Universe

20 pages, 1263 KiB

Open AccessArticle

Magnetized and Magnetically Charged Particles Motion around Regular Bardeen Black Hole in 4D Einstein Gauss–Bonnet Gravity

by Javlon Rayimbaev, Dilshodbek Bardiev, Farrux Abdulxamidov, Ahmadjon Abdujabbarov and Bobomurat Ahmedov

Universe 2022, 8(10), 549; https://doi.org/10.3390/universe8100549 - 21 Oct 2022

Cited by 13 | Viewed by 1689

Abstract

In this paper, we study the horizon properties and scalar invariants of the spacetime around a regular black hole (BH) in 4D Einstein Gauss-Bonnet (4D EGB) gravity. It is observed that the presence of both Gauss-Bonnet (GB) coupling and magnetic charge parameters causes [...] Read more.

In this paper, we study the horizon properties and scalar invariants of the spacetime around a regular black hole (BH) in 4D Einstein Gauss-Bonnet (4D EGB) gravity. It is observed that the presence of both Gauss-Bonnet (GB) coupling and magnetic charge parameters causes the shrinking of the outer horizon. We find that the range of the GB parameter

α / M^{2} \in (- 0.15869, 1)

, and the extreme value of magnetic charge reaches up to

g_{extr} = 0.886 M

, which allows for the existence of a BH horizon, while it is

g_{extr} = 0.7698 M

for pure Bardeen BH. We also investigate the dynamics of magnetized particles around the magnetically charged Bardeen BH, assuming the particle’s motion occurs in the equatorial plane in the proper observation frame, and the direction of the magnetic dipole moment of the particles is always kept radially and its magnitude is constant. Moreover, the dynamics of magnetically charged particles are also studied, and it is shown that both the energy and angular momentum of the particles corresponding to circular orbits increases with the increase of their magnetic charge. Finally, we also study collisions of magnetized, electrically neutral, and magnetically charged particles around the Bardeen BHs, where we provide analyses of critical angular momentum that may allow collision of the particles near-horizon radius, producing enormous values of center of mass energy of the collisions. Full article

(This article belongs to the Special Issue Black Holes in Einstein–Gauss–Bonnet Theories)

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20 pages, 1608 KiB

Open AccessArticle

Quintom Fields from Chiral K-Essence Cosmology

by José Socorro, Sinuhé Pérez-Payán, Rafael Hernández-Jiménez, Abraham Espinoza-García and Luis Rey Díaz-Barrón

Universe 2022, 8(10), 548; https://doi.org/10.3390/universe8100548 - 21 Oct 2022

Cited by 11 | Viewed by 1478

Abstract

In this paper, we present an analysis of a chiral cosmological scenario from the perspective of K-essence formalism. In this setup, several scalar fields interact within the kinetic and potential sectors. However, we only consider a flat Friedmann–Robertson–Lamaître–Walker universe coupled minimally to two [...] Read more.

In this paper, we present an analysis of a chiral cosmological scenario from the perspective of K-essence formalism. In this setup, several scalar fields interact within the kinetic and potential sectors. However, we only consider a flat Friedmann–Robertson–Lamaître–Walker universe coupled minimally to two quintom fields: one quintessence and one phantom. We examine a classical cosmological framework, where analytical solutions are obtained. Indeed, we present an explanation of the “big-bang” singularity by means of a “big-bounce”. Moreover, having a barotropic fluid description and for a particular set of parameters, the phantom line is in fact crossed. Additionally, for the quantum counterpart, the Wheeler–DeWitt equation is analytically solved for various instances, where the factor-ordering problem has been taken into account (measured by the factor Q). Hence, this approach allows us to compute the probability density of the previous two classical subcases. It turns out that its behavior is in effect damped as the scale factor and the scalar fields evolve. It also tends towards the phantom sector when the factor ordering constant

Q ≪ 0

. Full article

(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)

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15 pages, 528 KiB

Open AccessArticle

A PeVatron Candidate: Modeling the Boomerang Nebula in X-ray Band

by Xuan-Han Liang, Chao-Ming Li, Qi-Zuo Wu, Jia-Shu Pan and Ruo-Yu Liu

Universe 2022, 8(10), 547; https://doi.org/10.3390/universe8100547 - 21 Oct 2022

Cited by 9 | Viewed by 2075

Abstract

Pulsar wind nebula (PWN) Boomerang and the associated supernova remnant (SNR) G106.3+2.7 are among candidates for the ultra-high-energy (UHE) gamma-ray counterparts published by LHAASO. Although the centroid of the extended source, LHAASO J2226+6057, deviates from the pulsar’s position by about

{0.3}^{\circ}

, [...] Read more.

Pulsar wind nebula (PWN) Boomerang and the associated supernova remnant (SNR) G106.3+2.7 are among candidates for the ultra-high-energy (UHE) gamma-ray counterparts published by LHAASO. Although the centroid of the extended source, LHAASO J2226+6057, deviates from the pulsar’s position by about

{0.3}^{\circ}

, the source partially covers the PWN. Therefore, we cannot totally exclude the possibility that part of the UHE emission comes from the PWN. Previous studies mainly focus on whether the SNR is a PeVatron, while neglecting the energetic PWN. Here, we explore the possibility of the Boomerang Nebula being a PeVatron candidate by studying its X-ray radiation. By modeling the diffusion of relativistic electrons injected in the PWN, we fit the radial profiles of X-ray surface brightness and photon index. The solution with a magnetic field

B = 140 μ G

can well reproduce the observed profiles and implies a severe suppression of IC scattering of electrons. Hence, if future observations reveal part of the UHE emission originating from the PWN, we propose to introduce a proton component to account for the UHE emission in light of the recent LHAASO measurement on Crab Nebula. In this sense, Boomerang Nebula would be a hadronic PeVatron. Full article

(This article belongs to the Special Issue Advances in Astrophysics and Cosmology – in Memory of Prof. Tan Lu)

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7 pages, 448 KiB

Open AccessArticle

Frame-Dragging in Extrasolar Circumbinary Planetary Systems

by Lorenzo Iorio

Universe 2022, 8(10), 546; https://doi.org/10.3390/universe8100546 - 21 Oct 2022

Cited by 5 | Viewed by 1553

Abstract

Extrasolar circumbinary planets are so called because they orbit two stars instead of just one; to date, an increasing number of such planets have been discovered with a variety of techniques. If the orbital frequency of the hosting stellar pair is much higher [...] Read more.

Extrasolar circumbinary planets are so called because they orbit two stars instead of just one; to date, an increasing number of such planets have been discovered with a variety of techniques. If the orbital frequency of the hosting stellar pair is much higher than the planetary one, the tight stellar binary can be considered as a matter ring current generating its own post-Newtonian stationary gravitomagnetic field through its orbital angular momentum. It affects the orbital motion of a relatively distant planet with Lense-Thirring-type precessional effects which, under certain circumstances, may amount to a significant fraction of the static, gravitoelectric ones, analogous to the well known Einstein perihelion precession of Mercury, depending only on the masses of the system’s bodies. Instead, when the gravitomagnetic field is due solely to the spin of each of the central star(s), the Lense-Thirring shifts are several orders of magnitude smaller than the gravitoelectric ones. In view of the growing interest in the scientific community about the detection of general relativistic effects in exoplanets, the perspectives of finding new scenarios for testing such a further manifestation of general relativity might be deemed worth of further investigations. Full article

(This article belongs to the Special Issue Frame-Dragging and Gravitomagnetism)

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3 pages, 170 KiB

Open AccessEditorial

Shedding Light to the Dark Sides of the Universe: Cosmology from Strong Interactions

by Roman Pasechnik and Michal Šumbera

Universe 2022, 8(10), 545; https://doi.org/10.3390/universe8100545 - 19 Oct 2022

Viewed by 1210

Abstract

The basic aim of this Special Issue was to reflect upon the modern status of research on strong interactions and their implications in Cosmology [...] Full article

(This article belongs to the Special Issue Shedding Light to the Dark Sides of the Universe: Cosmology from Strong Interactions)

19 pages, 384 KiB

Open AccessArticle

Revisiting Dudas-Mourad Compactifications

by Ivano Basile, Salvatore Raucci and Sylvain Thomée

Universe 2022, 8(10), 544; https://doi.org/10.3390/universe8100544 - 19 Oct 2022

Cited by 16 | Viewed by 1391

Abstract

Superstring theories in ten dimensions allow spacetime supersymmetry breaking at the string scale at the expense of controlled Minkowski backgrounds. The next-to-maximally symmetric backgrounds, found by Dudas and Mourad, involve a warped compactification on an interval associated with codimension-one defects. We generalize these [...] Read more.

Superstring theories in ten dimensions allow spacetime supersymmetry breaking at the string scale at the expense of controlled Minkowski backgrounds. The next-to-maximally symmetric backgrounds, found by Dudas and Mourad, involve a warped compactification on an interval associated with codimension-one defects. We generalize these solutions by varying the effective field theory parameters, and we discuss the dimensional reduction on the interval. In particular, we show that scalars and form fields decouple in a certain range of dimensions, yielding Einstein-Yang-Mills theory. Moreover, we find that the breakdown of this effective description due to light Kaluza-Klein modes reflects the swampland distance conjecture, supporting the consistency of the picture at least qualitatively. Full article

(This article belongs to the Special Issue Probing the Quantum Space-Time)

31 pages, 626 KiB

Open AccessArticle

Dirac Observables in the 4-Dimensional Phase Space of Ashtekar’s Variables and Spherically Symmetric Loop Quantum Black Holes

by Geeth Ongole, Hongchao Zhang, Tao Zhu, Anzhong Wang and Bin Wang

Universe 2022, 8(10), 543; https://doi.org/10.3390/universe8100543 - 19 Oct 2022

Cited by 5 | Viewed by 1392

Abstract

In this paper, we study a proposal put forward recently by Bodendorfer, Mele and Münch and García-Quismondo and Marugán, in which the two polymerization parameters of spherically symmetric black hole spacetimes are the Dirac observables of the four-dimensional Ashtekar’s variables. In this model, [...] Read more.

In this paper, we study a proposal put forward recently by Bodendorfer, Mele and Münch and García-Quismondo and Marugán, in which the two polymerization parameters of spherically symmetric black hole spacetimes are the Dirac observables of the four-dimensional Ashtekar’s variables. In this model, black and white hole horizons in general exist and naturally divide the spacetime into the external and internal regions. In the external region, the spacetime can be made asymptotically flat by properly choosing the dependence of the two polymerization parameters on the Ashtekar variables. Then, we find that the asymptotical behavior of the spacetime is universal, and, to the leading order, the curvature invariants are independent of the mass parameter m. For example, the Kretschmann scalar approaches zero as

K ≃ A_{0} r^{- 4}

asymptotically, where

A_{0}

is generally a non-zero constant and independent of m, and r the geometric radius of the two-spheres. In the internal region, all the physical quantities are finite, and the Schwarzschild black hole singularity is replaced by a transition surface whose radius is always finite and non-zero. The quantum gravitational effects are negligible near the black hole horizon for very massive black holes. However, the behavior of the spacetime across the transition surface is significantly different from all loop quantum black holes studied so far. In particular, the location of the maximum amplitude of the curvature scalars is displaced from the transition surface and depends on m; so does the maximum amplitude. In addition, the radius of the white hole is much smaller than that of the black hole, and its exact value sensitively depends on m, too. Full article

(This article belongs to the Special Issue Loop Quantum Gravity: A Themed Issue in Honor of Prof. Abhay Ashtekar)

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9 pages, 475 KiB

Open AccessArticle

Nonanalytic Relativistic r-Modes of Slowly Rotating Nonbarotropic Neutron Stars

by Kirill Y. Kraav, Mikhail E. Gusakov and Elena M. Kantor

Universe 2022, 8(10), 542; https://doi.org/10.3390/universe8100542 - 19 Oct 2022

Cited by 4 | Viewed by 1114

Abstract

We show that the r-modes of slowly rotating nonbarotropic neutron stars are described by nonanalytic functions of stellar angular velocity, which makes the perturbation techniques, used so far in the r-mode theoretical studies, inapplicable. In contrast to those studies and in [...] Read more.

We show that the r-modes of slowly rotating nonbarotropic neutron stars are described by nonanalytic functions of stellar angular velocity, which makes the perturbation techniques, used so far in the r-mode theoretical studies, inapplicable. In contrast to those studies and in accordance with numerical calculations beyond the slow rotation approximation, the obtained r-mode spectrum is discrete, which resolves the continuous spectrum problem, lasting since 1997. Our findings imply that the relativistic r-modes in slowly rotating neutron stars dramatically differ from their Newtonian cousins, which may have important implications for the detectability of r-mode signatures in observations, in particular for the r-mode excitation efficiency during the neutron star inspirals. Full article

(This article belongs to the Section Universe Letters)

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10 pages, 294 KiB

Open AccessArticle

Sign Switching Dark Energy from a Running Barrow Entropy

by Sofia Di Gennaro and Yen Chin Ong

Universe 2022, 8(10), 541; https://doi.org/10.3390/universe8100541 - 19 Oct 2022

Cited by 40 | Viewed by 1646

Abstract

Barrow proposed that the area law of the entropy associated with a horizon might receive a “fractal correction” due to quantum gravitational effects—in place of

S \propto A

, we have instead

S \propto A^{1 + δ / 2}

, where [...] Read more.

Barrow proposed that the area law of the entropy associated with a horizon might receive a “fractal correction” due to quantum gravitational effects—in place of

S \propto A

, we have instead

S \propto A^{1 + δ / 2}

, where

0 ⩽ δ ⩽ 1

measures the deviation from the standard area law (

δ = 0

). Based on black hole thermodynamics, we argue that the Barrow entropy should run (i.e., energy scale dependent), which is reasonable given that quantum gravitational corrections are expected to be important only in the high-energy regime. When applied to the Friedmann equation, we demonstrate the possibility that such a running Barrow entropy index could give rise to a dynamical effective dark energy, which is asymptotically positive and vanishing, but negative at the Big Bang. Such a sign switching dark energy could help to alleviate the Hubble tension. Other cosmological implications are discussed. Full article

(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)

12 pages, 588 KiB

Open AccessArticle

On a Class of Harko-Kovacs-Lobo Wormholes

by Ramis Kh. Karimov, Ramil N. Izmailov and Kamal K. Nandi

Universe 2022, 8(10), 540; https://doi.org/10.3390/universe8100540 - 18 Oct 2022

Cited by 1 | Viewed by 1243

Abstract

The Harko, Kovács, and Lobo wormhole (HKLWH) metric contains two free parameters: one is the wormhole throat

r_{0}

, and the other is a dimensionless deviation parameter

γ

with values

0 < γ < 1

, the latter ensuring the needed violation [...] Read more.

The Harko, Kovács, and Lobo wormhole (HKLWH) metric contains two free parameters: one is the wormhole throat

r_{0}

, and the other is a dimensionless deviation parameter

γ

with values

0 < γ < 1

, the latter ensuring the needed violation of the null energy condition at the throat. In this paper, we study the energetics of the HKLWH and the influence of

γ

on the tidal forces in the Lorentz-boosted frame. Finally, we apply a new concept, namely, the probabilistic identity of the object observed by different external observers in terms of the Fresnel coefficients derived by Tangherlini. The intriguing result is that observations can differ depending on the location of the observer, i.e., there is a nonzero probability that the HKLWH will be identified as a black hole even when

γ \neq 0

. Full article

(This article belongs to the Special Issue Recent Advances in Wormhole Physics)

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13 pages, 450 KiB

Open AccessArticle

A Machine Learning Approach for Predicting Black Hole Mass in Blazars Using Broadband Emission Model Parameters

by Krishna Kumar Singh, Anilkumar Tolamatti, Sandeep Godiyal, Atul Pathania and Kuldeep Kumar Yadav

Universe 2022, 8(10), 539; https://doi.org/10.3390/universe8100539 - 18 Oct 2022

Cited by 2 | Viewed by 1488

Abstract

Blazars are observed to emit non-thermal radiation across the entire electromagnetic spectrum from the radio to the very-high-energy

γ

-ray region. The broadband radiation measured from a blazar is dominated by emission from a relativistic plasma jet which is assumed to be powered [...] Read more.

Blazars are observed to emit non-thermal radiation across the entire electromagnetic spectrum from the radio to the very-high-energy

γ

-ray region. The broadband radiation measured from a blazar is dominated by emission from a relativistic plasma jet which is assumed to be powered by a spinning supermassive black hole situated in the central region of the host galaxy. The formation of jets, their mode of energy transport, actual power budget, and connection with the central black hole are among the most fundamental open problems in blazar research. However, the observed broadband spectral energy distribution from blazars is generally explained by a simple one-zone leptonic emission model. The model parameters place constraints on the contributions from the magnetic field, radiation field, and kinetic power of particles to the emission region in the jet. This in turn constrains the minimum power transported by the jet from the central engine. In this work, we explore the potential of machine learning frameworks including linear regression, support vector machine, adaptive boosting, bagging, gradient boosting, and random forests for the estimation of the mass of the supermassive black hole at the center of the host galaxy of blazars using the best-fit emission model parameters derived from the broadband spectral energy distribution modeling in the literature. Our study suggests that the support vector machine, adaptive boosting, bagging, and random forest algorithms can predict black hole masses with reasonably good accuracy. Full article

(This article belongs to the Special Issue Multi-Messengers of Black Hole Accretion and Emission)

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13 pages, 3748 KiB

Open AccessArticle

Nighttime Cloud Cover Estimation Method at the Saishiteng 3850 m Site

by Baoquan Gao, Yiding Ping, Yao Lu and Chen Zhang

Universe 2022, 8(10), 538; https://doi.org/10.3390/universe8100538 - 18 Oct 2022

Cited by 1 | Viewed by 1726

Abstract

Cloud cover is critical for astronomical sites because it can be used to assess the observability of the local sky and further the fractional photometric time. For cloud monitoring in site-testing campaigns with all-sky cameras, previous studies have mainly focused on moonless images, [...] Read more.

Cloud cover is critical for astronomical sites because it can be used to assess the observability of the local sky and further the fractional photometric time. For cloud monitoring in site-testing campaigns with all-sky cameras, previous studies have mainly focused on moonless images, while the automatic processing methods for moonlight images are explored quite few. This paper proposes an automatic estimation method for cloud cover, which takes all cases of nighttime gray-scale all-sky images into account. For moonless images, the efficient Otsu algorithm is directly used to detect clouds. For moonlight images, they are transformed into cloud feature image using a colorization procedure, and then the Otsu algorithm is used to distinguish cloud pixels from sky pixels on the cloud feature image. The reliability of this method was evaluated on manually labeled images. The results show that the cloud cover error of this method is less than 9% in all scenarios. The fractional photometric time derived from this method is basically consistent with the published result of the Lenghu site. Full article

(This article belongs to the Section Space Science)

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12 pages, 271 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Quantum Clones inside Black Holes

by Gerard ’t Hooft

Universe 2022, 8(10), 537; https://doi.org/10.3390/universe8100537 - 18 Oct 2022

Cited by 20 | Viewed by 25066

Abstract

A systematic procedure is proposed for better understanding the evolution laws of black holes in terms of pure quantum states. We start with the two opposed regions I and

I I

in the Penrose diagram, and study the evolution of matter in these [...] Read more.

A systematic procedure is proposed for better understanding the evolution laws of black holes in terms of pure quantum states. We start with the two opposed regions I and

I I

in the Penrose diagram, and study the evolution of matter in these regions, using the algebra derived earlier from the Shapiro effect in quantum particles. Since this spacetime has two distinct asymptotic regions, one must assume that there is a mechanism that reduces the number of states. In earlier work we proposed that region

I I

describes the angular antipodes of region I, the ‘antipodal identification’, but this eventually leads to contradictions. Our much simpler proposal is now that all states defined in region

I I

are exact quantum clones of those in region I. This indicates more precisely how to restore unitarity by making all quantum states observable, and in addition suggests that generalisations towards other black hole structures will be possible. An apparent complication is that the wave function must evolve with a purely antisymmetric, imaginary-valued Hamiltonian, but this complication can be well-understood in a realistic interpretation of quantum mechanics. Full article

(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)

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43 pages, 541 KiB

Open AccessReview

Basic Notions of Poisson and Symplectic Geometry in Local Coordinates, with Applications to Hamiltonian Systems

by Alexei A. Deriglazov

Universe 2022, 8(10), 536; https://doi.org/10.3390/universe8100536 - 17 Oct 2022

Cited by 8 | Viewed by 1366

Abstract

This work contains a brief and elementary exposition of the foundations of Poisson and symplectic geometries, with an emphasis on applications for Hamiltonian systems with second-class constraints. In particular, we clarify the geometric meaning of the Dirac bracket on a symplectic manifold and [...] Read more.

This work contains a brief and elementary exposition of the foundations of Poisson and symplectic geometries, with an emphasis on applications for Hamiltonian systems with second-class constraints. In particular, we clarify the geometric meaning of the Dirac bracket on a symplectic manifold and provide a proof of the Jacobi identity on a Poisson manifold. A number of applications of the Dirac bracket are described: applications for proof of the compatibility of a system consisting of differential and algebraic equations, as well as applications for the problem of the reduction of a Hamiltonian system with known integrals of motion. Full article

(This article belongs to the Section Mathematical Physics)

16 pages, 748 KiB

Open AccessArticle

Long-Term Monitoring of Blazar PKS 0208-512: A Change of γ-Ray Baseline Activity from EGRET to Fermi Era

by Krishna Mohana Ammenadka, Debbijoy Bhattacharya, Subir Bhattacharyya, Nilay Bhatt and Chelliah Subramonian Stalin

Universe 2022, 8(10), 534; https://doi.org/10.3390/universe8100534 - 17 Oct 2022

Cited by 2 | Viewed by 1467

Abstract

The blazar PKS 0208-512 was in the lowest

γ

-ray brightness state during the initial 10 years of observations with the Fermi Gamma-ray Space Telescope (Fermi), which was an order of magnitude lower than its flux state during the EGRET era [...] Read more.

The blazar PKS 0208-512 was in the lowest

γ

-ray brightness state during the initial 10 years of observations with the Fermi Gamma-ray Space Telescope (Fermi), which was an order of magnitude lower than its flux state during the EGRET era (1991–2000). The weekly averaged maximum

γ

-ray flux of this source during the first 10 years of Fermi observation is nearly a factor of 3 lower than the highest flux observed by EGRET in a single epoch. During the period 2018–2020, the source showed a large

γ

-ray flare, with the average brightness similar to the period 1991–2000. We observed the source with AstroSat, during its low and high activity states, respectively. We carried out broad-band spectral energy distribution (SED) modeling of the source using a one-zone leptonic emission model during its various brightness states. From the SED modeling, we found that there was an inefficient conversion from the bulk energy to the particle energy during the long-term low-activity states as compared to the high flux state during the EGRET era and the later part of Fermi observation. Full article

(This article belongs to the Special Issue Multi-Messengers of Black Hole Accretion and Emission)

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12 pages, 3749 KiB

Open AccessArticle

Testing the Wave-Particle Duality of Gravitational Wave Using the Spin-Orbital-Hall Effect of Structured Light

by Qianfan Wu, Weishan Zhu and Longlong Feng

Universe 2022, 8(10), 535; https://doi.org/10.3390/universe8100535 - 16 Oct 2022

Cited by 5 | Viewed by 1783

Abstract

Probing the polarization of gravitational waves (GWs) would provide evidence of graviton, indicating the quantization of gravity. Motivated by the next generation of gravitational wave detectors, we make an attempt to study the possible helicity coupling of structured lights to GWs. With the [...] Read more.

Probing the polarization of gravitational waves (GWs) would provide evidence of graviton, indicating the quantization of gravity. Motivated by the next generation of gravitational wave detectors, we make an attempt to study the possible helicity coupling of structured lights to GWs. With the analog between gravitational fields and the generic electromagnetic media, we present a 4-vector optical Dirac equation based on the Maxwell theory under the paraxial approximation. It is found that twisted lights propagating in a gravitational field can be viewed as a non-Hermitian system with

PT

symmetry. We further demonstrate that the coupling effect between angular momentums of the GWs and twisted lights may make photons undergo both dipole and quadrupole transitions between different orbital-angular-momentum (OAM) eigenstates and lead to some measurable optical features, including the central intensity brightening and macroscopic rotation of the intensity pattern for twisted lights. The former is spin-independent, while the latter is a spin-dependent phenomenon, both of which can be viewed alternatively as the spin-orbital-Hall effect of structured lights in the GWs and can serve as an indicator of the particle nature of GWs. Full article

(This article belongs to the Special Issue Advances in Astrophysics and Cosmology – in Memory of Prof. Tan Lu)

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41 pages, 502 KiB

Open AccessArticle

Metric Gravity in the Hamiltonian Form—Canonical Transformations—Dirac’s Modifications of the Hamilton Method and Integral Invariants of the Metric Gravity

by Alexei M. Frolov

Universe 2022, 8(10), 533; https://doi.org/10.3390/universe8100533 - 14 Oct 2022

Cited by 1 | Viewed by 1399

Abstract

Two different Hamiltonian formulations of the metric gravity are discussed and applied to describe a free gravitational field in the d dimensional Riemann space-time. Theory of canonical transformations, which relates equivalent Hamiltonian formulations of the metric gravity, is investigated in detail. In particular, [...] Read more.

Two different Hamiltonian formulations of the metric gravity are discussed and applied to describe a free gravitational field in the d dimensional Riemann space-time. Theory of canonical transformations, which relates equivalent Hamiltonian formulations of the metric gravity, is investigated in detail. In particular, we have formulated the conditions of canonicity for transformation between the two sets of dynamical variables used in our Hamiltonian formulations of the metric gravity. Such conditions include the ordinary condition of canonicity known in classical Hamilton mechanics, i.e., the exact coincidence of the Poisson (or Laplace) brackets which are determined for both the new and old dynamical Hamiltonian variables. However, in addition to this, any true canonical transformations defined in the metric gravity, which is a constrained dynamical system, must also guarantee the exact conservation of the total Hamiltonians

H_{t}

(in both formulations) and preservation of the algebra of first-class constraints. We show that Dirac’s modifications of the classical Hamilton method contain a number of crucial advantages, which provide an obvious superiority of this method in order to develop various non-contradictory Hamiltonian theories of many physical fields, when a number of gauge conditions are also important. Theory of integral invariants and its applications to the Hamiltonian metric gravity are also discussed. For Hamiltonian dynamical systems with first-class constraints this theory leads to a number of peculiarities some of which have been investigated. Full article

(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)

21 pages, 859 KiB

Open AccessArticle

k-Essence Inflation Evading Swampland Conjectures and Inflationary Parameters

by Abdul Jawad, Shamaila Rani, Abdul Malik Sultan and Kashaf Embreen

Universe 2022, 8(10), 532; https://doi.org/10.3390/universe8100532 - 14 Oct 2022

Cited by 6 | Viewed by 1322

Abstract

In this paper, we study the inflationary scenario in the realm of k-essence model and swampland conjectures. Taking into account three models of Chaplygin gas, such as generalized, modified, and generalized cosmic Chaplygin gas models, we discuss the equation of state (EoS) [...] Read more.

In this paper, we study the inflationary scenario in the realm of k-essence model and swampland conjectures. Taking into account three models of Chaplygin gas, such as generalized, modified, and generalized cosmic Chaplygin gas models, we discuss the equation of state (EoS) parameter

ω

, slow roll parameters

(ϵ (ϕ), η (ϕ))

, curvature perturbation

(P_{s})

, tensor to scalar ratio

(r)

, and scalar spectral index

(n_{s})

. As regards the k-essence model, the coupling function as a function of scalar field

L (ϕ)

is used. We investigate the swampland conjecture and then find the value of

ζ (ϕ)

, i.e., bound of second conjecture for these three models by unifying swampland conjecture and k-essence. We plot the EoS parameter

ω

, inflationary parameters plane

r - n_{s}

and bound of swampland conjecture

ζ (ϕ) - ϕ

, which determine that the values of

ω < - 1

for each model, r, are

r < 0.0094, r \leq 0.0065, r \leq 0.0067

, and ranges for

n_{s}

are

[0.934, 0.999], [0.9, 0.999], [0.9, 0.992]

for generalized, modified, and generalized cosmic Chaplygin gas models, respectively, and compare their compatibility with the Planck data from 2018. Furthermore, we determine the bound for swampland conjecture as

ζ (ϕ) \leq 0.992, ζ (ϕ) \leq 0.964, ζ (ϕ) \leq 0.964

for generalized, modified and generalized cosmic Chaplygin gas models, respectively. Full article

(This article belongs to the Special Issue Dark Matter and Dark Energy: Particle Physics, Cosmology, and Experimental Searches)

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2 pages, 155 KiB

Open AccessEditorial

Quantum Models for Cosmology

by Jean-Pierre Gazeau and Przemysław Małkiewicz

Universe 2022, 8(10), 531; https://doi.org/10.3390/universe8100531 - 13 Oct 2022

Viewed by 1262

Abstract

This Special Issue presents a collection of review and original papers on various aspects and applications of quantum cosmological models [...] Full article

(This article belongs to the Special Issue Quantum Models for Cosmology)

12 pages, 3454 KiB

Open AccessArticle

A 2D Lithospheric Magnetic Anomaly Field over Egypt Using Gradient Data of Swarm Mission

by Asmaa Abdellatif, Essam Ghamry, Mohamed Sobh and Adel Fathy

Universe 2022, 8(10), 530; https://doi.org/10.3390/universe8100530 - 12 Oct 2022

Cited by 1 | Viewed by 1492

Abstract

The current work makes use of the geometrical configuration of the two lower-altitude Swarm satellites (Swarm A and C), moving side by side with a longitudinal distance of 1.4°, to estimate a two-dimensional (2D) model of the lithospheric magnetic anomaly field over Egypt [...] Read more.

The current work makes use of the geometrical configuration of the two lower-altitude Swarm satellites (Swarm A and C), moving side by side with a longitudinal distance of 1.4°, to estimate a two-dimensional (2D) model of the lithospheric magnetic anomaly field over Egypt using gradient data. The gradient in both the north–south and the east–west directions have been inverted using the weighted damping least-squares fit technique to estimate the best model coefficients of the 2D model. The best model coefficients have been obtained under the expansion of the Legendre polynomial from degree n = 7 to n = 56. Results showed that the gradient of the field in the north–south direction is always much smoother than that in the east–west direction. The noise in the east–west direction is attributed to the different environmental conditions surrounding both satellites. The modeled field always showed smoother variations than the observed data, even for the horizontal components (B_x and B_y). Full article

(This article belongs to the Section Space Science)

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24 pages, 1493 KiB

Open AccessReview

Black Hole Hyperaccretion in Collapsars: A Review

by Yun-Feng Wei and Tong Liu

Universe 2022, 8(10), 529; https://doi.org/10.3390/universe8100529 - 12 Oct 2022

Cited by 3 | Viewed by 2362

Abstract

The collapsar model is widely accepted as one of the standard scenarios for gamma-ray bursts (GRBs). In the massive collapsar scenario, the core will collapse to a black hole (BH) surrounded by a temporary hyperaccretion disk with a very high accretion rate. The [...] Read more.

The collapsar model is widely accepted as one of the standard scenarios for gamma-ray bursts (GRBs). In the massive collapsar scenario, the core will collapse to a black hole (BH) surrounded by a temporary hyperaccretion disk with a very high accretion rate. The newborn BH hyperaccretion system would launch the relativistic jets via neutrino annihilation and Blandford-Znajek (BZ) mechanism. At the initial accretion stage, the accretion disk should be a neutrino-dominated accretion flow (NDAF). If the jets can break out from the envelope and circumstellar medium, then a GRB will be triggered. In this review, we summarize the theoretical progress on the multimessenger astronomy of the BH hyperaccretion in the center of collapsars. The main topics include: jet propagation in collapsar, MeV neutrinos from NDAFs and proto-neutron stars, gravitational waves from collapsars. Full article

(This article belongs to the Special Issue Advances in Astrophysics and Cosmology – in Memory of Prof. Tan Lu)

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11 pages, 1598 KiB

Open AccessArticle

The Validation of FORMOSAT-3/COSMIC Measurements in the Middle Latitude Region of China with Ionosonde Observations during 2015–2018

by Liangchen Hu, Fanfan Su, Fuying Zhu and Xinxing Li

Universe 2022, 8(10), 528; https://doi.org/10.3390/universe8100528 - 11 Oct 2022

Viewed by 1081

Abstract

We used ground-based ionosonde observations at Ganzi (31.2° N, 100.4° E) to validate the COSMIC measurement in the middle latitude region of China during low solar activity. First, eligible data pairs from two kinds of techniques were selected for the validation. Then, we [...] Read more.

We used ground-based ionosonde observations at Ganzi (31.2° N, 100.4° E) to validate the COSMIC measurement in the middle latitude region of China during low solar activity. First, eligible data pairs from two kinds of techniques were selected for the validation. Then, we investigated the consistency of the ionospheric parameters’ F layer peak density (NmF2) from selected data pairs at different local times in different seasons, and we also investigated the F layer peak height (hmF2). The correlation of the parameters (including NmF2 and hmF2) were good in general. The correlation coefficients of the NmF2 and hmF2 from all selected data pairs were 0.94 and 0.77, respectively. The correlation coefficients were higher in the daytime than those at night for both the NmF2 and hmF2. The correlation coefficients in different seasons were close to each other for both the NmF2 and hmF2. The NmF2 from the COSMIC tends to be overestimated during the whole day except in the morning; the hmF2 from the COSMIC tends to be overestimated in the morning and underestimated in the afternoon. Full article

(This article belongs to the Special Issue Planetary Plasma Environment)

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33 pages, 2518 KiB

Open AccessReview

Software and Techniques for VLBI Data Processing and Analysis

by Michael Janssen, Jack F. Radcliffe and Jan Wagner

Universe 2022, 8(10), 527; https://doi.org/10.3390/universe8100527 - 10 Oct 2022

Cited by 9 | Viewed by 3155

Abstract

Very-long-baseline interferometry (VLBI) is a challenging observational technique, which requires in-depth knowledge about radio telescope instrumentation, interferometry, and the handling of noisy data. The reduction in raw data is mostly left to the scientists and demands the use of complex algorithms implemented in [...] Read more.

Very-long-baseline interferometry (VLBI) is a challenging observational technique, which requires in-depth knowledge about radio telescope instrumentation, interferometry, and the handling of noisy data. The reduction in raw data is mostly left to the scientists and demands the use of complex algorithms implemented in comprehensive software packages. The correct application of these algorithms necessitates a good understanding of the underlying techniques and physics that are at play. The verification of the processed data produced by the algorithms demands a thorough understanding of the underlying interferometric VLBI measurements. This review describes the latest techniques and algorithms that scientists should know about when analyzing VLBI data. Full article

(This article belongs to the Special Issue VLBI Science Applications)

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7 pages, 254 KiB

Open AccessArticle

A Covariant Polymerized Scalar Field in Semi-Classical Loop Quantum Gravity

by Rodolfo Gambini, Florencia Benítez and Jorge Pullin

Universe 2022, 8(10), 526; https://doi.org/10.3390/universe8100526 - 10 Oct 2022

Cited by 12 | Viewed by 1292

Abstract

We propose a new polymerization scheme for scalar fields coupled to gravity. It has the advantage of being a (non-bijective) canonical transformation of the fields, and therefore ensures the covariance of the theory. We study it in detail in spherically symmetric situations and [...] Read more.

We propose a new polymerization scheme for scalar fields coupled to gravity. It has the advantage of being a (non-bijective) canonical transformation of the fields, and therefore ensures the covariance of the theory. We study it in detail in spherically symmetric situations and compare to other approaches. Full article

(This article belongs to the Special Issue Loop Quantum Gravity: A Themed Issue in Honor of Prof. Abhay Ashtekar)

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12 pages, 333 KiB

Open AccessArticle

Damour–Solodukhin Wormhole as a Black Hole Mimicker: The Role of Observers’ Location

by Kamal K. Nandi, Ramis Kh. Karimov, Ramil N. Izmailov and Alexander A. Potapov

Universe 2022, 8(10), 525; https://doi.org/10.3390/universe8100525 - 9 Oct 2022

Cited by 1 | Viewed by 1394

Abstract

It has been recently argued that in semi-classical gravity, a minimal 2-sphere is not a horizon but a tiny throat of a wormhole, such as the Damour–Solodukhin wormhole (DSWH), with a free parameter

λ \neq 0

separating it from a Schwarxzschild black hole [...] Read more.

It has been recently argued that in semi-classical gravity, a minimal 2-sphere is not a horizon but a tiny throat of a wormhole, such as the Damour–Solodukhin wormhole (DSWH), with a free parameter

λ \neq 0

separating it from a Schwarxzschild black hole (BH) (

λ = 0

). As shown by DS, their horizonless WH can mimic many properties of a black hole (BH). Assuming that observing a BH mimicker is equivalent to observing a BH itself, we ask the question as to which identity of the object, a WH or a BH, an observer is likely to observe in a single experiment. To answer this, we introduce Tangherlini’s new concept of indeterminacy in the gravitational field by portraying the field as a refractive medium. We then postulate that the identity of the observed object will depend on the probabilistic outcome of photon motion probing the object. The probabilities will be described by Fresnel reflection (R) and transmission (T) coefficients derived by Tangherlini on the basis of a non-quantum statistical indeterminacy of photon motion in ordinary optical media. By adapting this approach to a gravitational “effective optical medium,” we obtain two intriguing results: (i) The Fresnel coefficients at the DSWH throat are independent of mass M but dependent solely on the parameter

λ \neq 0

. (ii) Depending on the location of the observer, what is a DSWH to one observer may appear as a BH to another observer for the same value of

λ \neq 0

. Full article

(This article belongs to the Section Gravitation)

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10 pages, 268 KiB

Open AccessArticle

Noncommutative Corrections to the Minimal Surface Areas of the Pure AdS Spacetime and Schwarzschild-AdS Black Hole

by Zhang-Cheng Liu and Yan-Gang Miao

Universe 2022, 8(10), 524; https://doi.org/10.3390/universe8100524 - 9 Oct 2022

Viewed by 1279

Abstract

Based on the perturbation expansion, we compute the noncommutative corrections to the minimal surface areas of the pure AdS spacetime and Schwarzschild-AdS black hole, where the noncommutative background is suitably constructed in terms of the Poincaré coordinate system. In particular, we find a [...] Read more.

Based on the perturbation expansion, we compute the noncommutative corrections to the minimal surface areas of the pure AdS spacetime and Schwarzschild-AdS black hole, where the noncommutative background is suitably constructed in terms of the Poincaré coordinate system. In particular, we find a reasonable tetrad with subtlety, which not only matches the metrics of the pure AdS spacetime and Schwarzschild-AdS black hole in the commutative case, but also makes the corrections real rather than complex in the noncommutative case. For the pure AdS spacetime, the nocommutative effect is only a logarithmic term, while for the Schwarzschild-AdS black hole, it contains a logarithmic contribution plus both a mass term and a noncommutative parameter related term. Furthermore, we show that the holographic entanglement entropy with noncommutativity obeys a relation which is similar to the first law of thermodynamics in the pure AdS spacetime. Full article

(This article belongs to the Collection Probing Quantum Gravity)

7 pages, 260 KiB

Open AccessArticle

The Possibility of Mirror Planet as Planet Nine in the Solar System

by Pei Wang, Yuchen Tang, Lei Zu, Yuanyuan Chen and Lei Feng

Universe 2022, 8(10), 523; https://doi.org/10.3390/universe8100523 - 8 Oct 2022

Cited by 1 | Viewed by 1375

Abstract

A series of dynamical anomalies in the orbits of distant trans-Neptunian objects points to a new celestial body (usually named Planet Nine) in the solar system. In this draft, we point out that a mirror planet captured from the outer solar system or [...] Read more.

A series of dynamical anomalies in the orbits of distant trans-Neptunian objects points to a new celestial body (usually named Planet Nine) in the solar system. In this draft, we point out that a mirror planet captured from the outer solar system or formed in the solar system is also a possible candidate. The introduction of the mirror matter model is due to an unbroken parity symmetry and is a potential explanation for dark matter. This mirror planet has null or fainter electromagnetic counterparts with a smaller optical radius and might be explored through gravitational effects. Full article

(This article belongs to the Special Issue Advances in Astrophysics and Cosmology – in Memory of Prof. Tan Lu)

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16 pages, 806 KiB

Open AccessArticle

The Effect of a Spiral Density Wave on the Galaxy’s Rotation Curve, as Applied to the Andromeda Galaxy (M31)

by Miroslava Vukcevic

Universe 2022, 8(10), 522; https://doi.org/10.3390/universe8100522 - 8 Oct 2022

Viewed by 1714

Abstract

The rotational velocity curve, which is the circular velocity profile of the stars and gas in a spiral galaxy as a function of their distance from the galactic center, plays an important role in the kinematic and dynamic investigation of spiral galaxies. There [...] Read more.

The rotational velocity curve, which is the circular velocity profile of the stars and gas in a spiral galaxy as a function of their distance from the galactic center, plays an important role in the kinematic and dynamic investigation of spiral galaxies. There are observations of approximately flat rotation curves (RC) at large distances that have introduced mass discrepancy between the theoretically derived RC and the observed one. In this paper, we derive a rotational velocity expression using a nonlinear spiral density wave solution for the surface mass density (SMD) within the disk. We show that the proposed nonlinear spiral solution is able to support the observed flat rotational velocity curve for large distances with no mass deficiency. The aim of the paper is to confirm the crucial importance of the mass distribution on the rotation curve profile. Although the model is limited by the fluid description of the galactic disk, it provides an improved rotational velocity expression and a rotation curve with no mass discrepancy in the outer part of the disk due to the inclusion of the spiral mass distribution. The disk mass has not been averaged within the exponential disk approximation, but it rather follows the observed spiral pattern given by the analytical solution of the nonlinear equation. The M31 galaxy has been chosen as the closest and well mapped spiral galaxy, similar in many aspects to our host galaxy, in order to apply a rotational velocity expression that accounts for nonlinear effects and derive RC. The obtained result can have a strong influence on large-scale gravity dynamics, as well. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)

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12 pages, 624 KiB

Open AccessArticle

Outliers in Spectral Time Lag-Selected Gamma Ray Bursts

by Fei-Fei Wang and Yuan-Chuan Zou

Universe 2022, 8(10), 521; https://doi.org/10.3390/universe8100521 - 8 Oct 2022

Viewed by 1413

Abstract

It is possible that astrophysical samples are polluted by some outliers, which might belong to a different sub-class. By removing the outliers, the underlying statistical features may be revealed. A more reliable correlation can be used as a standard candle relationship for cosmological [...] Read more.

It is possible that astrophysical samples are polluted by some outliers, which might belong to a different sub-class. By removing the outliers, the underlying statistical features may be revealed. A more reliable correlation can be used as a standard candle relationship for cosmological study. We present outlier searching for gamma ray bursts with the Partitioning Around Medoids (PAM) method. In this work, we choose three parameters from the sample, with all of them having rest-frame spectral time lag (

τ_{lag, i}

). In most cases, the outliers are GRBs 980425B and 030528A. Linear regression is carried out for the sample without the outliers. Some of them have passed hypothesis testing, while others have not. However, even for the passed sample, the correlation is not very significant. More parameter combinations should be considered in future work. Full article

(This article belongs to the Special Issue Advances in Astrophysics and Cosmology – in Memory of Prof. Tan Lu)

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21 pages, 2007 KiB

Open AccessArticle

The Phase Space Analysis of Interacting K-Essence Dark Energy Models in Loop Quantum Cosmology

by Bohai Chen, Yabo Wu, Jianan Chi, Wenzhong Liu and Yiliang Hu

Universe 2022, 8(10), 520; https://doi.org/10.3390/universe8100520 - 6 Oct 2022

Cited by 4 | Viewed by 2536

Abstract

The present work deals with two kinds of k-essence dark energy models within the framework of loop quantum cosmology (LQC). The two kinds of k-essence models originates from two forms of Lagrangians, i.e., [...] Read more.

The present work deals with two kinds of k-essence dark energy models within the framework of loop quantum cosmology (LQC). The two kinds of k-essence models originates from two forms of Lagrangians, i.e.,

L_{1} = F (X) V (ϕ)

and

L_{2} = F (X) - V (ϕ)

, where

F (X)

and

V (ϕ)

stand for the kinetic term and potential of the scalar field

ϕ

, respectively. Two models are based on different phase variables settings, and the general form of autonomous dynamical system is deduced for each Lagrangian. Then, the dynamical stabilities of the critical points in each model are analysed in different forms of

F (X)

and

V (ϕ)

. Model I is a 3-dim system with four stable points, and Model II is a 4-dim system but reduced to a 3-dim system using the symmetry analysis, which has five stable points. Moreover, the corresponding cosmological quantities, such as

Ω_{ϕ}

,

w_{ϕ}

and q, are calculated at each critical point. To compare these with the case of the classical Einstein cosmology (EC), the dynamical evolutionary trajectories in the phase space and evolutionary curves of the cosmological quantities are drawn for both EC and LQC cases, which shows that the loop quantum gravity effects diminish in the late-time universe but are significant in the early time. Further, the effects of interaction

Q = α H ρ_{m}

on the evolutions of the universe are discussed. With the loop quantum gravity effects, bouncing universe is achieved in both models for different initial values of

ϕ_{0}

,

{\dot{ϕ}}_{0}

,

H_{0}

,

ρ_{0}

and coupling parameter

α

, which helps to avoid singularities. However, the interaction has little effect on bounce, although it is important to the stability of some critical points. Full article

(This article belongs to the Special Issue Recent Advances in Quantum Cosmology)

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