arXiv subjects

R. A. Konoplya

Publications and source records attributed to R. A. Konoplya.

At least 55 records · Page 3Linked to original sources

arXiv2022

Traversable Wormholes in General Relativity

In [J. Blazquez-Salcedo, C. Knoll, E. Radu, Phys. Rev. Lett. 126 (2021) no.10, 101102] asymptotically flat traversable wormhole solutions were obtained in the Einstein-Dirac-Maxwell theory without using phantom matter. The normalizable numerical solutions found therein require a peculiar behavior at the throat: the mirror symmetry relatively the throat leads to the nonsmoothness of gravitational and matter fields. In particular, one must postulate changing of the sign of the fermionic charge density at the throat, requiring coexistence of particle and antiparticles without annihilation and posing a membrane of matter at the throat with specific properties. Apparently this kind of configuration could not exist in nature. We show that there are wormhole solutions, which are asymmetric relative the throat and endowed by smooth gravitational and matter fields, thereby being free from all the above problems. This indicates that such wormhole configurations could also be supported in a realistic scenario.

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arXiv2021

Blandford-Znajek mechanism in the general stationary axially-symmetric black-hole spacetime

We consider the Blandford-Znajek process of electromagnetic extraction of energy from a general axially symmetric asymptotically flat slowly rotating black hole. Using the general parametrization of the black-hole spacetime we construct formulas for the flux of the magnetic field and the rate of energy extraction, which are valid not only for the Kerr spacetime, but also for its arbitrary axially symmetric deformations. We show that in the dominant order these quantities depend only on a single deformation parameter, which relates the spin frequency of a black hole with its rotation parameter.

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arXiv2021

Black holes in galactic centers: quasinormal ringing, grey-body factors and Unruh temperature

Recently, Cardoso et al. [arXiv:2109.00005] found an exact solution describing the black hole immersed in a galactic-like distribution of matter. There, the properties of gravitational radiation were studied. Here we continue analysis of properties of this geometry via consideration of the electromagnetic radiation. We calculate quasinormal modes, asymptotic tails and grey-body factors for electromagnetic radiation. In addition, we discuss the Unruh temperature for this spacetime. Estimations made in the regime which is best fitting the galaxies behavior show that influence of the environment on classical and quantum radiation around such black holes must be relatively small.

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arXiv2021

Wormholes without exotic matter: quasinormal modes, echoes and shadows

An analytical solution representing traversable asymptotically flat and symmetric wormholes was obtained without adding exotic matter in two different theories independently: in the Einstein-Maxwell-Dirac theory and in the second Randall-Sundrum brane-world model. Further, a smooth normalizable asymmetric wormhole solution has been recently obtained numerically in the Einstein-Maxwell-Dirac theory. Using the time-domain integration method we study quasinormal ringing of all these wormholes with emphasis to the regime of mimicking the near extremal Reissner-Nordström black holes, which is characterised by echoes. In addition, we calculate radius of shadows cast by these wormholes.

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arXiv2021

Non-evident stability of a Dirac field in Schwarzschild-de Sitter spacetime

Perturbations of the Dirac field in the Schwarzschild spacetime are characterized by two wave equations (for two chiralities) with effective potentials which are iso-spectral and one of which is positive definite. Therefore, the stability of a Dirac field in the Schwarzschild background is straightforward. This is not so for the Schwarzschild-de Sitter case, because potentials for both chiralities have negative gaps and the fact that the stability is not automatically guaranteed in the asymptotically de Sitter case was apparently omitted in the literature. Performing the time-domain integration of the wave equations and, thereby, taking into consideration all the quasinormal modes of the spectrum, we demonstrate stability of the Dirac field in the Schwarzschild-de Sitter spacetime. The analysis of stability is extended also to the Reissner-Nordstrom-de Sitter solution.

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arXiv2021

Quasinormal modes and Hawking radiation of black holes in cubic gravity

We consider quasinormal modes and Hawking radiation of four-dimensional asymptotically flat black holes in the most general up to-cubic-order-in-curvature dimension-independent Einsteinian theory of gravity that shares its graviton spectrum with the Einstein theory on constant curvature backgrounds. We show that damping rate and real oscillation frequencies of quasinormal modes for scalar, electromagnetic and Dirac fields are suppressed once the coupling with the cubic term is on. The intensity of Hawking radiation is suppressed as well, leading to, roughly, one order longer lifetime at a sufficiently large coupling constant.

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arXiv2021

Hawking radiation of non-Schwarzschild black holes in higher derivative gravity: a crucial role of grey-body factors

The higher derivative gravity includes corrections of the second order in curvature and allows for both Schwarzschild and non-Schwarzschild asymptotically flat black-hole solutions. Here we find the grey-body factors and energy emission rates for Hawking radiation of test Dirac and electromagnetic fields in the vicinity of such a non-Schwarzschild black hole. The temperature and mass of the black hole monotonically decrease from its Schwarzschild value to zero when the coupling constant is increased up to its extremal value. Nevertheless, for small and moderate values of the coupling constant the Hawking radiation is enhanced, and only in the regime of large coupling it is suppressed, as one could expect. The reason for such counter-intuitive behavior is the important role of the grey-body factors: for small and moderate couplings the temperature falls relatively slowly, while the effective potentials for black holes of the same mass become considerably lower, allowing for much higher transmission rates. We have also estimated the lifetime of such black holes and shown that the range of black-hole masses at which ultra-relativistic emission of massive electrons and positrons starts is shifted towards smaller black-hole masses when the coupling constant is large.

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arXiv2021

Shadows of parametrized axially symmetric black holes allowing for separation of variables

Metric of axially symmetric asymptotically flat black holes in an arbitrary metric theory of gravity can be represented in the general form which depends on infinite number of parameters. We constrain this general class of metrics by requiring the existence of additional symmetries, which lead to the separation of variables in the Hamilton-Jacobi and Klein-Gordon equations, and show that once the metric functions change sufficiently moderately in some region near the black hole, the black-hole shadow depends on a few deformation parameters only. We analyze the influence of these parameters on the black-hole shadow. We also show that the shadow of the rotating black hole in the Einstein-dilaton-Gauss-Bonnet theory is well approximated if the terms violating the separation of variables are neglected in the metric.

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arXiv2021

Conformal Weyl gravity via two stages of quasinormal ringing and late-time behavior

Black hole (BH) solution in the conformal Weyl gravity is a generalization of the Schwarzschild spacetime which includes two additional constants appearing when integrating the third order differential equations for gravitational field. One constant looks like the effective cosmological constant providing the de Sitter asymptotic of the solution. The other constant allows one to describe flat rotation of galaxies without introducing of the dark matter. Here we show that the effective "dark matter" term in the metric function drastically changes the asymptotic behavior of the evolution of the wave function of a scalar field: after the Schwarzschild-like ringing phase, the ringing at another, non-Schwarzschildian, longer-lived frequency takes place before the beginning of the exponential asymptotic tail. Thus the evolution of the scalar field consists of the three qualitatively different stages: the Schwarzschild-like ringing phase, the effective dark matter ringing phase and the de Sitter phase characterized by exponential tails. The late time behavior of the electromagnetic field is qualitatively different as well: the exponential tails appear even in the absence of the effective de Sitter term.

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arXiv2020

Massive particles in the Einstein-Lovelock-anti-de Sitter black hole spacetime

An interpretation to the physics of stable geodesics of massive particles in black hole backgrounds has been recently proposed in the context of the AdS/CFT correspondence. It was argued that the existence of stable orbits indicates that the dual state does not thermalize on a thermal time scale and the bulk excitations can be interpreted as metastable states in the dual field theory. Here we study motion of massive particles in the background of the D-dimensional asymptotically anti-de Sitter (AdS) black holes in the Einstein-Lovelock theory. We show that, unlike the asymptotically flat case, for any kind of higher curvature Lovelock corrections there appear a stable circular orbit at a distance from the black hole. We find the general analytical expressions for the frequencies of distant circular orbits and radial oscillation frequencies. We show that the corresponding correction has the same power as in the Schwarzschild-AdS case, implying a universal scaling with the temperature for any Lovelock theory.

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arXiv2020

Quasinormal modes, stability and shadows of a black hole in the 4D Einstein-Gauss-Bonnet gravity

Recently a $D$-dimensional regularization approach leading to the non-trivial $(3+1)$-dimensional Einstein-Gauss-Bonnet (EGB) effective description of gravity was formulated which was claimed to bypass the Lovelock's theorem and avoid Ostrogradsky instability. Later it was shown that the regularization is possible only for some broad, but limited, class of metrics and Aoki, Gorji and Mukohyama [arXiv:2005.03859] formulated a well-defined four-dimensional EGB theory, which breaks the Lorentz invariance in a theoretically consistent and observationally viable way. The black-hole solution of the first naive approach proved out to be also the exact solution of the well-defined theory. Here we calculate quasinormal modes of scalar, electromagnetic and gravitational perturbations and find the radius of shadow for spherically symmetric and asymptotically flat black holes with Gauss-Bonnet corrections. We show that the black hole is gravitationally stable when ($-16 M^2<α\lessapprox 0.6 M^2$). The instability in the outer range is the eikonal one and it develops at high multipole numbers. The radius of the shadow $R_{Sh}$ obeys the linear law with a remarkable accuracy.

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arXiv2020

Simply rotating higher dimensional black holes in Einstein-Gauss-Bonnet theory

Using perturbative expansion in terms of powers of the rotation parameter $a$ we construct the axisymmetric and asymptotically flat black-hole metric in the $D$-dimensional Einstein-Gauss-Bonnet theory. In five-dimensional spacetime we find two solutions to the field equations, describing the asymptotically flat black holes, though only one of them is perturbative in mass, that is, goes over into the Minkowski spacetime when the black-hole mass goes to zero. We obtain the perturbative black-hole solution up to the order $O(αa^3)$ for any $D$, where $α$ is the Gauss-Bonnet coupling, while the $D=5$ solution which is nonperturbative in mass is found in analytic form up to the order $O(αa^7)$. In order to check the convergence of the expansion in $a$ we analyze characteristics of photon orbits in this spacetime and compute frequencies of the photon orbits and radius of the photon sphere.

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arXiv2020

Black holes in the four-dimensional Einstein-Lovelock gravity

A $(3+1)$-dimensional Einstein-Gauss-Bonnet theory of gravity has been recently formulated in [D. Glavan and C. Lin, Phys. Rev. Lett. {\bf 124}, 081301 (2020)] which is different from the pure Einstein theory, i.e., bypasses the Lovelock's theorem and avoids Ostrogradsky instability. The theory was formulated in $D > 4$ dimensions and its action consists of the Einstein-Hilbert term with a cosmological constant, while the Gauss-Bonnet term multiplied by a factor $1/(D-4)$. Then, the four-dimensional theory is defined as the limit $D \to 4$. Here we generalize this approach to the four-dimensional Einstein-Lovelock theory and formulate the most general static $4D$ black-hole solution allowing for a $Λ$-term (either positive or negative) and the electric charge $Q$. As metric functions cannot be found in a closed form in the general case, we develop and share publicly the code which constructs the metric functions for every given set of parameters.

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arXiv2020

BTZ black holes with higher curvature corrections in the 3D Einstein-Lovelock theory

The regularization procedure for getting the four-dimensional nontrivial Einstein-Gauss-Bonnet effective description of gravity and its Lovelock generalization has been recently developed. Here we propose the regularization for the three-dimensional gravity, which is based on the rescaling of the coupling constants and, afterward, taking the limit $D \to 3$. We obtain the generalization of the Bañados-Teitelboim-Zanelli solution in the presence of the higher curvature (Gauss-Bonnet and Lovelock) corrections of any order. The obtained general solution shows a peculiar behavior: The event horizon is allowed not only for asymptotically anti-de Sitter spacetimes, but also for the de-Sitter and flat cases, when the Gauss-Bonnet coupling constant is negative. The factor of the electric charge is analyzed as well for various branches of the solution and the Hawking temperature is obtained.

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arXiv2020

Shadow of the magnetically and tidally deformed black hole

We consider a simple model of a black hole surrounded by a toroidal structure and experiencing deformation due to magnetic field and the tidal force. For this system we construct the shadows of Preston-Poisson black hole, apply it to supermassive black hole $M87^*$ and compare it with the shadows of corresponding Schwarzschild and Kerr black holes. We find that for large deviation parameters $B$ and $E$ the shadow of Preston-Poisson black hole is clearly distinguishable from Kerr black hole provided the angular resolution of measurements is of order $μ\mathrm{arcsec}$.

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arXiv2020

(In)stability of black holes in the 4D Einstein-Gauss-Bonnet and Einstein-Lovelock gravities

A (3+1)-dimensional Einstein-Gauss-Bonnet effective description of gravity has been recently formulated as the $D \to 4$ limit of the higher dimensional field equations after the rescaling of the coupling constant. This approach has been recently extended to the four-dimensional Einstein-Lovelock gravity. Although validity of the regularization procedure has not been shown for the general case, but only for a wide class of metrics, the black-hole solution obtained as a result of such a regularization is also an exact solution in the well defined 4D Einstein-Gauss-Bonnet theory suggested by Aoki, Gorji and Mukohyama [arXiv:2005.03859] and in the scalar-tensor effective classical theories. Here we study the eikonal gravitational instability of asymptotically flat, de Sitter and anti-de Sitter black holes in the four dimensional Einstein-Gauss-Bonnet and Einstein-Lovelock theories. We find parametric regions of the eikonal instability for various orders of the Lovelock gravity, values of coupling and cosmological constants, and share the code which allows one to construct the instability region for an arbitrary set of parameters. For the four-dimensional Gauss-Bonnet black holes we obtain the region of stability in analytic form. Unlike the higher dimensional Einstein-Lovelock case, the eikonal instability serves as an effective cut-off of higher curvature Lovelock terms for the 4D black holes.

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arXiv2020

4D Einstein-Lovelock black holes: Hierarchy of orders in curvature

The Einstein-Lovelock theory contains an infinite series of corrections to the Einstein term with an increasing power of the curvature. It is well-known that for large black holes the lowest (Gauss-Bonnet) term is the dominant one, while for smaller black holes higher curvature corrections become important. We will show that if one is limited by positive values of the coupling constants, then the dynamical instability of black holes serves as an effective cut-off of influence of higher curvature corrections in the 4D Einstein-Lovelock approach: the higher is the order of the Lovelock term, the smaller is the maximal value of the coupling constant allowing for stability, so that effectively only a first few orders can deform the observable values seemingly. For negative values of coupling constants this is not so, and, despite some suppression of higher order terms also occurs due to the decreasing threshold values of the coupling constant, this does not lead to an noticeable opportunity to neglect higher order corrections. In the case a lot of orders of Lovelock theory are taken into account, so that the black-hole solution depends on a great number of coupling constants, we propose a compact description of it in terms of only two or three parameters encoding all the observable values.

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arXiv2020

General parametrization of black holes: the only parameters that matter

The general parametrization of a black-hole spacetime in arbitrary metric theories of gravity includes an infinite set of parameters. It is natural to suppose that essential astrophysically observable quantities, such as quasinormal modes, parameters of shadow, electromagnetic radiation and accreting matter in the vicinity of a black hole, must depend mostly on a few of these parameters. Starting from the parametrization for spherically symmetric configurations in the form of infinite continued fraction, we suggest a compact representation of the asymptotically flat spherically symmetric and slowly rotating black holes in terms of only three and four parameters respectively. A subclass of arbitrarily rotating black holes belonging to the Carter family can also be parametrized by only four parameters. This approximate representation of a black-hole metric should allow one to describe physical observables in the region of strong gravity.

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