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Universe, Volume 9, Issue 3 (March 2023) – 44 articles

Cover Story (view full-size image): The Bayesian evidence is a key tool in model selection, allowing a comparison of models with different numbers of parameters. Its use in the analysis of cosmological models has been limited by difficulties in calculating it, with current numerical algorithms requiring supercomputers. In this paper, we give exact formulae for the Bayesian evidence in the case of Gaussian likelihoods with arbitrary correlations and top-hat priors, and approximate formulae for the case of likelihood distributions with leading non-Gaussianities (skewness and kurtosis). We find that the analytical results have slightly lower precision than the thermodynamic numerical integration, while still being good enough to be useful. The main advantage is that the computational time is reduced by many orders of magnitude, thanks to the analytical expressions. View this paper
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11 pages, 270 KiB  
Communication
Optical Soliton Perturbation with Parabolic Law Nonlinearity
by Ahmed H. Arnous, Islam Samir, Anjan Biswas, Oswaldo González-Gaxiola, Luminita Moraru, Catalina Iticescu, Simona Moldovanu and Abdulah A. Alghamdi
Universe 2023, 9(3), 155; https://doi.org/10.3390/universe9030155 - 21 Mar 2023
Cited by 3 | Viewed by 834
Abstract
This paper recovers a broad spectrum of optical solitons for the perturbed nonlinear Schrödinger’s equation having a dual-power law of nonlinearity. The perturbation terms are from inter-modal dispersion and self-frequency shift. The integration scheme is the improved extended tanh function approach. The parameter [...] Read more.
This paper recovers a broad spectrum of optical solitons for the perturbed nonlinear Schrödinger’s equation having a dual-power law of nonlinearity. The perturbation terms are from inter-modal dispersion and self-frequency shift. The integration scheme is the improved extended tanh function approach. The parameter constraints that naturally emerge are also enumerated. Full article
(This article belongs to the Special Issue Research on Optical Soliton Perturbation)
9 pages, 264 KiB  
Communication
Hawking Radiation from the Boundary Scalar Field and the Information Loss Paradox
by Jingbo Wang
Universe 2023, 9(3), 154; https://doi.org/10.3390/universe9030154 - 18 Mar 2023
Viewed by 894
Abstract
Hawking radiation is an essential property of the quantum black hole. It results in the information loss paradox and provides an important clue with regard to the unification of quantum mechanics and general relativity. In previous work, the boundary scalar fields on the [...] Read more.
Hawking radiation is an essential property of the quantum black hole. It results in the information loss paradox and provides an important clue with regard to the unification of quantum mechanics and general relativity. In previous work, the boundary scalar fields on the horizon of black holes were used to determine the microstates of BTZ black holes and Kerr black holes. They account for Bekenstein–Hawking entropy. In this paper, we show that the Hawking radiation can also be derived from those scalar fields. Hawking radiation is a mixture of the thermal radiation of right- and left-moving sectors at different temperatures. Based on this result, for static BTZ black holes and Schwarzschild black holes, we propose a simple solution for the information loss paradox; i.e., the Hawking radiation is pure due to its entanglement between the left-moving sector and the right-moving sector. This entanglement may be detected in an analogue black hole in the near future. Full article
(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)
18 pages, 361 KiB  
Review
Geometric Outlines of the Gravitational Lensing and Its Astronomic Applications
by Bin Shen and Mingyang Yu
Universe 2023, 9(3), 153; https://doi.org/10.3390/universe9030153 - 17 Mar 2023
Viewed by 1270
Abstract
Gravitational lensing is a topic of great application value in the field of astronomy. The properties and research methods of gravitational lensing are closely related to the geometric and relativistic characteristics of the background universe. This review focuses on the theoretical research and [...] Read more.
Gravitational lensing is a topic of great application value in the field of astronomy. The properties and research methods of gravitational lensing are closely related to the geometric and relativistic characteristics of the background universe. This review focuses on the theoretical research and application of strong lenses and weak lenses. We first introduce the basic principles of gravitational lensing, focusing on the geometric basis of geometric lensing, the representation of deflection angles, and the curvature relationship in different geometric spaces. In addition, we summarize the wide range of applications of gravitational lensing, including the application of strong gravitational lensing in Schwarzschild black holes, time delay, the cosmic shearing based on weak lensing, the applications in signal extraction, dark matter, and dark energy. In astronomy, through the use of advanced astronomical instruments and computers, analyzing gravitational lensing effects to understand the structure of galaxies in the universe is an important topic at present. It is foreseeable that gravitational lensing will continue to play an important role in the study of cosmology and will enrich our understanding of the universe. Full article
(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)
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17 pages, 343 KiB  
Article
Generalized Helical Hypersurface with Space-like Axis in Minkowski 5-Space
by Erhan Güler
Universe 2023, 9(3), 152; https://doi.org/10.3390/universe9030152 - 15 Mar 2023
Cited by 1 | Viewed by 928
Abstract
We introduce the generalized helical hypersurface having a space-like axis in five-dimensional Minkowski space. We compute the first and second fundamental form matrices, Gauss map, and shape operator matrix of the hypersurface. Additionally, we compute the curvatures of the hypersurface by using the [...] Read more.
We introduce the generalized helical hypersurface having a space-like axis in five-dimensional Minkowski space. We compute the first and second fundamental form matrices, Gauss map, and shape operator matrix of the hypersurface. Additionally, we compute the curvatures of the hypersurface by using the Cayley–Hamilton theorem. Moreover, we give some relations for the mean and the Gauss–Kronecker curvatures of the hypersurface. Finally, we obtain the Laplace–Beltrami operator of the hypersurface. Full article
(This article belongs to the Section Mathematical Physics)
10 pages, 310 KiB  
Communication
Point Charge Subject to an Attractive Inverse-Square-Type Potential and Anharmonic-Type Potentials
by Jardel de Carvalho Veloso and Knut Bakke
Universe 2023, 9(3), 151; https://doi.org/10.3390/universe9030151 - 15 Mar 2023
Viewed by 743
Abstract
By applying the WKB (Wentzel, Kramers, Brillouin) approximation, we search for bound state solutions to the time-independent Schrödinger equation for an attractive inverse-square potential and anharmonic oscillators that stem from the interaction of a point charge with radial electric fields. We focus on [...] Read more.
By applying the WKB (Wentzel, Kramers, Brillouin) approximation, we search for bound state solutions to the time-independent Schrödinger equation for an attractive inverse-square potential and anharmonic oscillators that stem from the interaction of a point charge with radial electric fields. We focus on the bound states associated with the s-waves. Further, we obtain the revival time associated with each case studied. Full article
(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)
10 pages, 287 KiB  
Article
A New Perspective on Doubly Special Relativity
by J. M. Carmona, J. L. Cortés, J. J. Relancio and M. A. Reyes
Universe 2023, 9(3), 150; https://doi.org/10.3390/universe9030150 - 15 Mar 2023
Cited by 1 | Viewed by 1376
Abstract
Doubly special relativity considers a deformation of the special relativistic kinematics parametrized by a high-energy scale, in such a way that it preserves a relativity principle. When this deformation is assumed to be applied to any interaction between particles, one faces some inconsistencies. [...] Read more.
Doubly special relativity considers a deformation of the special relativistic kinematics parametrized by a high-energy scale, in such a way that it preserves a relativity principle. When this deformation is assumed to be applied to any interaction between particles, one faces some inconsistencies. In order to avoid them, we propose a new perspective where the deformation affects only the interactions between elementary particles. A consequence of this proposal is that the deformation cannot modify the special relativistic energy–momentum relation of a particle. Full article
(This article belongs to the Special Issue Quantum Gravity Phenomenology II)
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11 pages, 301 KiB  
Article
Periodic Orbits of Quantised Restricted Three-Body Problem
by Elbaz I. Abouelmagd, Juan Luis García Guirao and Jaume Llibre
Universe 2023, 9(3), 149; https://doi.org/10.3390/universe9030149 - 15 Mar 2023
Cited by 4 | Viewed by 1082
Abstract
In this paper, perturbed third-body motion is considered under quantum corrections to analyse the existence of periodic orbits. These orbits are studied through two approaches to identify the first (second) periodic-orbit types. The essential conditions are given in order to prove that the [...] Read more.
In this paper, perturbed third-body motion is considered under quantum corrections to analyse the existence of periodic orbits. These orbits are studied through two approaches to identify the first (second) periodic-orbit types. The essential conditions are given in order to prove that the circular (elliptical) periodic orbits of the rotating Kepler problem (RKP) can continue to the perturbed motion of the third body under quantum corrections where a massive primary body has excessive gravitational force over the smaller primary body. The primaries moved around each other in circular (elliptical) orbits, and the mass ratio was assumed to be sufficiently small. We prove the existence of the two types of orbits by using the terminologies of Poincaré for quantised perturbed motion. Full article
(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)
19 pages, 1320 KiB  
Article
Dual-Driven Solver for Reconstructing the Point Sources of Elastic Wave Based on Far-Field Data
by Pinchao Meng, Yuanyuan Chai and Weishi Yin
Universe 2023, 9(3), 148; https://doi.org/10.3390/universe9030148 - 12 Mar 2023
Viewed by 809
Abstract
Aiming at the inverse source problem of an elastic wave, a dual-driver solver is considered to reconstruct the point sources. In this way, the number, location, and magnitude of the point sources can be reconstructed from far-field measurement data. The solver is composed [...] Read more.
Aiming at the inverse source problem of an elastic wave, a dual-driver solver is considered to reconstruct the point sources. In this way, the number, location, and magnitude of the point sources can be reconstructed from far-field measurement data. The solver is composed of a data-driven module and a physical-driven module, which is coupled by a loss. The loss of the data-driven module and the physical-driven module are both the driving force of the solver evolution. The solver takes the far-field data as the input, and the number, location, and magnitude of the point sources as the output. It is trained by the Adam algorithm. Numerical experiments show that this method is effective for reconstructing the multi-sources. Full article
(This article belongs to the Section Mathematical Physics)
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30 pages, 2037 KiB  
Article
Probing Modified Gravity Theories with Scalar Fields Using Black-Hole Images
by Georgios Antoniou, Alexandros Papageorgiou and Panagiota Kanti
Universe 2023, 9(3), 147; https://doi.org/10.3390/universe9030147 - 11 Mar 2023
Cited by 8 | Viewed by 1593
Abstract
We study a number of well-motivated theories of modified gravity with the common overarching theme that they predict the existence of compact objects, such as black holes and wormholes endowed with scalar hair. We compute the shadow radius of the resulting compact objects [...] Read more.
We study a number of well-motivated theories of modified gravity with the common overarching theme that they predict the existence of compact objects, such as black holes and wormholes endowed with scalar hair. We compute the shadow radius of the resulting compact objects and demonstrate that black hole images, such as that of M87* or the more recent SgrA* by the Event Horizon Telescope (EHT) collaboration, could provide a powerful way to constrain deviations of the metric functions from what is expected from general relativity (GR) solutions. We focus our attention on Einstein-scalar-Gauss–Bonnet (EsGB) theory with three well-motivated couplings, including the dilatonic and Z2 symmetric cases. We then analyze the shadow radius of black holes in the context of the spontaneous scalarization scenario within EsGB theory with an additional coupling to the Ricci scalar (EsRGB). Finally, we turn our attention to spontaneous scalarization in the Einstein–Maxwell-Scalar (EMS) theory and demonstrate the impact of the parameters on the black hole shadow. Our results show that black hole imaging is an important tool for constraining black holes with scalar hair, and, for some part of the parameter space, black hole solutions with scalar hair may be marginally favored compared to solutions of GR. Full article
(This article belongs to the Section Gravitation)
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12 pages, 374 KiB  
Article
The General Property of Tracking and Thawing Models and Their Observational Constraints
by Yujie You, Qichao Qiang and Qing Gao
Universe 2023, 9(3), 146; https://doi.org/10.3390/universe9030146 - 11 Mar 2023
Viewed by 1065
Abstract
We study the general property of the evolution of a class of scalar fields with tracking and thawing behaviors. For the tracking solutions, we show explicitly with three different potentials that, independent of initial conditions, there exists a general relation between the equation [...] Read more.
We study the general property of the evolution of a class of scalar fields with tracking and thawing behaviors. For the tracking solutions, we show explicitly with three different potentials that, independent of initial conditions, there exists a general relation between the equation of state wϕ and the fractional energy density Ωϕ, so that the scalar field follows the same wϕΩϕ trajectory during the evolution. The analytical approximations of the wϕΩϕ trajectories are derived even though the analytical expression depends upon the particular form of the potential. For thawing solutions, a universal wϕΩϕ relation exists and the relation is independent of both the particular form of the potential and the initial condition of the scalar field. Based on the derived wϕΩϕ relation for the thawing models, we derive a tighter upper limit on wϕ=dwϕ/dlna. The observational data is also used to constrain the thawing potential with the help of the universal wϕΩϕ relation. Full article
(This article belongs to the Section Cosmology)
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17 pages, 570 KiB  
Article
Stability Analysis of the Inhomogeneous Perturbed Einstein Universe in Energy–Momentum Squared Gravity
by Muhammad Sharif and Muhammad Zeeshan Gul
Universe 2023, 9(3), 145; https://doi.org/10.3390/universe9030145 - 10 Mar 2023
Cited by 7 | Viewed by 884
Abstract
The main objective of this article is to examine the stability of Einstein static universe using inhomogeneous perturbations in the context of energy–momentum squared gravity. For this purpose, we used FRW spacetime with perfect matter distribution and formulated static as well as perturbed [...] Read more.
The main objective of this article is to examine the stability of Einstein static universe using inhomogeneous perturbations in the context of energy–momentum squared gravity. For this purpose, we used FRW spacetime with perfect matter distribution and formulated static as well as perturbed field equations. We took a minimal model of this theory to investigate the stable regions of the Einstein universe for conserved and non-conserved energy–momentum tensors. We found that stable modes of the Einstein universe appeared in both conserved and non-conserved cases for all values of the equation of state and model parameters corresponding to both open and closed cosmic models. We found that stable solutions in this modified theory were obtained for a broader ω-region compared to other modified theories. Full article
(This article belongs to the Section Cosmology)
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21 pages, 373 KiB  
Article
On Born’s Reciprocal Relativity, Algebraic Extensions of the Yang and Quaplectic Algebra, and Noncommutative Curved Phase Spaces
by Carlos Castro Perelman
Universe 2023, 9(3), 144; https://doi.org/10.3390/universe9030144 - 09 Mar 2023
Cited by 1 | Viewed by 789
Abstract
After a brief introduction of Born’s reciprocal relativity theory is presented, we review the construction of the deformed quaplectic group that is given by the semi-direct product of U(1,3) with the [...] Read more.
After a brief introduction of Born’s reciprocal relativity theory is presented, we review the construction of the deformed quaplectic group that is given by the semi-direct product of U(1,3) with the deformed (noncommutative) Weyl–Heisenberg group corresponding to noncommutative fiber coordinates and momenta [Xa,Xb]0; [Pa,Pb]0. This construction leads to more general algebras given by a two-parameter family of deformations of the quaplectic algebra, and to further algebraic extensions involving antisymmetric tensor coordinates and momenta of higher ranks [Xa1a2an,Xb1b2bn]0; [Pa1a2an,Pb1b2bn]0. We continue by examining algebraic extensions of the Yang algebra in extended noncommutative phase spaces and compare them with the above extensions of the deformed quaplectic algebra. A solution is found for the exact analytical mapping of the noncommuting xμ,pμ operator variables (associated to an 8D curved phase space) to the canonical YA,ΠA operator variables of a flat 12D phase space. We explore the geometrical implications of this mapping which provides, in the classical limit, the embedding functions YA(x,p),ΠA(x,p) of an 8D curved phase space into a flat 12D phase space background. The latter embedding functions determine the functional forms of the base spacetime metric gμν(x,p), the fiber metric of the vertical space hab(x,p), and the nonlinear connection Naμ(x,p) associated with the 8D cotangent space of the 4D spacetime. Consequently, we find a direct link between noncommutative curved phase spaces in lower dimensions and commutative flat phase spaces in higher dimensions. Full article
(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)
11 pages, 2840 KiB  
Article
The Dynamics of Earth’s Cusp in Response to the Interplanetary Shock
by Jie Ren, Qiugang Zong, Suiyan Fu, Huigen Yang, Zejun Hu, Xiaoxin Zhang, Xuzhi Zhou, Chao Yue, Lynn Kistler, Patrick Daly, Elena Kronberg and Robert Rankin
Universe 2023, 9(3), 143; https://doi.org/10.3390/universe9030143 - 08 Mar 2023
Cited by 1 | Viewed by 1256
Abstract
The Earth’s magnetospheric cusp, a region with an off-equatorial magnetic field minimum, is an important place which directly transports plasma and energy from the solar wind into the magnetosphere and ionosphere. Its magnetic topology and charged particles therein are known to respond to [...] Read more.
The Earth’s magnetospheric cusp, a region with an off-equatorial magnetic field minimum, is an important place which directly transports plasma and energy from the solar wind into the magnetosphere and ionosphere. Its magnetic topology and charged particles therein are known to respond to the solar wind and the interplanetary magnetic field. However, its dynamics in response to the interplanetary (IP) shock are still unknown, due to lack of direct spacecraft observations. This study first reports the observations of the cusp’s motion under the drive of an IP shock and both strong electric fields and outflowing energetic ions in the moving cusp. After an IP shock arrival on 7 September 2017, triple cusps were observed by Cluster C4 when it was crossing the high-altitude northern polar region to the sub-solar magnetosphere. The multiple cusps had a one-to-one correspondence with the dayside magnetosphere compression and relaxation detected by THEMIS E, indicating that one cusp moved back and forth three times due to the IP shock’s impact. In the moving cusp, there were strong impulsive electric fields with a peak of up to ∼40 mV/m and an ionospheric source population of upward propagating ions (O+, He+ and H+) with energies extending to MeV. However, the outflowing ions outside the cusp had energies of no more than 1 keV. An enhancement of energetic O+ appeared inside the cusp with the flux ratio of O+/H+ increasing from 10 keV to ∼ MeV, which implies the efficient acceleration of O+. These observations are shown to be consistent with the prompt acceleration by the impulsive electric fields, which is mass-dependent. This finding suggests a new acceleration mechanism for cusp energetic ions, especially for O+. Full article
(This article belongs to the Special Issue Auroral Physics)
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16 pages, 739 KiB  
Review
Solar Radio Emissions and Ultralight Dark Matter
by Haipeng An, Shuailiang Ge and Jia Liu
Universe 2023, 9(3), 142; https://doi.org/10.3390/universe9030142 - 07 Mar 2023
Cited by 6 | Viewed by 1222
Abstract
Ultralight axions and dark photons are well-motivated dark matter candidates. Inside the plasma, once the mass of ultralight dark matter candidates equals the plasma frequency, they can resonantly convert into electromagnetic waves, due to the coupling between the ultralight dark matter particles and [...] Read more.
Ultralight axions and dark photons are well-motivated dark matter candidates. Inside the plasma, once the mass of ultralight dark matter candidates equals the plasma frequency, they can resonantly convert into electromagnetic waves, due to the coupling between the ultralight dark matter particles and the standard model photons. The converted electromagnetic waves are monochromatic. In this article, we review the development of using radio detectors to search for ultralight dark matter conversions in the solar corona and solar wind plasma. Full article
(This article belongs to the Special Issue Solar Radio Emissions)
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61 pages, 650 KiB  
Review
Introducing the Random Phase Approximation Theory
by Giampaolo Co’
Universe 2023, 9(3), 141; https://doi.org/10.3390/universe9030141 - 07 Mar 2023
Cited by 4 | Viewed by 3312
Abstract
Random Phase Approximation (RPA) is the theory most commonly used to describe the excitations of many-body systems. In this article, the secular equations of the theory are obtained by using three different approaches: the equation of motion method, the Green function perturbation theory [...] Read more.
Random Phase Approximation (RPA) is the theory most commonly used to describe the excitations of many-body systems. In this article, the secular equations of the theory are obtained by using three different approaches: the equation of motion method, the Green function perturbation theory and the time-dependent Hartree–Fock theory. Each approach emphasizes specific aspects of the theory overlooked by the other methods. Extensions of the RPA secular equations to treat the continuum part of the excitation spectrum and also the pairing between the particles composing the system are presented. Theoretical approaches which overcome the intrinsic approximations of RPA are outlined. Full article
(This article belongs to the Special Issue Many Body Theory)
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3 pages, 167 KiB  
Editorial
Editorial to the Special Issue “Propagation of Coronal Mass Ejections”
by Mateja Dumbović and Fang Shen
Universe 2023, 9(3), 140; https://doi.org/10.3390/universe9030140 - 07 Mar 2023
Viewed by 603
Abstract
Coronal mass ejections (CMEs) and their associated shocks are one of the main drivers of heliosphere variability, causing both interplanetary and planetary perturbations [...] Full article
(This article belongs to the Special Issue Propagation of Coronal Mass Ejections)
12 pages, 356 KiB  
Article
Random Walk on a Rough Surface: Renormalization Group Analysis of a Simple Model
by Nikolay V. Antonov, Nikolay M. Gulitskiy, Polina I. Kakin and Dmitriy A. Kerbitskiy
Universe 2023, 9(3), 139; https://doi.org/10.3390/universe9030139 - 07 Mar 2023
Cited by 1 | Viewed by 869
Abstract
The field-theoretic renormalization group is applied to a simple model of a random walk on a rough fluctuating surface. We consider the Fokker–Planck equation for a particle in a uniform gravitational field. The surface is modeled by the generalized Edwards–Wilkinson linear stochastic equation [...] Read more.
The field-theoretic renormalization group is applied to a simple model of a random walk on a rough fluctuating surface. We consider the Fokker–Planck equation for a particle in a uniform gravitational field. The surface is modeled by the generalized Edwards–Wilkinson linear stochastic equation for the height field. The full stochastic model is reformulated as a multiplicatively renormalizable field theory, which allows for the application of the standard renormalization theory. The renormalization group equations have several fixed points that correspond to possible scaling regimes in the infrared range (long times and large distances); all the critical dimensions are found exactly. As an example, the spreading law for the particle’s cloud is derived. It has the form R2(t)t2/Δω with the exactly known critical dimension of frequency Δω and, in general, differs from the standard expression R2(t)t for an ordinary random walk. Full article
(This article belongs to the Section Field Theory)
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65 pages, 3005 KiB  
Review
Quiescent and Active Galactic Nuclei as Factories of Merging Compact Objects in the Era of Gravitational Wave Astronomy
by Manuel Arca Sedda, Smadar Naoz and Bence Kocsis
Universe 2023, 9(3), 138; https://doi.org/10.3390/universe9030138 - 06 Mar 2023
Cited by 14 | Viewed by 1365
Abstract
Galactic nuclei harbouring a central supermassive black hole (SMBH), possibly surrounded by a dense nuclear cluster (NC), represent extreme environments that house a complex interplay of many physical processes that uniquely affect stellar formation, evolution, and dynamics. The discovery of gravitational waves (GWs) [...] Read more.
Galactic nuclei harbouring a central supermassive black hole (SMBH), possibly surrounded by a dense nuclear cluster (NC), represent extreme environments that house a complex interplay of many physical processes that uniquely affect stellar formation, evolution, and dynamics. The discovery of gravitational waves (GWs) emitted by merging black holes (BHs) and neutron stars (NSs), funnelled a huge amount of work focused on understanding how compact object binaries (COBs) can pair up and merge together. Here, we review from a theoretical standpoint how different mechanisms concur with the formation, evolution, and merger of COBs around quiescent SMBHs and active galactic nuclei (AGNs), summarising the main predictions for current and future (GW) detections and outlining the possible features that can clearly mark a galactic nuclei origin. Full article
(This article belongs to the Special Issue Binary Evolution in Galactic Nuclei)
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19 pages, 515 KiB  
Article
Leptogenesis and Dark Matter–Nucleon Scattering Cross Section in the SE6SSM
by Roman Nevzorov
Universe 2023, 9(3), 137; https://doi.org/10.3390/universe9030137 - 06 Mar 2023
Cited by 2 | Viewed by 902
Abstract
The E6-inspired extension of the minimal supersymmetric (SUSY) standard model (MSSM) with an extra U(1)N gauge symmetry, under which right-handed neutrinos have zero charge, involves exotic matter beyond the MSSM to ensure anomaly cancellation. We consider the [...] Read more.
The E6-inspired extension of the minimal supersymmetric (SUSY) standard model (MSSM) with an extra U(1)N gauge symmetry, under which right-handed neutrinos have zero charge, involves exotic matter beyond the MSSM to ensure anomaly cancellation. We consider the variant of this extension (SE6SSM) in which the cold dark matter is composed of the lightest neutral exotic fermion and gravitino. The observed baryon asymmetry can be induced in this case via the decays of the lightest right-handed neutrino/sneutrino into exotic states even for relatively low reheating temperatures TR1067GeV. We argue that there are some regions of the SE6SSM parameter space, which are safe from all current constraints, and discuss the implications of this model for collider phenomenology. Full article
(This article belongs to the Special Issue Advances in Cosmology and Subatomic Particle Physics)
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27 pages, 1086 KiB  
Article
Realization of Bounce in a Modified Gravity Framework and Information Theoretic Approach to the Bouncing Point
by Sanghati Saha and Surajit Chattopadhyay
Universe 2023, 9(3), 136; https://doi.org/10.3390/universe9030136 - 06 Mar 2023
Cited by 5 | Viewed by 1328
Abstract
In this work, we report a study on bouncing cosmology with modified generalized Chaplygin Gas (mgCG) in a bulk viscosity framework. Reconstruction schemes were demonstrated in Einstein and modified f(T) gravity framework under the purview of viscous cosmological settings. We [...] Read more.
In this work, we report a study on bouncing cosmology with modified generalized Chaplygin Gas (mgCG) in a bulk viscosity framework. Reconstruction schemes were demonstrated in Einstein and modified f(T) gravity framework under the purview of viscous cosmological settings. We also took non-viscous cases into account. We studied the equation of state (EoS) parameter under various circumstances and judged the stability of the models through the sign of the squared speed of sound. We observed the mgCG behaving like avoidance of Big Rip in the presence of bulk viscosity at the turnaround point and in non-viscous cases, a phantom-like behavior appears. The turnaround point equation of state parameter crosses the phantom boundary, violating NEC. The role of the mgCG’s model parameters was also investigated before and after the bounce. A Hubble flow dynamics was carried out and, it was revealed that mgCG is capable of realizing an inflationary phase as well as an exit from inflation. An f(T) gravitational paradigm was also considered, where the mgCG density was reconstructed in the presence of bulk viscosity. The role of the parameters associated with the bouncing scale factor, describing how fast the bounce takes place, was also studied in this framework. Finally, the reconstructed mgCG turned out to be stable against small perturbations irrespective of the presence of bulk viscosity and modified gravity scenario. Finally, the reconstruction scheme was assessed using statistical analysis, Shannon entropy. Full article
(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)
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22 pages, 1271 KiB  
Article
Magnetized Particles with Electric Charge around Schwarzschild Black Holes in External Magnetic Fields
by Javlon Rayimbaev, Sanjar Shaymatov, Farrux Abdulxamidov, Saidmuhammad Ahmedov and Dilfuza Begmatova
Universe 2023, 9(3), 135; https://doi.org/10.3390/universe9030135 - 06 Mar 2023
Cited by 6 | Viewed by 1500
Abstract
We investigate the dynamics of test particles endowed with both electric charge and a magnetic dipole moment around a Schwarzschild black hole (BH) immersed in an externally asymptotically uniform magnetic field. We further analyse the effective potential and specific angular momentum and energy [...] Read more.
We investigate the dynamics of test particles endowed with both electric charge and a magnetic dipole moment around a Schwarzschild black hole (BH) immersed in an externally asymptotically uniform magnetic field. We further analyse the effective potential and specific angular momentum and energy of the particles. Furthermore, we show that the upper limit for magnetic interaction parameter β increases with increasing cyclotron frequency ωB, while the radius of the innermost stable circular orbit (ISCO) for charged test particles decreases for the upper value of β=βupper. Furthermore, we show that the energy efficiency released from the BH increases up to about 90% due to the presence of the magnetic dipole moment of the test particle. We explore a degeneracy between the spin parameter of rotating Kerr BH and the magnetic parameter for the values of the ISCO radius and energy efficiency. We study in detail the centre of mass energy for collisions of charged and magnetized particles in the environment surrounding the Schwarzchild BH. Finally, as an astrophysical application, we explore the magnetized parameter and cyclotron frequency numerically for a rotating magnetized neutron star. Interestingly, we show that the corresponding values of the above-mentioned parameters for the magnetar PSR J1745-2900 that orbits around the supermassive black hole (SMBH) that exists at the centre of the Milky Way galaxy are ωB5 and β0.67, respectively, for the magnetic field is about 10 G. Full article
(This article belongs to the Special Issue Galactic Center with Gravity)
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31 pages, 387 KiB  
Article
Contemporary Philosophical Perspectives on the Cosmological Constant
by Adam Koberinski, Bridget Falck and Chris Smeenk
Universe 2023, 9(3), 134; https://doi.org/10.3390/universe9030134 - 05 Mar 2023
Cited by 1 | Viewed by 1565
Abstract
The (re)introduction of Λ into cosmology has spurred debates that touch on central questions in philosophy of science, as well as the foundations of general relativity and particle physics. We provide a systematic assessment of the often implicit philosophical assumptions guiding the methodology [...] Read more.
The (re)introduction of Λ into cosmology has spurred debates that touch on central questions in philosophy of science, as well as the foundations of general relativity and particle physics. We provide a systematic assessment of the often implicit philosophical assumptions guiding the methodology of precision cosmology in relation to dark energy. We start by briefly introducing a recent account of scientific progress in terms of risky and constrained lines of inquiry. This allows us to contrast aspects of Λ that make it relevantly different from other theoretical entities in science, such as its remoteness from direct observation or manipulability. We lay out a classification for possible ways to explain apparent accelerated expansion but conclude that these conceptually clear distinctions may blur heavily in practice. Finally, we consider the important role played in cosmology by critical tests of background assumptions, approximation techniques, and core principles, arguing that the weak anthropic principle fits into this category. We argue that some core typicality assumptions—such as the Copernican principle and the cosmological principle—are necessary though not provable, while others—such as the strong anthropic principle and appeals to naturalness or probability in the multiverse—are not similarly justifiable. Full article
(This article belongs to the Special Issue Cosmological Constant)
12 pages, 329 KiB  
Article
Tidal Effects and Clock Comparison Experiments
by Cheng-Gang Qin, Tong Liu, Jin-Zhuang Dong, Xiao-Yi Dai, Yu-Jie Tan and Cheng-Gang Shao
Universe 2023, 9(3), 133; https://doi.org/10.3390/universe9030133 - 04 Mar 2023
Cited by 1 | Viewed by 922
Abstract
Einstein’s general relativity theory provides a successful understanding of the flow of time in the gravitational field. From Einstein’s equivalence principle, the influence of the Sun and Moon masses on clocks is given in the form of tidal potentials. Two clocks fixed on [...] Read more.
Einstein’s general relativity theory provides a successful understanding of the flow of time in the gravitational field. From Einstein’s equivalence principle, the influence of the Sun and Moon masses on clocks is given in the form of tidal potentials. Two clocks fixed on the surface of the Earth, compared to each other, can measure the tidal effects of the Sun and Moon. The measurement of tidal effects can provide a test for general relativity. Based on the standard general relativity method, we rigorously derive the formulas for clock comparison in the Barycentric Celestial Reference System and Geocentric Celestial Reference System, and demonstrate the tidal effects on clock comparison experiments. The unprecedented performance of atomic clocks makes it possible to measure the tidal effects on clock comparisons. We propose to test tidal effects with the laboratory clock comparisons and some international missions, and give the corresponding estimations. By comparing the state-of-the-art clocks over distances of 1000 km, the laboratory may test tidal effects with a level of 1%. Future space missions, such as the China space station and FOCOS mission, can also be used to test tidal effects, and the best accuracy may reach 0.3%. Full article
(This article belongs to the Special Issue Tidal Effects in General Relativity)
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18 pages, 1041 KiB  
Article
Approximate Analytical Solutions of the Schrödinger Equation with Hulthén Potential in the Global Monopole Spacetime
by Saulo S. Alves, Márcio M. Cunha, Hassan Hassanabadi and Edilberto O. Silva
Universe 2023, 9(3), 132; https://doi.org/10.3390/universe9030132 - 04 Mar 2023
Cited by 7 | Viewed by 1286
Abstract
In this paper, we studied the nonrelativistic quantum mechanics of an electron in a spacetime containing a topological defect. We also considered that the electron is influenced by the Hulthén potential. In particular, we dealt with the Schrödinger equation in the presence of [...] Read more.
In this paper, we studied the nonrelativistic quantum mechanics of an electron in a spacetime containing a topological defect. We also considered that the electron is influenced by the Hulthén potential. In particular, we dealt with the Schrödinger equation in the presence of a global monopole. We obtained approximate solutions for the problem, determined the scattering phase shift and the S-matrix, and analyzed bound states. Full article
(This article belongs to the Section Mathematical Physics)
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26 pages, 408 KiB  
Review
Unimodular Approaches to the Cosmological Constant Problem
by Pavel Jiroušek
Universe 2023, 9(3), 131; https://doi.org/10.3390/universe9030131 - 04 Mar 2023
Cited by 9 | Viewed by 726
Abstract
We review selected aspects of unimodular gravity and we discuss its viability as a solution of the old cosmological constant problem. In unimodular gravity, the cosmological constant is promoted to a global degree of freedom. We highlight the importance of correctly setting up [...] Read more.
We review selected aspects of unimodular gravity and we discuss its viability as a solution of the old cosmological constant problem. In unimodular gravity, the cosmological constant is promoted to a global degree of freedom. We highlight the importance of correctly setting up its initial data in order to achieve a resolution of the cosmological constant problem on a semi-classical level. We review recent path integral analysis of quantum aspects of unimodular gravity to note that the semi-classical findings carry over to the quantum level as well. We point out that a resolution of the problem inherently relies on a global constraint on the spacetime four-volume. This makes the theory closely related to the vacuum energy sequester, which operates in a similar way. We discuss possible avenues of extending unimodular gravity that preserve the resolution of the cosmological constant problem. Full article
(This article belongs to the Special Issue Cosmological Constant)
16 pages, 2659 KiB  
Article
Strong Deflection Gravitational Lensing for the Photons Coupled to the Weyl Tensor in a Conformal Gravity Black Hole
by Ghulam Abbas, Ali Övgün, Asif Mahmood and Muhammad Zubair
Universe 2023, 9(3), 130; https://doi.org/10.3390/universe9030130 - 02 Mar 2023
Viewed by 1029
Abstract
In the present paper, strong deflection gravitational lensing is studied in a conformal gravity black hole. With the help of geometric optics limits, we have formulated the light cone conditions for the photons coupled to the Weyl tensor in a conformal gravity black [...] Read more.
In the present paper, strong deflection gravitational lensing is studied in a conformal gravity black hole. With the help of geometric optics limits, we have formulated the light cone conditions for the photons coupled to the Weyl tensor in a conformal gravity black hole. It is explicitly found that strong deflection gravitational lensing depends on the coupling with the Weyl tensor, the polarization directions, and the black hole configuration parameters. We have applied the results of the strong deflection gravitational lensing to the supermassive black holes SgrA* and M87* and studied the possibility of encountering quantum improvement. It is not practicable to recognize similar black holes through the strong deflection gravitational lensing observables in the near future, except for the possible size of the black hole’s shadow. We also notice that by directly adopting the constraint of the measured shadow of M87*, the quantum effect demands immense care. Full article
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21 pages, 459 KiB  
Review
A Light Shed on Lepton Flavor Universality in B Decays
by Sonali Patnaik and Rajeev Singh
Universe 2023, 9(3), 129; https://doi.org/10.3390/universe9030129 - 01 Mar 2023
Cited by 4 | Viewed by 1017
Abstract
Behind succeeding measurements of anomalies in semileptonic decays at LHCb and several collider experiments hinting at the possible violation of lepton flavor universality, we undertake a concise review of theoretical foundations of the tree- and loop-level b-hadron decays, [...] Read more.
Behind succeeding measurements of anomalies in semileptonic decays at LHCb and several collider experiments hinting at the possible violation of lepton flavor universality, we undertake a concise review of theoretical foundations of the tree- and loop-level b-hadron decays, bclνl and bsl+l along with experimental environments. We revisit the world averages for RD(D*), RK(K*), RJ/ψ, and Rηc, for the semileptonic transitions and provide results within the framework of the relativistic independent quark model in addition to the results from model-independent studies. If the ongoing evaluation of the data of LHC Run 2 confirms the measurements of Run 1, then the statistical significance of the effect in each decay channel is likely to reach 5 σ. A confirmation of these measurements would soon turn out to be the first remarkable observation of physics beyond the Standard Model, providing a wider outlook on the understanding of new physics. Full article
(This article belongs to the Special Issue Search for New Physics at the LHC and Future Colliders)
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24 pages, 516 KiB  
Review
Aspects of Quantum Gravity Phenomenology and Astrophysics
by Arundhati Dasgupta and José Fajardo-Montenegro
Universe 2023, 9(3), 128; https://doi.org/10.3390/universe9030128 - 01 Mar 2023
Cited by 2 | Viewed by 1441
Abstract
With the discovery of gravitational waves, the search for the quantum of gravity, the graviton, is imminent. We discuss the current status of the bounds on graviton mass from experiments as well as the theoretical understanding of these particles. We provide an overview [...] Read more.
With the discovery of gravitational waves, the search for the quantum of gravity, the graviton, is imminent. We discuss the current status of the bounds on graviton mass from experiments as well as the theoretical understanding of these particles. We provide an overview of current experiments in astrophysics such as the search for Hawking radiation in gamma-ray observations and neutrino detectors, which will also shed light on the existence of primordial black holes. Finally, the semiclassical corrections to the image of the event horizon are discussed. Full article
(This article belongs to the Special Issue Quantum Gravity Phenomenology II)
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10 pages, 267 KiB  
Article
Kerr–Schild Tetrads and the Nijenhuis Tensor
by José Wadih Maluf, Fernando Lessa Carneiro, Sérgio Ulhoa and José Francisco Da Rocha-Neto
Universe 2023, 9(3), 127; https://doi.org/10.3390/universe9030127 - 28 Feb 2023
Viewed by 834
Abstract
We write the Kerr–Schild tetrads in terms of the flat space–time tetrads and of a (1, 1) tensor Sμλ. This tensor can be considered as a projection operator, since it transforms (i) flat space–time tetrads into non-flat tetrads, and vice-versa, [...] Read more.
We write the Kerr–Schild tetrads in terms of the flat space–time tetrads and of a (1, 1) tensor Sμλ. This tensor can be considered as a projection operator, since it transforms (i) flat space–time tetrads into non-flat tetrads, and vice-versa, and (ii) the Minkowski space–time metric tensor into a non-flat metric tensor, and vice-versa. The Sμλ tensor and its inverse are constructed in terms of the standard null vector field lμ that defines the Kerr–Schild form of the metric tensor in general relativity, and that yields black holes and non-linear gravitational waves as solutions of the vacuum Einstein’s field equations. We demonstrate that the condition for the vanishing of the Ricci tensor obtained by Kerr and Schild, in empty space–time, is also a condition for the vanishing of the Nijenhuis tensor constructed out of Sμλ. Thus, a theory based on the Nijenhuis tensor yields an important class of solutions of the Einstein’s field equations, namely, black holes and non-linear gravitational waves. We also demonstrate that the present mathematical framework can easily admit modifications of the Newtonian potential that may explain the long range gravitational effects related to galaxy rotation curves. Full article
(This article belongs to the Section Gravitation)
11 pages, 1070 KiB  
Article
Stark Broadening of Al IV Spectral Lines
by Milan S. Dimitrijević and Magdalena D. Christova
Universe 2023, 9(3), 126; https://doi.org/10.3390/universe9030126 - 28 Feb 2023
Cited by 1 | Viewed by 733
Abstract
Stark widths for 23 transitions in Al IV have been calculated by employing the modified semiempirical method. The results are obtained for an electron density of 1017 cm3 and temperatures from 10,000 K to 160,000 K. The results obtained in [...] Read more.
Stark widths for 23 transitions in Al IV have been calculated by employing the modified semiempirical method. The results are obtained for an electron density of 1017 cm3 and temperatures from 10,000 K to 160,000 K. The results obtained in this investigation are used for the examination of the influence of Stark broadening in Al IV stellar spectra, as well as to check the Stark width regular behavior and similarites within the Al IV spectrum. Full article
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