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S. O. Alexeyev

Publications and source records attributed to S. O. Alexeyev.

16 recordsLinked to original sources
arXiv2020

Shadows from spinning black holes in extended gravity

When the the shadow image of a supermassive black hole in the center of M87 was obtained a new era in the observational astrophysics began. The resolution improvement would allow to test extended gravity models at event horizon scales. We develop the method allowing to take into account the black hole rotation in black hole shadow modelling. It is demonstrated that the shadow of a black hole with a diameter smaller than $4R_{Sw}$ ($R_{Sw} = 2M$) cannot be represented in the terms of pure Kerr-Newman metric. Therefore, the detection of a shadow of such size would indicate that further expansions must be taken into account. This fact could give new indications on the detailed form of the theory of gravity at the considered scales.}

gr-qc↗
arXiv2019

Post-Newtonian limit of hybrid metric-Palatini f(R)-gravity

Using the latest most accurate values of post-Newtonian parameters $γ$ and $β$ obtained by MESSENGER we impose restrictions on the recently proposed hybrid f(R)-gravity model in its scalar-tensor representation. We show that the presence of a light scalar field in this theory does not contradict the experimental data based not only on the $ γ$ parameter (as was shown earlier), but also on all other PPN parameters. The application of parameterized post-Newtonian formalism to gravitational theories with massive fields is also discussed.

gr-qc↗
arXiv2018

Phenomenological Extension for Tidal Charge Black Hole

A simple phenomenological extension of the black hole solution with tidal charge is proposed. Empirical data on the Sgr A* is consistent with the suggested metric which serves as a generalisation of the Reissner-Nordstrom one. Such a generalisation includes the leading effects beyond general relativity so, the discussed metric can explain wider range of gravitational effects. We discuss physical features of an object described by the proposed metric, namely, the size of its shadow and the innermost stable circular orbit radius.

gr-qc↗
arXiv2018

Horndeski gravity without screening in binary pulsars

We test the subclasses of Horndeski gravity without Vainshtein mechanism in the strong field regime of binary pulsars. We find the rate of energy losses via the gravitational radiation predicted by such theories and compare our results with observational data from quasi-circular binaries PSR J1738+0333, PSR J0737-3039, PSR J1012+5307. In addition, we consider few specific cases: the hybrid metric-Palatini f(R)-gravity and massive Brans-Dicke theory.

astro-ph.HE↗
arXiv2017

Gravity Induced Non-Local Effects in the Standard Model

We show that the non-locality recently identified in quantum gravity using resummation techniques propagates to the matter sector of the theory. We describe these non-local effects using effective field theory techniques. We derive the complete set of non-local effective operators at order $N G^2$ for theories involving scalar, spinor, and vector fields. We then use recent data from the Large Hadron Collider to set a bound on the scale of space-time non-locality and find $M_\star> 3 \times 10^{-11}$ GeV.

hep-th↗
arXiv2016

Wormholes and Naked Singularities in Brans-Dicke cosmology

We perform analytical and numerical study of static spherically symmetric solutions in the context of Brans-Dicke-like cosmological model by Elizalde et al. with an exponential potential. In this model the phantom regime arises without the appearance of any ghost degree of freedom due to the specific form of coupling. For the certain parameter ranges the model contains a regular solution which we interpret as a wormhole in an otherwise dS Universe. We put several bounds on the parameter values: $ω<0 ,\,\, α^2/|ω|<10^{-5},22.7\lesssim\!ϕ_0\!\lesssim25\,$. The numerical solution could mimic the Schwarzschild one, so the original model is consistent with astrophysical and cosmological observational data. However differences between our solution and the Schwarzschild one can be quite large, so black hole candidate observations could probably place further limits on the $ϕ_0$ value.

gr-qc↗
arXiv2016

Observational Sequences of Randall-Sundrum Black Holes

We consider the spherically symmetric black hole solution of the Randall-Sundrum (RS) model. This asymptotically-Schwarzschild solution was found by Abdolrahimi, Cation, Page and Yaghoobpour-Tari (ACPY-solution). We investigate the specific properties of the accretion disk around ACPY black hole having astrophysical size and mass. The energy flux and temperature distribution are obtained and compared with the Schwarzschild ones. We also present the spectral energy distribution of the accretion disk around RS black holes, when the transfer function is taken into account. Thus we confirm a full agreement of the accretion rate in RS model with the GR predictions. Using recent data on the high-mass X-ray binary Cygnus X-1 and on the Galactic centre black hole candidate Sgr A* we model the accretion in the RS model.

gr-qc↗
arXiv2015

Maeda-Dadhich Solutions as Real Black Holes

A four-dimensional static Schwarzschild-like solution obtained in [3]-[6] in the frames of the Einstein-Gauss-Bonnet gravity at the Kaluza-Klein split is analyzed. The matter in these solutions is created by auxiliary dimensions. The main goal of our work is to study physically sensible characteristics, which could be observable. Study of the perturbed equations demonstrates their stability under linear perturbations. The specific combinations of the parameters, permitting to construct black hole-like objects with one or two horizons or naked singularities are determined. Stable orbits of test particles around these black holes are presented. We show the exotic thermodynamical properties of the solution, when the Hawking evaporation law has the behavior opposite to usual one in General Relativity.

gr-qc↗
arXiv2015

Observational Limits on Gauss-Bonnet and Randall-Sundrum Gravities

We discuss the possibilities of experimental search for new physics predicted by the Gauss-Bonnet and the Randall-Sundrum theories of gravity. The effective four-dimensional spherically-symmetrical solutions of these theories are analyzed. We consider these solutions in the weak-field limit and in the process of the primordial black holes evaporation. We show that the predictions of discussed models are the same as of General Relativity. So, current experiments are not applicable for such search therefore different methods of observation and higher accuracy are required.

gr-qc↗
arXiv2004

Gauss-Bonnet Black Holes at the LHC : Beyond the Dimensionality of Space

The Gauss - Bonnet invariant is one of the most promising candidates for a quadratic curvature correction to the Einstein action in expansions of supersymmetric string theory. We study the evaporation of such Schwarzschild - Gauss - Bonnet black holes which could be formed at future colliders if the Planck scale is of order a TeV, as predicted by some modern brane world models. We show that, beyond the dimensionality of space, the corresponding coupling constant could be measured by the LHC. This opens new windows for physics investigation in spite of the possible screening of microphysics due to the event horizon.

hep-ph↗
arXiv1999

Non-Singular Cosmological Models in String Gravity with Second Order Curvature Corrections

We investigate FRW cosmological solutions in the theory of modulus field coupled to gravity through a Gauss-Bonnet term. The explicit analytical forms of nonsingular asymptotics are presented for power-law and exponentially steep modulus coupling functions. We study the influence of modulus field potential on these asymptotical regimes and find some forms of the potential which do not destroy the nonsingular behavior. In particular, we obtain that exponentially steep coupling functions arising from the string theory do not allow nonsingular past asymptotic unless modulus field potential tends to zero for modulus field $ψ\to \pm \infty$. Finally, the modification of the chaotic dynamics in the closed FRW universe due to presence of the Gauss-Bonnet term is discussed.

gr-qc↗
arXiv1999

Black Holes of a Minimal Size in String Gravity

A lower limit for a neutral black hole size is obtained in the frames of the string gravity model with the second order curvature correction. It is shown that this effect remains when the third order curvature correction is also taken into account and argued that such restriction does exist in all perturbative orders of curvature expansions.

gr-qc↗
arXiv1997

Singular Regions in Black Hole Solutions in Higher Order Curvature Gravity

Four-dimensional black hole solutions generated by the low energy string effective action are investigated outside and inside the event horizon. A restriction for a minimal black hole size is obtained in the frame of the model discussed. Intersections, turning points and other singular points of the solution are investigated. It is shown that the position and the behavior of these particular points are definded by various kinds of zeros of the main system determinant. Some new aspects of the $r_s$ singularity are discussed.

gr-qc↗
arXiv1997

Internal Structure of a Gauss-Bonnet Black Hole

Black holes are studied in the frames of superstring theory using a non-trivial numerical integration method. A low energy string action containing graviton, dilaton, Gauss-Bonnet and Maxwell contributions is considered. Four-dimensional black hole solutions are studied inside and outside the event horizon. The internal part of the solutions is shown to have a non-trivial topology.

gr-qc↗
arXiv1996

Black hole solutions with dilatonic hair in higher curvature gravity

A new numerical integration method for examining a black hole structure was realized. Black hole solutions with dilatonic hair of 4D low energy effective SuperString Theory action with Gauss-Bonnet quadratic curvature contribution were studied, using this method, inside and outside the event horizon. Thermodynamical properties of this solution were also studied.

hep-th↗