and future infinity. The latter is directly related to the polarization asymmetry of Hawking radiation for massless spinning particles. We also calculate the Chern-Simons currents for both gravitational and electromagnetic chiral anomalies in the black hole spacetime, and discuss the properties of the chirality fluxes associated with these currents.

hep-th↗
arXiv2021

Bouncing cosmology in the limiting curvature theory of gravity

In this paper we discuss models satisfying the limiting curvature condition. For this purpose we modify the Einstein-Hilbert action by adding a term which restricts the growth of curvature. We analyze cosmological solutions in such models. Namely, we consider a closed contracting homogeneous isotropic universe filled with thermal radiation. We demonstrate that for properly chosen curvature constraints such a universe has a bounce. As a result its evolution is nonsingular and contains a "de Sitter-type" supercritical stage connecting contracting and expanding phases. Possible generalizations of these results are briefly discussed.

hep-th↗
arXiv2021

Spherically symmetric black holes in the limiting curvature theory of gravity

In this paper we describe a model of a four-dimensional spherically symmetric black hole in a limiting curvature theory of gravity. In this theory the Einstein-Hilbert action is modified by adding to the action terms providing inequality constraints on chosen curvature invariants. We demonstrated that in such a model, instead of formation of the curvature singularity, the spacetime remains regular in the black hole interior. For black holes with gravitational radius much larger than the radius $\ell$ of the critical curvature the obtained solutions describe a space exponentially expanding in one direction and periodically oscillating in the other two (spherical) directions.

gr-qc↗
arXiv2021

Two-dimensional black holes in the limiting curvature theory of gravity

In this paper we discuss modified gravity models in which growth of the curvature is dynamically restricted. To illustrate interesting features of such models we consider a modification of two-dimensional dilaton gravity theory which satisfies the limiting curvature condition. We show that such a model describes two-dimensional black holes which contain the de Sitter-like core instead of the singularity of the original non-modified theory. In the second part of the paper we study Vaidya type solutions of the model of the limiting curvature theory of gravity and used them to analyse properties of black holes which are created by the collapse of null fluid. We also apply these solutions to study interesting features of a black hole evaporation.

hep-th↗
arXiv2021

Resonant particle creation by a time-dependent potential in a nonlocal theory

Considering an exactly solvable local quantum theory of a scalar field interacting with a $δ$-shaped time-dependent potential we calculate the Bogoliubov coefficients analytically and determine the spectrum of created particles. We then show how these considerations, when suitably generalized to a specific nonlocal "infinite-derivative" quantum theory, are impacted by the presence of nonlocality. In this model, nonlocality leads to a significant resonant amplification of certain modes, leaving its imprint not only in the particle spectrum but also in the total number density of created particles.

hep-th↗
arXiv2020

Self-force on a static particle near a black hole

We study the self-force acting on a static charged point-like particle near a Schwarzschild black hole. We obtain the point-like particle as a limit of a spacetime describing a big neutral black hole with a small charged massive object nearby. The massive object is modeled by a black hole or a naked singularity. In this fully interacting system the massive object is supported above the black hole by a strut. Such a strut has a non-zero tension which corresponds to the external force compensating the gravitational force and the electromagnetic self-force acting on the massive object. We discuss details of the limiting procedure leading to the point-like particle situation. As a result, we obtain the standard gravitational force in the static frame of the Schwarzschild spacetime and the electromagnetic self-force. The electromagnetic-self force differs slightly from the classical results in a domain near the horizon. The difference is due to taking into account an influence of the strut on the electromagnetic field. We also demonstrate that higher order corrections to the gravitational force, a sort of the gravitational self-force, are not uniquely defined and they depend on details of the limiting procedure.

gr-qc↗
arXiv2020

Thermodynamics of two black holes

We study a system of two charged non-rotating black holes separated by a strut. Using the exact solution of the Einstein-Maxwell equations, which describes this system, we construct a consistent form of the first law of thermodynamics. We derive thermodynamic parameters related to the strut in an explicit form. The intensive thermodynamical quantity associated with the strut is its tension. We call the corresponding extensive quantity the thermodynamical length and we provide an explicit expression and interpretation for it.

gr-qc↗
arXiv2019

Ghost-free modification of the Polyakov action and Hawking radiation

In this paper we discuss possible effects of non-locality in black hole spacetimes. We consider a two-dimensional theory in which the action describing matter is a ghost-free modification of the Polyakov action. For this purpose we write the Polyakov action in a local form by using an auxiliary scalar field and modify its kinetic term by including into it a non-local ghost-free form factor. We demonstrate that the effective stress-energy tensor is modified and we study its properties in a background of a two-dimensional black hole. We obtain the expression for the contribution of the ghost-free auxiliary field to the entropy of the black hole. We also demonstrate that if the back-reaction effects are not taken into account, such a ghost-free modification of the theory does not change the energy flux of the Hawking radiation measured at infinity. We illustrate the discussed properties for black hole solution of a 2D dilaton gravity model which admits a rather complete analytical study.

hep-th↗
arXiv2019

On thermal field fluctuations in ghost-free theories

We study the response of a scalar thermal field to a $δ$-probe in the context of non-local ghost-free theories. In these theories a non-local form factor is inserted into the kinetic part of the action which does not introduce new poles. For the case of a static $δ$-potential we obtain an explicit expression for the thermal Hadamard function and use it for the calculation of the thermal fluctuations. We then demonstrate how the presence of non-locality modifies the amplitude of these fluctuations. Finally, we also discuss the fluctuation-dissipation theorem in the context of ghost-free quantum field theories at finite temperature.

hep-th↗
arXiv2019

Probing the vacuum fluctuations in scalar ghost-free theories

We discuss the response of vacuum fluctuations to a static potential in the context of massive, ghost-free infinite-derivative scalar field theories in two dimensions. For the special case of a $δ$-like potential, $V=λδ(x)$, the problem is exactly solvable and we calculate the corresponding Hadamard function for this quantum field. Using this exact result we determine the renormalized value of the vacuum polarization $\langle \hatφ^2(x)\rangle_\text{ren}$ as a function of the distance $x$ from the position of the potential. This expression depends on the amplitude of the potential as well as the scale of non-locality $\ell$; for distances $x\gg\ell$ the non-local and local results agree, whereas for distances $x < \ell$ there is a difference.

hep-th↗
arXiv2018

Superradiance in a ghost-free scalar theory

We study superradiance effect in the ghost-free theory. We consider a scattering of a ghost-free scalar massless field on a rotating cylinder. We assume that cylinder is thin and empty inside, so that its interaction with the field is described by a delta-like potential. This potential besides the real factor, describing its height, contains also imaginary part, responsible for the absorption of the field. By calculating the scattering amplitude we obtained the amplification coefficient both in the local and non-local (ghost-free) models and demonstrated that in the both cases it is greater than 1 when the standard superradiance condition is satisfied. We also demonstrated that dependence of the amplification coefficient on the frequency of the scalar field wave may be essentially modified in the non-local case.

hep-th↗
arXiv2018

Quantum scattering on a delta potential in ghost-free theory

We discuss the quantum-mechanical scattering of a massless scalar field on a $δ$-potential in a ghost-free theory and obtain analytic solutions for the scattering coefficients. Due to the non-locality of the ghost-free theory the transmission coefficient tends to unity for frequencies much larger than the inverse scale of non-locality, even for infinitely strong potentials. At the same time there exists a critical strength of the $δ$-potential barrier below which there is always a frequency that is totally reflected. These scattering properties in ghost-free theories are quite generic and distinguish them from local field theories. Moreover, we study quasi-normal states that are present for the $δ$-potential well. In the limit of vanishing non-locality, we recover the standard results of local field theory.

hep-th↗