Charged Particle Motions near Non-Schwarzschild Black Holes with External Magnetic Fields in Modified Theories of Gravity

A small deformation to the Schwarzschild metric controlled by four free parameters could be referred to as a nonspinning black hole solution in alternative theories of gravity. Since such a non-Schwarzschild metric can be changed into a Kerr-like black hole metric via a complex coordinate transformation, the recently proposed time-transformed, explicit symplectic integrators for the Kerr-type spacetimes are suitable for a Hamiltonian system describing the motion of charged particles around the non-Schwarzschild black hole surrounded with an external magnetic field. The obtained explicit symplectic methods are based on a time-transformed Hamiltonian split into seven parts, whose analytical solutions are explicit functions of new coordinate time. Numerical tests show that such explicit symplectic integrators for intermediate time steps perform well long-term when stabilizing Hamiltonian errors, regardless of regular or chaotic orbits. One of the explicit symplectic integrators with the techniques of Poincaré sections and fast Lyapunov indicators is applied to investigate the effects of the parameters, including the four free deformation parameters, on the orbital dynamical behavior. From the global phase-space structure, chaotic properties are typically strengthened under some circumstances, as the magnitude of the magnetic parameter or any one of the negative deformation parameters increases. However, they are weakened when the angular momentum or any one of the positive deformation parameters increases.

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INFLUENCE OF MAGNETIC CHARGE ON THE SAGNAC EFFECT IN THE BARDEEN SPACETIME

Izvestia Ufimskogo Nauchnogo Tsentra RAN ◽

10.31040/2222-8349-2021-0-1-92-96 ◽

2021 ◽

Vol 0 (1) ◽

pp. 92-96

Author(s):

R.KH. KARIMOV ◽

◽

K.K. NANDI ◽

Keyword(s):

Black Hole ◽

Light Source ◽

Magnetic Charge ◽

Sagnac Effect ◽

Modified Theories Of Gravity ◽

Circular Orbits ◽

The Earth ◽

Astrophysical Objects ◽

Interesting Task ◽

Theories Of Gravity

This paper investigates one of the most interesting effects associated with the rotation of astrophysical objects (the Sagnac effect). The effect was first confirmed in laboratory experiments by Georges Sagnac with a rotating ring interferometer in 1913. Later, the effect was also confirmed within the framework of the Earth in the "Around-the-World" experiment conducted by J. Hafele and R. Kitting, in which they twice circled the Earth with an atomic cesium clock on board and compared the "flying" clock with those remaining static on the Earth. As a result, a non-zero difference in the clock rate was found as a confirmation of the Sagnac effect. Subsequently, more precise satellite experiments have been carried out to measure the Sagnac effect within the Earth. The effect was also considered in general relativity and modified theories of gravity, where many works were carried out to study the influence of such parameters as angular momentum, cosmological constant, Ricci scalar, etc. on the Sagnac effect. An interesting task is to study the influence of a magnetic charge on the effect, since the solution with rotation described by a black hole with mass M and magnetic charge g is the Bardeen nonsingular black hole. The work will calculate the Sagnac effect in the space-time of the rotating Bardeen black hole for both geodesic and non-geodesic circular orbits of the light source / receiver (assuming that the light source and receiver are defined at the same point). Two types of circular orbits describe the opposing influence on the Sagnac effect: the Sagnac delay increases with an increase in the magnetic charge in the case of non-geodesic circular orbits and decreases in the case of geodesic circular orbits. However, the farther is the orbit of the light source / receiver, the less the magnetic charge affects the Sagnac delay. It is also assumed that the gravity of the Earth and the Sun near the surface is well described by the Bardeen metric.

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Constraints on alternative theories of gravity with observations of the Galactic Center

EPJ Web of Conferences ◽

10.1051/epjconf/201819101010 ◽

2018 ◽

Vol 191 ◽

pp. 01010 ◽

Cited By ~ 5

Author(s):

Alexander Zakharov

Keyword(s):

Black Hole ◽

Gravitational Wave ◽

Galactic Center ◽

Alternative Theories Of Gravity ◽

Significant Activity ◽

Graviton Mass ◽

The Future ◽

Theories Of Gravity ◽

Theory Of Gravity ◽

Alternative Theories

To evaluate a potential usually one analyzes trajectories of test particles. For the Galactic Center case astronomers use bright stars or photons, so there are two basic observational techniques to investigate a gravitational potential, namely, (a) monitoring the orbits of bright stars near the Galactic Center as it is going on with 10m Keck twin and four 8m VLT telescopes equipped with adaptive optics facilities (in addition, recently the IR interferometer GRAVITY started to operate with VLT); (b) measuring the size and shape of shadows around black hole with VLBI-technique using telescopes operating in mm-band. At the moment, one can use a small relativistic correction approach for stellar orbit analysis, however, in the future the approximation will not be precise enough due to enormous progress of observational facilities and recently the GRAVITY team found that the first post-Newtonian correction has to be taken into account for the gravitational redshift in the S2 star orbit case. Meanwhile for smallest structure analysis in VLBI observations one really needs a strong gravitational field approximation. We discuss results of observations and their interpretations. In spite of great efforts there is a very slow progress to resolve dark matter (DM) and dark energy (DE) puzzles and in these circumstances in last years a number of alternative theories of gravity have been proposed. Parameters of these theories could be effectively constrained with of observations of the Galactic Center. We show some cases of alternative theories of gravity where their parameters are constrained with observations, in particular, we consider massive theory of gravity. We choose the alternative theory of gravity since there is a significant activity in this field and in the last years theorists demonstrated an opportunity to create such theories without ghosts, on the other hand, recently, the joint LIGO & Virgo team presented an upper limit on graviton mass such as mg< 1:2 × 10-22eV [1] analyzing gravitational wave signal in their first paper where they reported about the discovery of gravitational waves from binary black holes as it was suggested by C. Will [2]. So, the authors concluded that their observational data do not indicate a significant deviation from classical general relativity. We show that an analysis of bright star trajectories could estimate a graviton mass with a commensurable accuracy in comparison with an approach used in gravitational wave observations and the estimates obtained with these two approaches are consistent. Therefore, such an analysis gives an opportunity to treat observations of bright stars near the Galactic Center as a useful tool to obtain constraints on the fundamental gravity law. We showed that in the future graviton mass estimates obtained with analysis of trajectories of bright stars would be better than current LIGO bounds on the value, therefore, based on a potential reconstruction at the Galactic Center we obtain bounds on a graviton mass and these bounds are comparable with LIGO constraints. Analyzing size of shadows around the supermassive black hole at the Galactic Center (or/and in the center of M87) one could constrain parameters of different alternative theories of gravity as well.

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ON GENERATING SOME KNOWN BLACK HOLE SOLUTIONS

Modern Physics Letters A ◽

10.1142/s0217732310031440 ◽

2010 ◽

Vol 25 (10) ◽

pp. 835-842 ◽

Cited By ~ 7

Author(s):

F. RAHAMAN ◽

MUBASHER JAMIL ◽

A. GHOSH ◽

K. CHAKRABORTY

Keyword(s):

General Relativity ◽

Black Hole ◽

Alternative Theories Of Gravity ◽

Dimensional Parameters ◽

Theories Of Gravity ◽

Alternative Theories ◽

Black Hole Solutions

In this paper, we have presented an algorithm to generate various black hole solutions in general relativity and alternative theories of gravity. The algorithm involves few dimensional parameters that are assigned suitable values to specify the required black hole.

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On the black holes in alternative theories of gravity: The case of nonlinear massive gravity

International Journal of Modern Physics D ◽

10.1142/s0218271815500224 ◽

2015 ◽

Vol 24 (03) ◽

pp. 1550022 ◽

Cited By ~ 16

Author(s):

Ivan Arraut

Keyword(s):

Black Holes ◽

Black Hole ◽

Degrees Of Freedom ◽

Massive Gravity ◽

Alternative Theories Of Gravity ◽

De Sitter ◽

Theories Of Gravity ◽

Bound Orbits ◽

Black Hole Temperature ◽

Alternative Theories

I derive general conditions in order to explain the origin of the Vainshtein radius inside dRGT. The set of equations, which I have called "Vainshtein" conditions are extremal conditions of the dynamical metric (gμν) containing all the degrees of freedom of the theory. The Vainshtein conditions are able to explain the coincidence between the Vainshtein radius in dRGT and the scale [Formula: see text], obtained naturally from the Schwarzschild de-Sitter (S-dS) space inside general relativity (GR). In GR, this scale was interpreted as the maximum distance in order to get bound orbits. The same scale corresponds to the static observer position if we want to define the black hole temperature in an asymptotically de-Sitter space. In dRGT, the scale marks a limit after which the extra degrees of freedom of the theory become relevant.

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An exploration of the black hole entropy in Gauss–Bonnet gravity via the Weyl tensor

Modern Physics Letters A ◽

10.1142/s0217732321501935 ◽

2021 ◽

pp. 2150193

Author(s):

Taha A. Malik ◽

Rafael Lopez-Mobilia

Keyword(s):

General Relativity ◽

Black Hole ◽

Weyl Tensor ◽

Black Hole Entropy ◽

Entropy Density ◽

Modified Theories Of Gravity ◽

Bonnet Gravity ◽

Gravitational Entropy ◽

Theories Of Gravity ◽

Almost All

Various proposals for gravitational entropy densities have been constructed from the Weyl tensor. In almost all cases, though, these studies have been restricted to general relativity, and little has been done in modified theories of gravity. However, in this paper, we investigate the simplest proposal for an entropy density constructed from the Weyl tensor in five-dimensional Gauss–Bonnet gravity and find that it fails to reproduce the expected entropy of a black hole.

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The black hole at the Galactic Center: Observations and models

International Journal of Modern Physics D ◽

10.1142/s0218271818410092 ◽

2018 ◽

Vol 27 (06) ◽

pp. 1841009 ◽

Cited By ~ 12

Author(s):

Alexander F. Zakharov

Keyword(s):

Black Hole ◽

Theoretical Analysis ◽

Observational Data ◽

Galactic Center ◽

Massive Gravity ◽

Alternative Theories Of Gravity ◽

Graviton Mass ◽

Vlbi Observations ◽

Theories Of Gravity ◽

Alternative Theories

One of the most interesting astronomical objects is the Galactic Center. It is a subject of intensive astronomical observations in different spectral bands in recent years. We concentrate our discussion on a theoretical analysis of observational data of bright stars in the IR-band obtained with large telescopes. We also discuss the importance of VLBI observations of bright structures which could characterize the shadow at the Galactic Center. If we adopt general relativity (GR), there are a number of theoretical models for the Galactic Center, such as a cluster of neutron stars, boson stars, neutrino balls, etc. Some of these models were rejected or the range of their parameters is significantly constrained with consequent observations and theoretical analysis. In recent years, a number of alternative theories of gravity have been proposed because there are dark matter (DM) and dark energy (DE) problems. An alternative theory of gravity may be considered as one possible solution for such problems. Some of these theories have black hole solutions, while other theories have no such solutions. There are attempts to describe the Galactic Center with alternative theories of gravity and in this case one can constrain parameters of such theories with observational data for the Galactic Center. In particular, theories of massive gravity are intensively developing and theorists have overcome pathologies presented in the initial versions of these theories. In theories of massive gravity, a graviton is massive in contrast with GR where a graviton is massless. Now these theories are considered as an alternative to GR. For example, the LIGO–Virgo collaboration obtained the graviton mass constraint of about [Formula: see text] eV in their first publication about the discovery of the first gravitational wave detection event that resulted of the merger of two massive black holes. Surprisingly, one could obtain a consistent and comparable constraint of graviton mass at a level around [Formula: see text][Formula: see text]eV from the analysis of observational data on the trajectory of the star S2 near the Galactic Center. Therefore, observations of bright stars with existing and forthcoming telescopes such as the European extremely large telescope (E-ELT) and the thirty meter telescope (TMT) are extremely useful for investigating the structure of the Galactic Center in the framework of GR, but these observations also give a tool to confirm, rule out or constrain alternative theories of gravity. As we noted earlier, VLBI observations with current and forthcoming global networks (like the Event Horizon Telescope) are used to check the hypothesis about the presence of a supermassive black hole at the Galactic Center.

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Scalarized black holes

Arabian Journal of Mathematics ◽

10.1007/s40065-021-00349-7 ◽

2021 ◽

Author(s):

Jose Luis Blázquez-Salcedo ◽

Burkhard Kleihaus ◽

Jutta Kunz

Keyword(s):

Black Holes ◽

General Relativity ◽

Black Hole ◽

Scalar Field ◽

Alternative Theories Of Gravity ◽

Gravitational Action ◽

Theories Of Gravity ◽

Bonnet Term ◽

Alternative Theories ◽

Black Hole Solutions

AbstractBlack holes represent outstanding astrophysical laboratories to test the strong gravity regime, since alternative theories of gravity may predict black hole solutions whose properties may differ distinctly from those of general relativity. When higher curvature terms are included in the gravitational action as, for instance, in the form of the Gauss–Bonnet term coupled to a scalar field, scalarized black holes result. Here we discuss several types of scalarized black holes and some of their properties.

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Testing the Anomalous Growth of the Black Hole Radius from AGN

Galaxies ◽

10.3390/galaxies6040107 ◽

2018 ◽

Vol 6 (4) ◽

pp. 107

Author(s):

Andrea Addazi ◽

Antonino Marciano ◽

Chenyang Qian

Keyword(s):

Black Hole ◽

Galactic Nuclei ◽

Phenomenological Approach ◽

Current Data ◽

Kerr Metric ◽

Alternative Theories Of Gravity ◽

X Rays ◽

Hole Radius ◽

Theories Of Gravity ◽

Anomalous Growth

We analyze constraints on the anomalous growth of the black hole radius or the black hole spin from the X-rays spectrum data of Active Galactic Nuclei (AGN) in NGC 5506. The anomalous growth of the mass or of the spin of a black hole may be unveiled within the framework of models of alternative gravity, including f ( R ) -gravity. Our phenomenological analysis is based on an effective parametrization for the black hole Kerr metric, which is inspired by the antievaporating solutions discovered by Nojiri and Odintsov. We find tight constraints on the parameter space of anomalous metrics. Intriguingly, we find that a more than secularly growing solution can better fit current data. Our result opens a pathway towards a new phenomenological approach for testing predictions of general relativity and alternative theories of gravity.

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