Universe

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 1625

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))

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 2158

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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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 2212

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 1978

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 1932

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 2120

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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15 pages, 794 KiB

Open AccessArticle

Search for Double Beta Decay of ¹⁰⁶Cd with an Enriched ¹⁰⁶CdWO₄ Crystal Scintillator in Coincidence with CdWO₄ Scintillation Counters

by Pierluigi Belli, R. Bernabei, V.B. Brudanin, F. Cappella, V. Caracciolo, R. Cerulli, F. A. Danevich, Antonella Incicchitti, D.V. Kasperovych, V.R. Klavdiienko, V.V. Kobychev, Vittorio Merlo, O.G. Polischuk, V.I. Tretyak and M.M. Zarytskyy

Universe 2020, 6(10), 182; https://doi.org/10.3390/universe6100182 - 16 Oct 2020

Cited by 15 | Viewed by 2396

Abstract

Studies on double beta decay processes in

^{106}

Cd were performed by using a cadmium tungstate scintillator enriched in

^{106}

Cd at 66% (

^{106}

CdWO

_{4}

) with two CdWO

_{4}

scintillation counters (with natural Cd composition). No effect was observed in [...] Read more.

Studies on double beta decay processes in

^{106}

Cd were performed by using a cadmium tungstate scintillator enriched in

^{106}

Cd at 66% (

^{106}

CdWO

_{4}

) with two CdWO

_{4}

scintillation counters (with natural Cd composition). No effect was observed in the data that accumulated over 26,033 h. New improved half-life limits were set on the different channels and modes of the

^{106}

Cd double beta decay at level of

lim T_{1 / 2} \sim 10^{20} - 10^{22}

yr. The limit for the two neutrino electron capture with positron emission in

^{106}

Cd to the ground state of

^{106}

Pd,

T_{1 / 2}^{2 ν EC β^{+}} \geq 2.1 \times 10^{21}

yr, was set by the analysis of the

^{106}

CdWO

_{4}

data in coincidence with the energy release 511 keV in both CdWO

_{4}

counters. The sensitivity approaches the theoretical predictions for the decay half-life that are in the range

T_{1 / 2} \sim 10^{21} - 10^{22}

yr. The resonant neutrinoless double-electron capture to the 2718 keV excited state of

^{106}

Pd is restricted at the level of

T_{1 / 2}^{0 ν 2 K} \geq 2.9 \times 10^{21}

yr. Full article

(This article belongs to the Special Issue Neutrinoless Double Beta Decay)

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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 2097

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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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 2975

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, 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 1976

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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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 1880

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 2376

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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8 pages, 249 KiB

Open AccessArticle

The Functional Schrödinger Equation in the Semiclassical Limit of Quantum Gravity with a Gaussian Clock Field

by Marcello Rotondo

Universe 2020, 6(10), 176; https://doi.org/10.3390/universe6100176 - 13 Oct 2020

Cited by 5 | Viewed by 1899

Abstract

We derive the functional Schrödinger equation for quantum fields in curved spacetime in the semiclassical limit of quantum geometrodynamics with a Gaussian incoherent dust acting as a clock field. We perform the semiclassical limit using a WKB-type expansion of the wave functional in [...] Read more.

We derive the functional Schrödinger equation for quantum fields in curved spacetime in the semiclassical limit of quantum geometrodynamics with a Gaussian incoherent dust acting as a clock field. We perform the semiclassical limit using a WKB-type expansion of the wave functional in powers of the squared Planck mass. The functional Schrödinger equation that we obtain exhibits a functional time derivative that completes the usual definition of WKB time for curved spacetime, and the usual Schrödinger-type evolution is recovered in Minkowski spacetime. Full article

(This article belongs to the Special Issue Universe: 5th Anniversary)

18 pages, 398 KiB

Open AccessArticle

A Test of Gravitational Theories Including Torsion with the BepiColombo Radio Science Experiment

by Giulia Schettino, Daniele Serra, Giacomo Tommei and Vincenzo Di Pierri

Universe 2020, 6(10), 175; https://doi.org/10.3390/universe6100175 - 12 Oct 2020

Cited by 4 | Viewed by 1913

Abstract

Within the framework of the relativity experiment of the ESA/JAXA BepiColombo mission to Mercury, which was launched at the end of 2018, we describe how a test of alternative theories of gravity, including torsion can be set up. Following March et al. (2011), [...] Read more.

Within the framework of the relativity experiment of the ESA/JAXA BepiColombo mission to Mercury, which was launched at the end of 2018, we describe how a test of alternative theories of gravity, including torsion can be set up. Following March et al. (2011), the effects of a non-vanishing spacetime torsion have been parameterized by three torsion parameters,

t_{1}

,

t_{2}

, and

t_{3}

. These parameters can be estimated within a global least squares fit, together with a number of parameters of interest, such as post-Newtonian parameters

γ

and

β

, and the orbits of Mercury and the Earth. The simulations have been performed by means of the ORBIT14 orbit determination software, which was developed by the Celestial Mechanics Group of the University of Pisa for the analysis of the BepiColombo radio science experiment. We claim that the torsion parameters can be determined by means of the relativity experiment of BepiColombo at the level of some parts in

10^{- 4}

, which is a significant result for constraining gravitational theories that allow spacetime torsion. Full article

(This article belongs to the Special Issue Universe: 5th Anniversary)

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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 2028

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 2015

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 9 | Viewed by 2488

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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17 pages, 329 KiB

Open AccessArticle

Singularity Theorems in the Effective Field Theory for Quantum Gravity at Second Order in Curvature

by Folkert Kuipers and Xavier Calmet

Universe 2020, 6(10), 171; https://doi.org/10.3390/universe6100171 - 10 Oct 2020

Cited by 8 | Viewed by 1941

Abstract

In this paper, we discuss singularity theorems in quantum gravity using effective field theory methods. To second order in curvature, the effective field theory contains two new degrees of freedom which have important implications for the derivation of these theorems: a massive spin-2 [...] Read more.

In this paper, we discuss singularity theorems in quantum gravity using effective field theory methods. To second order in curvature, the effective field theory contains two new degrees of freedom which have important implications for the derivation of these theorems: a massive spin-2 field and a massive spin-0 field. Using an explicit mapping of this theory from the Jordan frame to the Einstein frame, we show that the massive spin-2 field violates the null energy condition, while the massive spin-0 field satisfies the null energy condition, but may violate the strong energy condition. Due to this violation, classical singularity theorems are no longer applicable, indicating that singularities can be avoided, if the leading quantum corrections are taken into account. Full article

(This article belongs to the Special Issue Quantum Effects in General Relativity)

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 1931

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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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 1924

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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16 pages, 21099 KiB

Open AccessArticle

Using Unreal Engine to Visualize a Cosmological Volume

by Christopher Marsden and Francesco Shankar

Universe 2020, 6(10), 168; https://doi.org/10.3390/universe6100168 - 6 Oct 2020

Cited by 7 | Viewed by 4780

Abstract

In this work we present “Astera’’, a cosmological visualization tool that renders a mock universe in real time using Unreal Engine 4. The large scale structure of the cosmic web is hard to visualize in two dimensions, and a 3D real time projection [...] Read more.

In this work we present “Astera’’, a cosmological visualization tool that renders a mock universe in real time using Unreal Engine 4. The large scale structure of the cosmic web is hard to visualize in two dimensions, and a 3D real time projection of this distribution allows for an unprecedented view of the large scale universe, with visually accurate galaxies placed in a dynamic 3D world. The underlying data are based on empirical relations assigned using results from N-Body dark matter simulations, and are matched to galaxies with similar morphologies and sizes, images of which are extracted from the Sloan Digital Sky Survey. Within Unreal Engine 4, galaxy images are transformed into textures and dynamic materials (with appropriate transparency) that are applied to static mesh objects with appropriate sizes and locations. To ensure excellent performance, these static meshes are “instanced’’ to utilize the full capabilities of a graphics processing unit. Additional components include a dynamic system for representing accelerated-time active galactic nuclei. The end result is a visually realistic large scale universe that can be explored by a user in real time, with accurate large scale structure. Astera is not yet ready for public release, but we are exploring options to make different versions of the code available for both research and outreach applications. Full article

(This article belongs to the Section Galaxies and Clusters)

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23 pages, 598 KiB

Open AccessArticle

Study on Anisotropic Strange Stars in f ( T , T ) Gravity

by Ines G. Salako, M. Khlopov, Saibal Ray, M. Z. Arouko, Pameli Saha and Ujjal Debnath

Universe 2020, 6(10), 167; https://doi.org/10.3390/universe6100167 - 3 Oct 2020

Cited by 40 | Viewed by 3156

Abstract

In this work, we study the existence of strange stars in the background of

f (T, T)

gravity in the Einstein spacetime geometry, where

T

is the torsion tensor and

T

is the trace of the energy-momentum tensor. The equations [...] Read more.

In this work, we study the existence of strange stars in the background of

f (T, T)

gravity in the Einstein spacetime geometry, where

T

is the torsion tensor and

T

is the trace of the energy-momentum tensor. The equations of motion are derived for anisotropic pressure within the spherically symmetric strange star. We explore the physical features like energy conditions, mass-radius relations, modified Tolman–Oppenheimer–Volkoff (TOV) equations, principal of causality, adiabatic index, redshift and stability analysis of our model. These features are realistic and appealing to further investigation of properties of compact objects in

f (T, T)

gravity as well as their observational signatures. Full article

(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)

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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 2170

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))

28 pages, 438 KiB

Open AccessReview

Soft Anomalous Dimensions and Resummation in QCD

by Nikolaos Kidonakis

Universe 2020, 6(10), 165; https://doi.org/10.3390/universe6100165 - 1 Oct 2020

Cited by 12 | Viewed by 1572

Abstract

I discuss and review soft anomalous dimensions in QCD that describe soft-gluon threshold resummation for a wide range of hard-scattering processes. The factorization properties of the cross section in moment space and renormalization-group evolution are implemented to derive a general form for differential [...] Read more.

I discuss and review soft anomalous dimensions in QCD that describe soft-gluon threshold resummation for a wide range of hard-scattering processes. The factorization properties of the cross section in moment space and renormalization-group evolution are implemented to derive a general form for differential resummed cross sections. Detailed expressions are given for the soft anomalous dimensions at one, two, and three loops, including some new results, for a large number of partonic processes involving top quarks, electroweak bosons, Higgs bosons, and other particles in the standard model and beyond. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

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 2189

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 16 | Viewed by 2101

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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Graphical abstract

38 pages, 1202 KiB

Open AccessReview

Cosmogenic Activation in Double Beta Decay Experiments

by Susana Cebrián

Universe 2020, 6(10), 162; https://doi.org/10.3390/universe6100162 - 29 Sep 2020

Cited by 13 | Viewed by 3671

Abstract

Double beta decay is a very rare nuclear process and, therefore, experiments intended to detect it must be operated deep underground and in ultra-low background conditions. Long-lived radioisotopes produced by the previous exposure of materials to cosmic rays on the Earth’s surface or [...] Read more.

Double beta decay is a very rare nuclear process and, therefore, experiments intended to detect it must be operated deep underground and in ultra-low background conditions. Long-lived radioisotopes produced by the previous exposure of materials to cosmic rays on the Earth’s surface or even underground can become problematic for the required sensitivity. Here, the studies developed to quantify and reduce the activation yields in detectors and materials used in the set-up of these experiments will be reviewed, considering target materials like germanium, tellurium and xenon together with other ones commonly used like copper, lead, stainless steel or argon. Calculations following very different approaches and measurements from irradiation experiments using beams or directly cosmic rays will be considered for relevant radioisotopes. The effect of cosmogenic activation in present and future double beta decay projects based on different types of detectors will be analyzed too. Full article

(This article belongs to the Special Issue Neutrinoless Double Beta Decay)

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17 pages, 1311 KiB

Open AccessArticle

Predicting >10 MeV SEP Events from Solar Flare and Radio Burst Data

by Marlon Núñez and Daniel Paul-Pena

Universe 2020, 6(10), 161; https://doi.org/10.3390/universe6100161 - 28 Sep 2020

Cited by 19 | Viewed by 2986

Abstract

The prediction of solar energetic particle (SEP) events or solar radiation storms is one of the most important problems in the space weather field. These events may have adverse effects on technology infrastructures and humans in space; they may also irradiate passengers and [...] Read more.

The prediction of solar energetic particle (SEP) events or solar radiation storms is one of the most important problems in the space weather field. These events may have adverse effects on technology infrastructures and humans in space; they may also irradiate passengers and flight crews in commercial aircraft flying at polar latitudes. This paper explores the use of ≥ M2 solar flares and radio burst observations as proxies for predicting >10 MeV SEP events on Earth. These observations are manifestations of the parent event at the sun associated with the SEP event. As a consequence of processing data at the beginning of the physical process that leads to the radiation storm, the model may provide its predictions with large anticipation. The main advantage of the present approach is that the model analyzes solar data that are updated every 30 min and, as such, it may be operational; however, a disadvantage is that those SEP events associated with strong well-connected flares cannot be predicted. For the period from November 1997 to February 2014, we obtained a probability of detection of 70.2%, a false alarm ratio of 40.2%, and an average anticipation time of 9 h 52 min. In this study, the prediction model was built using decision trees, an interpretable machine learning technique. This approach leads to outputs and results comparable to those derived by the Empirical model for Solar Proton Event Real Time Alert (ESPERTA) model. The obtained decision tree shows that the best criteria to differentiate pre-SEP scenarios and non-pre-SEP scenarios are the peak and integrated flux for soft X-ray flares and the radio type III bursts. Full article

(This article belongs to the Section Solar and Stellar Physics)

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Graphical abstract

19 pages, 345 KiB

Open AccessArticle

Gravity-Induced Geometric Phases and Entanglement in Spinors and Neutrinos: Gravitational Zeeman Effect

by Banibrata Mukhopadhyay and Soumya Kanti Ganguly

Universe 2020, 6(10), 160; https://doi.org/10.3390/universe6100160 - 27 Sep 2020

Cited by 7 | Viewed by 2136

Abstract

We show Zeeman-like splitting in the energy of spinors propagating in a background gravitational field, analogous to the spinors in an electromagnetic field, otherwise termed the Gravitational Zeeman Effect. These spinors are also found to acquire a geometric phase, in a similar way [...] Read more.

We show Zeeman-like splitting in the energy of spinors propagating in a background gravitational field, analogous to the spinors in an electromagnetic field, otherwise termed the Gravitational Zeeman Effect. These spinors are also found to acquire a geometric phase, in a similar way as they do in the presence of magnetic fields. However, in a gravitational background, the Aharonov-Bohm type effect, in addition to Berry-like phase, arises. Based on this result, we investigate geometric phases acquired by neutrinos propagating in a strong gravitational field. We also explore entanglement of neutrino states due to gravity, which could induce neutrino-antineutrino oscillation in the first place. We show that entangled states also acquire geometric phases which are determined by the relative strength between gravitational field and neutrino masses. Full article

(This article belongs to the Section Gravitation)

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16 pages, 365 KiB

Open AccessReview

Precise Half-Life Values for Two-Neutrino Double-β Decay: 2020 Review

by Alexander Barabash

Universe 2020, 6(10), 159; https://doi.org/10.3390/universe6100159 - 27 Sep 2020

Cited by 98 | Viewed by 4312

Abstract

All existing positive results on two-neutrino double beta decay and two-neutrino double electron capture in different nuclei have been analyzed. Weighted average and recommended half-life values for

^{48}

Ca,

^{76}

Ge,

^{82}

Se,

^{96}

Zr,

^{100}

Mo,

^{100}

Mo -

^{100}

Ru (0 [...] Read more.

All existing positive results on two-neutrino double beta decay and two-neutrino double electron capture in different nuclei have been analyzed. Weighted average and recommended half-life values for

^{48}

Ca,

^{76}

Ge,

^{82}

Se,

^{96}

Zr,

^{100}

Mo,

^{100}

Mo -

^{100}

Ru (0₁⁺),

^{116}

Cd,

^{128}

Te,

^{130}

Te,

^{136}

Xe,

^{150}

Nd,

^{150}

Nd -

^{150}

Sm (0₁⁺),

^{238}

U,

^{78}

Kr,

^{124}

Xe and

^{130}

Ba have been obtained. Given the measured half-life values, effective nuclear matrix elements for all these transitions were calculated. Full article

(This article belongs to the Special Issue Neutrinoless Double Beta Decay)

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