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    Analytical solution for charged fluid pressure profiles – circulation in combined electromagnetic field

    by Zdeněk Stuchlík· Prosinec 01, 2017· in Fyzika· 0 comments
    We introduce a general transformation leading to an integral form of pressure equations characterizing equilibrium configurations of charged perfect fluid circling in strong gravitational and combined electromagnetic fields. The transformation generalizes our recent analytical treatment applicable to electric or magnetic fields treated separately along with the gravitational one. As an example, we present a particular solution for a fluid circling close to a charged rotating black hole immersed in an asymptotically uniform magnetic field.
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    The motion of the test particle near the black hole embedded into dust: The flat space case

    by Zdeněk Stuchlík· Prosinec 01, 2017· in Fyzika· 0 comments
    In this work we investigate the motion of free particle in the field of strongly gravitating object which is embedded into dust cosmological background. We use newly obtained exact solution of Einstein equations in comoving coordinates for the system under consideration in case of zero spatial curvature. Observable velocity of the particle moving relatively to the observer comoving with cosmological expansion is found from geodesic equations.
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    Particle collisions and optical effects in the mining Kerr-Newman spacetimes

    by Zdeněk Stuchlík· Listopad 01, 2017· in Fyzika· 0 comments
    We study ultrahigh-energy particle collisions and optical effects in the extraordinary class of mining braneworld Kerr-Newman (KN) naked singularity spacetimes, predicting extremely high efficiency of Keplerian accretion, and compare the results to those related to the other classes of the KN naked singularity and black hole spacetimes. We demonstrate that in the mining KN spacetimes the ultrahigh center-of- mass energy occurs for collisions of particles following the extremely- low-energy stable circular geodesics of the "mining regime," colliding with large family of incoming particles, e.g., those infalling from the marginally stable counter-rotating circular geodesics. This is qualitatively different situation in comparison to the standard KN naked singularity or black hole spacetimes where the collisional ultrahigh center-of-mass energy can be obtained only in the near-extreme spacetimes. We also show that observers following the stable circular geodesics of the mining regime can register extremely blue-shifted radiation incoming from the Universe, and see strongly deformed sky due to highly relativistic motion along such stable orbits. The strongly blue-shifted radiation could be thus a significant source of energy for such orbiting observers.
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    Classical corrections to black hole entropy in d dimensions: A rear window to quantum gravity?

    by Zdeněk Stuchlík· Listopad 01, 2017· in Fyzika· 0 comments
    We provide a simple derivation of the corrections for Schwarzschild and Schwarzschild-Tangherlini black hole entropy without knowing the details of quantum gravity. We will follow the ideas of Bekenstein, Wheeler, and Jaynes, using summation techniques without calculus approximations, to directly find logarithmic corrections to the well-known entropy formula for black holes. Our approach is free from pathological behavior giving negative entropy for small values of black hole mass M . With the aid of the "universality" principle, we will argue that this purely classical approach could open a window for exploring properties of quantum gravity.
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    Super-spinning compact objects and models of high-frequency quasi-periodic oscillations observed in Galactic microquasars. II. Forced resonances

    by Zdeněk Stuchlík· Listopad 01, 2017· in Fyzika· 0 comments
    In our previous work (Paper I) we applied several models of high- frequency quasi-periodic oscillations (HF QPOs) to estimate the spin of the central compact object in three Galactic microquasars assuming the possibility that the central compact body is a super-spinning object (or a naked singularity) with external spacetime described by Kerr geometry with a dimensionless spin parameter a ≡ cJ/GM2 > 1. Here we extend our consideration, and in a consistent way investigate implications of a set of ten resonance models so far discussed only in the context of a < 1. The same physical arguments as in Paper I are applied to these models, I.e. only a small deviation of the spin estimate from a = 1, a ≳ 1, is assumed for a favoured model. For five of these models that involve Keplerian and radial epicyclic oscillations we find the existence of a unique specific QPO excitation radius. Consequently, there is a simple behaviour of dimensionless frequency M × νU(a) represented by a single continuous function having solely one maximum close to a ≳ 1. Only one of these models is compatible with the expectation of a ≳ 1. The other five models that involve the radial and vertical epicyclic oscillations imply the existence of multiple resonant radii. This signifies a more complicated behaviour of M × νU(a) that cannot be represented by single functions. Each of these five models is compatible with the expectation of a ≳ 1.
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    Scalar perturbations of nonsingular nonrotating black holes in conformal gravity

    by Zdeněk Stuchlík· Září 01, 2017· in Fyzika· 0 comments
    We study scalar and electromagnetic perturbations of a family of nonsingular nonrotating black hole spacetimes that are solutions in a large class of conformally invariant theories of gravity. The effective potential for scalar perturbations depends on the exact form of the scaling factor. Electromagnetic perturbations do not feel the scaling factor, and the corresponding quasinormal mode spectrum is the same as in the Schwarzschild metric. We find that these black hole metrics are stable under scalar and electromagnetic perturbations. Assuming that the quasinormal mode spectrum for scalar perturbations is not too different from that for gravitational perturbations, we can expect that the calculation of the quasinormal mode spectrum and the observation with gravitational wave detectors of quasinormal modes from astrophysical black holes can constrain the scaling factor and test these solutions.
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    Are eikonal quasinormal modes linked to the unstable circular null geodesics?

    by Zdeněk Stuchlík· Srpen 01, 2017· in Fyzika· 0 comments
    In Cardoso et al. [6] it was claimed that quasinormal modes which any stationary, spherically symmetric and asymptotically flat black hole emits in the eikonal regime are determined by the parameters of the circular null geodesic: the real and imaginary parts of the quasinormal mode are multiples of the frequency and instability timescale of the circular null geodesics respectively. We shall consider asymptotically flat black hole in the Einstein-Lovelock theory, find analytical expressions for gravitational quasinormal modes in the eikonal regime and analyze the null geodesics. Comparison of the both phenomena shows that the expected link between the null geodesics and quasinormal modes is violated in the Einstein-Lovelock theory. Nevertheless, the correspondence exists for a number of other cases and here we formulate its actual limits.
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    Super-Spinning Compact Objects and Rapid Variability of Galactic Microquasars

    by Zdeněk Stuchlík· Srpen 01, 2017· in Fyzika· 0 comments
    In our previous work we applied several models of high-frequency quasi- periodic oscillations to estimate the spin of the central compact object in three Galactic microquasars. We also assumed the possibility that the central compact body is a super-spinning object. Here we extend our consideration and investigate in a consistent way the implications of several resonance models so far discussed only in the context of black holes.
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    Energy conditions of non-singular black hole spacetimes in conformal gravity

    by Zdeněk Stuchlík· Srpen 01, 2017· in Fyzika· 0 comments
    Conformal gravity can elegantly solve the problem of spacetime singularities present in Einstein's gravity. For every physical spacetime, there is an infinite family of conformally equivalent singularity-free metrics. In the unbroken phase, every non-singular metric is equivalent and can be used to infer the physical properties of the spacetime. In the broken phase, a Higgs-like mechanism should select a certain vacuum, which thus becomes the physical one. However, in the absence of the complete theoretical framework we do not know how to select the right vacuum. In this paper, we study the energy conditions of non-singular black hole spacetimes obtained in conformal gravity assuming they are solutions of Einstein's gravity with an effective energy-momentum tensor. We check whether such conditions can be helpful to select the vacuum of the broken phase.
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    Comments on “Casimir effect in the Kerr spacetime with quintessence”

    by Zdeněk Stuchlík· Červenec 01, 2017· in Fyzika· 0 comments
    This comment is devoted to the recalculation of the Casimir energy of a massless scalar field in the Kerr black hole surrounded by quintessence derived in [B. Toshmatov, Z. Stuchlík and B. Ahmedov, Eur. Phys. J. Plus 132, 98 (2017)] and its comparison with the results recently obtained in [V. B. Bezerra, M. S. Cunha, L. F. F. Freitas and C. R. Muniz, Mod. Phys. Lett. A 32, 1750005 (2017)] in the spacetime [S. G. Ghosh, Eur. Phys. J. C 76, 222 (2016)]. We have shown that in the more realistic spacetime which does not have the failures illustrated here, the Casimir energy is significantly bigger than that derived in [V. B. Bezerra, M. S. Cunha, L. F. F. Freitas and C. R. Muniz, Mod. Phys. Lett. A 32, 1750005 (2017)], and the difference becomes crucial especially in the regions of near horizons of the spacetime.
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