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Roman A. Konoplya

Publications and source records attributed to Roman A. Konoplya.

15 recordsLinked to original sources
arXiv2026

Perturbations of Charged Black Holes with Higher-Order Interactions

We study linear gravitational perturbations and the corresponding quasinormal-mode spectra of electrically and magnetically charged black holes in the presence of higher-order corrections to the Einstein-Hilbert action. In particular, we consider a four-derivative interaction that couples the Riemann tensor to the electromagnetic field strength in the unique (Horndeski) combination that preserves second-order equations of motion. We provide a comprehensive analysis of the stability conditions. We numerically confirm the absence of exponentially growing modes throughout the physically admissible region of parameter space where the theory remains free of pathologies. We then investigate in detail the properties of the quasinormal-mode spectra and identify several distinctive features induced by the higher-order interaction.

gr-qc↗
arXiv2026

Cesàro convergence of the high-order WKB method and its applications to black-hole overtones and long-lived modes

We develop a fully automatic Mathematica implementation of the black-hole WKB method at very high orders based on the Bender-Wu algorithm, which in principle is limited only by memory and computational time, and show that when pushed to sufficiently high order and improved by diagonal Padé approximants the method becomes efficient for two regimes which are usually regarded as difficult for the standard low-order WKB treatment: the first several overtones with n>l and the very long-lived quasinormal modes of massive fields. At the same time, we show that this efficiency has a nontrivial limitation: for black-hole metrics belonging to the non-moderate class, especially when higher coefficients of the near-horizon parametrization become large, the WKB sequence may exhibit an apparent convergence to values which are nevertheless far from the accurate quasinormal frequencies. Thus, numerical stabilization of the WKB output alone is not always a sufficient criterion of correctness. However, we observe that although the WKB method with diagonal or near-diagonal Padé approximants does not exhibit monotonic convergence order by order, the corresponding Cesàro means become monotonically convergent once a sufficiently high WKB order is reached. This behavior may serve as an internal WKB criterion for the convergence of the method.

gr-qc↗
arXiv2026

An efficient higher-order WKB code for quasinormal modes and greybody factors

The higher-order WKB Mathematica code for computing quasinormal modes, whose accuracy was significantly enhanced through extensions to higher orders and, in particular, through the use of Padé resummation, has been widely employed in numerous studies over the past several years. In this work, we present an updated and optimized version of the code. The main improvement consists in expanding the effective potential in a Taylor series around its maximum, rather than evaluating the full analytic expression of the WKB formula for each specific potential. This modification leads to a substantial reduction in computation time. In cases where the effective potential is complicated and involves non-rational functions, the speed gain can reach several orders of magnitude, while preserving the accuracy of the method.

gr-qc↗
arXiv2026

Dirty Black Holes, Clean Signals: Near-Horizon vs. Environmental Effects on Grey-Body Factors and Hawking Radiation

Grey-body factors are not only essential ingredients for computing the intensity of Hawking radiation, but also serve as characteristics of black hole's geometry that are closely related to their quasinormal modes. Importantly, they tend to be more stable under small deformations of the background spacetime. In this work, we carry out a detailed analysis of grey-body factors and Hawking radiation for a spherically symmetric black hole subject to localized deformations which do not alter the Hawking temperature: near-horizon modifications to simulate possible new physics or matter fields, and far-zone perturbations to model environmental or astrophysical effects. We show that environmental deformations have only a minor impact on the grey-body factors and Hawking radiation--unless the additional potential barrier created by the environment becomes comparable in height to the primary peak associated with the black hole itself, a scenario more relevant to nonlinear dynamics. In contrast, near-horizon deformations significantly affect the Hawking spectrum, particularly in the low-frequency regime.

gr-qc↗
arXiv2025

Regular Black Holes from Proper-Time flow in Quantum Gravity and their Quasinormal modes, Shadow and Hawking radiation

We derive a class of regular black holes from the proper-time renormalization group approach to asymptotically safe gravity. A central challenge is the robustness of physical predictions to the regularization scheme. We address this by computing key observables for our quantum-corrected black holes, which are non-singular and asymptotically Schwarzschild. We calculate the quasinormal mode spectrum, finding significant deviations from the classical case. The Hawking radiation spectrum is strongly suppressed, implying a slower evaporation rate and relaxed constraints on primordial black holes as dark matter. Shadows and ISCO radii remain consistent with observations. Our results demonstrate that the singularity resolution and its primary observational implications are robust physical outcomes.

gr-qc↗
arXiv2025

Time Evolution of Black Hole Perturbations in Quadratic Gravity

We study the full time-domain evolution of gravitational perturbations in black hole spacetimes arising in Einstein-Weyl gravity, a renormalizable extension of general relativity containing quadratic curvature corrections. We analyze both Schwarzschild and non-Schwarzschild solutions, focusing on monopole and higher multipole perturbations. Using semi-analytical methods based on the Rezzolla-Zhidenko parametrization for approximation of the black hole spacetime and time-domain integration for analysis of evolution of perturbations, we study the late-time behavior of gravitational perturbations. Our results show that the ringdown phase is followed by universal slowly decaying oscillatory tails with the envelope $ ψ\propto t^{-5/6} $. We also demonstrate the breakdown of the eikonal correspondence between quasinormal modes and unstable null geodesics, highlighting limitations of the WKB method in this context. Our analysis confirms the range of (in)stability of black holes in Einstein-Weyl gravity found in recent publications.

gr-qc↗
arXiv2025

Quasinormal Ringing and Shadows of Black Holes and Wormholes in Dark Matter-Inspired Weyl Gravity

Weyl gravity naturally generates effective dark matter and cosmological constant terms as integration constants, eliminating the need to explicitly introduce them into the theory. Additionally, the framework permits three intriguing solutions for compact objects: an asymptotically de Sitter Schwarzschild-like black hole described by the Mannheim-Kazanas solution, a non-Schwarzschild black hole, and a traversable wormhole that exists without exotic matter. In this work, we investigate the quasinormal spectra of all three solutions. We demonstrate that when the mass of the black hole corresponding to the Mannheim-Kazanas solution approaches zero, the perturbation equations yield an exact solution expressible through hypergeometric functions. The quasinormal modes of black holes in Weyl gravity can be classified into three distinct branches: Schwarzschild-like modes modified by effective dark matter and cosmological terms, and modes associated with empty spacetime (de Sitter and dark matter branches), which are further influenced by the black hole mass. Previous studies have shown that the dark matter term induces a secondary stage of quasinormal ringing following the initial Schwarzschild phase. Here, we compute the frequencies using convergent methods and elucidate how this unique time-domain behavior translates into the frequency domain. Furthermore, we demonstrate that the non-Schwarzschild black hole can be distinguished from both the Schwarzschild-like solution and the wormhole through their distinct quasinormal spectra. We also compute shadow radii for black holes and wormholes within Weyl gravity, revealing that wormholes with large throat radii can produce significantly smaller shadows compared to black holes of equivalent mass.

gr-qc↗
arXiv2021

General parametrization of wormhole spacetimes and its application to shadows and quasinormal modes

The general parametrization for spacetimes of spherically symmetric Lorentzian, traversable wormholes in an arbitrary metric theory of gravity is presented. The parametrization is similar in spirit to the post-Newtonian parametrized formalism, but with validity that extends beyond the weak field region and covers the whole space. Our method is based on a continued-fraction expansion in terms of a compactified radial coordinate. Calculations of shadows and quasinormal modes for various examples of parametrization of known wormhole metrics that we have performed show that, for most cases, the parametrization provides excellent accuracy already at the first order. Therefore, only a few parameters are dominant and important for finding potentially observable quantities in a wormhole background. We have also extended the analysis to the regime of slow rotation.

gr-qc↗
arXiv2021

Grey-body factors and Hawking radiation of black holes in $4D$ Einstein-Gauss-Bonnet gravity

The $(3+1)$-dimensional Einstein-Gauss-Bonnet theory of gravity which breaks the Lorentz invariance in a theoretically consistent and observationally viable way has been recently suggested by Aoki, Gorji and Mukohyama [arXiv:2005.03859]. Here we calculate grey-body factor for Dirac, electromagnetic and gravitational fields and estimate the intensity of Hawking radiation and lifetime for asymptotically flat black holes in this theory. Positive coupling constant leads to much smaller evaporation rate and longer life-time of a black hole, while the negative one enhances Hawking radiation. The grey-body factors for electromagnetic and Dirac fields are smaller for larger values of the coupling constant.

gr-qc↗
arXiv2021

General parametrization of higher-dimensional black holes and its application to Einstein-Lovelock theory

Here we have developed the general parametrization for spherically symmetric and asymptotically flat black-hole spacetimes in an arbitrary metric theory of gravity. The parametrization is similar in spirit to the parametrized post-Newtonian (PPN) approximation, but valid in the whole space outside the event horizon, including the near horizon region. This generalizes the continued-fraction expansion method in terms of a compact radial coordinate suggested by Rezzolla and Zhidenko [Phys.Rev.D 90 8, 084009 (2014)] for the four-dimensional case. As the first application of our higher-dimensional parametrization we have approximated black-hole solutions of the Einstein-Lovelock theory in various dimensions. This allows one to write down the black-hole solution which depends on many parameters (coupling constants in front of higher curvature terms) in a very compact analytic form, which depends only upon a few parameters of the parametrization. The approximate metric deviates from the exact (but extremely cumbersome) expressions by fractions of one percent even at the first order of the continued-fraction expansion, which is confirmed here by computation of observable quantities, such as quasinormal modes of the black hole.

gr-qc↗
arXiv2020

Einstein-scalar-Gauss-Bonnet black holes: Analytical approximation for the metric and applications to calculations of shadows

Recently, numerical solutions to the field equations of Einstein-scalar-Gauss-Bonnet gravity that correspond to black-holes with non-trivial scalar hair have been reported. Here, we employ the method of the continued-fraction expansion in terms of a compact coordinate in order to obtain an analytical approximation for the aforementioned solutions. For a wide variety of coupling functionals to the Gauss-Bonnet term we were able to obtain analytical expressions for the metric functions and the scalar field. In addition we estimated the accuracy of these approximations by calculating the black-hole shadows for such black holes. Excellent agreement between the numerical solutions and analytical approximations has been found.

gr-qc↗
arXiv2019

Echoes in brane worlds: ringing at a black hole--wormhole transition

Echoes are known as modifications of the usual quasinormal ringing of a black hole at late times because of the deviation of space-time from the initial black-hole geometry in a small region near its event horizon. We consider a class of brane-world model solutions of the Shiromizu-Maeda-Sasaki equations, which describe both black holes and wormholes and interpolate between them via a continuous parameter. In this way the brane-world scenario provides a natural model for wormholes mimicking the black hole behavior if the continuous parameter is chosen near the threshold with a black-hole solution. We show that in the vicinity of this threshold interpolating between black holes and wormholes, quasinormal ringing of the wormholes at the initial stage is indistinguishable from that of the black holes with nearby values of the above parameter, but at later times the signal is modified by intensive echoes. We notice that the black-hole mimickers that are wormholes near the threshold have the largest quality factor and are therefore the best oscillators among the considered examples.

gr-qc↗
arXiv2018

Quasinormal modes of massive fermions in Kerr spacetime: Long-lived modes and the fine structure

Quasinormal modes of a massive Dirac field were calculated for various static black hole backgrounds with the help of the WKB formula. These estimations, however, are rough and valid only for very small values of $μM$, where $M$ and $μ$ are mass of the black hole and field respectively. Thus, no accurate calculations of massive Dirac modes are known even for the Schwarzschild black hole and this is all the more so for the Kerr solution. Here we calculate quasinormal modes of a massive Dirac field in the Kerr background. We have shown that the infinitely long-lived quasinormal modes (quasiresonances), which exist for boson fields, appear also in the fermions' quasinormal spectrum. Two chiralities of massive fermions lead to an additional "fine structure" in the spectrum. We discuss the effect of this fine structure on the behavior of quasiresonances and the stability. The analysis is also extended to a charged massive field in the Kerr-Newman background.

gr-qc↗
arXiv2008

Evolution of perturbations of squashed Kaluza-Klein black holes: escape from instability

The squashed Kaluza-Klien (KK) black holes differ from the Schwarzschild black holes with asymptotic flatness or the black strings even at energies for which the KK modes are not excited yet, so that squashed KK black holes open a window in higher dimensions. Another important feature is that the squashed KK black holes are apparently stable and, thereby, let us avoid the Gregory-Laflamme instability. In the present paper, the evolution of scalar and gravitational perturbations in time and frequency domains is considered for these squashed KK black holes. The scalar field perturbations are analyzed for general rotating squashed KK black holes. Gravitational perturbations for the so called zero mode are shown to be decayed for non-rotating black holes, in concordance with the stability of the squashed KK black holes. The correlation of quasinormal frequencies with the size of extra dimension is discussed.

hep-th↗