Next Issue
Volume 6, May
Previous Issue
Volume 6, March

Table of Contents

Universe, Volume 6, Issue 4 (April 2020) – 13 articles

  • Issues are regarded as officially published after their release is announced to the table of contents alert mailing list.
  • You may sign up for e-mail alerts to receive table of contents of newly released issues.
  • PDF is the official format for papers published in both, html and pdf forms. To view the papers in pdf format, click on the "PDF Full-text" link, and use the free Adobe Readerexternal link to open them.
Cover Story (view full-size image) The article explores methods for testing Finsler gravity theories. A satellite with an on-board [...] Read more.
Order results
Result details
Select all
Export citation of selected articles as:
Open AccessArticle
Automated Classification of Massive Spectra Based on Enhanced Multi-Scale Coded Convolutional Neural Network
Universe 2020, 6(4), 60; https://doi.org/10.3390/universe6040060 - 23 Apr 2020
Viewed by 373
Abstract
The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) has produced massive medium-resolution spectra. Data mining for special and rare stars in massive LAMOST spectra is of great significance. Feature extraction plays an important role in the process of automatic spectra classification. The [...] Read more.
The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) has produced massive medium-resolution spectra. Data mining for special and rare stars in massive LAMOST spectra is of great significance. Feature extraction plays an important role in the process of automatic spectra classification. The proper classification network can extract most of the common spectral features with minimum noise and individual features. Such a network has better generalization capabilities and can extract sufficient features for classification. A variety of classification networks of one dimension and two dimensions are both designed and implemented systematically in this paper to verify whether spectra is easier to deal with in a 2D situation. The experimental results show that the fully connected neural network cannot extract enough features. Although convolutional neural network (CNN) with a strong feature extraction capability can quickly achieve satisfactory results on the training set, there is a tendency for overfitting. Signal-to-noise ratios also have effects on the network. To investigate the problems above, various techniques are tested and the enhanced multi-scale coded convolutional neural network (EMCCNN) is proposed and implemented, which can perform spectral denoising and feature extraction at different scales in a more efficient manner. In a specified search, eight known and one possible cataclysmic variables (CVs) in LAMOST MRS are identified by EMCCNN including four CVs, one dwarf nova and three novae. The result supplements the spectra of CVs. Furthermore, these spectra are the first medium-resolution spectra of CVs. The EMCCNN model can be easily extended to search for other rare stellar spectra. Full article
Show Figures

Figure 1

Open AccessArticle
On the Analyticity of Static Solutions of a Field Equation in Finsler Gravity
Universe 2020, 6(4), 59; https://doi.org/10.3390/universe6040059 - 22 Apr 2020
Viewed by 360
Abstract
It is well-known that static vacuum solutions of Einstein equations are analytic in suitable coordinates. We ask here for an extension of this result in the context of Finsler gravity. We consider Finsler spacetimes that retain several properties of static Lorentzian spacetimes, are [...] Read more.
It is well-known that static vacuum solutions of Einstein equations are analytic in suitable coordinates. We ask here for an extension of this result in the context of Finsler gravity. We consider Finsler spacetimes that retain several properties of static Lorentzian spacetimes, are Berwald and have vanishing Ricci scalar. Full article
(This article belongs to the Special Issue Finsler Modification of Classical General Relativity )
Open AccessEditorial
Editorial to the Special Issue “Probing New Physics with Black Holes”
Universe 2020, 6(4), 58; https://doi.org/10.3390/universe6040058 - 22 Apr 2020
Viewed by 603
Abstract
Black holes are fantastic laboratories for probing new physics. Both theoretically and experimentally, many new ideas are emerging to use them as tools for understanding better quantum gravity or classical gravity beyond general relativity. I briefly review some new results. Full article
(This article belongs to the Special Issue Probing New Physics with Black Holes)
Open AccessArticle
Numerical Modelling of Satellite Downlink Signals in a Finslerian-Perturbed Schwarzschild Spacetime
Universe 2020, 6(4), 57; https://doi.org/10.3390/universe6040057 - 20 Apr 2020
Viewed by 453
Abstract
The work presented in this paper aims to contribute to the problem of testing Finsler gravity theories by means of experiments and observations in the solar system. Within a class of spherically symmetric static Finsler spacetimes we consider a satellite with an on-board [...] Read more.
The work presented in this paper aims to contribute to the problem of testing Finsler gravity theories by means of experiments and observations in the solar system. Within a class of spherically symmetric static Finsler spacetimes we consider a satellite with an on-board atomic clock, orbiting in the Finslerian-perturbed gravitational field of the earth, whose time signal is transmitted to a ground station, where its receive time and frequency are measured with respect to another atomic clock. This configuration is realized by the Galileo 5 and 6 satellites that have gone astray and are now on non-circular orbits. Our method consists in the numerical integration of the satellite’s orbit, followed by an iterative procedure which provides the numerically integrated signals, i.e., null geodesics, from the satellite to the ground station. One of our main findings is that for orbits that are considerably more eccentric than the Galileo 5 and 6 satellite orbits, Finslerian effects can be separated from effects of perturbations of the Schwarzschild spacetime within the Lorentzian geometry. We also discuss the separation from effects of non-gravitational perturbations. This leads us to the conclusion that observations of this kind combined with appropriate numerical modelling can provide suitable tests of Finslerian modifications of general relativity. Full article
(This article belongs to the Special Issue Finsler Modification of Classical General Relativity )
Show Figures

Figure 1

Open AccessArticle
Generalised Uncertainty Relations for Angular Momentum and Spin in Quantum Geometry
Universe 2020, 6(4), 56; https://doi.org/10.3390/universe6040056 - 19 Apr 2020
Cited by 1 | Viewed by 396
Abstract
We derive generalised uncertainty relations (GURs) for orbital angular momentum and spin in the recently proposed smeared-space model of quantum geometry. The model implements a minimum length and a minimum linear momentum and recovers both the generalised uncertainty principle (GUP) and extended uncertainty [...] Read more.
We derive generalised uncertainty relations (GURs) for orbital angular momentum and spin in the recently proposed smeared-space model of quantum geometry. The model implements a minimum length and a minimum linear momentum and recovers both the generalised uncertainty principle (GUP) and extended uncertainty principle (EUP), previously proposed in the quantum gravity literature, within a single formalism. In this paper, we investigate the consequences of these results for particles with extrinsic and intrinsic angular momentum and obtain generalisations of the canonical so ( 3 ) and su ( 2 ) algebras. We find that, although SO ( 3 ) symmetry is preserved on three-dimensional slices of an enlarged phase space, corresponding to a superposition of background geometries, individual subcomponents of the generalised generators obey nontrivial subalgebras. These give rise to GURs for orbital angular momentum while leaving the canonical commutation relations intact except for a simple rescaling, ħ ħ + β . The value of the new parameter, β ħ × 10 61 , is determined by the ratio of the dark energy density to the Planck density, and its existence is required by the presence of both minimum length and momentum uncertainties. Here, we assume the former to be of the order of the Planck length and the latter to be of the order of the de Sitter momentum ħ Λ , where Λ is the cosmological constant, which is consistent with the existence of a finite cosmological horizon. In the smeared-space model, ħ and β are interpreted as the quantisation scales for matter and geometry, respectively, and a quantum state vector is associated with the spatial background. We show that this also gives rise to a rescaled Lie algebra for generalised spin operators, together with associated subalgebras that are analogous to those for orbital angular momentum. Remarkably, consistency of the algebraic structure requires the quantum state associated with a flat background to be fermionic, with spin eigenvalues ± β / 2 . Finally, the modified spin algebra leads to GURs for spin measurements. The potential implications of these results for cosmology and high-energy physics, and for the description of spin and angular momentum in relativistic theories of quantum gravity, including dark energy, are briefly discussed. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
Open AccessArticle
Foundations of Finsler Spacetimes from the Observers’ Viewpoint
Universe 2020, 6(4), 55; https://doi.org/10.3390/universe6040055 - 16 Apr 2020
Cited by 2 | Viewed by 400
Abstract
Physical foundations for relativistic spacetimes are revisited in order to check at what extent Finsler spacetimes lie in their framework. Arguments based on inertial observers (as in the foundations of special relativity and classical mechanics) are shown to correspond with a double linear [...] Read more.
Physical foundations for relativistic spacetimes are revisited in order to check at what extent Finsler spacetimes lie in their framework. Arguments based on inertial observers (as in the foundations of special relativity and classical mechanics) are shown to correspond with a double linear approximation in the measurement of space and time. While general relativity appears by dropping the first linearization, Finsler spacetimes appear by dropping the second one. The classical Ehlers–Pirani–Schild approach is carefully discussed and shown to be compatible with the Lorentz–Finsler case. The precise mathematical definition of Finsler spacetime is discussed by using the space of observers. Special care is taken in some issues such as the fact that a Lorentz–Finsler metric would be physically measurable only on the causal directions for a cone structure, the implications for models of spacetimes of some apparently innocuous hypotheses on differentiability, or the possibilities of measurement of a varying speed of light. Full article
(This article belongs to the Special Issue Finsler Modification of Classical General Relativity )
Show Figures

Figure 1

Open AccessArticle
High-Energy and Very High-Energy Constraints from Log-Parabolic Spectral Models in Narrow-Line Seyfert 1 Galaxies
Universe 2020, 6(4), 54; https://doi.org/10.3390/universe6040054 - 16 Apr 2020
Viewed by 362
Abstract
Narrow-line Seyfert 1 galaxies (NLSy1s) are a well established class of γ -ray sources, showing the presence of a jet like the more common flat-spectrum radio quasars. The evidence of γ -ray emission poses the issue of the location of the γ -ray [...] Read more.
Narrow-line Seyfert 1 galaxies (NLSy1s) are a well established class of γ -ray sources, showing the presence of a jet like the more common flat-spectrum radio quasars. The evidence of γ -ray emission poses the issue of the location of the γ -ray emitting zone and of the contribution of the γ - γ absorption within the broad-line region (BLR), since such objects have been detected by Fermi-LAT in the MeV-GeV energy range but not by imaging atmospheric Cherenkov telescopes beyond 100 GeV. We discuss how the spectral properties of three NLSy1s (SBS 0846+513, PMN J0948+0022, and PKS 1502+036) derived from the Fermi Large Area Telescope Fourth Source Catalog (4FGL) compared with theoretical models based on the observed properties of the BLR. In particular, we focus on the question on how simple power-law spectral models and log-parabolic ones could be disentangled in γ -ray narrow-line Seyfert 1 galaxies by means of current Fermi-LAT or future imaging atmospheric Cherenkov telescopes data. We found that the only possibility for a log-parabolic model to mimic a power-law model in the energy band above E 100 GeV is to have a very small value of the curvature parameter β 0.05 . Full article
(This article belongs to the Special Issue Seyfert Galaxies Astrophysics)
Show Figures

Figure 1

Open AccessArticle
Revisiting the 2PN Pericenter Precession in View of Possible Future Measurements
Universe 2020, 6(4), 53; https://doi.org/10.3390/universe6040053 - 13 Apr 2020
Cited by 1 | Viewed by 496
Abstract
At the second post-Newtonian (2PN) order, the secular pericenter precession ω ˙ 2 PN of either a full two-body system made of well-detached non-rotating monopole masses of comparable size and a restricted two-body system composed of a point particle orbiting a fixed central [...] Read more.
At the second post-Newtonian (2PN) order, the secular pericenter precession ω ˙ 2 PN of either a full two-body system made of well-detached non-rotating monopole masses of comparable size and a restricted two-body system composed of a point particle orbiting a fixed central mass have been analytically computed so far with a variety of approaches. We offer our contribution by analytically computing ω ˙ 2 PN in a perturbative way with the method of variation of elliptical elements by explicitly calculating both the direct contribution due to the 2PN acceleration A 2 PN , and also an indirect part arising from the self-interaction of the 1PN acceleration A 1 PN in the orbital average accounting for the instantaneous shifts induced by A 1 PN itself. Explicit formulas are straightforwardly obtained for both the point particle and full two-body cases without recurring to simplifying assumptions on the eccentricity e. Two different numerical integrations of the equations of motion confirm our analytical results for both the direct and indirect precessions. The values of the resulting effects for Mercury and some binary pulsars are confronted with the present-day level of experimental accuracies in measuring/constraining their pericenter precessions. The supermassive binary black hole in the BL Lac object OJ 287 is considered as well. A comparison with some of the results appeared in the literature is made. Full article
(This article belongs to the Special Issue Current and Future Tests of General Relativity)
Show Figures

Figure 1

Open AccessEditor’s ChoiceReview
Present and Future Contributions of Reactor Experiments to Mass Ordering and Neutrino Oscillation Studies
Universe 2020, 6(4), 52; https://doi.org/10.3390/universe6040052 - 08 Apr 2020
Viewed by 397
Abstract
After a long a glorious history, marked by the first direct proofs of neutrino existence and of the mixing between the first and third neutrino generations, the reactor antineutrino experiments are still well alive and will continue to give important contributions to the [...] Read more.
After a long a glorious history, marked by the first direct proofs of neutrino existence and of the mixing between the first and third neutrino generations, the reactor antineutrino experiments are still well alive and will continue to give important contributions to the development of elementary particle physics and astrophysics. In parallel to the SBL (short baseline) experiments, that will be dedicated mainly to the search for sterile neutrinos, a new kind of experiments will start playing an important role: reactor experiments with a “medium” value, around 50 km, of the baseline, somehow in the middle between the SBL and the LBL (long baselines), like KamLAND, which in the recent past gave essential contributions to the developments of neutrino physics. These new medium baseline reactor experiments can be very important, mainly for the study of neutrino mass ordering. The first example of this kind, the liquid scintillator JUNO experiment, characterized by a very high mass and an unprecedented energy resolution, will soon start data collecting in China. Its main aspects are discussed here, together with its potentialities for what concerns the mass ordering investigation and also the other issues that can be studied with this detector, spanning from the accurate oscillation parameter determination to the study of solar neutrinos, geoneutrinos, atmospheric neutrinos and neutrinos emitted by supernovas and to the search for signals of potential Lorentz invariance violation. Full article
(This article belongs to the collection Neutrino Oscillations)
Show Figures

Figure 1

Open AccessArticle
Comparison of Strain Effect between Aluminum and Palladium Gated MOS Quantum Dot Systems
Universe 2020, 6(4), 51; https://doi.org/10.3390/universe6040051 - 06 Apr 2020
Viewed by 363
Abstract
As nano-scale metal-oxide-semiconductor devices are cooled to temperatures below 1 K, detrimental effects due to unintentional dots become apparent. The reproducibility of the location of these unintentional dots suggests that there are other mechanisms in play, such as mechanical strains in the semiconductor [...] Read more.
As nano-scale metal-oxide-semiconductor devices are cooled to temperatures below 1 K, detrimental effects due to unintentional dots become apparent. The reproducibility of the location of these unintentional dots suggests that there are other mechanisms in play, such as mechanical strains in the semiconductor introduced by metallic gates. Here, we investigate the formation of strain-induced dots on aluminum and palladium gated metal oxide semiconductor (MOS) quantum devices using COMSOL Multiphysics. Simulation results show that the strain effect on the electrochemical potential of the system can be minimized by replacing aluminum with palladium as the gate material and increasing the thickness of the gate oxide. Full article
(This article belongs to the Special Issue Quantum Dynamics and Applications)
Show Figures

Figure 1

Open AccessArticle
Quantum Cosmology of Fab Four John Theory with Conformable Fractional Derivative
Universe 2020, 6(4), 50; https://doi.org/10.3390/universe6040050 - 31 Mar 2020
Viewed by 419
Abstract
We study a quantization via fractional derivative of a nonminimal derivative coupling cosmological theory, namely, the Fab Four John theory. Its Hamiltonian version presents the issue of fractional powers in the momenta. That problem is solved here by the application of the so-called [...] Read more.
We study a quantization via fractional derivative of a nonminimal derivative coupling cosmological theory, namely, the Fab Four John theory. Its Hamiltonian version presents the issue of fractional powers in the momenta. That problem is solved here by the application of the so-called conformable fractional derivative. This leads to a Wheeler–DeWitt equation of second order, showing that a Bohm–de Broglie interpretation can be constructed. That combination of fractional quantization and Bohmian interpretation provides us a new quantization method, in which the quantum potential is the criterion to say if a quantum solution is acceptable or not to be further studied. We show that a wide range of solutions for the scale factor is possible. Among all of those, a bouncing solution analogous to the perfect fluid cosmology seems to deserve special attention. Full article
(This article belongs to the Special Issue Quantum Models for Cosmology)
Show Figures

Figure 1

Open AccessArticle
Model Independent Diagnostics in Interacting Dark Energy Models
Universe 2020, 6(4), 49; https://doi.org/10.3390/universe6040049 - 30 Mar 2020
Viewed by 375
Abstract
In this paper, we have explored non-gravitational interaction dark energy model between dark energy and cold dark matter (CDM). The main purpose of this work is to explore the deviations from Λ C D M and to distinguish the evolution trajectory of the [...] Read more.
In this paper, we have explored non-gravitational interaction dark energy model between dark energy and cold dark matter (CDM). The main purpose of this work is to explore the deviations from Λ C D M and to distinguish the evolution trajectory of the interaction dark energy model with different parameters. Six forms of interaction were focused on. We have analyzed the deceleration parameters of the coupled model and explored the theoretical models through s t a t e f i n d e r h i e r a r c h y and the O m diagnostic. We have considered that the Equation of State (EoS) parameter w and parameter ξ take different values on the interaction dark energy model respectively and keep another parameter unchanged. It was found that q ( z ) is the worst of the four diagnoses. O m , S 3 ( 1 ) and S 4 ( 1 ) are good tools to distinguish the interaction dark energy model from Λ C D M . S 3 ( 1 ) is a better diagnostic tool when the state parameter w changes and the coupling parameter ξ is fixed, while S 4 ( 1 ) works better when the coupling parameter ξ changes and the state parameter w is fixed. Full article
(This article belongs to the Special Issue Probing the Dark Universe with Theory and Observations)
Show Figures

Figure 1

Open AccessArticle
Stable Wormholes in the Background of an Exponential f(R) Gravity
Universe 2020, 6(4), 48; https://doi.org/10.3390/universe6040048 - 26 Mar 2020
Viewed by 336
Abstract
The current paper is devoted to investigating wormhole solutions with an exponential gravity model in the background of f ( R ) theory. Spherically symmetric static spacetime geometry is chosen to explore wormhole solutions with anisotropic fluid source. The behavior of the traceless [...] Read more.
The current paper is devoted to investigating wormhole solutions with an exponential gravity model in the background of f ( R ) theory. Spherically symmetric static spacetime geometry is chosen to explore wormhole solutions with anisotropic fluid source. The behavior of the traceless matter is studied by employing a particular equation of state to describe the important properties of the shape-function of the wormhole geometry. Furthermore, the energy conditions and stability analysis are done for two specific shape-functions. It is seen that the energy condition are to be violated for both of the shape-functions chosen here. It is concluded that our results are stable and realistic. Full article
(This article belongs to the Special Issue Relativistic Astrophysics)
Show Figures

Figure 1

Previous Issue
Next Issue
Back to TopTop