Universe

Editorial

5 pages, 184 KiB

Open AccessEditorial

Editorial to the Special Issue “Selected Papers from the 17th Russian Gravitational Conference—International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17)”

by Vladimir M. Mostepanenko, Alexei A. Starobinsky and Elena N. Velichko

Universe 2021, 7(8), 296; https://doi.org/10.3390/universe7080296 - 12 Aug 2021

Viewed by 1654

Abstract

This Special Issue consists of selected papers reflecting the plenary and sectional talks presented at the 17th Russian Gravitational Conference—International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17) [...] Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

Research

Jump to: Editorial, Review

15 pages, 308 KiB

Open AccessCommunication

Generalized Rest Mass and Dirac’s Monopole in 5D Theory and Cosmology

by Boris G. Aliyev

Universe 2021, 7(8), 295; https://doi.org/10.3390/universe7080295 - 11 Aug 2021

Cited by 1 | Viewed by 1529

Abstract

It is shown that the 5D geodetic equations and 5D Ricci identities give us a way to create a new viewpoint on some problems of modern physics, astrophysics, and cosmology. Specifically, the application of the 5D geodetic equations in (4+1) and (3+1+1) splintered [...] Read more.

It is shown that the 5D geodetic equations and 5D Ricci identities give us a way to create a new viewpoint on some problems of modern physics, astrophysics, and cosmology. Specifically, the application of the 5D geodetic equations in (4+1) and (3+1+1) splintered forms obtained with the help of the monad and dyad methods made it possible to introduce a new, effective generalized concept of the rest mass of the elementary particle. The latter leads one to novel connections between the general relativity and quantum field theories, and all that, including the (4+1) splitting of the 5D Ricci identities, brings about a better understanding of the magnetic monopole problem and the vital difference in the origins of the Maxwell equations and gives rise to surprising connections between them. The obtained results also provide new insight into the mechanism of the 4D universe’s expansion and its following acceleration. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

9 pages, 668 KiB

Open AccessCommunication

Quantization of Gravitationally Bound Systems

by Michael Fil’chenkov and Yuri Laptev

Universe 2021, 7(2), 30; https://doi.org/10.3390/universe7020030 - 30 Jan 2021

Cited by 1 | Viewed by 1882

Abstract

Some of the approaches to quantization in gravity theory concerning gravitationally bound systems are considered. Grades of quantization applicable to these systems have been classified in terms of quantum mechanics, quantum field theory, and quantum geometrodynamics. Energy levels for the graviatom, Lemaître’s atom, [...] Read more.

Some of the approaches to quantization in gravity theory concerning gravitationally bound systems are considered. Grades of quantization applicable to these systems have been classified in terms of quantum mechanics, quantum field theory, and quantum geometrodynamics. Energy levels for the graviatom, Lemaître’s atom, quantum gravitational collapse have been calculated, and relationships for the masses of bound system components, as well as Universe’s birth probabilities, are presented to exemplify the properties of gravitationally bound systems. Objects and processes in them have been analyzed to construct quantum models of compact astrophysical objects and the early Universe. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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10 pages, 290 KiB

Open AccessCommunication

Gravitation in Unified Scalar Field Theory

by Alexander A. Chernitskii

Universe 2021, 7(1), 11; https://doi.org/10.3390/universe7010011 - 9 Jan 2021

Cited by 6 | Viewed by 2054

Abstract

The scalar field of space-time film is considered as unified fundamental field. The field model under consideration is the space-time generalization of the model for a two-dimensional thin film. The force and metrical interactions between solitons are considered. These interactions correspond to the [...] Read more.

The scalar field of space-time film is considered as unified fundamental field. The field model under consideration is the space-time generalization of the model for a two-dimensional thin film. The force and metrical interactions between solitons are considered. These interactions correspond to the electromagnetic and gravitational interactions respectively. The metrical interaction and its correspondence to the gravitational one are considered in detail. The practical applications of this approach are briefly discussed. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

8 pages, 2551 KiB

Open AccessCommunication

Comparison of Gravitational and Light Frequency Shifts in Rubidium Atomic Clock

by Alexey Baranov, Sergey Ermak, Roman Lozov and Vladimir Semenov

Universe 2021, 7(1), 3; https://doi.org/10.3390/universe7010003 - 24 Dec 2020

Cited by 3 | Viewed by 2798

Abstract

The article presents the results of an experimental study of the external magnetic field orientation and magnitude influence on the rubidium atomic clock, simulating the influence of the geomagnetic field on the onboard rubidium atomic clock of navigation satellites. The tensor component value [...] Read more.

The article presents the results of an experimental study of the external magnetic field orientation and magnitude influence on the rubidium atomic clock, simulating the influence of the geomagnetic field on the onboard rubidium atomic clock of navigation satellites. The tensor component value of the atomic clock frequency light shift on the rubidium cell was obtained, and this value was

~

2 Hz. The comparability of the relative light shift

(~ 10^{- 9})

and the regular gravitational correction

(4 \times 10^{- 10})

to the frequency of the rubidium atomic clock was shown. The experimental results to determine the orientational shift influence on the rubidium atomic clock frequency were presented. A significant effect on the relative frequency instability of a rubidium atomic clock at a level of

10^{- 12}

(10^{- 13})

for rotating external magnetic field amplitudes of 1.5 A/m and 3 A/m was demonstrated. This magnitude corresponds to the geomagnetic field in the orbit of navigation satellites. The necessity of taking into account various factors (satellite orbit parameters and atomic clock characteristics) is substantiated for correct comparison of corrections to the rubidium onboard atomic clock frequency associated with the Earth’s gravitational field action and the satellite orientation in the geomagnetic field. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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9 pages, 721 KiB

Open AccessCommunication

The Solution of the Cosmological Constant Problem: The Cosmological Constant Exponential Decrease in the Super-Early Universe

by Ol’ga Babourova and Boris Frolov

Universe 2020, 6(12), 230; https://doi.org/10.3390/universe6120230 - 4 Dec 2020

Cited by 6 | Viewed by 1805

Abstract

The stage of a super-early (primordial) scale-invariant Universe is considered on the basis of the Poincaré–Weyl gauge theory of gravity in a Cartan–Weyl space-time. An approximate solution has been found that demonstrates an inflationary behavior of the scale factor and, at the same [...] Read more.

The stage of a super-early (primordial) scale-invariant Universe is considered on the basis of the Poincaré–Weyl gauge theory of gravity in a Cartan–Weyl space-time. An approximate solution has been found that demonstrates an inflationary behavior of the scale factor and, at the same time, a sharp exponential decrease in the effective cosmological constant from a huge value at the beginning of the Big Bang to an extremely small (but not zero) value in the modern era, which solves the well-known “cosmological constant problem.” Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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26 pages, 342 KiB

Open AccessArticle

Density Operator Approach to Turbulent Flows in Plasma and Atmospheric Fluids

by Konstantin G. Zloshchastiev

Universe 2020, 6(11), 216; https://doi.org/10.3390/universe6110216 - 20 Nov 2020

Cited by 3 | Viewed by 2110

Abstract

We formulate a statistical wave-mechanical approach to describe dissipation and instabilities in two-dimensional turbulent flows of magnetized plasmas and atmospheric fluids, such as drift and Rossby waves. This is made possible by the existence of Hilbert space, associated with the electric potential of [...] Read more.

We formulate a statistical wave-mechanical approach to describe dissipation and instabilities in two-dimensional turbulent flows of magnetized plasmas and atmospheric fluids, such as drift and Rossby waves. This is made possible by the existence of Hilbert space, associated with the electric potential of plasma or stream function of atmospheric fluid. We therefore regard such turbulent flows as macroscopic wave-mechanical phenomena, driven by the non-Hermitian Hamiltonian operator we derive, whose anti-Hermitian component is attributed to an effect of the environment. Introducing a wave-mechanical density operator for the statistical ensembles of waves, we formulate master equations and define observables: such as the enstrophy and energy of both the waves and zonal flow as statistical averages. We establish that our open system can generally follow two types of time evolution, depending on whether the environment hinders or assists the system’s stability and integrity. We also consider a phase-space formulation of the theory, including the geometrical-optic limit and beyond, and study the conservation laws of physical observables. It is thus shown that the approach predicts various mechanisms of energy and enstrophy exchange between drift waves and zonal flow, which were hitherto overlooked in models based on wave kinetic equations. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

22 pages, 3350 KiB

Open AccessArticle

The Line-of-Sight Analysis of Spatial Distribution of Galaxies in the COSMOS2015 Catalogue

by Maxim Nikonov, Mikhail Chekal, Stanislav Shirokov, Andrey Baryshev and Vladimir Gorokhov

Universe 2020, 6(11), 215; https://doi.org/10.3390/universe6110215 - 20 Nov 2020

Cited by 1 | Viewed by 2356

Abstract

New observations of high-redshift objects are crucial for the improvement of the standard

Λ

CDM cosmological model and our understanding of the Universe. One of the main directions of modern observational cosmology is the analysis of the large-scale structure of Universe, in particular, [...] Read more.

New observations of high-redshift objects are crucial for the improvement of the standard

Λ

CDM cosmological model and our understanding of the Universe. One of the main directions of modern observational cosmology is the analysis of the large-scale structure of Universe, in particular, in deep fields. We study the large-scale structure of the Universe along the line of sight using the latest version of the COSMOS2015 catalogue, which contains 518,404 high quality photometric redshifts of galaxies selected in the optical range of the COSMOS field (

2 \times 2

deg

^{2}

), with depth up to the redshift

z \sim 6

. We analyze large-scale fluctuations in the number of galaxies along the line of sight and provide an estimate of the average linear sizes of the self-correlating fluctuations (structures) in independent redshift bins of

Δ z = 0.1

along with the estimate of the standard deviation from homogeneity (the observed cosmic variance). We suggest a new method of the line-of-sight analysis based on previous works and formulate further prospects of method development. For the case of the theoretical form of approximation of homogeneity in the

Λ

CDM framework, the average standard deviation of detected structures from homogeneity is

σ_{mean}^{Λ CDM} = 0.09 \pm 0.02

, and the average characteristic size of structures is

R_{mean}^{Λ CDM} = 790 \pm 150

Mpc. For the case of the empirical approximation of homogeneity, the average standard deviation of detected structures from homogeneity is

σ_{mean}^{empiric} = 0.08 \pm 0.01

, and the average characteristic size of structures is

R_{mean}^{empiric} = 640 \pm 140

Mpc. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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9 pages, 233 KiB

Open AccessCommunication

On the Collision of Relativistic Shock Waves and the Large Scale Structure of the Universe

by Alexander Golubiatnikov and Daniil Lyuboshits

Universe 2020, 6(11), 214; https://doi.org/10.3390/universe6110214 - 20 Nov 2020

Cited by 1 | Viewed by 1605

Abstract

The solution to the problem of symmetric collision of two relativistic shock waves is given and limiting cases are investigated: Newtonian mechanics and ultrarelativistic mechanics. The results are correlated with the presence of known superclusters and "walls" in the Universe. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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18 pages, 373 KiB

Open AccessArticle

Quantum Analysis of BTZ Black Hole Formation Due to the Collapse of a Dust Shell

by Alexander A. Andrianov, Artem Starodubtsev and Yasser Elmahalawy

Universe 2020, 6(11), 201; https://doi.org/10.3390/universe6110201 - 30 Oct 2020

Cited by 3 | Viewed by 2361

Abstract

We perform Hamiltonian reduction of a model in which 2 + 1 dimensional gravity with negative cosmological constant is coupled to a cylindrically symmetric dust shell. The resulting action contains only a finite number of degrees of freedom. The phase space consists of [...] Read more.

We perform Hamiltonian reduction of a model in which 2 + 1 dimensional gravity with negative cosmological constant is coupled to a cylindrically symmetric dust shell. The resulting action contains only a finite number of degrees of freedom. The phase space consists of two copies of

A D S^{2}

—both coordinate and momentum space are curved. Different regions in the Penrose diagram can be identified with different patches of

A D S^{2}

momentum space. Quantization in the momentum representation becomes particularly simple in the vicinity of the horizon, where one can neglect momentum non-commutativity. In this region, we calculate the spectrum of the shell radius. This spectrum turns out to be continuous outside the horizon and becomes discrete inside the horizon with eigenvalue spacing proportional to the square root of the black hole mass. We also calculate numerically quantum transition amplitudes between different regions of the Penrose diagram in the vicinity of the horizon. This calculation shows a possibility of quantum tunneling of the shell into classically forbidden regions of the Penrose diagram, although with an exponentially damped rate away from the horizon. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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23 pages, 861 KiB

Open AccessArticle

Exact and Slow-Roll Solutions for Exponential Power-Law Inflation Connected with Modified Gravity and Observational Constraints

by Igor Fomin and Sergey Chervon

Universe 2020, 6(11), 199; https://doi.org/10.3390/universe6110199 - 29 Oct 2020

Cited by 13 | Viewed by 3125

Abstract

We investigate the ability of the exponential power-law inflation to be a phenomenologically correct model of the early universe. We study General Relativity (GR) scalar cosmology equations in Ivanov–Salopek–Bond (or Hamilton–Jacobi like) representation where the Hubble parameter H is the function of a [...] Read more.

We investigate the ability of the exponential power-law inflation to be a phenomenologically correct model of the early universe. We study General Relativity (GR) scalar cosmology equations in Ivanov–Salopek–Bond (or Hamilton–Jacobi like) representation where the Hubble parameter H is the function of a scalar field

ϕ

. Such approach admits calculation of the potential for given

H (ϕ)

and consequently reconstruction of

f (R)

gravity in parametric form. By this manner the Starobinsky potential and non-minimal Higgs potential (and consequently the corresponding

f (R)

gravity) were reconstructed using constraints on the model’s parameters. We also consider methods for generalising the obtained solutions to the case of chiral cosmological models and scalar-tensor gravity. Models based on the quadratic relationship between the Hubble parameter and the function of the non-minimal interaction of the scalar field and curvature are also considered. Comparison to observation (PLANCK 2018) data shows that all models under consideration give correct values for the scalar spectral index and tensor-to-scalar ratio under a wide range of exponential-power-law model’s parameters. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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16 pages, 333 KiB

Open AccessArticle

Black Holes and Complexity via Constructible Universe

by Jerzy Król and Paweł Klimasara

Universe 2020, 6(11), 198; https://doi.org/10.3390/universe6110198 - 27 Oct 2020

Cited by 5 | Viewed by 2345

Abstract

The relation of randomness and classical algorithmic computational complexity is a vast and deep subject by itself. However, already, 1-randomness sequences call for quantum mechanics in their realization. Thus, we propose to approach black hole’s quantum computational complexity by classical computational classes and [...] Read more.

The relation of randomness and classical algorithmic computational complexity is a vast and deep subject by itself. However, already, 1-randomness sequences call for quantum mechanics in their realization. Thus, we propose to approach black hole’s quantum computational complexity by classical computational classes and randomness classes. The model of a general black hole is proposed based on formal tools from Zermelo–Fraenkel set theory like random forcing or minimal countable constructible model

L_{α}

. The Bekenstein–Hawking proportionality rule is shown to hold up to a multiplicative constant. Higher degrees of randomness and algorithmic computational complexity are derived in the model. Directions for further studies are also formulated. The model is designed for exploring deep quantum regime of spacetime. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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21 pages, 1464 KiB

Open AccessArticle

Qualitative Analysis of the Dynamics of a Two-Component Chiral Cosmological Model

by Viktor Zhuravlev and Sergey Chervon

Universe 2020, 6(11), 195; https://doi.org/10.3390/universe6110195 - 24 Oct 2020

Cited by 8 | Viewed by 2107

Abstract

We present a qualitative analysis of chiral cosmological model (CCM) dynamics with two scalar fields in the spatially flat Friedman–Robertson–Walker Universe. The asymptotic behavior of chiral models is investigated based on the characteristics of the critical points of the selfinteraction potential and zeros [...] Read more.

We present a qualitative analysis of chiral cosmological model (CCM) dynamics with two scalar fields in the spatially flat Friedman–Robertson–Walker Universe. The asymptotic behavior of chiral models is investigated based on the characteristics of the critical points of the selfinteraction potential and zeros of the metric components of the chiral space. The classification of critical points of CCMs is proposed. The role of zeros of the metric components of the chiral space in the asymptotic dynamics is analysed. It is shown that such zeros lead to new critical points of the corresponding dynamical systems. Examples of models with different types of zeros of metric components are represented. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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14 pages, 436 KiB

Open AccessArticle

On a Crucial Role of Gravity in the Formation of Elementary Particles

by Ahmed Alharthy and Vladimir V. Kassandrov

Universe 2020, 6(11), 193; https://doi.org/10.3390/universe6110193 - 23 Oct 2020

Cited by 8 | Viewed by 5348

Abstract

We consider the model of minimally interacting electromagnetic, gravitational and massive scalar fields free of any additional nonlinearities. In the dimensionless form, the Lagranginan contains only one parameter

γ = {(m \sqrt{G} / e)}^{2}

which corresponds to the ratio of [...] Read more.

We consider the model of minimally interacting electromagnetic, gravitational and massive scalar fields free of any additional nonlinearities. In the dimensionless form, the Lagranginan contains only one parameter

γ = {(m \sqrt{G} / e)}^{2}

which corresponds to the ratio of gravitational and electromagnetic interactions and, for a typical elementary particle, is about

10^{- 40}

in value. However, regular (soliton-like) solutions can exist only for

γ \neq 0

, so that gravity would be necessary to form the structure of an (extended) elementary particle. Unfortunately (in the stationary spherically symmetrical case), the numerical procedure breaks in the range

γ \leq 0.9

so that whether the particle-like solutions actually exist in the model remains unclear. Nonetheless, for

γ \sim 1

we obtain, making use of the minimal energy requirement, a discrete set of (horizon-free) electrically charged regular solutions of the Planck’s range mass and dimensions (“maximons”, “planckeons”, etc.). In the limit

γ \to \infty

, the model reduces to the well-known coupled system of the Einstein and Klein–Gordon equations. We obtain—to our knowledge—for the first time, the discrete spectrum of neutral soliton-like solutions (“mini-boson stars”, “soliton stars”, etc.) Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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10 pages, 262 KiB

Open AccessCommunication

Is the Axionic Dark Matter an Equilibrium System?

by Alexander B. Balakin and Amir F. Shakirzyanov

Universe 2020, 6(11), 192; https://doi.org/10.3390/universe6110192 - 22 Oct 2020

Cited by 15 | Viewed by 1747

Abstract

We consider an axionic dark matter model with a modified periodic potential for the pseudoscalar field in the framework of the axionic extension of the Einstein-aether theory. The modified potential is assumed to be equipped by the guiding function, which depends on the [...] Read more.

We consider an axionic dark matter model with a modified periodic potential for the pseudoscalar field in the framework of the axionic extension of the Einstein-aether theory. The modified potential is assumed to be equipped by the guiding function, which depends on the expansion scalar constructed as the trace of the covariant derivative of the aether velocity four-vector. The equilibrium state of the axion field is defined as the state, for which the modified potential itself and its first derivative with respect to the pseudoscalar field are equal to zero. We apply the developed formalism to the homogeneous isotropic cosmological model, and find the basic function, which describes the equilibrium state of the axionic dark matter in the expanding Universe. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

20 pages, 13075 KiB

Open AccessArticle

Is OJ 287 a Single Supermassive Black Hole?

by Marina S. Butuzova and Alexander B. Pushkarev

Universe 2020, 6(11), 191; https://doi.org/10.3390/universe6110191 - 22 Oct 2020

Cited by 21 | Viewed by 3151

Abstract

Light curves for more than century optical photometric observations of the blazar OJ 287 reveals strong flares with a quasi-period of about 12 years. For a long time, this period has been interpreted by processes in a binary black hole system. We propose [...] Read more.

Light curves for more than century optical photometric observations of the blazar OJ 287 reveals strong flares with a quasi-period of about 12 years. For a long time, this period has been interpreted by processes in a binary black hole system. We propose an alternative explanation for this period, which is based on Doppler factor periodic variations of the emitting region caused by jet helicity. Using multi-epoch very large baseline interferometry (VLBI) observations carried out in a framework of the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) program and other VLBA (Very Long Baseline Array) archival experiments at the observing frequency of 15 GHz, we derived geometrical parameters of the jet helix. To reach an agreement between the VLBI and photometric optical observation data, the jet component motion at a small angle to the radial direction is necessary. Such non-radial motion is observed and, together with the jet helical shape, can be naturally explained by the development of the Kelvin–Helmholtz instability in the parsec-scale outflow. In this case, the true precession of the OJ 287 jet may manifest itself in differences between the peak flux values of the 12-year optical flares. A possibility to create this precession due to Lense–Thirring effect of a single supermassive black hole is also discussed. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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11 pages, 352 KiB

Open AccessCommunication

Resonant Effect for Breit–Wheeler Process in the Field of an X-ray Pulsar

by Vitalii D. Serov, Sergei P. Roshchupkin and Victor V. Dubov

Universe 2020, 6(11), 190; https://doi.org/10.3390/universe6110190 - 22 Oct 2020

Cited by 6 | Viewed by 2315

Abstract

The resonant process of the creation of an ultrarelativistic electron–positron pair by two hard gamma quanta in the field of an X-ray pulsar (the Breit–Wheeler process modified by an external field) was theoretically studied. Under resonance conditions, the intermediate virtual electron (positron) in [...] Read more.

The resonant process of the creation of an ultrarelativistic electron–positron pair by two hard gamma quanta in the field of an X-ray pulsar (the Breit–Wheeler process modified by an external field) was theoretically studied. Under resonance conditions, the intermediate virtual electron (positron) in the external field becomes a real particle. As a result, there are four reaction channels for the process instead of two. For each of those channels, the initial process of the second order in the fine structure constant in the field of an X-ray pulsar effectively reduces into two successive processes of the first order: X-ray-stimulated Breit–Wheeler process and X-ray-stimulated Compton effect. The resonant kinematics of the process was also studied in detail. The process had characteristic threshold energy, and all initial and final particles had to be ultrarelativistic and propagate in a narrow cone. Furthermore, the resonant energy spectrum of the electron-positron pair significantly depended on emission angles. Clearly, there was a qualitative difference between resonant and nonresonant cases. Lastly, the resonant differential probability of studied process was obtained. The resonant differential probability significantly exceeded the nonresonant one without the external field of an X-ray pulsar. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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7 pages, 227 KiB

Open AccessCommunication

Gravitational Radiation as the Bremsstrahlung of Superheavy Particles in the Early Universe

by Andrey A. Grib and Yuri V. Pavlov

Universe 2020, 6(10), 188; https://doi.org/10.3390/universe6100188 - 20 Oct 2020

Cited by 2 | Viewed by 1647

Abstract

The number of superheavy particles with the mass of the Grand Unification scale with trans-Planckian energy created at the epoch of superheavy particle creation from the vacuum by the gravitation of the expanding Universe is calculated. In later collisions of these particles, gravitational [...] Read more.

The number of superheavy particles with the mass of the Grand Unification scale with trans-Planckian energy created at the epoch of superheavy particle creation from the vacuum by the gravitation of the expanding Universe is calculated. In later collisions of these particles, gravitational radiation is radiated playing the role of bremsstrahlung for gravity. The effective background radiation of the Universe is evaluated. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

11 pages, 445 KiB

Open AccessArticle

Collapsing Wormholes Sustained by Dustlike Matter

by Pavel E. Kashargin and Sergey V. Sushkov

Universe 2020, 6(10), 186; https://doi.org/10.3390/universe6100186 - 18 Oct 2020

Cited by 7 | Viewed by 2267

Abstract

It is well known that static wormhole configurations in general relativity (GR) are possible only if matter threading the wormhole throat is “exotic”—i.e., violates a number of energy conditions. For this reason, it is impossible to construct static wormholes supported only by dust-like [...] Read more.

It is well known that static wormhole configurations in general relativity (GR) are possible only if matter threading the wormhole throat is “exotic”—i.e., violates a number of energy conditions. For this reason, it is impossible to construct static wormholes supported only by dust-like matter which satisfies all usual energy conditions. However, this is not the case for non-static configurations. In 1934, Tolman found a general solution describing the evolution of a spherical dust shell in GR. In this particular case, Tolman’s solution describes the collapsing dust ball; the inner space-time structure of the ball corresponds to the Friedmann universe filled by a dust. In the present work we use the general Tolman’s solution in order to construct a dynamic spherically symmetric wormhole solution in GR with dust-like matter. The solution constructed represents the collapsing dust ball with the inner wormhole space-time structure. It is worth noting that, with the dust-like matter, the ball is made of satisfies the usual energy conditions and cannot prevent the collapse. We discuss in detail the properties of the collapsing dust wormhole. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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16 pages, 341 KiB

Open AccessArticle

On the Discrete Version of the Schwarzschild Problem

by Vladimir Khatsymovsky

Universe 2020, 6(10), 185; https://doi.org/10.3390/universe6100185 - 17 Oct 2020

Cited by 5 | Viewed by 2046

Abstract

We consider a Schwarzschild type solution in the discrete Regge calculus formulation of general relativity quantized within the path integral approach. Earlier, we found a mechanism of a loose fixation of the background scale of Regge lengths. This elementary length scale is defined [...] Read more.

We consider a Schwarzschild type solution in the discrete Regge calculus formulation of general relativity quantized within the path integral approach. Earlier, we found a mechanism of a loose fixation of the background scale of Regge lengths. This elementary length scale is defined by the Planck scale and some free parameter of such a quantum extension of the theory. Besides, Regge action was reduced to an expansion over metric variations between the tetrahedra and, in the main approximation, is a finite-difference form of the Hilbert–Einstein action. Using for the Schwarzschild problem a priori general non-spherically symmetrical ansatz, we get finite-difference equations for its discrete version. This defines a solution which at large distances is close to the continuum Schwarzschild geometry, and the metric and effective curvature at the center are cut off at the elementary length scale. Slow rotation can also be taken into account (Lense–Thirring-like metric). Thus, we get a general approach to the classical background in the quantum framework in zero order: it is an optimal starting point for the perturbative expansion of the theory, finite-difference equations are classical, and the elementary length scale has quantum origin. Singularities, if any, are resolved. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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16 pages, 350 KiB

Open AccessArticle

Gravitational Interaction of Cosmic String with Spinless Particle

by Pavel Spirin

Universe 2020, 6(10), 184; https://doi.org/10.3390/universe6100184 - 16 Oct 2020

Cited by 3 | Viewed by 1966

Abstract

We consider the gravitational interaction of spinless relativistic particle and infinitely thin cosmic string within the classical linearized-theory framework. We compute the particle’s motion in the transverse (to the unperturbed string) plane. The reciprocal action of the particle on the cosmic string is [...] Read more.

We consider the gravitational interaction of spinless relativistic particle and infinitely thin cosmic string within the classical linearized-theory framework. We compute the particle’s motion in the transverse (to the unperturbed string) plane. The reciprocal action of the particle on the cosmic string is also investigated. We derive the retarded solution which includes the longitudinal (with respect to the unperturbed-particle motion) and totally-transverse string perturbations. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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15 pages, 677 KiB

Open AccessArticle

Null and Timelike Geodesics near the Throats of Phantom Scalar Field Wormholes

by Ivan Potashov, Julia Tchemarina and Alexander Tsirulev

Universe 2020, 6(10), 183; https://doi.org/10.3390/universe6100183 - 16 Oct 2020

Cited by 8 | Viewed by 2162

Abstract

We study geodesic motion near the throats of asymptotically flat, static, spherically symmetric traversable wormholes supported by a self-gravitating minimally coupled phantom scalar field with an arbitrary self-interaction potential. We assume that any such wormhole possesses the reflection symmetry with respect to the [...] Read more.

We study geodesic motion near the throats of asymptotically flat, static, spherically symmetric traversable wormholes supported by a self-gravitating minimally coupled phantom scalar field with an arbitrary self-interaction potential. We assume that any such wormhole possesses the reflection symmetry with respect to the throat, and consider only its observable “right half”. It turns out that the main features of bound orbits and photon trajectories close to the throats of such wormholes are very different from those near the horizons of black holes. We distinguish between wormholes of two types, the first and second ones, depending on whether the redshift metric function has a minimum or maximum at the throat. First, it turns out that orbits located near the centre of a wormhole of any type exhibit retrograde precession, that is, the angle of pericentre precession is negative. Second, in the case of high accretion activity, wormholes of the first type have the innermost stable circular orbit at the throat while those of the second type have the resting-state stable circular orbit in which test particles are at rest at all times. In our study, we have in mind the possibility that the strongly gravitating objects in the centres of galaxies are wormholes, which can be regarded as an alternative to black holes, and the scalar field can be regarded as a realistic model of dark matter surrounding galactic centres. In this connection, we discuss qualitatively some observational aspects of results obtained in this article. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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25 pages, 421 KiB

Open AccessArticle

An Alternative to Dark Matter and Dark Energy: Scale-Dependent Gravity in Superfluid Vacuum Theory

by Konstantin G. Zloshchastiev

Universe 2020, 6(10), 180; https://doi.org/10.3390/universe6100180 - 15 Oct 2020

Cited by 16 | Viewed by 3088

Abstract

We derive an effective gravitational potential, induced by the quantum wavefunction of a physical vacuum of a self-gravitating configuration, while the vacuum itself is viewed as the superfluid described by the logarithmic quantum wave equation. We determine that gravity has a multiple-scale pattern, [...] Read more.

We derive an effective gravitational potential, induced by the quantum wavefunction of a physical vacuum of a self-gravitating configuration, while the vacuum itself is viewed as the superfluid described by the logarithmic quantum wave equation. We determine that gravity has a multiple-scale pattern, to such an extent that one can distinguish sub-Newtonian, Newtonian, galactic, extragalactic and cosmological terms. The last of these dominates at the largest length scale of the model, where superfluid vacuum induces an asymptotically Friedmann–Lemaître–Robertson–Walker-type spacetime, which provides an explanation for the accelerating expansion of the Universe. The model describes different types of expansion mechanisms, which could explain the discrepancy between measurements of the Hubble constant using different methods. On a galactic scale, our model explains the non-Keplerian behaviour of galactic rotation curves, and also why their profiles can vary depending on the galaxy. It also makes a number of predictions about the behaviour of gravity at larger galactic and extragalactic scales. We demonstrate how the behaviour of rotation curves varies with distance from a gravitating center, growing from an inner galactic scale towards a metagalactic scale: A squared orbital velocity’s profile crosses over from Keplerian to flat, and then to non-flat. The asymptotic non-flat regime is thus expected to be seen in the outer regions of large spiral galaxies. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

17 pages, 354 KiB

Open AccessArticle

Matter Accretion Versus Semiclassical Bounce in Schwarzschild Interior

by Kirill Bronnikov, Sergey Bolokhov and Milena Skvortsova

Universe 2020, 6(10), 178; https://doi.org/10.3390/universe6100178 - 14 Oct 2020

Cited by 7 | Viewed by 1930

Abstract

We discuss the properties of the previously constructed model of a Schwarzschild black hole interior where the singularity is replaced by a regular bounce, ultimately leading to a white hole. We assume that the black hole is young enough so that the Hawking [...] Read more.

We discuss the properties of the previously constructed model of a Schwarzschild black hole interior where the singularity is replaced by a regular bounce, ultimately leading to a white hole. We assume that the black hole is young enough so that the Hawking radiation may be neglected. The model is semiclassical in nature and uses as a source of gravity the effective stress-energy tensor (SET) corresponding to vacuum polarization of quantum fields, and the minimum spherical radius is a few orders of magnitude larger than the Planck length, so that the effects of quantum gravity should still be negligible. We estimate the other quantum contributions to the effective SET, caused by a nontrivial topology of spatial sections and particle production from vacuum due to a nonstationary gravitational field and show that these contributions are negligibly small as compared to the SET due to vacuum polarization. The same is shown for such classical phenomena as accretion of different kinds of matter to the black hole and its further motion to the would-be singularity. Thus, in a clear sense, our model of a semiclassical bounce instead of a Schwarzschild singularity is stable under both quantum and classical perturbations. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

17 pages, 864 KiB

Open AccessArticle

Relativistic Effects in Orbital Motion of the S-Stars at the Galactic Center

by Rustam Gainutdinov and Yurij Baryshev

Universe 2020, 6(10), 177; https://doi.org/10.3390/universe6100177 - 14 Oct 2020

Cited by 8 | Viewed by 2399

Abstract

The Galactic Center star cluster, known as S-stars, is a perfect source of relativistic phenomena observations. The stars are located in the strong field of relativistic compact object Sgr A* and are moving with very high velocities at pericenters of their orbits. In [...] Read more.

The Galactic Center star cluster, known as S-stars, is a perfect source of relativistic phenomena observations. The stars are located in the strong field of relativistic compact object Sgr A* and are moving with very high velocities at pericenters of their orbits. In this work we consider motion of several S-stars by using the Parameterized Post-Newtonian (PPN) formalism of General Relativity (GR) and Post-Newtonian (PN) equations of motion of the Feynman’s quantum-field gravity theory, where the positive energy density of the gravity field can be measured via the relativistic pericenter shift. The PPN parameters

β

and

γ

are constrained using the S-stars data. The positive value of the

T_{g}^{00}

component of the gravity energy–momentum tensor is confirmed for condition of S-stars motion. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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5 pages, 262 KiB

Open AccessCommunication

Energy Conservation Law in the Closed Universe and a Concept of the Proper Time

by Natalia Gorobey, Alexander Lukyanenko and Pavel Drozdov

Universe 2020, 6(10), 174; https://doi.org/10.3390/universe6100174 - 12 Oct 2020

Cited by 2 | Viewed by 2064

Abstract

To define time in the homogeneous anisotropic Bianchi-IX model of the universe, we propose a classical equation of motion of the proper time of the universe as an additional gauge condition. This equation is the law of conservation of energy. As a result, [...] Read more.

To define time in the homogeneous anisotropic Bianchi-IX model of the universe, we propose a classical equation of motion of the proper time of the universe as an additional gauge condition. This equation is the law of conservation of energy. As a result, a new parameter, called a “mass” of the universe, appears. This parameter is added to the anisotropy energy and regarded as an observed quantity. The “mass” of the universe is decisive when it comes to the dynamics of its origin. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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18 pages, 358 KiB

Open AccessArticle

Energy–Momentum Pseudotensor and Superpotential for Generally Covariant Theories of Gravity of General Form

by Roman Ilin and Sergey Paston

Universe 2020, 6(10), 173; https://doi.org/10.3390/universe6100173 - 11 Oct 2020

Cited by 3 | Viewed by 2052

Abstract

The current paper is devoted to the investigation of the general form of the energy–momentum pseudotensor (pEMT) and the corresponding superpotential for the wide class of theories. The only requirement for such a theory is the general covariance of the action without any [...] Read more.

The current paper is devoted to the investigation of the general form of the energy–momentum pseudotensor (pEMT) and the corresponding superpotential for the wide class of theories. The only requirement for such a theory is the general covariance of the action without any restrictions on the order of derivatives of the independent variables in it or their transformation laws. As a result of the generalized Noether procedure, we obtain a recurrent chain of the equations, which allows one to express canonical pEMT as a divergence of the superpotential. The explicit expression for this superpotential is also given. We discuss the structure of the obtained expressions and the conditions for the derived pEMT conservation laws to be satisfied independently (fully or partially) by the equations of motion. Deformations of the superpotential form for theories with a change in the independent variables in action are also considered. We apply these results to some interesting particular cases: general relativity and its modifications, particularly mimetic gravity and Regge–Teitelboim embedding gravity. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

19 pages, 508 KiB

Open AccessArticle

Hybrid Metric-Palatini Gravity: Regular Stringlike Configurations

by Kirill Bronnikov, Sergey Bolokhov and Milena Skvortsova

Universe 2020, 6(10), 172; https://doi.org/10.3390/universe6100172 - 11 Oct 2020

Cited by 7 | Viewed by 2573

Abstract

We discuss static, cylindrically symmetric vacuum solutions of hybrid metric-Palatini gravity (HMPG), a recently proposed theory that has been shown to successfully pass the local observational tests and produce a certain progress in cosmology. We use HMPG in its well-known scalar-tensor representation. The [...] Read more.

We discuss static, cylindrically symmetric vacuum solutions of hybrid metric-Palatini gravity (HMPG), a recently proposed theory that has been shown to successfully pass the local observational tests and produce a certain progress in cosmology. We use HMPG in its well-known scalar-tensor representation. The latter coincides with general relativity containing, as a source of gravity, a conformally coupled scalar field

ϕ

and a self-interaction potential

V (ϕ)

. The

ϕ

field can be canonical or phantom, and, accordingly, the theory splits into canonical and phantom sectors. We seek solitonic (stringlike) vacuum solutions of HMPG, that is, completely regular solutions with Minkowski metric far from the symmetry axis, with a possible angular deficit. A transition of the theory to the Einstein conformal frame is used as a tool, and many of the results apply to the general Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories as well as

f (R)

theories of gravity. One of these results is a one-to-one correspondence between stringlike solutions in the Einstein and Jordan frames if the conformal factor that connects them is everywhere regular. An algorithm for the construction of stringlike solutions in HMPG and scalar-tensor theories is suggested, and some examples of such solutions are obtained and discussed. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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6 pages, 250 KiB

Open AccessCommunication

How Extra Symmetries Affect Solutions in General Relativity

by Aroonkumar Beesham and Fisokuhle Makhanya

Universe 2020, 6(10), 170; https://doi.org/10.3390/universe6100170 - 9 Oct 2020

Cited by 4 | Viewed by 1987

Abstract

To get exact solutions to Einstein’s field equations in general relativity, one has to impose some symmetry requirements. Otherwise, the equations are too difficult to solve. However, sometimes, the imposition of too much extra symmetry can cause the problem to become somewhat trivial. [...] Read more.

To get exact solutions to Einstein’s field equations in general relativity, one has to impose some symmetry requirements. Otherwise, the equations are too difficult to solve. However, sometimes, the imposition of too much extra symmetry can cause the problem to become somewhat trivial. As a typical example to illustrate this, the effects of conharmonic flatness are studied and applied to Friedmann–Lemaitre–Robertson–Walker spacetime. Hence, we need to impose some symmetry to make the problem tractable, but not too much so as to make it too simple. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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Graphical abstract

13 pages, 814 KiB

Open AccessCommunication

On the Energy of a Non-Singular Black Hole Solution Satisfying the Weak Energy Condition

by Irina Radinschi, Theophanes Grammenos, Farook Rahaman, Marius-Mihai Cazacu, Andromahi Spanou and Joydeep Chakraborty

Universe 2020, 6(10), 169; https://doi.org/10.3390/universe6100169 - 7 Oct 2020

Cited by 5 | Viewed by 1956

Abstract

The energy-momentum localization for a new four-dimensional and spherically symmetric, charged black hole solution that through a coupling of general relativity with non-linear electrodynamics is everywhere non-singular while it satisfies the weak energy condition, is investigated. The Einstein and Møller energy-momentum complexes have [...] Read more.

The energy-momentum localization for a new four-dimensional and spherically symmetric, charged black hole solution that through a coupling of general relativity with non-linear electrodynamics is everywhere non-singular while it satisfies the weak energy condition, is investigated. The Einstein and Møller energy-momentum complexes have been employed in order to calculate the energy distribution and the momenta for the aforesaid solution. It is found that the energy distribution depends explicitly on the mass and the charge of the black hole, on two parameters arising from the space-time geometry considered, and on the radial coordinate. Further, in both prescriptions all the momenta vanish. In addition, a comparison of the results obtained by the two energy-momentum complexes is made, whereby some limiting and particular cases are pointed out. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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10 pages, 275 KiB

Open AccessCommunication

Universal Constants and Natural Systems of Units in a Spacetime of Arbitrary Dimension

by Anton Sheykin and Sergey Manida

Universe 2020, 6(10), 166; https://doi.org/10.3390/universe6100166 - 1 Oct 2020

Cited by 4 | Viewed by 2215

Abstract

We study the properties of fundamental physical constants using the threefold classification of dimensional constants proposed by J.-M. Lévy-Leblond: constants of objects (masses, etc.), constants of phenomena (coupling constants), and “universal constants” (such as c and ℏ). We show that all of [...] Read more.

We study the properties of fundamental physical constants using the threefold classification of dimensional constants proposed by J.-M. Lévy-Leblond: constants of objects (masses, etc.), constants of phenomena (coupling constants), and “universal constants” (such as c and ℏ). We show that all of the known “natural” systems of units contain at least one non-universal constant. We discuss the possible consequences of such non-universality, e.g., the dependence of some of these systems on the number of spatial dimensions. In the search for a “fully universal” system of units, we propose a set of constants that consists of c, ℏ, and a length parameter and discuss its origins and the connection to the possible kinematic groups discovered by Lévy-Leblond and Bacry. Finally, we give some comments about the interpretation of these constants. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

12 pages, 549 KiB

Open AccessCommunication

Resonant Production of an Ultrarelativistic Electron–Positron Pair at the Gamma Quantum Scattering by a Field of the X-ray Pulsar

by Vadim A. Yelatontsev, Sergei P. Roshchupkin and Viktor V. Dubov

Universe 2020, 6(10), 164; https://doi.org/10.3390/universe6100164 - 1 Oct 2020

Cited by 3 | Viewed by 2234

Abstract

The process of a resonant production of an ultrarelativistic electron–positron pair in the process of gamma-quantum scattering in the X-ray field of a pulsar is theoretically studied. This process has two reaction channels. Under resonant conditions, an intermediate electron (for a channel A) [...] Read more.

The process of a resonant production of an ultrarelativistic electron–positron pair in the process of gamma-quantum scattering in the X-ray field of a pulsar is theoretically studied. This process has two reaction channels. Under resonant conditions, an intermediate electron (for a channel A) or a positron (for a channel B) enters the mass shell. As a result, the initial second-order process of the fine-structure constant in the X-ray field effectively splits into two first-order processes: the X-ray field-stimulated Breit–Wheeler process and the the X-ray field-stimulated Compton effect on an intermediate electron or a positron. The resonant kinematics of the process is studied in detail. It is shown that for the initial gamma quantum there is a threshold energy, which for the X-ray photon energy (1–

10^{2}

) keV has the order of magnitude (

10^{3}

–10) MeV. In this case, all the final particles (electron, positron, and final gamma quantum) fly in a narrow cone along the direction of the initial gamma quantum momentum. It is important to note that the energies of the electron–positron pair and the final gamma quantum depend significantly on their outgoing angles. The obtained resonant probability significantly exceeds the non-resonant one. The obtained results can be used to explain the spectrum of positrons near pulsars. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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18 pages, 357 KiB

Open AccessFeature PaperArticle

Non-Relativistic Limit of Embedding Gravity as General Relativity with Dark Matter

by Sergey Paston

Universe 2020, 6(10), 163; https://doi.org/10.3390/universe6100163 - 29 Sep 2020

Cited by 17 | Viewed by 2149

Abstract

Regge-Teitelboim embedding gravity is the modified gravity based on a simple string-inspired geometrical principle—our spacetime is considered here as a 4-dimensional surface in a flat bulk. This theory is similar to the recently popular theory of mimetic gravity—the modification of gravity appears in [...] Read more.

Regge-Teitelboim embedding gravity is the modified gravity based on a simple string-inspired geometrical principle—our spacetime is considered here as a 4-dimensional surface in a flat bulk. This theory is similar to the recently popular theory of mimetic gravity—the modification of gravity appears in both theories as a result of the change of variables in the action of General Relativity. Embedding gravity, as well as mimetic gravity, can be used in explaining the dark matter mystery since, in both cases, the modified theory can be presented as General Relativity with additional fictitious matter (embedding matter or mimetic matter). For the general case, we obtain the equations of motion of embedding matter in terms of embedding function as a set of first-order dynamical equations and constraints consistent with them. Then, we construct a non-relativistic limit of these equations, in which the motion of embedding matter turns out to be slow enough so that it can play the role of cold dark matter. The non-relativistic embedding matter turns out to have a certain self-interaction, which could be useful in the context of solving the core-cusp problem that appears in the

Λ

-Cold Dark Matter (

Λ

CDM) model. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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16 pages, 354 KiB

Open AccessArticle

Formation and Clustering of Primordial Black Holes in Brans-Dicke Theory

by Victor Berezin, Vyacheslav Dokuchaev, Yury Eroshenko and Alexey Smirnov

Universe 2020, 6(10), 158; https://doi.org/10.3390/universe6100158 - 24 Sep 2020

Cited by 3 | Viewed by 1921

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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12 pages, 253 KiB

Open AccessCommunication

The Negative Energy in Generalized Vaidya Spacetime

by Vitalii Vertogradov

Universe 2020, 6(9), 155; https://doi.org/10.3390/universe6090155 - 22 Sep 2020

Cited by 9 | Viewed by 1908

Abstract

In this paper we consider the negative energy problem in generalized Vaidya spacetime. We consider several models where we have the naked singularity as a result of the gravitational collapse. In these models we investigate the geodesics for particles with negative energy when [...] Read more.

In this paper we consider the negative energy problem in generalized Vaidya spacetime. We consider several models where we have the naked singularity as a result of the gravitational collapse. In these models we investigate the geodesics for particles with negative energy when the II type of the matter field satisfies the equation of the state

P = α ρ

(

α \in [0, 1]

). Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

10 pages, 287 KiB

Open AccessCommunication

Spinors in Cylindrically Symmetric Space–Time

by Bijan Saha

Universe 2020, 6(9), 152; https://doi.org/10.3390/universe6090152 - 15 Sep 2020

Cited by 5 | Viewed by 2002

Abstract

We studied the behavior of nonlinear spinor field within the scope of a static cylindrically symmetric space–time. It is found that the energy-momentum tensor (EMT) of the spinor field in this case possesses nontrivial non-diagonal components. The presence of non-diagonal components of the [...] Read more.

We studied the behavior of nonlinear spinor field within the scope of a static cylindrically symmetric space–time. It is found that the energy-momentum tensor (EMT) of the spinor field in this case possesses nontrivial non-diagonal components. The presence of non-diagonal components of the EMT imposes three-way restrictions either on the space–time geometry or on the components of the spinor field or on both. It should be noted that the analogical situation occurs in cosmology when the nonlinear spinor field is exploited as a source of gravitational field given by the Bianchi type-I cosmological model. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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20 pages, 691 KiB

Open AccessArticle

The State of the Art in Constraining Axion-to-Nucleon Coupling and Non-Newtonian Gravity from Laboratory Experiments

by Vladimir M. Mostepanenko and Galina L. Klimchitskaya

Universe 2020, 6(9), 147; https://doi.org/10.3390/universe6090147 - 8 Sep 2020

Cited by 20 | Viewed by 2386

Abstract

Constraints on the Yukawa-type corrections to Newton’s gravitational law and on the coupling constant of axionlike particles to nucleons obtained from different laboratory experiments are reviewed and compared. The constraints on non-Newtonian gravity under discussion cover the wide interaction range from nanometers to [...] Read more.

Constraints on the Yukawa-type corrections to Newton’s gravitational law and on the coupling constant of axionlike particles to nucleons obtained from different laboratory experiments are reviewed and compared. The constraints on non-Newtonian gravity under discussion cover the wide interaction range from nanometers to millimeters and follow from the experiments on neutron scattering, measuring the Casimir force and Cavendish-type experiments. The constraints on the axion-to-nucleon coupling constant following from the magnetometer measurements, Cavendish-type experiments, Casimir physics, and experiments with beams of molecular hydrogen are considered, which refer to the region of axion masses from

10^{- 10}

to 200 eV. Particular attention is given to the recent constraints obtained from measuring the Casimir force at nanometer separation distance between the test bodies. Several proposed experiments focussed on constraining the non-Newtonian gravity, axionlike particles and other hypothetical weakly interacting particles, such as chameleons and symmetrons, are discussed. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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17 pages, 577 KiB

Open AccessArticle

Gravitational Interaction in a Null String Gas and Its Possible Consequences

by Alexander Lelyakov

Universe 2020, 6(9), 142; https://doi.org/10.3390/universe6090142 - 2 Sep 2020

Cited by 4 | Viewed by 4839

Abstract

In this paper, we investigated the possible consequences of gravitational interaction in a gas of null strings. The structural elements of this gas are closed null strings in the form of a circle (thin closed tubes of massless scalar field). A possibility to [...] Read more.

In this paper, we investigated the possible consequences of gravitational interaction in a gas of null strings. The structural elements of this gas are closed null strings in the form of a circle (thin closed tubes of massless scalar field). A possibility to qualitatively take into account the mutual influence on motion for gravitationally interacting null strings is proposed. It is shown that the result of gravitational interaction is the self-consistent motion of two null strings inside a space-limited region. Such systems of gravitationally interacting null strings can be considered as primary particles in a gas of null strings with an effective nonzero rest mass. It is noted that the “lifetime” of such particles should depend on external conditions. Long-term existence (“lifetime”) of primary particles is possible if they are combined into more complex structures. The possibility of such a union depends on the motion direction of the null strings forming the particles. The most interesting is the possibility of combining primary particles into spherically symmetric formations—“macro” objects. A feature of such “macro” formations in a gas of null strings is the fundamental impossibility to have finally formed structure. In a gas of null strings, processes leading to a random (dynamic) change in the number of null strings gravitationally belonging to a “macro” object are inevitable. By averaging over time the various spatial distributions of the “macro” formations, the concepts of “substance” and “interaction field” can be introduced in a gas of null strings. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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14 pages, 523 KiB

Open AccessArticle

Resonant Effects in a Photoproduction of Ultrarelativistic Electron-Positron Pairs on a Nucleus in the Field of the X-ray Pulsar

by Nikita R. Larin, Sergei P. Roshchupkin and Victor V. Dubov

Universe 2020, 6(9), 141; https://doi.org/10.3390/universe6090141 - 2 Sep 2020

Cited by 7 | Viewed by 1950

Abstract

The resonant photoproduction of the electron-positron pairs on a nucleus near a surface of the X-ray pulsar was studied theoretically. The main feature of the processes, which are responsible for the formation of the electron-positron fluxes, is a capability to occur in a [...] Read more.

The resonant photoproduction of the electron-positron pairs on a nucleus near a surface of the X-ray pulsar was studied theoretically. The main feature of the processes, which are responsible for the formation of the electron-positron fluxes, is a capability to occur in a resonant way in the electromagnetic field of the X-ray pulsar. One of the properties of the resonant case is that the initial process of second order in the fine structure constant in an external field effectively reduces into two successive processes of the first order due to the fact that in the resonant conditions intermediate virtual electron (positron) becomes a real particle. It is shown that the resonances are possible only when the initial gamma quantum energy is more than the threshold energy, which significantly depends on the number of absorbed photons of an external electromagnetic field. Additionally, in the resonant conditions, the energies of the particles depend on the outgoing angle of a positron (channel A) or an electron (channel B). It is shown that the resonant differential cross section has an extremely large magnitude in units

α Z^{2} r_{e}^{2}

. A mechanism to explain the presence of anomalous fluxes of ultrarelativistic positrons near the surface of an X-ray pulsar was proposed. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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8 pages, 458 KiB

Open AccessCommunication

The Resonant Effect of an Annihilation Channel in the Interaction of the Ultrarelativistic Electron and Positron in the Field of an X-ray Pulsar

by Dmitriy V. Doroshenko, Sergei P. Roshchupkin and Victor V. Dubov

Universe 2020, 6(9), 137; https://doi.org/10.3390/universe6090137 - 28 Aug 2020

Cited by 4 | Viewed by 2131

Abstract

We investigated the effects that occur during the circulation of ultrarelativistic electrons and positrons in the field of an X-ray pulsar. A resonant process in annihilation and the subsequent production of the electron–positron pairs were studied theoretically. Under the resonance, the second-order process [...] Read more.

We investigated the effects that occur during the circulation of ultrarelativistic electrons and positrons in the field of an X-ray pulsar. A resonant process in annihilation and the subsequent production of the electron–positron pairs were studied theoretically. Under the resonance, the second-order process in an original fine-structure constant process effectively decays to two first order processes of the fine-structure constant: single-photon annihilation of the electron–positron pair stimulated by the external field, and the Breit–Wheeler process (single-photon birth of the electron–positron pair) stimulated by the external field. We show that resonance has a threshold energy for a certain combinational energy of the initial electron and positron. Furthermore, there is a definite small angle between initial ultrarelativistic particles’ momenta, in which resonance takes place. Initial and final electron–positron pairs fly in a narrow cone. We noticed that electron (positron) emission angle defines the energy of the final pair. We show that the resonant cross-section in the field of the X-ray pulsar may significantly exceed the corresponding cross-section without the field (Bhabha cross-section). Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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13 pages, 3655 KiB

Open AccessCommunication

Resonant Ultrarelativistic Electron–Positron Pair Production by High-Energy Electrons in the Field of an X-ray Pulsar

by Georgii K. Sizykh, Sergei P. Roshchupkin and Victor V. Dubov

Universe 2020, 6(9), 132; https://doi.org/10.3390/universe6090132 - 22 Aug 2020

Cited by 5 | Viewed by 2371

Abstract

The process of resonant high-energy electron–positron pair production by an ultrarelativistic electron colliding with the field of an X-ray pulsar is theoretically investigated. Resonant kinematics of the process is studied in detail. Under the resonance condition, the intermediate virtual photon in the X-ray [...] Read more.

The process of resonant high-energy electron–positron pair production by an ultrarelativistic electron colliding with the field of an X-ray pulsar is theoretically investigated. Resonant kinematics of the process is studied in detail. Under the resonance condition, the intermediate virtual photon in the X-ray pulsar field becomes a real particle. As a result, the initial process of the second order in the fine structure constant effectively reduces into two successive processes of the first order: X-ray-stimulated Compton effect and X-ray-stimulated Breit–Wheeler process. For a high-energy initial electron all the final ultrarelativistic particles propagate in a narrow cone along the direction of the initial electron momentum. The presence of threshold energy for the initial electron which is of order of 100 MeV for 1-KeV-frequency field is shown. At the same time, the energy spectrum of the final particles (two electrons and a positron) highly depends on their exit angles and on the initial electron energy. This result significantly distinguishes the resonant process from the non-resonant one. It is shown that the resonant differential probability significantly exceeds the non-resonant one. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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Graphical abstract

12 pages, 663 KiB

Open AccessCommunication

Radiation from an Inertial Mirror Horizon

by Michael Good and Ernazar Abdikamalov

Universe 2020, 6(9), 131; https://doi.org/10.3390/universe6090131 - 20 Aug 2020

Cited by 11 | Viewed by 2364

Abstract

The purpose of this study is to investigate radiation from asymptotic zero acceleration motion where a horizon is formed and subsequently detected by an outside witness. A perfectly reflecting moving mirror is used to model such a system and compute the energy and [...] Read more.

The purpose of this study is to investigate radiation from asymptotic zero acceleration motion where a horizon is formed and subsequently detected by an outside witness. A perfectly reflecting moving mirror is used to model such a system and compute the energy and spectrum. The trajectory is asymptotically inertial (zero proper acceleration)—ensuring negative energy flux (NEF), yet approaches light-speed with a null ray horizon at a finite advanced time. We compute the spectrum and energy analytically. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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20 pages, 286 KiB

Open AccessArticle

Is the Copenhagen Interpretation Inapplicable to Quantum Cosmology?

by Tatyana P. Shestakova

Universe 2020, 6(9), 128; https://doi.org/10.3390/universe6090128 - 19 Aug 2020

Cited by 5 | Viewed by 2410

Abstract

It is generally accepted that the Copenhagen interpretation is inapplicable to quantum cosmology, by contrast with the many worlds interpretation. I shall demonstrate that the two basic principles of the Copenhagen interpretation, the principle of wholeness and the principle of complementarity, do make [...] Read more.

It is generally accepted that the Copenhagen interpretation is inapplicable to quantum cosmology, by contrast with the many worlds interpretation. I shall demonstrate that the two basic principles of the Copenhagen interpretation, the principle of wholeness and the principle of complementarity, do make sense in quantum gravity, since we can judge about quantum gravitational processes in the very early Universe by their vestiges in our macroscopic Universe. I shall present the extended phase space approach to quantum gravity and show that it can be interpreted in the spirit of the Everett’s “relative states” formulation, while there is no contradiction between the “relative states” formulation and the mentioned basic principles of the Copenhagen interpretation. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

15 pages, 1024 KiB

Open AccessArticle

Quark Stars in Massive Brans–Dicke Gravity with Tolman–Kuchowicz Spacetime

by Amal Majid and M. Sharif

Universe 2020, 6(8), 124; https://doi.org/10.3390/universe6080124 - 13 Aug 2020

Cited by 31 | Viewed by 2431

Abstract

In this paper, we construct anisotropic model representing salient features of strange stars in the framework of massive Brans–Dicke gravity. We formulate the field equations for Tolman–Kuchowicz ansatz by incorporating the MIT bag model. Junction conditions are applied on the boundary of the [...] Read more.

In this paper, we construct anisotropic model representing salient features of strange stars in the framework of massive Brans–Dicke gravity. We formulate the field equations for Tolman–Kuchowicz ansatz by incorporating the MIT bag model. Junction conditions are applied on the boundary of the stellar model to evaluate the unknown constants in terms of mass and radius of the star. The radius of the strange star candidate PSR J1614-2230 is predicted by assuming maximum anisotropy at the surface of the star for different values of the coupling parameter, mass of the scalar field and bag constant. We examine various properties as well as the viability and stability of the anisotropic sphere. We conclude that the astrophysical model agrees with the essential criteria of a physically realistic model for higher values of the coupling parameter as well as mass of the scalar field. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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6 pages, 270 KiB

Open AccessCommunication

Possible Wormhole Candidates in Active Galactic Nuclei

by Mikhail Piotrovich, Serguei Krasnikov, Stanislava Buliga and Tinatin Natsvlishvili

Universe 2020, 6(8), 120; https://doi.org/10.3390/universe6080120 - 11 Aug 2020

Cited by 3 | Viewed by 1910

Abstract

The hypothesis is considered that the active galactic nuclei (AGNs) are wormhole (WH) mouths rather than supermassive black holes (SMBHs). We study the difference in the physical properties of such objects from those of AGNs with SMBH, as well as the the corresponding [...] Read more.

The hypothesis is considered that the active galactic nuclei (AGNs) are wormhole (WH) mouths rather than supermassive black holes (SMBHs). We study the difference in the physical properties of such objects from those of AGNs with SMBH, as well as the the corresponding difference in observational data. Firstly, the radiative efficiency for some types of WHs (both static and rotating) can be significantly larger than the theoretical maximal value for the Kerr SMBHs. A number of AGNs is presented, for which the observational data can be interpreted as the result of the presence of WHs in them. Secondly, a sufficiently strong gamma radiation with a characteristic spectrum noticeably differing from that of AGNs jets, can be emitted from a static WH as a result of a collision of accreting flows of matter inside the WH. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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9 pages, 295 KiB

Open AccessArticle

Intensity and Polarization of Radiation Emerging from Rotating Accretion Disc with Increasing Height

by Nikolai Silant’ev, Galina Alekseeva, Yulia Ananjevskaja and Viktor Novikov

Universe 2020, 6(8), 117; https://doi.org/10.3390/universe6080117 - 10 Aug 2020

Cited by 2 | Viewed by 1888

Abstract

We consider the radiation emission in continuum and spectral lines from rotating accretion disc with the progressive increasing height. It is known that for the plane accretion disc with homogeneous atmosphere the wave electric field

E

is perpendicular to the plane between line [...] Read more.

We consider the radiation emission in continuum and spectral lines from rotating accretion disc with the progressive increasing height. It is known that for the plane accretion disc with homogeneous atmosphere the wave electric field

E

is perpendicular to the plane between line of sight

n

and the normal to the disc

N

. For the expanding accretion disc the wave electric field from every inclined cone-like part with given azimuthal angle

φ

has direction perpendicular to the plane between

n

and local normal

N^{'}

to the inclined surface. This behaviour is the consequence of homogeneity of inclined atmosphere and has purely geometrical origin. The geometrical consideration shows that the position angles of polarized radiation in the right and left parts relative to plane (nN) of the inclined accretion disc have opposite values. Therefore, for inclined accretion disc the integral continuum radiation has the usual polarization angle perpendicular to the plane

(nN)

, but smaller degree of polarization and less alongated along the normal

N

than that for the plane accretion disc. For spectral line, due to the Doppler effect, the polarization (position) angles have opposite signs for the red and blue wings. Such behaviour is frequently observed in H

α

-radiation. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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19 pages, 894 KiB

Open AccessArticle

Guessing the Riddle of a Black Hole

by Boris E. Meierovich

Universe 2020, 6(8), 113; https://doi.org/10.3390/universe6080113 - 7 Aug 2020

Cited by 6 | Viewed by 2561

Abstract

A static structure of matter, extremely compressed to the state of a Bose–Einstein condensate by its own gravitational field, is considered. Instead of the widely spread restriction

det g_{i k} < 0

, I used a weaker condition of regularity: all invariants [...] Read more.

A static structure of matter, extremely compressed to the state of a Bose–Einstein condensate by its own gravitational field, is considered. Instead of the widely spread restriction

det g_{i k} < 0

, I used a weaker condition of regularity: all invariants of

g_{i k}

are finite. This makes it possible to find regular static solutions to Einstein equations for a spherically symmetric distribution of matter with no restriction on total mass. In these regular static solutions, the metric component

g^{r r}

changes its sign twice:

g^{r r} (r) = 0

at

r = r_{g}

and at

r = r_{h} > r_{g}

. The signature of the metric tensor is changed to (+,+,−,−) within the spherical layer

r_{g} < r < r_{h}

. Though the gravitation dominates at extremely high density, I assume that it does not violate the exchange interaction of elementary particles of the Standard Model. The found regular static solution to Einstein equations, having no limitation on mass, pretends to describe the state of a black hole to which the gravitational collapse leads. The features of a collapsed black hole, its internal composition depending on total mass and the relation with surrounding dark matter, are considered. An astrophysical application: The pressure balance at the interface between a black hole and dark matter determines the plateau velocity of a galaxy rotation curve as a function of the black hole mass. The plateau velocity is inversely proportional to the black hole mass. The speed of rotation of a star at the periphery of a galaxy is proportional to the square root of the black hole mass (direct attraction to the center) and inversely proportional to the mass of the same black hole (as the influence of dark matter). For a condensate of massive bosons in the Standard Model, the direct attraction to the black hole and the influence of dark matter are equal if the black hole mass is about

\tilde{M}

∼

4.24 \times 10^{37}

g. In galaxies with black hole masses

M ≳ M_{⊙} = 1.989 \times 10^{33}

g (like UMa: NGC 3726 and UMa: NGC 3769 of the Ursa Major cluster), the motion of stars is driven by dark matter. Their rotation curves should have a well-defined plateau. On the contrary, in galaxies with black hole masses

M > > \tilde{M}

(like in our Milky Way with the black hole mass

M = 8.6 \times 10^{39}

g), the motion of stars is regulated by the black hole in the center. Dark matter does not play a significant role in our Milky Way Galaxy. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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Review

Jump to: Editorial, Research

92 pages, 1446 KiB

Open AccessReview

Einstein’s Geometrical versus Feynman’s Quantum-Field Approaches to Gravity Physics: Testing by Modern Multimessenger Astronomy

by Yurij Baryshev

Universe 2020, 6(11), 212; https://doi.org/10.3390/universe6110212 - 18 Nov 2020

Cited by 14 | Viewed by 4521

Abstract

Modern multimessenger astronomy delivers unique opportunity for performing crucial observations that allow for testing the physics of the gravitational interaction. These tests include detection of gravitational waves by advanced LIGO-Virgo antennas, Event Horizon Telescope observations of central relativistic compact objects (RCO) in active [...] Read more.

Modern multimessenger astronomy delivers unique opportunity for performing crucial observations that allow for testing the physics of the gravitational interaction. These tests include detection of gravitational waves by advanced LIGO-Virgo antennas, Event Horizon Telescope observations of central relativistic compact objects (RCO) in active galactic nuclei (AGN), X-ray spectroscopic observations of Fe

K_{α}

line in AGN, Galactic X-ray sources measurement of masses and radiuses of neutron stars, quark stars, and other RCO. A very important task of observational cosmology is to perform large surveys of galactic distances independent on cosmological redshifts for testing the nature of the Hubble law and peculiar velocities. Forthcoming multimessenger astronomy, while using such facilities as advanced LIGO-Virgo, Event Horizon Telescope (EHT), ALMA, WALLABY, JWST, EUCLID, and THESEUS, can elucidate the relation between Einstein’s geometrical and Feynman’s quantum-field approaches to gravity physics and deliver a new possibilities for unification of gravitation with other fundamental quantum physical interactions. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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22 pages, 330 KiB

Open AccessReview

Some Issues on the Foundations of Modern Cosmology, Gravitation and Quantum Physics

by Emilio Elizalde

Universe 2020, 6(11), 189; https://doi.org/10.3390/universe6110189 - 22 Oct 2020

Cited by 3 | Viewed by 2595

Abstract

After the first clear evidence of the recession—at very high speeds—of the spiral nebulae was announced by V.M. Slipher in 1914, as a result of his work started in 1912, it still took several decades to properly understand the phenomenon in terms of [...] Read more.

After the first clear evidence of the recession—at very high speeds—of the spiral nebulae was announced by V.M. Slipher in 1914, as a result of his work started in 1912, it still took several decades to properly understand the phenomenon in terms of an expansion of the Universe. Some historical issues around that crucial discovery and the contemporary attempts at determining the scale of the visible Universe will be discussed. Presently, very important questions to answer are: What is the precise value of the expansion rate? What drives the acceleration of the Universe’s expansion? The latter is called dark energy, but what is it actually? The possibility that this could be the result of a sort of Casimir effect at the cosmological level has not been discarded, yet. One of the main technical problems in tackling this issue is constituted by the regularization and corresponding renormalization procedures. Beautiful but rather non-trivial mathematics, involving the zeta function of pseudodifferential operators (associated with physical quantities), are key in this respect. A discussion of those items is provided here. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

21 pages, 499 KiB

Open AccessReview

Gravity with Higher Derivatives in D-Dimensions

by Sergey G. Rubin, Arkadiy Popov and Polina M. Petriakova

Universe 2020, 6(10), 187; https://doi.org/10.3390/universe6100187 - 20 Oct 2020

Cited by 4 | Viewed by 2187

Abstract

The aim of this review is to discuss the ways to obtain results based on gravity with higher derivatives in D-dimensional world. We considered the following ways: (1) reduction to scalar tensor gravity, (2) direct solution of the equations of motion, (3) derivation [...] Read more.

The aim of this review is to discuss the ways to obtain results based on gravity with higher derivatives in D-dimensional world. We considered the following ways: (1) reduction to scalar tensor gravity, (2) direct solution of the equations of motion, (3) derivation of approximate equations in the presence of a small parameter in the system, and (4) the method of test functions. Some applications are presented to illustrate each method. The unification of two necessary elements of a future theory is also kept in mind—the extra dimensions and the extended form of the gravity. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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35 pages, 716 KiB

Open AccessReview

Quantum Vacuum Effects in Braneworlds on AdS Bulk

by Aram A. Saharian

Universe 2020, 6(10), 181; https://doi.org/10.3390/universe6100181 - 15 Oct 2020

Cited by 6 | Viewed by 2125

Abstract

We review the results of investigations for brane-induced effects on the local properties of quantum vacuum in background of AdS spacetime. Two geometries are considered: a brane parallel to the AdS boundary and a brane intersecting the AdS boundary. For both cases, the [...] Read more.

We review the results of investigations for brane-induced effects on the local properties of quantum vacuum in background of AdS spacetime. Two geometries are considered: a brane parallel to the AdS boundary and a brane intersecting the AdS boundary. For both cases, the contribution in the vacuum expectation value (VEV) of the energy–momentum tensor is separated explicitly and its behavior in various asymptotic regions of the parameters is studied. It is shown that the influence of the gravitational field on the local properties of the quantum vacuum is essential at distance from the brane larger than the AdS curvature radius. In the geometry with a brane parallel to the AdS boundary, the VEV of the energy–momentum tensor is considered for scalar field with the Robin boundary condition, for Dirac field with the bag boundary condition and for the electromagnetic field. In the latter case, two types of boundary conditions are discussed. The first one is a generalization of the perfect conductor boundary condition and the second one corresponds to the confining boundary condition used in QCD for gluons. For the geometry of a brane intersecting the AdS boundary, the case of a scalar field is considered. The corresponding energy–momentum tensor, apart from the diagonal components, has nonzero off-diagonal component. As a consequence of the latter, in addition to the normal component, the Casimir force acquires a component parallel to the brane. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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17 pages, 414 KiB

Open AccessReview

The Higgs Mechanism and Spacetime Symmetry

by Irina Dymnikova

Universe 2020, 6(10), 179; https://doi.org/10.3390/universe6100179 - 15 Oct 2020

Cited by 3 | Viewed by 2053

Abstract

In this review, we summarize the results of the analysis of the inherent relation between the Higgs mechanism and spacetime symmetry provided by generic incorporation of the de Sitter vacuum as a false vacuum with the equation of state

p = - ρ

[...] Read more.

In this review, we summarize the results of the analysis of the inherent relation between the Higgs mechanism and spacetime symmetry provided by generic incorporation of the de Sitter vacuum as a false vacuum with the equation of state

p = - ρ

. This relation has been verified by the application for the interpretation of the experimental results on the negative mass squares for neutrinos, and of the appearance of the minimal length in the annihilation reaction

e^{+} e^{-} \to γ γ (γ)

. An additional verification is expected for the dark matter candidates with the interior de Sitter vacuum of the GUT scale, whose predicted observational signatures include the induced proton decay in the matter of an underground detector, such as IceCUBE. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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27 pages, 2623 KiB

Open AccessReview

Visible Shapes of Black Holes M87* and SgrA*

by Vyacheslav I. Dokuchaev and Natalia O. Nazarova

Universe 2020, 6(9), 154; https://doi.org/10.3390/universe6090154 - 21 Sep 2020

Cited by 38 | Viewed by 4294

Abstract

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 [...] Read more.

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 ≳ 0.75

. Full article

(This article belongs to the Special Issue Selected Papers from the 17th Russian Gravitational Conference —International Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17))

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