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Universe, Volume 7, Issue 4 (April 2021) – 35 articles

Cover Story (view full-size image): The Casimir force acts between two parallel uncharged metallic plates spaced in vacuum. It was predicted by Dutch physicist Hendrik Casimir in 1948. This force is of entirely quantum nature: there is no such force in classical electrodynamics. It is explained by existence of the zero-point and thermal fluctuations of the electromagnetic field, which have an infinitely large energy density. The fluctuations are modified by the presence of plates, and the Casimir force originates from the difference between two infinities. After 1956, when Evgenii Lifshitz created the general theory of the Casimir force, it was believed that the problem was solved. However, in the early twenty-first century it was found that the Lifshitz theory contradicts the measurement data and the principles of thermodynamics. This was called the Casimir puzzle, or the Casimir conundrum, which is as yet unresolved. View this paper
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19 pages, 2245 KiB  
Article
The I-Love-Q Relations for Superfluid Neutron Stars
by Cheung-Hei Yeung, Lap-Ming Lin, Nils Andersson and Greg Comer
Universe 2021, 7(4), 111; https://doi.org/10.3390/universe7040111 - 20 Apr 2021
Cited by 18 | Viewed by 1874
Abstract
The I-Love-Q relations are approximate equation-of-state independent relations that connect the moment of inertia, the spin-induced quadrupole moment, and the tidal deformability of neutron stars. In this paper, we study the I-Love-Q relations for superfluid neutron stars for a general relativistic two-fluid model: [...] Read more.
The I-Love-Q relations are approximate equation-of-state independent relations that connect the moment of inertia, the spin-induced quadrupole moment, and the tidal deformability of neutron stars. In this paper, we study the I-Love-Q relations for superfluid neutron stars for a general relativistic two-fluid model: one fluid being the neutron superfluid and the other a conglomerate of all charged components. We study to what extent the two-fluid dynamics might affect the robustness of the I-Love-Q relations by using a simple two-component polytropic model and a relativistic mean field model with entrainment for the equation-of-state. Our results depend crucially on the spin ratio Ωn/Ωp between the angular velocities of the neutron superfluid and the normal component. We find that the I-Love-Q relations can still be satisfied to high accuracy for superfluid neutron stars as long as the two fluids are nearly co-rotating Ωn/Ωp1. However, the deviations from the I-Love-Q relations increase as the spin ratio deviates from unity. In particular, the deviation of the Q-Love relation can be as large as O(10%) if Ωn/Ωp differ from unity by a few tens of percent. As Ωn/Ωp1 is expected for realistic neutron stars, our results suggest that the two-fluid dynamics should not affect the accuracy of any gravitational waveform models for neutron star binaries that employ the relation to connect the spin-induced quadrupole moment and the tidal deformability. Full article
(This article belongs to the Special Issue Superfluidity and Superconductivity in Neutron Stars)
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46 pages, 1135 KiB  
Review
Bouncing Quantum Cosmology
by Nelson Pinto-Neto
Universe 2021, 7(4), 110; https://doi.org/10.3390/universe7040110 - 20 Apr 2021
Cited by 10 | Viewed by 2027
Abstract
The goal of this contribution is to present the properties of a class of quantum bouncing models in which the quantum bounce originates from the Dirac canonical quantization of a midi-superspace model composed of a homogeneous and isotropic background, together with small inhomogeneous [...] Read more.
The goal of this contribution is to present the properties of a class of quantum bouncing models in which the quantum bounce originates from the Dirac canonical quantization of a midi-superspace model composed of a homogeneous and isotropic background, together with small inhomogeneous perturbations. The resulting Wheeler-DeWitt equation is interpreted in the framework of the de Broglie-Bohm quantum theory, enormously simplifying the calculations, conceptually and technically. It is shown that the resulting models are stable and they never get to close to the Planck energy, where another more involved quantization scheme would have to be evoked, and they are compatible with present observations. Some physical effects around the bounce are discussed, like baryogenesis and magnetogenesis, and the crucial role of dark matter and dark energy is also studied. Full article
(This article belongs to the Special Issue Bounce Cosmology)
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19 pages, 373 KiB  
Article
Dirac’s Formalism for Time-Dependent Hamiltonian Systems in the Extended Phase Space
by Angel Garcia-Chung, Daniel Gutiérrez-Ruiz and J. David Vergara
Universe 2021, 7(4), 109; https://doi.org/10.3390/universe7040109 - 20 Apr 2021
Viewed by 1723
Abstract
Dirac’s formalism for constrained systems is applied to the analysis of time-dependent Hamiltonians in the extended phase space. We show that the Lewis invariant is a reparametrization invariant, and we calculate the Feynman propagator using the extended phase space description. We show that [...] Read more.
Dirac’s formalism for constrained systems is applied to the analysis of time-dependent Hamiltonians in the extended phase space. We show that the Lewis invariant is a reparametrization invariant, and we calculate the Feynman propagator using the extended phase space description. We show that the Feynman propagator’s quantum phase is given by the boundary term of the canonical transformation of the extended phase space. We propose a new canonical transformation within the extended phase space that leads to a Lewis invariant generalization, and we sketch some possible applications. Full article
(This article belongs to the Section Field Theory)
20 pages, 581 KiB  
Article
No-Slip Boundary Conditions for Electron Hydrodynamics and the Thermal Casimir Pressure
by Mandy Hannemann, Gino Wegner and Carsten Henkel
Universe 2021, 7(4), 108; https://doi.org/10.3390/universe7040108 - 20 Apr 2021
Cited by 11 | Viewed by 1988
Abstract
We derive modified reflection coefficients for electromagnetic waves in the THz and far infrared range. The idea is based on hydrodynamic boundary conditions for metallic conduction electrons. The temperature-dependent part of the Casimir pressure between metal plates is evaluated. The results should shed [...] Read more.
We derive modified reflection coefficients for electromagnetic waves in the THz and far infrared range. The idea is based on hydrodynamic boundary conditions for metallic conduction electrons. The temperature-dependent part of the Casimir pressure between metal plates is evaluated. The results should shed light on the “thermal anomaly,” where measurements deviate from the standard fluctuation electrodynamics for conducting metals. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
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18 pages, 1604 KiB  
Article
Our Peculiar Motion Inferred from Number Counts of Mid Infra Red AGNs and the Discordance Seen with the Cosmological Principle
by Ashok K. Singal
Universe 2021, 7(4), 107; https://doi.org/10.3390/universe7040107 - 20 Apr 2021
Cited by 17 | Viewed by 2303
Abstract
According to the Cosmological Principle, the Universe is isotropic and no preferred direction would be seen by an observer that might be stationary with respect to the expanding cosmic fluid. However, because of observer’s partaking in the solar system peculiar motion, there would [...] Read more.
According to the Cosmological Principle, the Universe is isotropic and no preferred direction would be seen by an observer that might be stationary with respect to the expanding cosmic fluid. However, because of observer’s partaking in the solar system peculiar motion, there would appear in some of the observed properties of the Cosmos a dipole anisotropy, which could in turn be exploited to determine the peculiar motion of the solar system. The dipole anisotropy in the Cosmic Microwave Background Radiation (CMBR) has given a peculiar velocity vector 370 km s1 along l=264,b=48. However, some other dipoles, for instance, from the number counts, sky brightness or redshift distributions in large samples of distant Active Galactic Nuclei (AGNs), have yielded values of the peculiar velocity many times larger than that from the CMBR, though surprisingly, in all cases the directions agreed with the CMBR dipole. Here we determine our peculiar motion from a sample of 0.28 million AGNs, selected from the Mid Infra Red Active Galactic Nuclei (MIRAGN) sample comprising more than a million sources. From this, we find a peculiar velocity, which is more than four times the CMBR value, although the direction seems to be within ∼2σ of the CMBR dipole. A genuine value of the solar peculiar velocity should be the same irrespective of the data or the technique employed to estimate it. Therefore, such discordant dipole amplitudes might mean that the explanation for these dipoles, including that of the CMBR, might in fact be something else. The observed fact that the direction in all cases is the same, though obtained from completely independent surveys using different instruments and techniques, by different sets of people employing different computing routines, might nonetheless indicate that these dipoles are not merely due to some systematics, otherwise why would they all be pointing along the same direction. It might instead suggest a preferred direction in the Universe, implying a genuine anisotropy, which would violate the Cosmological Principle, the core of the modern cosmology. Full article
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20 pages, 3534 KiB  
Article
Van der Waals Interactions of Moving Particles with Surfaces of Cylindrical Geometry
by George V. Dedkov
Universe 2021, 7(4), 106; https://doi.org/10.3390/universe7040106 - 19 Apr 2021
Cited by 2 | Viewed by 1668
Abstract
General nonrelativistic theory has been developed and the expressions obtained for the tangential (dissipative) and radial (conservative) image forces and van der Waals forces (vdW) acting on charged and neutral particles when they move parallel to the axis of a cylinder with circular [...] Read more.
General nonrelativistic theory has been developed and the expressions obtained for the tangential (dissipative) and radial (conservative) image forces and van der Waals forces (vdW) acting on charged and neutral particles when they move parallel to the axis of a cylinder with circular cross-section, or in the space between coaxial cylinders. Numerical calculations of vdW forces have been performed for metal (Au) and dielectric (Si) materials of cylinders (filaments) and Cs atoms at velocities ~107m/s. A remarkable result is that in the case of metal cylinders (atomic filaments and chains) the dynamic vdW potential can be repulsive for certain values of the velocity–distance parameter and the characteristic atomic frequency. In the case of a Si material, the dynamic vdW potential increases relative to the static one (by modulus) when the velocity–distance parameter Vω0/R changes from zero to ~1.3 and then tends to zero. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
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23 pages, 369 KiB  
Article
Painlevé–Gullstrand form of the Lense–Thirring Spacetime
by Joshua Baines, Thomas Berry, Alex Simpson and Matt Visser
Universe 2021, 7(4), 105; https://doi.org/10.3390/universe7040105 - 19 Apr 2021
Cited by 35 | Viewed by 1849
Abstract
The standard Lense–Thirring metric is a century-old slow-rotation large-distance approximation to the gravitational field outside a rotating massive body, depending only on the total mass and angular momentum of the source. Although it is not an exact solution to the vacuum Einstein equations, [...] Read more.
The standard Lense–Thirring metric is a century-old slow-rotation large-distance approximation to the gravitational field outside a rotating massive body, depending only on the total mass and angular momentum of the source. Although it is not an exact solution to the vacuum Einstein equations, asymptotically the Lense–Thirring metric approaches the Kerr metric at large distances. Herein we shall discuss a specific variant of the standard Lense–Thirring metric, carefully chosen for simplicity, clarity, and various forms of improved mathematical and physical behaviour, (to be more carefully defined in the body of the article). We shall see that this Lense–Thirring variant can be viewed as arising from the linearization of a suitably chosen tetrad representing the Kerr spacetime. In particular, we shall construct an explicit unit-lapse Painlevé–Gullstrand variant of the Lense–Thirring spacetime, one that has flat spatial slices, a very simple and physically intuitive tetrad, and extremely simple curvature tensors. We shall verify that this variant of the Lense–Thirring spacetime is Petrov type I, (so it is not algebraically special), but nevertheless possesses some very straightforward timelike geodesics, (the “rain” geodesics). We shall also discuss on-axis and equatorial geodesics, ISCOs (innermost stable circular orbits) and circular photon orbits. Finally, we wrap up by discussing some astrophysically relevant estimates, and analyze what happens if we extrapolate down to small values of r; verifying that for sufficiently slow rotation we explicitly recover slowly rotating Schwarzschild geometry. This Lense–Thirring variant can be viewed, in its own right, as a “black hole mimic”, of direct interest to the observational astronomy community. Full article
(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)
59 pages, 770 KiB  
Review
Electron-Positron Vacuum Instability in Strong Electric Fields. Relativistic Semiclassical Approach
by Dmitry N. Voskresensky
Universe 2021, 7(4), 104; https://doi.org/10.3390/universe7040104 - 16 Apr 2021
Cited by 13 | Viewed by 1808
Abstract
The instability of electron-positron vacuum in strong electric fields is studied. First, falling to the Coulomb center is discussed at Z>137/2 for a spinless boson and at Z>137 for electron. Subsequently, focus is concentrated on description of [...] Read more.
The instability of electron-positron vacuum in strong electric fields is studied. First, falling to the Coulomb center is discussed at Z>137/2 for a spinless boson and at Z>137 for electron. Subsequently, focus is concentrated on description of deep electron levels and spontaneous positron production in the field of a finite-size nucleus with the charge Z>Zcr170. Next, these effects are studied in application to the low-energy heavy-ion collisions. Subsequently, we consider phenomenon of “electron condensation” on levels of upper continuum crossed the boundary of the lower continuum ϵ=m in the field of a supercharged nucleus with ZZcr. Finally, attention is focused on many-particle problems of polarization of the quantum electrodynamics (QED) vacuum and electron condensation at ultra-short distances from a source of charge. We argue for a principal difference of cases, when the size of the source is larger than the pole size rpole, at which the dielectric permittivity of the vacuum reaches zero and smaller rpole. Some arguments are presented in favor of the logical consistency of QED. All of the problems are considered within the same relativistic semiclassical approach. Full article
(This article belongs to the Section Field Theory)
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28 pages, 4693 KiB  
Review
On the Impact Monitoring of Near-Earth Objects: Mathematical Tools, Algorithms, and Challenges for the Future
by Giacomo Tommei
Universe 2021, 7(4), 103; https://doi.org/10.3390/universe7040103 - 16 Apr 2021
Cited by 1 | Viewed by 2795
Abstract
The Impact Monitoring (IM) of Near-Earth Objects (NEOs) is a young field of research, considering that 22 years ago precise algorithms to compute an impact probability with the Earth did not exist. On the other hand, the year 2020 just passed saw the [...] Read more.
The Impact Monitoring (IM) of Near-Earth Objects (NEOs) is a young field of research, considering that 22 years ago precise algorithms to compute an impact probability with the Earth did not exist. On the other hand, the year 2020 just passed saw the increase of IM operational systems: in addition to the two historical systems, CLOMON2 (University of Pisa/SpaceDyS) and Sentry (JPL/NASA), the European Space Agency (ESA) started its own system AstOD. Moreover, in the last five years three systems for the detection of imminent impactors (small asteroidal objects detected a few days before the possible impact with the Earth) have been developed: SCOUT (at JPL/NASA), NEORANGER (at University of Helsinki) and NEOScan (at University of Pisa/SpaceDyS). The IM science, in addition to being useful for the planetary protection, is a very fascinating field of research because it involves astronomy, physics, mathematics and computer science. In this paper I am going to review the mathematical tools and algorithms of the IM science, highlighting the historical evolution and the challenges to be faced in the future. Full article
(This article belongs to the Special Issue Small Bodies in the Solar System)
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7 pages, 253 KiB  
Communication
The Effect of Higher Dimensional QCD Operators on the Spectroscopy of Bottom-Up Holographic Models
by Sergey Afonin
Universe 2021, 7(4), 102; https://doi.org/10.3390/universe7040102 - 15 Apr 2021
Cited by 3 | Viewed by 1629
Abstract
Within the bottom-up holographic approach to QCD, the highly excited hadrons are identified with the bulk normal modes in the fifth “holographic” dimension. We show that additional states in the same mass range can appear also from taking into consideration the 5D fields [...] Read more.
Within the bottom-up holographic approach to QCD, the highly excited hadrons are identified with the bulk normal modes in the fifth “holographic” dimension. We show that additional states in the same mass range can appear also from taking into consideration the 5D fields dual to higher dimensional QCD operators. The possible effects of these operators have not been taken into account in virtually any phenomenological applications. Using the scalar case as the simplest example, we demonstrate that the additional higher dimensional operators lead to a large degeneracy of highly excited states in the soft wall holographic model, and in the hard wall holographic model, they result in a proliferation of excited states. The considered model can be viewed as the first analytical toy model predicting a one-to-one mapping of the excited meson states to definite QCD operators, to which they prefer to couple. Full article
(This article belongs to the Special Issue Cosmological Models, Quantum Theories and Astrophysical Observations)
16 pages, 32706 KiB  
Article
Gravitational Interaction in the Chimney Lattice Universe
by Maxim Eingorn, Andrew McLaughlin II, Ezgi Canay, Maksym Brilenkov and Alexander Zhuk
Universe 2021, 7(4), 101; https://doi.org/10.3390/universe7040101 - 15 Apr 2021
Cited by 3 | Viewed by 1749
Abstract
We investigate the influence of the chimney topology T×T×R of the Universe on the gravitational potential and force that are generated by point-like massive bodies. We obtain three distinct expressions for the solutions. One follows from Fourier expansion of [...] Read more.
We investigate the influence of the chimney topology T×T×R of the Universe on the gravitational potential and force that are generated by point-like massive bodies. We obtain three distinct expressions for the solutions. One follows from Fourier expansion of delta functions into series using periodicity in two toroidal dimensions. The second one is the summation of solutions of the Helmholtz equation, for a source mass and its infinitely many images, which are in the form of Yukawa potentials. The third alternative solution for the potential is formulated via the Ewald sums method applied to Yukawa-type potentials. We show that, for the present Universe, the formulas involving plain summation of Yukawa potentials are preferable for computational purposes, as they require a smaller number of terms in the series to reach adequate precision. Full article
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18 pages, 466 KiB  
Article
Perturbations of the Gravitational Energy in the TEGR: Quasinormal Modes of the Schwarzschild Black Hole
by José Wadih Maluf, Sérgio Ulhoa, Fernando Lessa Carneiro and Karlúcio H. C. Castello-Branco
Universe 2021, 7(4), 100; https://doi.org/10.3390/universe7040100 - 14 Apr 2021
Cited by 4 | Viewed by 1888
Abstract
We calculate the gravitational energy spectrum of the perturbations of a Schwarzschild black hole described by quasinormal modes, in the framework of the teleparallel equivalent of general relativity (TEGR). We obtain a general formula for the gravitational energy enclosed by a large surface [...] Read more.
We calculate the gravitational energy spectrum of the perturbations of a Schwarzschild black hole described by quasinormal modes, in the framework of the teleparallel equivalent of general relativity (TEGR). We obtain a general formula for the gravitational energy enclosed by a large surface of constant radius r, in the region m<<r<<, where m is the mass of the black hole. Considering the usual asymptotic expression for the perturbed metric components, we arrive at finite values for the energy spectrum. The perturbed energy depends on the two integers n and l that describe the quasinormal modes. In this sense, the energy perturbations are discretized. We also obtain a simple expression for the decrease of the flux of gravitational radiation of the perturbations. Full article
(This article belongs to the Special Issue Teleparallel Gravity: Foundations and Observational Constraints)
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17 pages, 309 KiB  
Article
Curved Momentum Space, Locality, and Generalized Space-Time
by José Manuel Carmona, José Luis Cortés and José Javier Relancio
Universe 2021, 7(4), 99; https://doi.org/10.3390/universe7040099 - 13 Apr 2021
Cited by 14 | Viewed by 1611
Abstract
We establish the correspondence between two apparently unrelated but in fact complementary approaches of a relativistic deformed kinematics: the geometric properties of momentum space and the loss of absolute locality in canonical space-time, which can be restored with the introduction of a generalized [...] Read more.
We establish the correspondence between two apparently unrelated but in fact complementary approaches of a relativistic deformed kinematics: the geometric properties of momentum space and the loss of absolute locality in canonical space-time, which can be restored with the introduction of a generalized space-time. This correspondence is made explicit for the case of κ-Poincaré kinematics and compared with its properties in the Hopf algebra framework. Full article
(This article belongs to the Collection Probing Quantum Gravity)
19 pages, 507 KiB  
Article
Nuclear Matrix Elements for Heavy Ion Sequential Double Charge Exchange Reactions
by Horst Lenske, Jessica Bellone, Maria Colonna and Danilo Gambacurta
Universe 2021, 7(4), 98; https://doi.org/10.3390/universe7040098 - 13 Apr 2021
Cited by 21 | Viewed by 1518
Abstract
The theoretical approach to a sequential heavy ion double charge exchange reaction is presented. A brief introduction into the formal theory of second-order nuclear reactions and their application to Double Single Charge Exchange (DSCE) reactions by distorted wave theory is given, thereby completing [...] Read more.
The theoretical approach to a sequential heavy ion double charge exchange reaction is presented. A brief introduction into the formal theory of second-order nuclear reactions and their application to Double Single Charge Exchange (DSCE) reactions by distorted wave theory is given, thereby completing the theoretical background to our recent work. Formally, the DSCE reaction amplitudes are shown to be separable into superpositions of distortion factors, accounting for initial and final state ion–ion interactions, and nuclear matrix elements. A broad space is given to the construction of nuclear DSCE response functions on the basis of polarization propagator theory. The nuclear response tensors resemble the nuclear matrix elements of 2νββ decay in structure but contain in general a considerable more complex multipole and spin structure. The QRPA theory is used to derive explicit expressions for nuclear matrix elements (NMEs). The differences between the NME of the first and the second interaction vertexes in a DSCE reaction is elucidated. Reduction schemes for the transition form factors are discussed by investigating the closure approximation and the momentum structure of form factors. DSCE unit strength cross sections are derived. Full article
(This article belongs to the Special Issue Nuclear Issues for Neutrino Physics)
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24 pages, 613 KiB  
Review
A Gravitational-Wave Perspective on Neutron-Star Seismology
by Nils Andersson
Universe 2021, 7(4), 97; https://doi.org/10.3390/universe7040097 - 11 Apr 2021
Cited by 23 | Viewed by 2314
Abstract
We provide a bird’s-eye view of neutron-star seismology, which aims to probe the extreme physics associated with these objects, in the context of gravitational-wave astronomy. Focussing on the fundamental mode of oscillation, which is an efficient gravitational-wave emitter, we consider the seismology aspects [...] Read more.
We provide a bird’s-eye view of neutron-star seismology, which aims to probe the extreme physics associated with these objects, in the context of gravitational-wave astronomy. Focussing on the fundamental mode of oscillation, which is an efficient gravitational-wave emitter, we consider the seismology aspects of a number of astrophysically relevant scenarios, ranging from transients (like pulsar glitches and magnetar flares), to the dynamics of tides in inspiralling compact binaries and the eventual merged object and instabilities acting in isolated, rapidly rotating, neutron stars. The aim is not to provide a thorough review, but rather to introduce (some of) the key ideas and highlight issues that need further attention. Full article
(This article belongs to the Special Issue Fundamental Processes in Neutron Stars and Supernovae)
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22 pages, 10048 KiB  
Review
20 Years of Indian Gamma Ray Astronomy Using Imaging Cherenkov Telescopes and Road Ahead
by Krishna Kumar Singh and Kuldeep Kumar Yadav
Universe 2021, 7(4), 96; https://doi.org/10.3390/universe7040096 - 10 Apr 2021
Cited by 10 | Viewed by 2325
Abstract
The field of ground-based γ-ray astronomy has made very significant advances over the last three decades with the extremely successful operations of several atmospheric Cherenkov telescopes worldwide. The advent of the imaging Cherenkov technique for indirect detection of cosmic γ rays has [...] Read more.
The field of ground-based γ-ray astronomy has made very significant advances over the last three decades with the extremely successful operations of several atmospheric Cherenkov telescopes worldwide. The advent of the imaging Cherenkov technique for indirect detection of cosmic γ rays has immensely contributed to this field with the discovery of more than 220 γ-ray sources in the Universe. This has greatly improved our understanding of the various astrophysical processes involved in the non-thermal emission at energies above 100 GeV. In this paper, we summarize the important results achieved by the Indian γ-ray astronomers from the GeV-TeV observations using imaging Cherenkov telescopes over the last two decades. We mainly emphasize the results obtained from the observations of active galactic nuclei with the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera) telescope, which has been operational since 1997 at Mount Abu, India. We also discuss the future plans of the Indian γ-ray astronomy program with special focus on the scientific objectives of the recently installed 21 m diameter MACE (Major Atmospheric Cherenkov Experiment) telescope at Hanle, India. Full article
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16 pages, 336 KiB  
Article
The Trans-Planckian Censorship Conjecture in Different Frameworks of Viable Inflation
by Bruno Sanna and Lorenzo Sebastiani
Universe 2021, 7(4), 95; https://doi.org/10.3390/universe7040095 - 9 Apr 2021
Cited by 1 | Viewed by 1849
Abstract
We review the recently proposed Trans-Planckian Censorship Conjecture (TCC) that stems from the trans-Planckian problem of cosmological perturbations. We analyze the implications and constraints that the TCC introduces in different frameworks of viable inflation. We revisit the case of slow-roll scalar field inflation [...] Read more.
We review the recently proposed Trans-Planckian Censorship Conjecture (TCC) that stems from the trans-Planckian problem of cosmological perturbations. We analyze the implications and constraints that the TCC introduces in different frameworks of viable inflation. We revisit the case of slow-roll scalar field inflation and we investigate the cases of slow-roll f(R) and f(R,ϕ)-gravity. Finally, we consider the conjecture in the context of constant-roll scalar field inflation. Full article
(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)
21 pages, 383 KiB  
Review
Heavy Tetraquarks in the Relativistic Quark Model
by Rudolf N. Faustov, Vladimir O. Galkin and Elena M. Savchenko
Universe 2021, 7(4), 94; https://doi.org/10.3390/universe7040094 - 8 Apr 2021
Cited by 66 | Viewed by 2183
Abstract
We give a review of the calculations of the masses of tetraquarks with two and four heavy quarks in the framework of the relativistic quark model based on the quasipotential approach and QCD. The diquark-antidiquark picture of heavy tetraquarks is used. The quasipotentials [...] Read more.
We give a review of the calculations of the masses of tetraquarks with two and four heavy quarks in the framework of the relativistic quark model based on the quasipotential approach and QCD. The diquark-antidiquark picture of heavy tetraquarks is used. The quasipotentials of the quark-quark and diquark-antidiquark interactions are constructed similarly to the previous consideration of mesons and baryons. Diquarks are considered in the colour triplet state. It is assumed that the diquark and antidiquark interact in the tetraquark as a whole and the internal structure of the diquarks is taken into account by the calculated form factor of the diquark-gluon interaction. All parameters of the model are kept fixed from our previous calculations of meson and baryon properties. A detailed comparison of the obtained predictions for heavy tetraquark masses with available experimental data is given. Many candidates for tetraquarks are found. It is argued that the structures in the di-J/ψ mass spectrum observed recently by the LHCb collaboration can be interpreted as ccc¯c¯ tetraquarks. Full article
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33 pages, 1060 KiB  
Article
Measurement of the Casimir Force between 0.2 and 8 μm: Experimental Procedures and Comparison with Theory
by Giuseppe Bimonte, Benjamin Spreng, Paulo A. Maia Neto, Gert-Ludwig Ingold, Galina L. Klimchitskaya, Vladimir M. Mostepanenko and Ricardo S. Decca
Universe 2021, 7(4), 93; https://doi.org/10.3390/universe7040093 - 7 Apr 2021
Cited by 45 | Viewed by 2634
Abstract
We present results on the determination of the differential Casimir force between an Au-coated sapphire sphere and the top and bottom of Au-coated deep silicon trenches performed by means of the micromechanical torsional oscillator in the range of separations from 0.2 to 8 [...] Read more.
We present results on the determination of the differential Casimir force between an Au-coated sapphire sphere and the top and bottom of Au-coated deep silicon trenches performed by means of the micromechanical torsional oscillator in the range of separations from 0.2 to 8 μm. The random and systematic errors in the measured force signal are determined at the 95% confidence level and combined into the total experimental error. The role of surface roughness and edge effects is investigated and shown to be negligibly small. The distribution of patch potentials is characterized by Kelvin probe microscopy, yielding an estimate of the typical size of patches, the respective r.m.s. voltage and their impact on the measured force. A comparison between the experimental results and theory is performed with no fitting parameters. For this purpose, the Casimir force in the sphere-plate geometry is computed independently on the basis of first principles of quantum electrodynamics using the scattering theory and the gradient expansion. In doing so, the frequency-dependent dielectric permittivity of Au is found from the optical data extrapolated to zero frequency by means of the plasma and Drude models. It is shown that the measurement results exclude the Drude model extrapolation over the region of separations from 0.2 to 4.8 μm, whereas the alternative extrapolation by means of the plasma model is experimentally consistent over the entire measurement range. A discussion of the obtained results is provided. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
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27 pages, 430 KiB  
Article
Vacuum Polarization with Zero-Range Potentials on a Hyperplane
by Davide Fermi
Universe 2021, 7(4), 92; https://doi.org/10.3390/universe7040092 - 7 Apr 2021
Cited by 3 | Viewed by 1704
Abstract
The quantum vacuum fluctuations of a neutral scalar field induced by background zero-range potentials concentrated on a flat hyperplane of co-dimension 1 in (d+1)-dimensional Minkowski spacetime are investigated. Perfectly reflecting and semitransparent surfaces are both taken into account, [...] Read more.
The quantum vacuum fluctuations of a neutral scalar field induced by background zero-range potentials concentrated on a flat hyperplane of co-dimension 1 in (d+1)-dimensional Minkowski spacetime are investigated. Perfectly reflecting and semitransparent surfaces are both taken into account, making reference to the most general local, homogeneous and isotropic boundary conditions compatible with the unitarity of the quantum field theory. The renormalized vacuum polarization is computed for both zero and non-zero mass of the field, implementing a local version of the zeta regularization technique. The asymptotic behaviors of the vacuum polarization for small and large distances from the hyperplane are determined to leading order. It is shown that boundary divergences are softened in the specific case of a pure Dirac delta potential. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
28 pages, 362 KiB  
Article
The Quantization of Gravity: Quantization of the Hamilton Equations
by Claus Gerhardt
Universe 2021, 7(4), 91; https://doi.org/10.3390/universe7040091 - 7 Apr 2021
Cited by 2 | Viewed by 1825
Abstract
We quantize the Hamilton equations instead of the Hamilton condition. The resulting equation has the simple form Δu=0 in a fiber bundle, where the Laplacian is the Laplacian of the Wheeler–DeWitt metric provided n4. Using then [...] Read more.
We quantize the Hamilton equations instead of the Hamilton condition. The resulting equation has the simple form Δu=0 in a fiber bundle, where the Laplacian is the Laplacian of the Wheeler–DeWitt metric provided n4. Using then separation of variables, the solutions u can be expressed as products of temporal and spatial eigenfunctions, where the spatial eigenfunctions are eigenfunctions of the Laplacian in the symmetric space SL(n,R)/SO(n). Since one can define a Schwartz space and tempered distributions in SL(n,R)/SO(n) as well as a Fourier transform, Fourier quantization can be applied such that the spatial eigenfunctions are transformed to Dirac measures and the spatial Laplacian to a multiplication operator. Full article
(This article belongs to the Special Issue Advances in Loop Quantum Cosmology)
24 pages, 10491 KiB  
Article
A Statistical Analysis of Plasma Bubbles Observed by Swarm Constellation during Different Types of Geomagnetic Storms
by Fayrouz Hussien, Essam Ghamry and Adel Fathy
Universe 2021, 7(4), 90; https://doi.org/10.3390/universe7040090 - 6 Apr 2021
Cited by 7 | Viewed by 2595
Abstract
Based on the observations of Ionospheric Bubble Index (IBI) data from the Swarm mission, the characteristics of plasma bubbles are investigated during different types of geomagnetic storms recorded from 2014 to 2020. The geometrical constellation of the Swarm mission enabled us to investigate [...] Read more.
Based on the observations of Ionospheric Bubble Index (IBI) data from the Swarm mission, the characteristics of plasma bubbles are investigated during different types of geomagnetic storms recorded from 2014 to 2020. The geometrical constellation of the Swarm mission enabled us to investigate the altitudinal profile of the IBIs during different activity levels in a statistical mean. Results show that the majority of IBIs associated with moderate storms are observed at low altitudes and the probability of observing IBIs at high altitudes (Swarm-B) increases with the increase in storm level. This is confirmed by observing the F2 layer peak height (hmF2) during super storm events at larger altitudes using COSMIC data. The maximum number of IBIs is recorded within the South Atlantic Anomaly (SAA) region with a long duration time and tends to increase only during dusk time. Both the large duration time and number of IBIs over the South Atlantic Anomaly (SAA) suggest that the gradient in the electron density and the depression in the magnetic field are the main factors controlling IBI events. Also, the IBIs at high altitudes are larger at sunset and at low altitudes pre-midnight. In addition, the occurrence of IBIs is always larger in the northern hemisphere than in the southern hemisphere irrespective of the type of storm, as well as during the summer months. Moreover, there is no correlation between the duration time of IBIs and both the altitudinal observation of the IBIs and the storm type. Seasonal occurrence of IBIs is larger during equinoxes and vice versa during solstices irrespective of both the type of storm and the altitude of the satellite. The large number of IBIs during equinoxes agrees with the previous studies, which expect that the large electron density is a developer of steeper n. Large occurrences of super storm IBIs observed within the pre-midnight during summer and at sunset during equinoxes are a novel observation that needs further investigation. Also, the majority of IBIs are observed a few hours after geomagnetic substorms, which reflects the role of the Disturbance Dynamo Electric Field (DDEF) as a main driver of IBIs. Full article
(This article belongs to the Special Issue Space Weather)
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31 pages, 473 KiB  
Article
Broken Scale Invariance, Gravity Mass, and Dark Energy inModified Einstein Gravity with Two Measure Finsler like Variables
by Panayiotis Stavrinos and Sergiu I. Vacaru
Universe 2021, 7(4), 89; https://doi.org/10.3390/universe7040089 - 3 Apr 2021
Cited by 10 | Viewed by 1908
Abstract
We study new classes of generic off-diagonal and diagonal cosmological solutions for effective Einstein equations in modified gravity theories (MGTs), with modified dispersion relations (MDRs), and encoding possible violations of (local) Lorentz invariance (LIVs). Such MGTs are constructed for actions and Lagrange densities [...] Read more.
We study new classes of generic off-diagonal and diagonal cosmological solutions for effective Einstein equations in modified gravity theories (MGTs), with modified dispersion relations (MDRs), and encoding possible violations of (local) Lorentz invariance (LIVs). Such MGTs are constructed for actions and Lagrange densities with two non-Riemannian volume forms (similar to two measure theories (TMTs)) and associated bimetric and/or biconnection geometric structures. For conventional nonholonomic 2 + 2 splitting, we can always describe such models in Finsler-like variables, which is important for elaborating geometric methods of constructing exact and parametric solutions. Examples of such Finsler two-measure formulations of general relativity (GR) and MGTs are considered for Lorentz manifolds and their (co) tangent bundles and abbreviated as FTMT. Generic off-diagonal metrics solving gravitational field equations in FTMTs are determined by generating functions, effective sources and integration constants, and characterized by nonholonomic frame torsion effects. By restricting the class of integration functions, we can extract torsionless and/or diagonal configurations and model emergent cosmological theories with square scalar curvature, R2, when the global Weyl-scale symmetry is broken via nonlinear dynamical interactions with nonholonomic constraints. In the physical Einstein–Finsler frame, the constructions involve: (i) nonlinear re-parametrization symmetries of the generating functions and effective sources; (ii) effective potentials for the scalar field with possible two flat regions, which allows for a unified description of locally anisotropic and/or isotropic early universe inflation related to acceleration cosmology and dark energy; (iii) there are “emergent universes” described by off-diagonal and diagonal solutions for certain nonholonomic phases and parametric cosmological evolution resulting in various inflationary phases; (iv) we can reproduce massive gravity effects in two-measure theories. Finally, we study a reconstructing procedure for reproducing off-diagonal FTMT and massive gravity cosmological models as effective Einstein gravity or Einstein–Finsler theories. Full article
(This article belongs to the Special Issue Modified Theories of Gravity and Cosmological Applications)
11 pages, 3055 KiB  
Article
Revisiting the Planet Mass and Stellar Metallicity Relation for Low-Mass Exoplanets Orbiting GKM Class Stars
by Jonathan H. Jiang, Daniel Zhao, Xuan Ji, Bohan Xie and Kristen A. Fahy
Universe 2021, 7(4), 88; https://doi.org/10.3390/universe7040088 - 2 Apr 2021
Cited by 1 | Viewed by 2059
Abstract
The growing database of exoplanets has shown us the statistical characteristics of various exoplanet populations, providing insight towards their origins. Observational evidence suggests that the process by which gas giants are conceived in the stellar disk may be disparate from that of smaller [...] Read more.
The growing database of exoplanets has shown us the statistical characteristics of various exoplanet populations, providing insight towards their origins. Observational evidence suggests that the process by which gas giants are conceived in the stellar disk may be disparate from that of smaller planets. Using NASA’s Exoplanet Archive, we analyzed the relationships between planet mass and stellar metallicity, as well as planet mass and stellar mass for low-mass exoplanets (MP < 0.13 MJ) orbiting spectral class G, K, and M stars. We performed further uncertainty analysis to confirm that the exponential law relationships found between the planet mass, stellar mass, and the stellar metallicity cannot be fully explained by observation biases alone. Full article
(This article belongs to the Section Planetary Sciences)
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42 pages, 1575 KiB  
Review
Variable Density Flows in Rotating Astrophysical Plasma. Linear Waves and Resonant Phenomena
by Maria Fedotova, Dmitry Klimachkov and Arakel Petrosyan
Universe 2021, 7(4), 87; https://doi.org/10.3390/universe7040087 - 1 Apr 2021
Cited by 6 | Viewed by 1956
Abstract
New observational data and modeling of physical processes constantly appear in the young and rapidly developing branch of science of plasma astrophysics. However, there is a lack of theoretical studies in the field of plasma astrophysics, that could unite the physics of various [...] Read more.
New observational data and modeling of physical processes constantly appear in the young and rapidly developing branch of science of plasma astrophysics. However, there is a lack of theoretical studies in the field of plasma astrophysics, that could unite the physics of various objects in the Universe, explain the observed phenomena and contribute to the improvement of numerical modeling schemes efficiency. This article makes up for this shortcoming by introducing different models, taking into account the various properties of plasma objects. We present a review of the latest magnetohydrodynamic theories of wave processes in rotating astrophysical plasma, taking into account important and common properties of astrophysical objects as compressibility and stratification. Full article
(This article belongs to the Section Space Science)
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10 pages, 3305 KiB  
Article
Quantum Physics Literacy Aimed at K12 and the General Public
by Caterina Foti, Daria Anttila, Sabrina Maniscalco and Maria Luisa Chiofalo
Universe 2021, 7(4), 86; https://doi.org/10.3390/universe7040086 - 1 Apr 2021
Cited by 15 | Viewed by 3145
Abstract
Educating K12 students and general public in quantum physics represents an evitable must no longer since quantum technologies are going to revolutionize our lives. Quantum literacy is a formidable challenge and an extraordinary opportunity for a massive cultural uplift, where citizens learn how [...] Read more.
Educating K12 students and general public in quantum physics represents an evitable must no longer since quantum technologies are going to revolutionize our lives. Quantum literacy is a formidable challenge and an extraordinary opportunity for a massive cultural uplift, where citizens learn how to engender creativity and practice a new way of thinking, essential for smart community building. Scientific thinking hinges on analyzing facts and creating understanding, and it is then formulated with the dense mathematical language for later fact checking. Within classical physics, learners’ intuition may in principle be educated via classroom demonstrations of everyday-life phenomena. Their understanding can even be framed with the mathematics suited to their instruction degree. For quantum physics, on the contrary, we have no experience of quantum phenomena and the required mathematics is beyond non-expert reach. Therefore, educating intuition needs imagination. Without rooting to experiments and some degree of formal framing, educators face the risk to provide only evanescent tales, often misled, while resorting to familiar analogies. Here, we report on the realization of QPlayLearn, an online platform conceived to explicitly address challenges and opportunities of massive quantum literacy. QPlayLearn’s mission is to provide multilevel education on quantum science and technologies to anyone, regardless of age and background. To this aim, innovative interactive tools enhance the learning process effectiveness, fun, and accessibility, while remaining grounded on scientific correctness. Examples are games for basic quantum physics teaching, on-purpose designed animations, and easy-to-understand explanations on terminology and concepts by global experts. As a strategy for massive cultural change, QPlayLearn offers diversified content for different target groups, from primary school all the way to university physics students. It is addressed also to companies wishing to understand the potential of the emergent quantum industry, journalists, and policymakers needing to seize what quantum technologies are about, as well as all quantum science enthusiasts. Full article
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33 pages, 8719 KiB  
Review
Probing the Universe with Fast Radio Bursts
by Shivani Bhandari and Chris Flynn
Universe 2021, 7(4), 85; https://doi.org/10.3390/universe7040085 - 1 Apr 2021
Cited by 19 | Viewed by 5591
Abstract
Fast Radio Bursts (FRBs) represent a novel tool for probing the properties of the universe at cosmological distances. The dispersion measures of FRBs, combined with the redshifts of their host galaxies, has very recently yielded a direct measurement of the baryon content of [...] Read more.
Fast Radio Bursts (FRBs) represent a novel tool for probing the properties of the universe at cosmological distances. The dispersion measures of FRBs, combined with the redshifts of their host galaxies, has very recently yielded a direct measurement of the baryon content of the universe, and has the potential to directly constrain the location of the “missing baryons”. The first results are consistent with the expectations of ΛCDM for the cosmic density of baryons, and have provided the first constraints on the properties of the very diffuse intergalactic medium (IGM) and circumgalactic medium (CGM) around galaxies. FRBs are the only known extragalactic sources that are compact enough to exhibit diffractive scintillation in addition to showing exponential tails which are typical of scattering in turbulent media. This will allow us to probe the turbulent properties of the circumburst medium, the host galaxy ISM/halo, and intervening halos along the path, as well as the IGM. Measurement of the Hubble constant and the dark energy parameter w can be made with FRBs, but require very large samples of localised FRBs (>103) to be effective on their own—they are best combined with other independent surveys to improve the constraints. Ionisation events, such as for He ii, leave a signature in the dispersion measure—redshift relation, and if FRBs exist prior to these times, they can be used to probe the reionisation era, although more than 103 localised FRBs are required. Full article
(This article belongs to the Special Issue Fast Radio Bursts)
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36 pages, 1237 KiB  
Review
Casimir Puzzle and Casimir Conundrum: Discovery and Search for Resolution
by Vladimir M. Mostepanenko
Universe 2021, 7(4), 84; https://doi.org/10.3390/universe7040084 - 1 Apr 2021
Cited by 43 | Viewed by 2490
Abstract
This paper provides a review of the complicated problems in Lifshitz theory describing the Casimir force between real material plates composed of metals and dielectrics, including different approaches to their resolution. For both metallic plates with perfect crystal lattices and any dielectric plates, [...] Read more.
This paper provides a review of the complicated problems in Lifshitz theory describing the Casimir force between real material plates composed of metals and dielectrics, including different approaches to their resolution. For both metallic plates with perfect crystal lattices and any dielectric plates, we show that the Casimir entropy calculated in the framework of Lifshitz theory violates the Nernst heat theorem when the well-approved dielectric functions are used in computations. The respective theoretical Casimir forces are excluded by the measurement data of numerous precision experiments. In the literature, this situation has been called the Casimir puzzle and the Casimir conundrum for the cases of metallic and dielectric plates, respectively. This review presents a summary of both the main theoretical and experimental findings on this subject. Next, a discussion is provided of the main approaches proposed in the literature to bring the Lifshitz theory into agreement with the measurement data and with the laws of thermodynamics. Special attention is paid to the recently suggested spatially nonlocal Drude-like response functions, which consider the relaxation properties of conduction electrons, as does the standard Drude model, but lead to the theoretical results being in agreement with both thermodynamics and the measurement data through the alternative response to quantum fluctuations of the mass shell. Further advances and trends in this field of research are discussed. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
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8 pages, 256 KiB  
Communication
On the Heating of AGN Magnetospheres
by Zaza Osmanov and Swadesh Mahajan
Universe 2021, 7(4), 83; https://doi.org/10.3390/universe7040083 - 31 Mar 2021
Cited by 2 | Viewed by 1320
Abstract
The Langmuir–Landau-Centrifugal Drive (LLCD), which can effectively “convert” gravitational energy into particles, is explored as a driving mechanism responsible for the extreme thermal luminosity acquired by some active galactic nuclei (AGN). For this purpose, we consider equations governing the process of heating of [...] Read more.
The Langmuir–Landau-Centrifugal Drive (LLCD), which can effectively “convert” gravitational energy into particles, is explored as a driving mechanism responsible for the extreme thermal luminosity acquired by some active galactic nuclei (AGN). For this purpose, we consider equations governing the process of heating of AGN magnetospheres. In particular, we examine the Fourier components of the momentum equation, the continuity equation and the Poisson equation in the linear approximation and estimate the growth rate of the centrifugally excited electrostatic waves and the increment of the Langmuir collapse. It is shown that the process of energy pumping is composed of three stages: in the first stage the energy is efficiently transferred from rotation to the electrostatic modes. In due course of time, the second regime-the Langmuir collapse-occurs, when energy pumping is even more efficient. This process is terminated by the Landau damping, when enormous energy is released in the form of heat. We show that the magnetospheres of the supermassive black holes with luminosities of the order of 104546 erg/s can be heated up to 10610 K. Full article
(This article belongs to the Special Issue Panchromatic View of the Life-Cycle of AGN)
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19 pages, 8031 KiB  
Article
Blocks Size Frequency Distribution in the Enceladus Tiger Stripes Area: Implications on Their Formative Processes
by Maurizio Pajola, Alice Lucchetti, Lara Senter and Gabriele Cremonese
Universe 2021, 7(4), 82; https://doi.org/10.3390/universe7040082 - 30 Mar 2021
Cited by 9 | Viewed by 2464
Abstract
We study the size frequency distribution of the blocks located in the deeply fractured, geologically active Enceladus South Polar Terrain with the aim to suggest their formative mechanisms. Through the Cassini ISS images, we identify ~17,000 blocks with sizes ranging from ~25 m [...] Read more.
We study the size frequency distribution of the blocks located in the deeply fractured, geologically active Enceladus South Polar Terrain with the aim to suggest their formative mechanisms. Through the Cassini ISS images, we identify ~17,000 blocks with sizes ranging from ~25 m to 366 m, and located at different distances from the Damascus, Baghdad and Cairo Sulci. On all counts and for both Damascus and Baghdad cases, the power-law fitting curve has an index that is similar to the one obtained on the deeply fractured, actively sublimating Hathor cliff on comet 67P/Churyumov-Gerasimenko, where several non-dislodged blocks are observed. This suggests that as for 67P, sublimation and surface stresses favor similar fractures development in the Enceladus icy matrix, hence resulting in comparable block disaggregation. A steeper power-law index for Cairo counts may suggest a higher degree of fragmentation, which could be the result of localized, stronger tectonic disruption of lithospheric ice. Eventually, we show that the smallest blocks identified are located from tens of m to 20–25 km from the Sulci fissures, while the largest blocks are found closer to the tiger stripes. This result supports the ejection hypothesis mechanism as the possible source of blocks. Full article
(This article belongs to the Section Planetary Sciences)
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