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Universe, Volume 6, Issue 2 (February 2020) – 16 articles

Cover Story (view full-size image): Gravity is the force that shapes the overall temporal and spatial structure of the Universe but it is also the least understood of the known interactions. In the last couple of decades, there has been tremendous progress in collecting cosmological data, which made mapping the properties of gravity possible at astrophysical and cosmological scales. However, to make sense of these results, one has to build a set of alternatives against which to compare the standard model. In this context, scalar–tensor theories á-la Horndeski display many non-Einsteinian effects, for instance, generalized Poisson equation, Yukawa interactions, and anisotropic stress. To test gravity and guide future experiments, identifying properties that can be measured in a model-independent way is a fundamental step to improve our understanding of gravity.View this paper.
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Open AccessArticle
Bifurcation Analysis and Periodic Solutions of the HD 191408 System with Triaxial and Radiative Perturbations
Universe 2020, 6(2), 35; https://doi.org/10.3390/universe6020035 - 22 Feb 2020
Cited by 4 | Viewed by 817
Abstract
The nonlinear orbital dynamics of a class of the perturbed restricted three-body problem is studied. The two primaries considered here refer to the binary system HD 191408. The third particle moves under the gravity of the binary system, whose triaxial rate and radiation [...] Read more.
The nonlinear orbital dynamics of a class of the perturbed restricted three-body problem is studied. The two primaries considered here refer to the binary system HD 191408. The third particle moves under the gravity of the binary system, whose triaxial rate and radiation factor are also considered. Based on the dynamic governing equation of the third particle in the binary HD 191408 system, the motion state manifold is given. By plotting bifurcation diagrams of the system, the effects of various perturbation factors on the dynamic behavior of the third particle are discussed in detail. In addition, the relationship between the geometric configuration and the Jacobian constant is discussed by analyzing the zero-velocity surface and zero-velocity curve of the system. Then, using the Poincaré–Lindsted method and numerical simulation, the second- and third-order periodic orbits of the third particle around the collinear libration point in two- and three-dimensional spaces are analytically and numerically presented. This paper complements the results by Singh et al. [Singh et al., AMC, 2018]. It contains not only higher-order analytical periodic solutions in the vicinity of the collinear equilibrium points but also conducts extensive numerical research on the bifurcation of the binary system. Full article
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Open AccessArticle
Gravitoelectromagnetism, Solar System Tests, and Weak-Field Solutions in f (T,B) Gravity with Observational Constraints
Universe 2020, 6(2), 34; https://doi.org/10.3390/universe6020034 - 18 Feb 2020
Cited by 15 | Viewed by 905
Abstract
Gravitomagnetism characterizes phenomena in the weak-field limit within the context of rotating systems. These are mainly manifested in the geodetic and Lense-Thirring effects. The geodetic effect describes the precession of the spin of a gyroscope in orbit about a massive static central object, [...] Read more.
Gravitomagnetism characterizes phenomena in the weak-field limit within the context of rotating systems. These are mainly manifested in the geodetic and Lense-Thirring effects. The geodetic effect describes the precession of the spin of a gyroscope in orbit about a massive static central object, while the Lense-Thirring effect expresses the analogous effect for the precession of the orbit about a rotating source. In this work, we explore these effects in the framework of Teleparallel Gravity and investigate how these effects may impact recent and future missions. We find that teleparallel theories of gravity may have an important impact on these effects which may constrain potential models within these theories. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
Open AccessArticle
Squeezing of Primordial Gravitational Waves as Quantum Discord
Universe 2020, 6(2), 33; https://doi.org/10.3390/universe6020033 - 15 Feb 2020
Cited by 3 | Viewed by 818
Abstract
We investigate the squeezing of primordial gravitational waves (PGWs) in terms of quantum discord. We construct a classical state of PGWs without quantum discord and compare it with the Bunch–Davies vacuum. Then it is shown that the oscillatory behavior of the angular-power spectrum [...] Read more.
We investigate the squeezing of primordial gravitational waves (PGWs) in terms of quantum discord. We construct a classical state of PGWs without quantum discord and compare it with the Bunch–Davies vacuum. Then it is shown that the oscillatory behavior of the angular-power spectrum of the cosmic microwave background (CMB) fluctuations induced by PGWs can be the signature of the quantum discord of PGWs. In addition, we discuss the effect of quantum decoherence on the entanglement and the quantum discord of PGWs for super-horizon modes. For the state of PGWs with decoherence effect, we examine the decoherence condition and the correlation condition introduced by C. Kiefer et al. (Class. Quantum Grav. 24 (2007) 1699). We show that the decoherence condition is not sufficient for the separability of PGWs and the correlation condition implies that the PGWs in the matter-dominated era have quantum discord. Full article
(This article belongs to the Section Cosmology)
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Open AccessReview
Three-Flavor Oscillations with Accelerator Neutrino Beams
Universe 2020, 6(2), 32; https://doi.org/10.3390/universe6020032 - 11 Feb 2020
Cited by 2 | Viewed by 901
Abstract
The three-flavor neutrino oscillation paradigm is well established in particle physics thanks to the crucial contribution of accelerator neutrino beam experiments. In this paper, we review the most important contributions of these experiments to the physics of massive neutrinos after the discovery of [...] Read more.
The three-flavor neutrino oscillation paradigm is well established in particle physics thanks to the crucial contribution of accelerator neutrino beam experiments. In this paper, we review the most important contributions of these experiments to the physics of massive neutrinos after the discovery of θ 13 and future perspectives in such a lively field of research. Special emphasis is given to the technical challenges of high power beams and the oscillation results of T2K, OPERA, ICARUS, and NO ν A. We discuss in detail the role of accelerator neutrino experiments in the precision era of neutrino physics in view of DUNE and Hyper-Kamiokande, the program of systematic uncertainty reduction and the development of new beam facilities. Full article
(This article belongs to the Collection Neutrino Oscillations)
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Open AccessFeature PaperEditor’s ChoiceReview
Measuring Electromagnetic Fields in Rotating Frames of Reference
Universe 2020, 6(2), 31; https://doi.org/10.3390/universe6020031 - 11 Feb 2020
Cited by 2 | Viewed by 993
Abstract
We review the problem of transforming electromagnetic fields between inertial and rotating reference frames. We compare the method of straightforward tensor coordinate transformations adopted by Schiff in his well-known paper of 1939 with the method of Orthogonal Tetrads (OT) that was applied to [...] Read more.
We review the problem of transforming electromagnetic fields between inertial and rotating reference frames. We compare the method of straightforward tensor coordinate transformations adopted by Schiff in his well-known paper of 1939 with the method of Orthogonal Tetrads (OT) that was applied to this problem in 1964 by Irvine. Although both methods are mathematically rigorous, the transformed fields have different forms depending on the method adopted. We emphasize that the OT method is expected to predict the fields that would actually be measured by an observer in a rotating frame of reference. We briefly discuss existing experimental evidence that supports the OT approach, but point out that there appears to be little awareness in the physics community of this problem or its resolution. We use both methods to transform the electrostatic and magnetic fields generated by rotating charged spherical shells from an inertial into a co-rotating system. We also briefly describe how such an arrangement of shells could be used to measure rotation relative to the fixed stars. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
Open AccessReview
Neutrino Telescopes and High-Energy Cosmic Neutrinos
Universe 2020, 6(2), 30; https://doi.org/10.3390/universe6020030 - 10 Feb 2020
Cited by 5 | Viewed by 950
Abstract
In this review paper, we present the main aspects of high-energy cosmic neutrino astrophysics. We begin by describing the generic expectations for cosmic neutrinos, including the effects of propagation from their sources to the detectors. Then we introduce the operating principles of current [...] Read more.
In this review paper, we present the main aspects of high-energy cosmic neutrino astrophysics. We begin by describing the generic expectations for cosmic neutrinos, including the effects of propagation from their sources to the detectors. Then we introduce the operating principles of current neutrino telescopes, and examine the main features (topologies) of the observable events. After a discussion of the main background processes, due to the concomitant presence of secondary particles produced in the terrestrial atmosphere by cosmic rays, we summarize the current status of the observations with astrophysical relevance that have been greatly contributed by IceCube detector. Then, we examine various interpretations of these findings, trying to assess the best candidate sources of cosmic neutrinos. We conclude with a brief perspective on how the field could evolve within a few years. Full article
(This article belongs to the Collection Neutrino Oscillations)
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Open AccessArticle
Oscillating Neutrinos and Majorana Neutrino Masses
Universe 2020, 6(2), 29; https://doi.org/10.3390/universe6020029 - 06 Feb 2020
Viewed by 683
Abstract
We discuss the mass matrices with texture zeros for the quarks and leptons. The flavor mixing angles for the quarks are functions of the quark masses and can be calculated. The results agree with the experimental data. The texture zero mass matrices for [...] Read more.
We discuss the mass matrices with texture zeros for the quarks and leptons. The flavor mixing angles for the quarks are functions of the quark masses and can be calculated. The results agree with the experimental data. The texture zero mass matrices for the leptons and the see-saw mechanism are used to derive relations between the matrix elements of the lepton mixing matrix and the ratios of the neutrino masses. Using the measured neutrino mass differences, the neutrino masses can be calculated. The neutrinoless double beta decay is discussed. The effective Majorana neutrino mass, describing the neutrinoless double beta decay, can be calculated—it is about 4.6 meV. The present experimental limit is at least twenty times larger. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
Open AccessArticle
A Table-Top Pilot Experiment for Narrow Mass Range Light Cold Dark Matter Particle Searches
Universe 2020, 6(2), 28; https://doi.org/10.3390/universe6020028 - 03 Feb 2020
Viewed by 971
Abstract
This report presents the detection framework and a proposal for a pilot table-top experiment (supported by simulations and preliminary test results) for adoption into narrow mass range light Cold Dark Matter (CDM) searches, specifically for axions or Axion-Like Particles (ALPs) in a resonant [...] Read more.
This report presents the detection framework and a proposal for a pilot table-top experiment (supported by simulations and preliminary test results) for adoption into narrow mass range light Cold Dark Matter (CDM) searches, specifically for axions or Axion-Like Particles (ALPs) in a resonant cavity-based scheme. The novelty of this proposal lies in an attempt to concentrate searches corresponding to specific axion masses of interest (coinciding with recent proposals), using multiple cavities in a symmetric scheme, instead of using noisy and complicated tuning mechanisms, and in reduction of associated hardware by employing simpler underlying instrumentation instead of heterodyne mode of detection, by means of a low-noise ac amplification and dc phase-sensitive detection scheme, in order to make a viable and compact table-top experiment possible. These simplifications could possibly be valuable in substantially reducing detection hardware, experiment complexities (and associated noise) and long run-times, while maintaining low noise similar to conventional axion searches. The feasibility of proposed scheme and the experiment design are demonstrated with some calculations, simulations and preliminary tests with artificial axion signals injected into the cavities. The technique and ideas reported here have significant potential to be developed into a small-scale table-top, narrow-range, dark matter axion/ALP spectroscopy experiment, in addition to aiding in the on-going resonant cavity-based and broadband experiments. Full article
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Open AccessFeature PaperReview
Rotating Disc around a Schwarzschild Black Hole
Universe 2020, 6(2), 27; https://doi.org/10.3390/universe6020027 - 03 Feb 2020
Cited by 1 | Viewed by 797
Abstract
A stationary and axisymmetric (in fact circular) metric is reviewed which describes the first-order perturbation of a Schwarzschild black-hole space-time due to a rotating finite thin disc encircling the hole symmetrically. The key Green functions of the problem (corresponding to an infinitesimally thin [...] Read more.
A stationary and axisymmetric (in fact circular) metric is reviewed which describes the first-order perturbation of a Schwarzschild black-hole space-time due to a rotating finite thin disc encircling the hole symmetrically. The key Green functions of the problem (corresponding to an infinitesimally thin ring)—the one for the gravitational potential and the one for the dragging angular velocity—were already derived, in terms of infinite series, by Will in 1974, but we have now put them into closed forms using elliptic integrals. Such forms are more practical for numerical evaluation and for integration in problems involving extended sources. This last point mostly remains difficult, but we illustrate that it may be workable by using the simple case of a finite thin disc with constant Newtonian surface density. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
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Open AccessFeature PaperArticle
Influence of Cosmic Repulsion and Magnetic Fields on Accretion Disks Rotating around Kerr Black Holes
Universe 2020, 6(2), 26; https://doi.org/10.3390/universe6020026 - 29 Jan 2020
Cited by 36 | Viewed by 1101
Abstract
We present a review of the influence of cosmic repulsion and external magnetic fields on accretion disks rotating around rotating black holes and on jets associated with these rotating configurations. We consider both geometrically thin and thick disks. We show that the vacuum [...] Read more.
We present a review of the influence of cosmic repulsion and external magnetic fields on accretion disks rotating around rotating black holes and on jets associated with these rotating configurations. We consider both geometrically thin and thick disks. We show that the vacuum energy represented by the relic cosmological constant strongly limits extension of the accretion disks that is for supermassive black holes comparable to extension of largest galaxies, and supports collimation of jets at large distances from the black hole. We further demonstrate that an external magnetic field crucially influences the fate of ionized Keplerian disks causing creation of winds and jets, enabling simultaneously acceleration of ultra-high energy particles with energy up to 10 21 eV around supermassive black holes with M 10 10 M surrounded by sufficiently strong magnetic field with B 10 4 G. We also show that the external magnetic fields enable existence of “levitating” off-equatorial clouds or tori, along with the standard equatorial toroidal structures, if these carry a non-vanishing, appropriately distributed electric charge. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
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Open AccessReview
Black Holes and Wormholes in Extended Gravity
Universe 2020, 6(2), 25; https://doi.org/10.3390/universe6020025 - 27 Jan 2020
Cited by 1 | Viewed by 874
Abstract
We discuss black hole type solutions and wormhole type ones in the effective gravity models. Such models appear during the attempts to construct the quantum theory of gravity. The mentioned solutions, being, mostly, the perturbative generalisations of well-known ones in general relativity, carry [...] Read more.
We discuss black hole type solutions and wormhole type ones in the effective gravity models. Such models appear during the attempts to construct the quantum theory of gravity. The mentioned solutions, being, mostly, the perturbative generalisations of well-known ones in general relativity, carry out additional set of parameters and, therefore could help, for example, in the studying of the last stages of Hawking evaporation, in extracting the possibilities for the experimental or observational search and in helping to constrain by astrophysical data. Full article
(This article belongs to the Special Issue Quantum Effects in General Relativity)
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Open AccessReview
Rotation and Spin and Position Operators in Relativistic Gravity and Quantum Electrodynamics
Universe 2020, 6(2), 24; https://doi.org/10.3390/universe6020024 - 26 Jan 2020
Cited by 1 | Viewed by 733
Abstract
First, we examine how spin is treated in special relativity and the necessity of introducing spin supplementary conditions (SSC) and how they are related to the choice of a center-of-mass of a spinning particle. Next, we discuss quantum electrodynamics and the Foldy–Wouthuysen transformation [...] Read more.
First, we examine how spin is treated in special relativity and the necessity of introducing spin supplementary conditions (SSC) and how they are related to the choice of a center-of-mass of a spinning particle. Next, we discuss quantum electrodynamics and the Foldy–Wouthuysen transformation which we note is a position operator identical to the Pryce–Newton–Wigner position operator. The classical version of the operators are shown to be essential for the treatment of classical relativistic particles in general relativity, of special interest being the case of binary systems (black holes/neutron stars) which emit gravitational radiation. Full article
(This article belongs to the Special Issue Rotation Effects in Relativity)
Open AccessArticle
Shadow Images of a Rotating Dyonic Black Hole with a Global Monopole Surrounded by Perfect Fluid
Universe 2020, 6(2), 23; https://doi.org/10.3390/universe6020023 - 24 Jan 2020
Cited by 23 | Viewed by 1072
Abstract
In this paper, we revisit the rotating global monopole metric and extend the metric to a rotating dyonic global monopole in the presence of a perfect fluid. We then show that the surface topology at the event horizon, related to the metric computed, [...] Read more.
In this paper, we revisit the rotating global monopole metric and extend the metric to a rotating dyonic global monopole in the presence of a perfect fluid. We then show that the surface topology at the event horizon, related to the metric computed, is a 2-sphere using the Gauss-Bonnet theorem. By choosing ω = 1 / 3 , 0 , 1 / 3 we investigate the effect of dark matter, dust and radiation on the silhouette of a black hole. The presence of the global monopole parameter γ and the perfect fluid parameters υ also deform the shape of a black hole’s shadow, which has been depicted through graphical illustrations. Finally, we analyse the energy emission rate of a rotating dyonic global monopole surrounded by perfect fluid with respect to parameters. Full article
(This article belongs to the Special Issue Gravitational Lensing and Optical Geometry: A Centennial Perspective)
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Open AccessArticle
Curvature Invariants for Charged and Rotating Black Holes
Universe 2020, 6(2), 22; https://doi.org/10.3390/universe6020022 - 24 Jan 2020
Cited by 5 | Viewed by 1206
Abstract
Riemann curvature invariants are important in general relativity because they encode the geometrical properties of spacetime in a manifestly coordinate-invariant way. Fourteen such invariants are required to characterize four-dimensional spacetime in general, and Zakhary and McIntosh showed that as many as seventeen can [...] Read more.
Riemann curvature invariants are important in general relativity because they encode the geometrical properties of spacetime in a manifestly coordinate-invariant way. Fourteen such invariants are required to characterize four-dimensional spacetime in general, and Zakhary and McIntosh showed that as many as seventeen can be required in certain degenerate cases. We calculate explicit expressions for all seventeen of these Zakhary–McIntosh curvature invariants for the Kerr–Newman metric that describes spacetime around black holes of the most general kind (those with mass, charge, and spin), and confirm that they are related by eight algebraic conditions (dubbed syzygies by Zakhary and McIntosh), which serve as a useful check on our results. Plots of these invariants show richer structure than is suggested by traditional (coordinate-dependent) textbook depictions, and may repay further investigation. Full article
(This article belongs to the Special Issue Probing New Physics with Black Holes)
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Open AccessArticle
Black Hole Evaporation: A Perspective from Loop Quantum Gravity
Universe 2020, 6(2), 21; https://doi.org/10.3390/universe6020021 - 24 Jan 2020
Cited by 13 | Viewed by 917
Abstract
A personal perspective on the black hole evaporation process is presented using, as guidelines, inputs from: (i) loop quantum gravity, (ii) simplified models where concrete results have been obtained, and, (iii) semi-classical quantum general relativity. On the one hand, the final picture is [...] Read more.
A personal perspective on the black hole evaporation process is presented using, as guidelines, inputs from: (i) loop quantum gravity, (ii) simplified models where concrete results have been obtained, and, (iii) semi-classical quantum general relativity. On the one hand, the final picture is conservative in that there are concrete results that support each stage of the argument, and there are no large departures from general relativity or semi-classical gravity in tame regions outside macroscopic black holes. On the other hand, it argues against certain views that are commonly held in many quarters, such as persistence of a piece of singularity that constitutes a part of the final boundary of space–time; presence of an event horizon serving as an absolute barrier between the interior and the exterior, and the (often implicit) requirement that purification must be completed by the time the ‘last rays’ representing the extension of this event horizon reach I + . Full article
(This article belongs to the Special Issue Probing New Physics with Black Holes)
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Open AccessFeature PaperReview
Measuring Gravity at Cosmological Scales
Universe 2020, 6(2), 20; https://doi.org/10.3390/universe6020020 - 22 Jan 2020
Cited by 13 | Viewed by 1087
Abstract
This review is a pedagogical introduction to models of gravity and how they are constrained through cosmological observations. We focus on the Horndeski scalar-tensor theory and on the quantities that can be measured with a minimum of assumptions. Alternatives or extensions of general [...] Read more.
This review is a pedagogical introduction to models of gravity and how they are constrained through cosmological observations. We focus on the Horndeski scalar-tensor theory and on the quantities that can be measured with a minimum of assumptions. Alternatives or extensions of general relativity have been proposed ever since its early years. Because of the Lovelock theorem, modifying gravity in four dimensions typically means adding new degrees of freedom. The simplest way is to include a scalar field coupled to the curvature tensor terms. The most general way of doing so without incurring in the Ostrogradski instability is the Horndeski Lagrangian and its extensions. Testing gravity means therefore, in its simplest term, testing the Horndeski Lagrangian. Since local gravity experiments can always be evaded by assuming some screening mechanism or that baryons are decoupled, or even that the effects of modified gravity are visible only at early times, we need to test gravity with cosmological observations in the late universe (large-scale structure) and in the early universe (cosmic microwave background). In this work, we review the basic tools to test gravity at cosmological scales, focusing on model-independent measurements. Full article
(This article belongs to the Section Cosmology)
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