arXiv subjects

A. Zhidenko

Publications and source records attributed to A. Zhidenko.

At least 73 records · Page 4Linked to original sources

arXiv2014

Decay of a charged scalar and Dirac fields in the Kerr-Newman-de Sitter background

We find the quasinormal modes of the charged scalar and Dirac fields in the background of the rotating charged black holes, described by the Kerr-Newman-de Sitter solution. The dependence of the quasinormal spectrum upon the black hole parameters mass M, angular momentum a, charge Q, as well as on values of the Λ-term and field charge q is investigated. Special attention is given to the near extremal limit of the black hole charge. In particular, we find that for both scalar and Dirac fields, charged perturbations decay quicker for q>0 and slower for q<0 for values of black holes charge Q less than than some threshold value, which is close to the extremal value of charge and depend on parameters of the black holes.

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arXiv2014

Charged scalar field instability between the event and cosmological horizons

Recently, a new interesting instability of a charged scalar field in the Reissner-Nordström-de Sitter background has been found (arXiv:1405.4931v2) through the time-domain integration of the perturbation equation. We investigate further properties of this instability, confirm its existence by concordant frequency-domain and time-domain calculations and show that it occurs at however small value of the coupling eQ, where e and Q are charges of a scalar field and black hole respectively. We also investigate the parametric region of instability and show that the critical values of eQ at which the stabilization happens strongly depends on the value of cosmological constant $Λ$ and softly on Q. We show that all the unstable modes are superradiant, but not all the superradiant modes are unstable. We analytically prove that superradiance is necessary (but not sufficient) condition for the instability in this case and, thereby, demonstrate the superradiant origin of the instability.

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arXiv2014

Massive charged scalar field in the Kerr-Newman background II: Hawking radiation

We perform accurate calculations of the energy-, momentum-, and charge-emission rates of a charged scalar field in the background of the Kerr-Newman black hole at the range of parameters for which the effect is not negligibly small and, at the same time, the semiclassical regime is, at least marginally, valid. For black holes with charge below or not much higher than the charge accretion limit $Q \sim μM/e$ (where $e$ and $μ$ are the electron's mass and charge), the time between the consequent emitting of two charged particles is very large. For primordial black holes the transition between the increasing and decreasing of the ratio $Q/M$ occurs around the charge accretion limit. The rotation increases the intensity of radiation up to three orders, while the effect of the field's mass strongly suppresses the radiation.

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arXiv2013

Instability of D-dimensional extremally charged Reissner-Nordstrom(-de Sitter) black holes: Extrapolation to arbitrary D

In our earlier work (PRL 103 (2009) 161101) it was shown that nonextremal highly charged Reissner-Nordstrom-de Sitter black holes are gravitationally unstable in D>6-dimensional space-times. Here, we find accurate threshold values of the $Λ$-term at which the instability of the extremally charged black holes starts. The larger $D$ is, the smaller is the threshold value of $Λ$. We have shown that the ratio $ρ= r_{h}/r_{cos}$ (where $r_{cos}$ and $r_{h}$ are the cosmological and event horizons) is proportional to $e^{-(D-4)/2}$ at the onset of instability for D=7,8,...11, implying that the same law should fulfill for arbitrary D. This is numerical evidence that extremally charged Reissner-Nordstrom-de Sitter black holes are gravitationally unstable for D>6, while asymptotically flat extremally charged Reissner-Nordstrom black holes are stable for all D. The instability is not connected to the horizon instability discussed recently in the literature, and, unlike the later one, develops also outside the event horizon, that is, it can be seen by an external observer. In addition, for the nonextremal case through fitting of the numerical data we obtained an approximate analytical formula which relates values of charge and the $Λ$-term at the onset of instability.

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arXiv2013

A massive charged scalar field in the Kerr-Newman background I: quasinormal modes, late-time tails and stability

So far analysis of the quasinormal spectrum of a massive charged scalar field in the black hole background has been limited by the regime of small μM and qQ, where μ, q (M, Q) are mass and charge of the field (black hole). Here we shall present a comprehensive picture of quasinormal modes, late-time tails and stability of a massive charged scalar field around Kerr-Newman black holes for any physically meaningful values of the parameters. We shall show that despite presence of the two mechanisms of superradiance (owing to black hole's rotation and charge) and the massive term creating growing bound states, there is no indication of instability under quasinormal modes' boundary conditions. We have shown that for some moderate values of qQ dominant quasinormal modes may have arbitrarily small real oscillation frequencies Re(ω). An analytic formula for the quasinormal modes has been derived in the regime of large qQ. The larger the field's charge, the sooner asymptotic tails dominate in a signal, making it difficult to extract quasinormal frequencies from a time-domain profile. Analytic expressions for intermediate and asymptotically late-time tails have been found for the Reissner-Nordström black hole. For the near extremal Kerr-Newman black holes we have obtained a more general picture of the mode branching found recently for massless fields [arXiv:1212.3271] in the Kerr background.

gr-qc↗

arXiv2013

Instabilities of wormholes and regular black holes supported by a phantom scalar field

We test the stability of various wormholes and black holes supported by a scalar field with a negative kinetic term. The general axial perturbations and the monopole type of polar perturbations are considered in the linear approximation. Two classes of objects are considered: (i) wormholes with flat asymptotic behavior at one end and AdS on the other (M-AdS wormholes) and (ii) regular black holes with asymptotically de Sitter expansion far beyond the horizon (the so-called black universes). A difficulty in such stability studies is that the effective potential for perturbations forms an infinite wall at throats, if any. Its regularization is in general possible only by numerical methods, and such a method is suggested in a general form and used in the present paper. As a result, we have shown that all configurations under study are unstable under spherically symmetric perturbations, except for a special class of black universes where the event horizon coincides with the minimum of the area function. For this stable family, the frequencies of quasinormal modes of axial perturbations are calculated.

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arXiv2013

Radiation processes in the vicinity of non-Schwarzschild and non-Kerr black holes

Usually alternative theories of gravity imply deviations from the well-known Kerr space-time, a model of an isolated black hole in General Relativity. In the dominant order, the deformed Kerr metric, free of closed time-like curves outside the event horizon, has been suggested recently by Johannsen and Psaltis. It has a single deformation parameter which is not constrained by the current observations, allowing, thereby, for a kind of unified and simple phenomenological description of black holes in various theories of gravity. Here we consider a number of classical and quantum phenomena of radiation in the vicinity of such deformed Schwarzschild-like and Kerr-like black holes: spiralling of particles into black holes, decay of fields propagating in the black hole's background, Hawking radiation. In particular, we calculate some quantitative characteristics of the above phenomena, such as the binding energy of particles, quasinormal modes, late-time tails of fields of various spin, intensity of Hawking radiation. The binding energy released when a particle goes over from a given stable orbit in the equatorial plane to the innermost stable one is calculated for such non-Kerr black holes. Due to inseparability of the wave equations in the general case, the perturbations and stability of scalar, Dirac, and electromagnetic fields are analyzed for vanishing rotation only. The dependence of the radiation phenomena on the deformation parameter is discussed.

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arXiv2012

Stability of tardyons and tachyons in the rotating and expanding Universe

In the present paper we analyze the spectrum of quasinormal modes for massive scalar and Dirac fields, allowing for both tardyonic ($μ^2 >0$) and tachyonic ($μ^2 <0$) masses, in the expanding and rotating cosmological background. The spectrum found shows a number of peculiar features, which are absent in the Minkowski space-time. A hypothetical particle that moves faster than light, \emph{a tachyon}, is known to be classically unstable in the Minkowski space-time. This instability has its analog at the quantum level: small vacuum fluctuations of the field lead to the unbounded growth of the amplitude, so that the appearance of the real tachyons in the spectrum means that there is catastrophic instability in the theory. It has been conjectured a long time ago that possibly the lightest particles with nonzero mass, the neutrino, may be a tachyon. Here we shall show that in the rotating and expanding Universe tachyons are stable if their mass is less than some constant, which is related to the Universe's rotation and expansion scales. The current upper bound on the rotation scale gives us a very small upper bound on the tachyon's mass which is many orders less than the mass of the electron. This might explain why only very light particles have the chance of becoming tachyons. It is shown that the spectrum of the "normal" ($μ^2 >0$) Dirac field has a discontinuity as a function of the z-component of the wave vector $k_z$ at $k_z = 0$. Keywords: OPERA, superluminal neutrino.

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arXiv2011

Long life of Gauss-Bonnet corrected black holes

Dictated by the string theory and various higher dimensional scenarios, black holes in $D>4$-dimensional space-times must have higher curvature corrections. The first and dominant term is quadratic in curvature, and called the Gauss-Bonnet (GB) term. We shall show that although the Gauss-Bonnet correction changes black hole's geometry only softly, the emission of gravitons is suppressed by many orders even at quite small values of the GB coupling. The huge suppression of the graviton emission is due to the multiplication of the two effects: the quick cooling of the black hole when one turns on the GB coupling and the exponential decreasing of the grey-body factor of the tensor type of gravitons at small and moderate energies. At higher $D$ the tensor gravitons emission is dominant, so that the overall lifetime of black holes with Gauss-Bonnet corrections is many orders larger than was expected. This effect should be relevant for the future experiments at the Large Hadron Collider (LHC). Keywords: Hawking radiation, black hole evaporation.

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arXiv2011

On the stability of scalar-vacuum space-times

We study the stability of static, spherically symmetric solutions to the Einstein equations with a scalar field as the source. We describe a general methodology of studying small radial perturbations of scalar-vacuum configurations with arbitrary potentials V(ϕ), and in particular space-times with throats (including wormholes), which are possible if the scalar is phantom. At such a throat, the effective potential for perturbations V_eff has a positive pole (a potential wall) that prevents a complete perturbation analysis. We show that, generically, (i) V_eff has precisely the form required for regularization by the known S-deformation method, and (ii) a solution with the regularized potential leads to regular scalar field and metric perturbations of the initial configuration. The well-known conformal mappings make these results also applicable to scalar-tensor and f(R) theories of gravity. As a particular example, we prove the instability of all static solutions with both normal and phantom scalars and V(ϕ) = 0 under spherical perturbations. We thus confirm the previous results on the unstable nature of anti-Fisher wormholes and Fisher's singular solution and prove the instability of other branches of these solutions including the anti-Fisher "cold black holes".

gr-qc↗

arXiv2011

AdS-like spectrum of the asymptotically Gödel space-times

A black hole immersed in a rotating Universe, described by the Gimon-Hashimoto solution, is tested on stability against scalar field perturbations. Unlike the previous studies on perturbations of this solution, which dealt only with the limit of slow Universe rotation j, we managed to separate variables in the perturbation equation for the general case of arbitrary rotation. This leads to qualitatively different dynamics of perturbations, because the exact effective potential does not allow for Schwarzschild-like asymptotic of the wave function in the form of purely outgoing waves. The Dirichlet boundary conditions are allowed instead, which result in a totally different spectrum of asymptotically Gödel black holes: the spectrum of quasinormal frequencies is similar to the one of asymptotically anti-de Sitter black holes. At large and intermediate overtones N, the spectrum is equidistant in N. In the limit of small black holes, quasinormal modes (QNMs) approach the normal modes of the empty Gödel space-time. There is no evidence of instability in the found frequencies, which supports the idea that the existence of closed time-like curves (CTCs) and the onset of instability correlate (if at all) not in a straightforward way.

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arXiv2011

Superradiance and instability of the charged Myers-Perry black holes in the Gödel Universe

We consider scalar field perturbations of the asymptotically Gödel 5-dimensional charged rotating black holes with two equal angular momenta. It is shown that the spectrum of proper oscillations of the perturbation includes superradiant unstable modes. The reason for the instability is the confining Dirichlet boundary condition at the asymptotically far region of the Gödel Universe. The confining box makes superradiant modes extract rotational energy from the black hole and, after repeated reflections from the black hole, grow unboundedly. A similar instability takes place for rotating black holes in the asymptotically anti-de Sitter (AdS) space-time.

hep-th↗

arXiv2011

Quasinormal modes of black holes: from astrophysics to string theory

Perturbations of black holes, initially considered in the context of possible observations of astrophysical effects, have been studied for the past ten years in string theory, brane-world models and quantum gravity. Through the famous gauge/gravity duality, proper oscillations of perturbed black holes, called quasinormal modes (QNMs), allow for the description of the hydrodynamic regime in the dual finite temperature field theory at strong coupling, which can be used to predict the behavior of quark-gluon plasmas in the nonperturbative regime. On the other hand, the brane-world scenarios assume the existence of extra dimensions in nature, so that multidimensional black holes can be formed in a laboratory experiment. All this stimulated active research in the field of perturbations of higher-dimensional black holes and branes during recent years. In this review recent achievements on various aspects of black hole perturbations are discussed such as decoupling of variables in the perturbation equations, quasinormal modes (with special emphasis on various numerical and analytical methods of calculations), late-time tails, gravitational stability, AdS/CFT interpretation of quasinormal modes, and holographic superconductors. We also touch on state-of-the-art observational possibilities for detecting quasinormal modes of black holes.

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arXiv2011

Quasinormal modes, scattering and Hawking radiation of Kerr-Newman black holes in a magnetic field

We perform a comprehensive analysis of the spectrum of proper oscillations (quasinormal modes), transmission/reflection coefficients and Hawking radiation for a massive charged scalar field in the background of the Kerr-Newman black hole immersed in an asymptotically homogeneous magnetic field. There are two main effects: the Zeeman shift of the particle energy in the magnetic field and the difference of values of an electromagnetic potential between the horizon and infinity, i.e. the Faraday induction. We have shown that "turning on" the magnetic field induces a stronger energy-emission rate and leads to "recharging" of the black hole. Thus, a black hole immersed in a magnetic field evaporates much quicker, achieving thereby an extremal state in a shorter period of time. Quasinormal modes are moderately affected by the presence of a magnetic field which is assumed to be relatively small compared to the gravitational field of the black hole.

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arXiv2010

Graviton Emission in the Bulk by a Simply Rotating Black Hole

In this work, we study the emission of tensor-type gravitational degrees of freedom from a higher-dimensional, simply rotating black hole in the bulk. The decoupled radial part of the corresponding field equation is first solved analytically in the limit of low-energy emitted particles and low-angular momentum of the black hole in order to derive the absorption probability. Both the angular and radial equations are then solved numerically, and the comparison of the analytical and numerical results show a very good agreement in the low and intermediate energy regimes. By using our exact, numerical results we compute the energy and angular momentum emission rates and their dependence on the spacetime parameters such as the number of additional spacelike dimensions and the angular momentum of the black hole. Particular care is given to the convergence of our results in terms of the number of modes taken into account in the calculation, and the multiplicity of graviton tensor modes that correspond to the same angular-momentum numbers.

hep-th↗

arXiv2010

Holographic conductivity of zero temperature superconductors

Using the recently found by G. Horowitz and M. Roberts (arXiv:0908.3677) numerical model of the ground state of holographic superconductors (at zero temperature), we calculate the conductivity for such models. The universal relation connecting conductivity with the reflection coefficient was used for finding the conductivity by the WKB approach. The dependence of the conductivity on the frequency and charge density is discussed. Numerical calculations confirm the general arguments of (arXiv:0908.3677) in favor of non-zero conductivity even at zero temperature. In addition to the Horowitz-Roberts solution we have found (probably infinite) set of extra solutions which are normalizable and reach the same correct RN-AdS asymptotic at spatial infinity. These extra solutions (which correspond to larger values of the grand canonical potential) lead to effective potentials that also vanish at the horizon and thus correspond to a non-zero conductivity at zero temperature.

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arXiv2009

Instability of higher dimensional charged black holes in the de-Sitter world

We have shown that higher dimensional Reissner-Nordström-de Sitter black holes are gravitationally unstable for large values of the electric charge and cosmological constant in $D \geq 7$ space-time dimensions. We have found the shape of the slightly perturbed black hole at the threshold point of instability. Why only $D=4, 5$ and 6 dimensional worlds are favorable as to the black stability remains unknown.

hep-th↗

arXiv2009

Gravitational instability of simply rotating AdS black holes in higher dimensions

We study the stability of AdS black hole holes rotating in a single two plane for tensor-type gravitational perturbations in $D > 6$ space-time dimensions. First, by an analytic method, we show that there exists no unstable mode when the magnitude $a$ of the angular momentum is smaller than $r_h^2/R$ where $r_h$ is the horizon radius, and $R$ is the AdS curvature radius. Then, by numerical calculations of quasinormal modes, using the separability of the relevant perturbation equations, we show that an instability occurs for rapidly rotating black holes with $a>r_h^2/R$, although the growth rate is tiny (of order $10^{-12}$ of the inverse horizon radius). We give numerical evidences indicating that this instability is caused by superradiance.

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