Searcharxiv⌕ Search

arXiv subjects

V. I. Dokuchaev

Publications and source records attributed to V. I. Dokuchaev.

At least 19 recordsLinked to original sources
arXiv2024

Vaidya Spacetime in the Diagonal Coordinates

We have analyzed the transformation from initial coordinates $(v,r)$ of the Vaidya metric with light coordinate $v$ to the most physical diagonal coordinates $(t,r)$. An exact solution has been obtained for the corresponding metric tensor in the case of a linear dependence of the mass function of the Vaidya metric on light coordinate $v$. In the diagonal coordinates, a narrow region (with a width proportional to the mass growth rate of a black hole) has been detected near the visibility horizon of the Vaidya accreting black hole, in which the metric differs qualitatively from the Schwarzschild metric and cannot be represented as a small perturbation. It has been shown that, in this case, a single set of diagonal coordinates $(t,r)$ is insufficient to cover the entire range of initial coordinates $(v,r)$ outside the visibility horizon; at least three sets of diagonal coordinates are required, the domains of which are separated by singular surfaces on which the metric components have singularities (either $g_{00}=0$ or $g_{00}=\infty$.). The energy-momentum tensor diverges on these surfaces; however, the tidal forces turn out to be finite, which follows from an analysis of the deviation equations for geodesics. Therefore, these singular surfaces are exclusively coordinate singularities that can be referred to as false firewalls because there are no physical singularities on them. We have also considered the transformation from the initial coordinates to other diagonal coordinates $(η,y)$, in which the solution is obtained in explicit form, and there is no energy-momentum tensor divergence.

gr-qc↗
arXiv2022

Cosmological particle creation in Weyl geometry

We investigated the possibility of the homogeneous and isotropic cosmological solution in Weyl geometry, which differs from the Riemannian geometry by adding the so called Weyl vector. The Weyl gravity is obtained by constructing the gravitational Lagrangian both to be quadratic in curvatures and conformal invariant. It is found that such solution may exist provided there exists the direct interaction between the Weyl vector and the matter fields. Assuming the matter Lagrangian is that of the perfect fluid, we found how such an interaction can be implemented. Due to the existence of quadratic curvature terms and the direct interaction the perfect fluid particles may be created straight from the vacuum, and we found the expression for the rate of their production which appeared to be conformal invariant. In the case of creating the universe ``from nothing'' in the vacuum state, we investigated the problem, whether this vacuum may persist or not. It is shown that the vacuum may persist with respect to producing the non-dust matter (with positive pressure), but cannot resist to producing the dust particles. These particles, being non-interactive, may be considered as the candidates for dark matter.

gr-qc↗
arXiv2022

Weyl cosmology

The homogeneous and isotropic cosmological model in the Weyl conformal geometry is considered. We showed that, despite the conformal invariance, the dust matter is allowed in such a universe. It is shown that the number of dust particles is not conserved, i. e., they are continuously produced. The general form of the law for their creation is found.

gr-qc↗
arXiv2021

Visible shapes of black holes M87* and SgrA*

We review the physical origins for possible visible images of the supermassive black hole M87* in the galaxy M87 and SgrA* in the Milky Way Galaxy. The classical dark black hole shadow of the maximal size is visible in the case of luminous background behind the black hole at the distance exceeding the so-called photon spheres. The notably smaller dark shadow (dark silhouette) of the black hole event horizon is visible if the black hole is highlighted by the inner parts of the luminous accreting matter inside the photon spheres. The first image of the supermassive black hole M87 *, obtained by the Event Horizon Telescope collaboration, shows the lensed dark image of the southern hemisphere of the black hole event horizon globe, highlighted by accreting matter, while the classical black hole shadow is invisible at all. A size of the dark spot on the Event Horizon Telescope (EHT) image agrees with a corresponding size of the dark event horizon silhouette in a thin accretion disk model in the case of either the high or moderate value of the black hole spin, $a\gtrsim0.75$.

astro-ph.HE↗
arXiv2021

Double layer from least action principle

We derived the equations for the double layers in Quadratic Gravity, using solely the least action principle. The advantage of our approach is that, in the process of calculation, the $δ'$-function does not appear at all, and the $δ$-functions appear for a moment and are mutually canceled prior to integration. We revealed the peculiar structure of the obtained equations, namely, that the surface energy-momentum tensor of the matter fields (constituents of the thin shells) does not play a role in the determination of the trajectory of the double layer. Also, we suggested that the space-like double layers may provide us with the adequate description of the creation of the universe from the black hole singularity. The related topics, including the Gauss-Bonnet term and $F(R)$-theories, are shortly discussed.

gr-qc↗
arXiv2020

Formation and clustering of primordial black holes in Brans-Dicke theory

The formation of primordial black holes in the early universe in the Brans-Dicke scalar-tensor theory of gravity is investigated. Corrections to the threshold value of density perturbations are found. Above the threshold, the gravitational collapse occurs after the cosmological horizon crossing. The corrections depend in a certain way on the evolving scalar field. They affect the probability of primordial black holes formation, and can lead to their clustering at large scales if the scalar field is inhomogeneous. The formation of the clusters, in turn, increases the probability of black holes merge and the corresponding rate of gravitational wave bursts. The clusters can provide a significant contribution to the LIGO/Virgo gravitational wave events, if part of the observed events are associated with primordial black holes.

astro-ph.CO↗
arXiv2019

Possible Explanation of the Geograv Detector Signal during the Explosion of SN 1987A in Modified Gravity Models

A change in gravity law in some regimes is predicted in the modified gravity models that are actively discussed at present. In this paper, we consider a possibility that the signal recorded by the Geograv resonant gravitational-wave detector in 1987 during the explosion of SN 1987A was produced by an abrupt change in the metric during the passage of a strong neutrino flux through the detector. Such an impact on the detector is possible, in particular, in extended scalar-tensor theories in which the local matter density gradient affects the gravitational force. The first short neutrino pulse emitted at the initial stage of stellar core collapse before the onset of neutrino opacity could exert a major influence on the detector by exiting the detector response at the main resonance frequency. In contrast, the influence of the subsequent broad pulse (with a duration of several seconds) in the resonant detector is exponentially suppressed, despite the fact that the second pulse carries an order-of-magnitude more neutrino energy, and it could generate a signal in the LSD neutrino detector. This explains the time delay of 1.4s between the Geograv and LSD signals. The consequences of this effect of modified gravity for LIGO/Virgo observations are discussed.

astro-ph.HE↗
arXiv2019

Event horizon image within black hole shadow

We argue that a genuine image of the black hole viewed by a distant observer is not its shadow, but a more compact event horizon image probed by the luminous matter plunging into black hole. The external border of the black hole shadow is washed out by radiation from matter plunging into black hole and approaching the event horizon. This effect will crucially influence the results of future observations by the Event Horizon Telescope. We show that gravitational lensing of the luminous matter plunging into black hole provides the possibility for visualization of the event horizon. The lensed image of the event horizon is formed by the highly red-shifted photons emitted by the plunging matter very near the black hole event horizon and detected by a distant observer. The resulting event horizon image is a gravitationally lensed projection on the celestial sphere of the whole black hole event horizon sphere. Seemingly, black holes are the unique objects in the Universe which may be viewed by distant observers at once from both the front and back sides.

astro-ph.HE↗
arXiv2018

Gravitational Lensing of a star by a rotating black hole

The gravitational lensing of a finite star moving around a rotating Kerr black hole has been numerically calculated. Calculations for the direct image of the star and for the first and second light echoes have been performed for the star moving with an orbital period of 3.22 h around the supermassive black hole SgrA* at the Galactic Center. Time dependences for the observed star position on the celestial sphere, radiation flux from the star, frequency of detected radiation, major and minor semiaxes of the lensed star image have been calculated and plotted. The detailed observation of such lensing requires a space interferometer such as the Russian Millimetron project.

astro-ph.HE↗
arXiv2017

Conformal invariance and phenomenology of cosmological particle production

Starting with the idea to describe phenomenologically the particle creation in the strong gravitational fields, we introduced explicitly the particle number nonconservation (= creation law) into the action integral with the corresponding Lagrange multiplier. Following the fundamental result by Ya. B. Zel'dovich and A. A. Starobinsky (1977) we then postulated that the rate of particle creation is proportional to the square of Weyl tensor. Concerning the conformal invariance, yet another question arises: how the scalar field could know about the surgery made on the metric tensor (if such an invariance is the fundamental law of Nature and not just the mathematical exercise)? The only way is that the scalar field is itself the part of metric, namely, the conformal factor. We showed, that such an identification results in the natural appearance of the quartic self-interaction term in the scalar field Lagrangian, which is needed to make particle massive. And it is just quartic, because our space-time is four-dimensional.

gr-qc↗
arXiv2017

Destruction of axion miniclusters in the Galaxy

Previously, it has been established that axion dark matter (DM) is clustered to form clumps (axion miniclusters) with masses $M\sim10^{-12}M_\odot$. The passages of such clumps through the Earth are very rare events occurring once in $10^5$ years. It has also been shown that the Earth's passage through DM streams, which are the remnants of clumps destroyed by tidal gravitational forces from Galactic stars, is a much more probable event occurring once in several years. In this paper we have performed details calculations of the destruction of miniclusters by taking into account their distribution in orbits in the Galactic halo. We have investigated two DM halo models, the Navarro-Frenk-White and isothermal density profiles. Apart from the Galactic disk stars, we have also taken into account the halo and bulge stars. We show that about 2-5% of the axion miniclusters are destroyed when passing near stars and transform into axion streams, while the clump destruction efficiency depends on the DM halo model. The expected detection rate of streams with an overdensity exceeding an order of magnitude is 1-2 in 20 years. The possibility of detecting streams by their tidal gravitational effect on gravitational-wave interferometers is also considered.

astro-ph.GA↗
arXiv2017

Recurrent fast radio bursts from collisions of neutron stars in the evolved stellar clusters

We propose the model describing the observed multiple fast radio bursts due to the close encounters and collisions of neutron stars in the central clusters of the evolved galactic nuclei. The subsystem of neutron star cluster may originate in the dense galactic nucleus evolutionary in the combined processes of stellar and dynamical evolution. The neutron stars in the compact cluster can produce the short living binaries with the highly eccentric orbits, and finally collide after several orbital revolutions. Fast radio bursts may be produced during the close periastron approach and at the process of the final binary merging. In the sufficiently dense star cluster the neutron stars collisions can be very frequent. Therefore, this model can explain in principle the observed recurrent (multiple) fast radio bursts, analogous to the observed ones from the source FRB 121102. Among the possible observational signatures of the proposed model may be the registration of the gravitational wave bursts by the laser interferometers LIGO/VIRGO or by the next generation of gravitational wave detectors.

astro-ph.HE↗
arXiv2017

Cosmological Horizons as They Are Looked from a Moving Frame

We consider the cosmological horizons in the expanding universe from the point of view of observer moving with respect to CMB frame. The deformation (non-sphericity) of cosmological horizons is demonstrated. Some principle consequences are discussed.

gr-qc↗
arXiv2016

On maximal analytical extension of the Vaidya metric

The classical Vaidya metric is transformed to the special diagonal coordinates in the case of the linear mass function allowing rather easy treatment. We find the exact analytical expressions for metric functions in these diagonal coordinates. Using these coordinates, we elaborate the maximum analytic extension of the Vaidya metric with a linear growth of the black hole mass and construct the corresponding Carter-Penrose diagrams for different specific cases. The derived global geometry seemingly is valid also for a more general behavior of the black hole mass in the Vaidya metric.

gr-qc↗
arXiv2016

Spherically symmetric conformal gravity and "gravitational bubbles"

The general structure of the spherically symmetric solutions in the Weyl conformal gravity is described. The corresponding Bach equations are derived for the special type of metrics, which can be considered as the representative of the general class. The complete set of the pure vacuum solutions is found. It consists of two classes. The first one contains the solutions with constant two-dimensional curvature scalar of our specific metrics, and the representatives are the famous Robertson--Walker metrics. One of them we called the "gravitational bubbles", which is compact and with zero Weyl tensor. Thus, we obtained the pure vacuum curved space-times (without any material sources, including the cosmological constant) what is absolutely impossible in General Relativity. Such a phenomenon makes it easier to create the universe from "nothing". The second class consists of the solutions with varying curvature scalar. We found its representative as the one-parameter family. It appears that it can be conformally covered by the thee-parameter Mannheim--Kazanas solution. We also investigated the general structure of the energy-momentum tensor in the spherical conformal gravity and constructed the vectorial equation that reveals clearly some features of non-vacuum solutions. Two of them are explicitly written, namely, the metrics à la Vaidya, and the electrovacuum space-time metrics.

gr-qc↗
arXiv2015

Physical laboratory at the center of the Galaxy

We review the physical processes that occur at the center of the Galaxy and that are related to the supermassive black hole Sgr A* residing there. The discovery of high-velocity S0 stars orbiting Sgr A* for the first time allowed measuring the mass of this supermassive black hole, the closest one to us, with a 10\% accuracy, with the result $M_h=(4.1\pm0.4)\times 10^6M_\odot$. Further monitoring can potentially discover the Newtonian precession of the S0 star orbits in the gravitational field of the black hole due to invisible distributed matter. This will yield the "weight" of the elusive dark matter concentrated there and provide new information for the identification of dark matter particles. The weak accretion activity of the "dormant quasar" at the Galactic center occasionally shows up as quasiperiodic X-ray and near-IR oscillations with mean periods of $11$ and $19$ min. These oscillations can possibly be interpreted as related to the rotation frequency of the Sgr A* event horizon and to the latitude oscillations of hot plasma spots in the accretion disk. Both these frequencies depend only on the black hole gravitational field and not on the accretion model. Using this interpretation yields quite the accurate values for both the mass $M_h$ and the spin $a$ (Kerr rotation parameter) of Sgr A*: $M_h=(4.2\pm0.2)\times 10^6M_\odot$ and $a=0.65\pm0.05$.

astro-ph.HE↗
arXiv2015

Conformal gravity and "gravitational bubbles"

We describe the general structure of the spherically symmetric solutions in the Weyl conformal gravity. The corresponding Bach equations are derived for the special type of metrics, which can be considered as the representative of the general class. The complete set of the pure vacuum solutions, consisting of two classes, is found. The first one contains the solutions with constant two-dimensional curvature scalar, and the representatives are the famous Robertson--Walker metrics. We called one of them the "gravitational bubbles", which is compact and with zero Weyl tensor. These "gravitational bubbles" are the pure vacuum curved space-times (without any material sources, including the cosmological constant), which are absolutely impossible in General Relativity. This phenomenon makes it easier to create the universe from "nothing". The second class consists of the solutions with varying curvature scalar. We found its representative as the one-parameter family, which can be conformally covered by the thee-parameter Mannheim--Kazanas solution. We describe the general structure of the energy-momentum tensor in the spherical conformal gravity and construct the vectorial equation that reveals clearly some features of non-vacuum solutions.

gr-qc↗