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Volume 7
Issue 5
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Universe, Volume 7, Issue 5 (May 2021) – 47 articles

Cover Story (view full-size image): The first gravitational wave signal was observed by the LIGO detectors in September 2014. This was followed by observations of tens of other signals by LIGO and Virgo projects. The signals observed originated from inspirals and mergers of black holes and neutrons stars—the most compact objects known in the Universe. This opened a new field gravitational wave astronomy. During the first half of the third observing run of LIGO and Virgo projects, 39 new gravitational wave signals were detected. Some of them originated from mergers of binary systems with significantly unequal masses and unusually high masses posed challenges for the theoretical models of the stellar evolution. Theoretical analysis of gravitational collapse and gravitational waves by the recipient of the Nobel Prize in physics in 2020, Sir Roger Penrose, provided a fundamental background for the LIGO and Virgo observations. View this paper
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19 pages, 440 KiB  
Article
Motion-Induced Radiation Due to an Atom in the Presence of a Graphene Plane
by César D. Fosco, Fernando C. Lombardo and Francisco D. Mazzitelli
Universe 2021, 7(5), 158; https://doi.org/10.3390/universe7050158 - 20 May 2021
Cited by 9 | Viewed by 1864
Abstract
We study the motion-induced radiation due to the non-relativistic motion of an atom, coupled to the vacuum electromagnetic field by an electric dipole term, in the presence of a static graphene plate. After computing the probability of emission for an accelerated atom in [...] Read more.
We study the motion-induced radiation due to the non-relativistic motion of an atom, coupled to the vacuum electromagnetic field by an electric dipole term, in the presence of a static graphene plate. After computing the probability of emission for an accelerated atom in empty space, we evaluate the corrections due to the presence of the plate. We show that the effect of the plate is to increase the probability of emission when the atom is near the plate and oscillates along a direction perpendicular to it. On the contrary, for parallel oscillations, there is a suppression. We also evaluate the quantum friction on an atom moving at constant velocity parallel to the plate. We show that there is a threshold for quantum friction: friction occurs only when the velocity of the atom is larger than the Fermi velocity of the electrons in graphene. 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, 4147 KiB  
Article
Moon Mapping Project Results on Solar Wind Ion Flux and Composition
by Francesco Nozzoli and Pietro Richelli
Universe 2021, 7(5), 157; https://doi.org/10.3390/universe7050157 - 19 May 2021
Cited by 1 | Viewed by 2289
Abstract
The “Moon Mapping” project is a collaboration between the Italian and Chinese Governments allowing cooperation and exchange between students from both countries. The main aim of the project is to analyze remotely-sensed data collected by the Chinese space missions Chang’E-1/2 over the Moon [...] Read more.
The “Moon Mapping” project is a collaboration between the Italian and Chinese Governments allowing cooperation and exchange between students from both countries. The main aim of the project is to analyze remotely-sensed data collected by the Chinese space missions Chang’E-1/2 over the Moon surface. The Italian Space Agency is responsible for the Italian side and the Center of Space Exploration, while the China Ministry of Education is responsible for the Chinese side. In this article, we summarize the results of the “Moon Mappining” project topic #1: “map of the solar wind ion” using the data collected by Chang’E-1 satellite. Chang’E-1 is a lunar orbiter, its revolution period lasts 2 h, and its orbit is polar. The satellite is equipped with two Solar Wind Ion Detectors (SWIDs) that are two perpendicular electrostatic spectrometers mapping the sky with a field of view of 15° × 6.7° × 24 ch. The spectrometers can measure solar wind flux in the range 40 eV/q–17 keV/q with an energy resolution of 8% and time resolution of ∼3 s. The data collected by the two Solar Wind Ion Detectors are analyzed to characterize the solar wind flux and composition on the Moon surface and to study the time variations due to the solar activity. The data measured by Chang’E-1 compared with the one measured in the same period by the electrostatic spectrometers onboard the ACE satellite, or with another solar activity indicator as the sunspot number, enrich the multi-messenger/multi-particle view of the Sun, gathering valuable information about the space weather outside the Earth magnetosphere. Full article
(This article belongs to the Special Issue Selected Papers from the 1st International Electronic Conference on Universe (ECU 2021))
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20 pages, 825 KiB  
Article
The Casimir Interaction between Spheres Immersed in Electrolytes
by Renan O. Nunes, Benjamin Spreng, Reinaldo de Melo e Souza, Gert-Ludwig Ingold, Paulo A. Maia Neto and Felipe S. S. Rosa
Universe 2021, 7(5), 156; https://doi.org/10.3390/universe7050156 - 18 May 2021
Cited by 15 | Viewed by 2714
Abstract
We investigate the Casimir interaction between two dielectric spheres immersed in an electrolyte solution. Since ionized solutions typically correspond to a plasma frequency much smaller than kBT/ħ at room temperature, only the contribution of the zeroth Matsubara frequency is [...] Read more.
We investigate the Casimir interaction between two dielectric spheres immersed in an electrolyte solution. Since ionized solutions typically correspond to a plasma frequency much smaller than kBT/ħ at room temperature, only the contribution of the zeroth Matsubara frequency is affected by ionic screening. We follow the electrostatic fluctuational approach and derive the zero-frequency contribution from the linear Poisson-Boltzmann (Debye-Hückel) equation for the geometry of two spherical surfaces of arbitrary radii. We show that a contribution from monopole fluctuations, which is reminiscent of the Kirkwood-Shumaker interaction, arises from the exclusion of ionic charge in the volume occupied by the spheres. Alongside the contribution from dipole fluctuations, such monopolar term provides the leading-order Casimir energy for very small spheres. Finally, we also investigate the large sphere limit and the conditions for validity of the proximity force (Derjaguin) approximation. Altogether, our results represent the first step towards a full scattering approach to the screening of the Casimir interaction between spheres that takes into account the nonlocal response of the electrolyte solution. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
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20 pages, 4111 KiB  
Article
The Genuine Resonance of Full-Charm Tetraquarks
by Xiaoyun Chen
Universe 2021, 7(5), 155; https://doi.org/10.3390/universe7050155 - 18 May 2021
Cited by 3 | Viewed by 1669
Abstract
In this work, the genuine resonance states of full-charm tetraquark systems with quantum numbers JPC=0++,1+−,2++ are searched in a nonrelativistic chiral quark model with the help of the Gaussian [...] Read more.
In this work, the genuine resonance states of full-charm tetraquark systems with quantum numbers JPC=0++,1+−,2++ are searched in a nonrelativistic chiral quark model with the help of the Gaussian Expansion Method. In this calculation, two structures, meson-meson and diquark–antidiquark, as well as their mixing with all possible color-spin configurations, are considered. The results show that no bound states can be formed. However, resonances are possible because of the color structure. The genuine resonances are identified by the stabilization method (real scaling method). Several resonances for the full-charm system are proposed, and some of them are reasonable candidates for the full-charm states recently reported by LHCb. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
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16 pages, 353 KiB  
Article
Dynamical Analysis of Charged Dissipative Cylindrical Collapse in Energy-Momentum Squared Gravity
by Muhammad Zeeshan Gul and Muhammad Sharif
Universe 2021, 7(5), 154; https://doi.org/10.3390/universe7050154 - 18 May 2021
Cited by 31 | Viewed by 1921
Abstract
This paper investigates the dynamics of charged cylindrical collapse with the dissipative matter configuration in f(R,TαβTαβ) theory. This newly formulated theory resolves the primordial singularity and provides feasible cosmological results in the early [...] Read more.
This paper investigates the dynamics of charged cylindrical collapse with the dissipative matter configuration in f(R,TαβTαβ) theory. This newly formulated theory resolves the primordial singularity and provides feasible cosmological results in the early universe.Moreover, its implications occur in high curvature regime where the deviations of energy-momentum squared gravity from general relativity is confirmed. We establish dynamical and transport equations through the Misner–Sharp and Mu¨ler–Israel Stewart techniques, respectively. We then couple these equations to examine the impact of effective fluid parameters and correction terms on the collapsing phenomenon. A connection between the modified terms, matter parameters, and Weyl tensor is also developed. To obtain conformal flatness, we choose a particular model of this theory and assume that dust matter with zero charge leads to conformal flatness and homogenous energy density. We found that the modified terms, dissipative matter, and electromagnetic field reduce the collapsing phenomenon. Full article
(This article belongs to the Special Issue Selected Papers from the 1st International Electronic Conference on Universe (ECU 2021))
19 pages, 394 KiB  
Article
Static Spherically Symmetric Black Holes in Weak f(T)-Gravity
by Christian Pfeifer and Sebastian Schuster
Universe 2021, 7(5), 153; https://doi.org/10.3390/universe7050153 - 17 May 2021
Cited by 31 | Viewed by 2570
Abstract
With the advent of gravitational wave astronomy and first pictures of the “shadow” of the central black hole of our milky way, theoretical analyses of black holes (and compact objects mimicking them sufficiently closely) have become more important than ever. The near future [...] Read more.
With the advent of gravitational wave astronomy and first pictures of the “shadow” of the central black hole of our milky way, theoretical analyses of black holes (and compact objects mimicking them sufficiently closely) have become more important than ever. The near future promises more and more detailed information about the observable black holes and black hole candidates. This information could lead to important advances on constraints on or evidence for modifications of general relativity. More precisely, we are studying the influence of weak teleparallel perturbations on general relativistic vacuum spacetime geometries in spherical symmetry. We find the most general family of spherically symmetric, static vacuum solutions of the theory, which are candidates for describing teleparallel black holes which emerge as perturbations to the Schwarzschild black hole. We compare our findings to results on black hole or static, spherically symmetric solutions in teleparallel gravity discussed in the literature, by comparing the predictions for classical observables such as the photon sphere, the perihelion shift, the light deflection, and the Shapiro delay. On the basis of these observables, we demonstrate that among the solutions we found, there exist spacetime geometries that lead to much weaker bounds on teleparallel gravity than those found earlier. Finally, we move on to a discussion of how the teleparallel perturbations influence the Hawking evaporation in these spacetimes. Full article
(This article belongs to the Special Issue Teleparallel Gravity: Foundations and Observational Constraints)
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19 pages, 2149 KiB  
Article
Mechanisms and Evolution of Geoeffective Large-Scale Plasma Jets in the Magnetosheath
by Alexei V. Dmitriev, Bhavana Lalchand and Sayantan Ghosh
Universe 2021, 7(5), 152; https://doi.org/10.3390/universe7050152 - 17 May 2021
Cited by 8 | Viewed by 2301
Abstract
Geoeffective magnetosheath plasma jets (those that interact with the magnetopause) are an important area of research and technology, since they affect the “space-weather” around the Earth. We identified such large-scale magnetosheath plasma jets with a duration of >30 s using plasma and magnetic [...] Read more.
Geoeffective magnetosheath plasma jets (those that interact with the magnetopause) are an important area of research and technology, since they affect the “space-weather” around the Earth. We identified such large-scale magnetosheath plasma jets with a duration of >30 s using plasma and magnetic data acquired from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) multi-spacecraft experiment during the years 2007 to 2009. We present a statistical survey of 554 of such geoeffective jets and elaborate on four mechanisms for the generation of these jets as the upstream solar wind structures of tangential discontinuities (TDs), rotational discontinuities (RDs), the quasi-radial interplanetary magnetic field (rIMF) and the collapsing foreshock (CFS) interrupting the rIMF intervals. We found that 69% of the jets are generated due to the interaction between interplanetary discontinuities (TD: 24%, RD: 25%, CFS: 20%) with the bow shock. Slow and weak jets due to the rIMF contributed to 31% of these jets. The CFS and rIMF were found to be similar in their characteristics. TDs and RDs contributed to most of the fast and powerful jets, with large spatial scales, which might be attributed to transient effects in the travelling foreshock. Full article
(This article belongs to the Special Issue Space Weather)
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11 pages, 339 KiB  
Article
On A. D. Sakharov’s Hypothesis of Cosmological Transitions with Changes in the Signature of the Metric
by Tatyana P. Shestakova
Universe 2021, 7(5), 151; https://doi.org/10.3390/universe7050151 - 17 May 2021
Cited by 1 | Viewed by 2174
Abstract
The paper discusses possible consequences of A. D. Sakharov’s hypothesis of cosmological transitions with changes in the signature of the metric, based on the path integral approach. This hypothesis raises a number of mathematical and philosophical questions. Mathematical questions concern the definition of [...] Read more.
The paper discusses possible consequences of A. D. Sakharov’s hypothesis of cosmological transitions with changes in the signature of the metric, based on the path integral approach. This hypothesis raises a number of mathematical and philosophical questions. Mathematical questions concern the definition of the path integral to include integration over spacetime regions with different signatures of the metric. One possible way to describe the changes in the signature is to admit time and space coordinates to be purely imaginary. It may look like a generalization of what we have in the case of pseudo-Riemannian manifolds with a non-trivial topology. The signature in these regions can be fixed by special gauge conditions on components of the metric tensor. The problem is what boundary conditions should be imposed on the boundaries of these regions and how they should be taken into account in the definition of the path integral. The philosophical question is what distinguishes the time coordinate among other coordinates but the sign of the corresponding principal value of the metric tensor. In particular, there is an attempt in speculating how the existence of the regions with different signature can affect the evolution of the Universe. Full article
(This article belongs to the Special Issue Selected Papers from the 1st International Electronic Conference on Universe (ECU 2021))
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13 pages, 279 KiB  
Article
Minisuperspace Quantization of f(T, B) Cosmology
by Andronikos Paliathanasis
Universe 2021, 7(5), 150; https://doi.org/10.3390/universe7050150 - 16 May 2021
Cited by 13 | Viewed by 2190
Abstract
We discuss the quantization in the minisuperspace for the generalized fourth-order teleparallel cosmological theory known as fT, B. Specifically we focus on the case where the theory is linear on the torsion scalar, in that consideration we are able [...] Read more.
We discuss the quantization in the minisuperspace for the generalized fourth-order teleparallel cosmological theory known as fT, B. Specifically we focus on the case where the theory is linear on the torsion scalar, in that consideration we are able to write the cosmological field equations with the use of a scalar field different from the scalar tensor theories, but with the same dynamical constraints as that of scalar tensor theories. We use the minisuperspace description to write for the first time the Wheeler-DeWitt equation. With the use of the theory of similarity transformations we are able to find exact solutions for the Wheeler-DeWitt equations as also to investigate the classical and semiclassical limit in the de Broglie -Bohm representation of quantum mechanics. Full article
(This article belongs to the Special Issue Teleparallel Gravity: Foundations and Observational Constraints)
13 pages, 343 KiB  
Article
De Sitter Solutions in Einstein–Gauss–Bonnet Gravity
by Sergey Vernov and Ekaterina Pozdeeva
Universe 2021, 7(5), 149; https://doi.org/10.3390/universe7050149 - 16 May 2021
Cited by 25 | Viewed by 2133
Abstract
De Sitter solutions play an important role in cosmology because the knowledge of unstable de Sitter solutions can be useful to describe inflation, whereas stable de Sitter solutions are often used in models of late-time acceleration of the Universe. The Einstein–Gauss–Bonnet gravity cosmological [...] Read more.
De Sitter solutions play an important role in cosmology because the knowledge of unstable de Sitter solutions can be useful to describe inflation, whereas stable de Sitter solutions are often used in models of late-time acceleration of the Universe. The Einstein–Gauss–Bonnet gravity cosmological models are actively used both as inflationary models and as dark energy models. To modify the Einstein equations one can add a nonlinear function of the Gauss–Bonnet term or a function of the scalar field multiplied on the Gauss–Bonnet term. The effective potential method essentially simplifies the search and stability analysis of de Sitter solutions, because the stable de Sitter solutions correspond to minima of the effective potential. Full article
(This article belongs to the Special Issue Selected Papers from the 1st International Electronic Conference on Universe (ECU 2021))
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17 pages, 303 KiB  
Article
Renormalizability of Alternative Theories of Gravity: Differences between Power Counting and Entropy Argument
by Francesco Bajardi, Francesco Bascone and Salvatore Capozziello
Universe 2021, 7(5), 148; https://doi.org/10.3390/universe7050148 - 14 May 2021
Cited by 10 | Viewed by 2198
Abstract
It is well known that General Relativity cannot be considered under the standard of a perturbatively renormalizable quantum field theory, but asymptotic safety is taken into account as a possibility for the formulation of gravity as a non-perturbative renormalizable theory. Recently, the entropy [...] Read more.
It is well known that General Relativity cannot be considered under the standard of a perturbatively renormalizable quantum field theory, but asymptotic safety is taken into account as a possibility for the formulation of gravity as a non-perturbative renormalizable theory. Recently, the entropy argument has however stepped into the discussion claiming for a “no-go” to the asymptotic safety argument. In this paper, we present simple counter-examples, considering alternative theories of gravity, to the entropy argument as further indications, among others, on the possible flows in the assumptions on which the latter is based. We considered different theories, namely curvature-based extensions of General Relativity as f(R), f(G), extensions of teleparallel gravity as f(T), and Horava–Lifshitz gravity, working out the explicit spherically symmetric solutions in order to make a comparison between power counting and the entropy argument. Even in these cases, inconsistencies were found. Full article
(This article belongs to the Special Issue Teleparallel Gravity: Foundations and Observational Constraints)
21 pages, 2748 KiB  
Article
Angular Distributions of Emitted Electrons in the Two-Neutrino ββ Decay
by Ovidiu Niţescu, Rastislav Dvornický, Sabin Stoica and Fedor Šimkovic
Universe 2021, 7(5), 147; https://doi.org/10.3390/universe7050147 - 14 May 2021
Cited by 8 | Viewed by 2618
Abstract
The two-neutrino double-beta decay (2νββ-decay) process is attracting more and more attention of the physics community due to its potential to explain nuclear structure aspects of involved atomic nuclei and to constrain new (beyond the Standard model) physics [...] Read more.
The two-neutrino double-beta decay (2νββ-decay) process is attracting more and more attention of the physics community due to its potential to explain nuclear structure aspects of involved atomic nuclei and to constrain new (beyond the Standard model) physics scenarios. Topics of interest are energy and angular distributions of the emitted electrons, which might allow the deduction of valuable information about fundamental properties and interactions of neutrinos once a new generation of the double-beta decay experiments will be realized. These tasks require an improved theoretical description of the 2νββ-decay differential decay rates, which is presented. The dependence of the denominators in nuclear matrix elements on lepton energies is taken into account via the Taylor expansion. Both the Fermi and Gamow-Teller matrix elements are considered. For nuclei of experimental interest, relevant phase-space factors are calculated by using exact Dirac wave functions with finite nuclear size and electron screening. The uncertainty of the angular correlation factor on nuclear structure parameters is discussed. It is emphasized that the effective axial-vector coupling constant gAeff can be determined more reliably by accurately measuring the angular correlation factor. Full article
(This article belongs to the Special Issue Nuclear Issues for Neutrino Physics)
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25 pages, 1196 KiB  
Article
Photon–Photon Interactions and the Opacity of the Universe in Gamma Rays
by Alberto Franceschini
Universe 2021, 7(5), 146; https://doi.org/10.3390/universe7050146 - 14 May 2021
Cited by 14 | Viewed by 3963
Abstract
We discuss the topic of the transparency of the Universe in gamma rays due to extragalactic background light, and its cosmological and physical implications. Rather than a review, this is a personal account on the development of 30 years of this branch of [...] Read more.
We discuss the topic of the transparency of the Universe in gamma rays due to extragalactic background light, and its cosmological and physical implications. Rather than a review, this is a personal account on the development of 30 years of this branch of physical science. Extensive analysis of the currently available information appears to us as revealing a global coherence among the astrophysical, cosmological, and fundamental physics data, or, at least, no evident need so far of substantial modification of our present understanding. Deeper data from future experiments will verify to what extent and in which directions this conclusion should be modified. Full article
(This article belongs to the Special Issue High-Energy Gamma-Ray Astronomy: Results on Fundamental Questions after 30 Years of Ground-Based Observations)
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13 pages, 436 KiB  
Article
Origin of Low- and High-Energy Monopole Collectivity in 132Sn
by Nikolay N. Arsenyev and Alexey P. Severyukhin
Universe 2021, 7(5), 145; https://doi.org/10.3390/universe7050145 - 13 May 2021
Cited by 7 | Viewed by 1845
Abstract
Beginning with the Skyrme interaction, we study the properties of the isoscalar giant monopole resonances (ISGMR) of 132Sn. Using the finite-rank separable approximation for the particle-hole interaction, the coupling between one- and two-phonon terms in the wave functions of excited states is [...] Read more.
Beginning with the Skyrme interaction, we study the properties of the isoscalar giant monopole resonances (ISGMR) of 132Sn. Using the finite-rank separable approximation for the particle-hole interaction, the coupling between one- and two-phonon terms in the wave functions of excited states is taken into account in very large configurational spaces. The inclusion of the phonon–phonon coupling (PPC) results in the formation of a low-energy 0+ state. The PPC inclusion leads to a fragmentation of the ISGMR strength to lower energy states and also to a higher energy tail. Using the same set of parameters, we describe the available experimental data for the ISGMR characteristics of 118,120,122,124Sn and give a prediction for 126,128,130,132Sn. Full article
(This article belongs to the Special Issue Nuclear Physics and Multimessenger Astrophysics)
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33 pages, 413 KiB  
Article
Landau Levels in a Gravitational Field: The Schwarzschild Spacetime Case
by Alexandre Landry and Fayçal Hammad
Universe 2021, 7(5), 144; https://doi.org/10.3390/universe7050144 - 12 May 2021
Cited by 7 | Viewed by 2078
Abstract
We investigate the gravitational effect on Landau levels. We show that the familiar infinite Landau degeneracy of the energy levels of a quantum particle moving inside a uniform and constant magnetic field is removed by the interaction of the particle with a gravitational [...] Read more.
We investigate the gravitational effect on Landau levels. We show that the familiar infinite Landau degeneracy of the energy levels of a quantum particle moving inside a uniform and constant magnetic field is removed by the interaction of the particle with a gravitational field. Two independent approaches are used to solve the relevant Schrödinger equation within the Newtonian approximation. It is found that both approaches yield qualitatively similar results within their respective approximations. With the goal of clarifying some results found in the literature concerning the use of a third independent approach for extracting the quantization condition based on a similar differential equation, we show that such an approach cannot yield a general and yet consistent result. We point out to the more accurate, but impractical, way to use such an approach; a way which does in principle yield a consistent quantization condition. We discuss how our results could be used to contribute in a novel way to the existing methods for testing gravity at the tabletop experiments level as well as at the astrophysical observational level by deriving the corrections brought by Yukawa-like and power-law deviations from the inverse-square law. The full relativistic regime is also examined in detail. Full article
16 pages, 301 KiB  
Article
Accidental Gauge Symmetries of Minkowski Spacetime in Teleparallel Theories
by Jose Beltrán Jiménez and Tomi S. Koivisto
Universe 2021, 7(5), 143; https://doi.org/10.3390/universe7050143 - 12 May 2021
Cited by 27 | Viewed by 1780
Abstract
In this paper, we provide a general framework for the construction of the Einstein frame within non-linear extensions of the teleparallel equivalents of General Relativity. These include the metric teleparallel and the symmetric teleparallel, but also the general teleparallel theories. We write the [...] Read more.
In this paper, we provide a general framework for the construction of the Einstein frame within non-linear extensions of the teleparallel equivalents of General Relativity. These include the metric teleparallel and the symmetric teleparallel, but also the general teleparallel theories. We write the actions in a form where we separate the Einstein–Hilbert term, the conformal mode due to the non-linear nature of the theories (which is analogous to the extra degree of freedom in f(R) theories), and the sector that manifestly shows the dynamics arising from the breaking of local symmetries. This frame is then used to study the theories around the Minkowski background, and we show how all the non-linear extensions share the same quadratic action around Minkowski. As a matter of fact, we find that the gauge symmetries that are lost by going to the non-linear generalisations of the teleparallel General Relativity equivalents arise as accidental symmetries in the linear theory around Minkowski. Remarkably, we also find that the conformal mode can be absorbed into a Weyl rescaling of the metric at this order and, consequently, it disappears from the linear spectrum so only the usual massless spin 2 perturbation propagates. These findings unify in a common framework the known fact that no additional modes propagate on Minkowski backgrounds, and we can trace it back to the existence of accidental gauge symmetries of such a background. Full article
(This article belongs to the Special Issue Teleparallel Gravity: Foundations and Observational Constraints)
58 pages, 5339 KiB  
Review
Feedback from Active Galactic Nuclei in Galaxy Groups
by Dominique Eckert, Massimo Gaspari, Fabio Gastaldello, Amandine M. C. Le Brun and Ewan O’Sullivan
Universe 2021, 7(5), 142; https://doi.org/10.3390/universe7050142 - 11 May 2021
Cited by 87 | Viewed by 6147
Abstract
The co-evolution between supermassive black holes and their environment is most directly traced by the hot atmospheres of dark matter halos. The cooling of the hot atmosphere supplies the central regions with fresh gas, igniting active galactic nuclei (AGN) with long duty cycles. [...] Read more.
The co-evolution between supermassive black holes and their environment is most directly traced by the hot atmospheres of dark matter halos. The cooling of the hot atmosphere supplies the central regions with fresh gas, igniting active galactic nuclei (AGN) with long duty cycles. Outflows from the central engine tightly couple with the surrounding gaseous medium and provide the dominant heating source preventing runaway cooling by carving cavities and driving shocks across the medium. The AGN feedback loop is a key feature of all modern galaxy evolution models. Here, we review our knowledge of the AGN feedback process in the specific context of galaxy groups. Galaxy groups are uniquely suited to constrain the mechanisms governing the cooling–heating balance. Unlike in more massive halos, the energy that is supplied by the central AGN to the hot intragroup medium can exceed the gravitational binding energy of halo gas particles. We report on the state-of-the-art in observations of the feedback phenomenon and in theoretical models of the heating-cooling balance in galaxy groups. We also describe how our knowledge of the AGN feedback process impacts galaxy evolution models and large-scale baryon distributions. Finally, we discuss how new instrumentation will answer key open questions on the topic. Full article
(This article belongs to the Special Issue The Physical Properties of the Groups of Galaxies)
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59 pages, 3140 KiB  
Review
Gamma Rays as Probes of Cosmic-Ray Propagation and Interactions in Galaxies
by Luigi Tibaldo, Daniele Gaggero and Pierrick Martin
Universe 2021, 7(5), 141; https://doi.org/10.3390/universe7050141 - 11 May 2021
Cited by 37 | Viewed by 4476
Abstract
Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in [...] Read more.
Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in unprecedented detail the properties of galactic cosmic rays. A variety of scales and environments are now accessible to us, from the local interstellar medium near the Sun and the vicinity of cosmic-ray accelerators, out to the Milky Way at large and beyond, with a growing number of gamma-ray emitting star-forming galaxies. Gamma-ray observations have been pushing forward our understanding of the life cycle of cosmic rays in galaxies and, combined with advances in related domains, they have been challenging standard assumptions in the field and have spurred new developments in modelling approaches and data analysis methods. We provide a review of the status of the subject and discuss perspectives on future progress. Full article
(This article belongs to the Special Issue High-Energy Gamma-Ray Astronomy: Results on Fundamental Questions after 30 Years of Ground-Based Observations)
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15 pages, 492 KiB  
Article
The SuSA Model for Neutrino Oscillation Experiments: From Quasielastic Scattering to the Resonance Region
by Maria B. Barbaro, Arturo De Pace and Luisa Fiume
Universe 2021, 7(5), 140; https://doi.org/10.3390/universe7050140 - 10 May 2021
Cited by 10 | Viewed by 1953
Abstract
High-precision studies of Beyond-Standard-Model physics through accelerator-based neutrino oscillation experiments require a very accurate description of neutrino–nucleus cross-sections in a broad energy region, going from quasielastic scattering up to deep, inelastic scattering. In this work, we focus on the following processes: quasielastic scattering, [...] Read more.
High-precision studies of Beyond-Standard-Model physics through accelerator-based neutrino oscillation experiments require a very accurate description of neutrino–nucleus cross-sections in a broad energy region, going from quasielastic scattering up to deep, inelastic scattering. In this work, we focus on the following processes: quasielastic scattering, two-particle-two-hole excitations, and the excitation of the first (Delta) and second (Roper) resonances of the nucleon. The nuclear model is fully relativistic and includes both one- and two-body currents. We compare our results with recent T2K and MicroBooNE data on carbon and argon targets, and present predictions for DUNE kinematics. Full article
(This article belongs to the Special Issue Nuclear Issues for Neutrino Physics)
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39 pages, 1448 KiB  
Review
Scaling Properties of Galaxy Groups
by Lorenzo Lovisari, Stefano Ettori, Massimo Gaspari and Paul A. Giles
Universe 2021, 7(5), 139; https://doi.org/10.3390/universe7050139 - 10 May 2021
Cited by 67 | Viewed by 3411
Abstract
Galaxy groups and poor clusters are more common than rich clusters, and host the largest fraction of matter content in the Universe. Hence, their studies are key to understand the gravitational and thermal evolution of the bulk of the cosmic matter. Moreover, because [...] Read more.
Galaxy groups and poor clusters are more common than rich clusters, and host the largest fraction of matter content in the Universe. Hence, their studies are key to understand the gravitational and thermal evolution of the bulk of the cosmic matter. Moreover, because of their shallower gravitational potential, galaxy groups are systems where non-gravitational processes (e.g., cooling, AGN feedback, star formation) are expected to have a higher impact on the distribution of baryons, and on the general physical properties, than in more massive objects, inducing systematic departures from the expected scaling relations. Despite their paramount importance from the astrophysical and cosmological point of view, the challenges in their detection have limited the studies of galaxy groups. Upcoming large surveys will change this picture, reassigning to galaxy groups their central role in studying the structure formation and evolution in the Universe, and in measuring the cosmic baryonic content. Here, we review the recent literature on various scaling relations between X-ray and optical properties of these systems, focusing on the observational measurements, and the progress in our understanding of the deviations from the self-similar expectations on groups’ scales. We discuss some of the sources of these deviations, and how feedback from supernovae and/or AGNs impacts the general properties and the reconstructed scaling laws. Finally, we discuss future prospects in the study of galaxy groups. Full article
(This article belongs to the Special Issue The Physical Properties of the Groups of Galaxies)
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13 pages, 1376 KiB  
Article
What Solar–Terrestrial Link Researchers Should Know about Interplanetary Drivers
by Yuri I. Yermolaev, Irina G. Lodkina, Lidia A. Dremukhina, Michael Y. Yermolaev and Alexander A. Khokhlachev
Universe 2021, 7(5), 138; https://doi.org/10.3390/universe7050138 - 10 May 2021
Cited by 20 | Viewed by 1761
Abstract
One of the most promising methods of research in solar–terrestrial physics is the comparison of the responses of the magnetosphere–ionosphere–atmosphere system to various types of interplanetary disturbances (so-called “interplanetary drivers”). Numerous studies have shown that different types of drivers result in different reactions [...] Read more.
One of the most promising methods of research in solar–terrestrial physics is the comparison of the responses of the magnetosphere–ionosphere–atmosphere system to various types of interplanetary disturbances (so-called “interplanetary drivers”). Numerous studies have shown that different types of drivers result in different reactions of the system for identical variations in the interplanetary magnetic field. In particular, the sheaths—compression regions before fast interplanetary CMEs (ICMEs)—have higher efficiency in terms of the generation of magnetic storms than ICMEs. The growing popularity of this method of research is accompanied by the growth of incorrect methodological approaches in such studies. These errors can be divided into four main classes: (i) using incorrect data with the identification of driver types published in other studies; (ii) using incorrect methods to identify the types of drivers and, as a result, misclassify the causes of magnetospheric-ionospheric disturbances; (iii) ignoring a frequent case with a complex, composite, nature of the driver (the presence of a sequence of several simple drivers) and matching the system response with only one of the drivers; for example, a magnetic storm is often generated by a sheath in front of ICME, although the authors consider these events to be a so-called “CME-induced” storm, rather than a “sheath-induced” storm; (iv) ignoring the compression regions before the fast CME in the case when there is no interplanetary shock (IS) in front of the compression region (“sheath without IS” or the so-called “lost driver”), although this type of driver generates about 10% of moderate and large magnetic storms. Possible ways of solving this problem are discussed. Full article
(This article belongs to the Section Solar and Stellar Physics)
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16 pages, 912 KiB  
Review
Recent Observations of Gravitational Waves by LIGO and Virgo Detectors
by Andrzej Królak and Paritosh Verma
Universe 2021, 7(5), 137; https://doi.org/10.3390/universe7050137 - 9 May 2021
Cited by 6 | Viewed by 4130
Abstract
In this paper we present the most recent observations of gravitational waves (GWs) by LIGO and Virgo detectors. We also discuss contributions of the recent Nobel prize winner, Sir Roger Penrose to understanding gravitational radiation and black holes (BHs). We make a short [...] Read more.
In this paper we present the most recent observations of gravitational waves (GWs) by LIGO and Virgo detectors. We also discuss contributions of the recent Nobel prize winner, Sir Roger Penrose to understanding gravitational radiation and black holes (BHs). We make a short introduction to GW phenomenon in general relativity (GR) and we present main sources of detectable GW signals. We describe the laser interferometric detectors that made the first observations of GWs. We briefly discuss the first direct detection of GW signal that originated from a merger of two BHs and the first detection of GW signal form merger of two neutron stars (NSs). Finally we present in more detail the observations of GW signals made during the first half of the most recent observing run of the LIGO and Virgo projects. Finally we present prospects for future GW observations. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
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15 pages, 1323 KiB  
Article
Astrophysical Wormholes
by Cosimo Bambi and Dejan Stojkovic
Universe 2021, 7(5), 136; https://doi.org/10.3390/universe7050136 - 8 May 2021
Cited by 74 | Viewed by 6824
Abstract
Wormholes are hypothetical topologically-non-trivial structures of spacetime. From the theoretical point of view, the possibility of their existence is challenging but cannot be ruled out. This article is a compact and non-exhaustive review of past and current efforts to search for astrophysical wormholes [...] Read more.
Wormholes are hypothetical topologically-non-trivial structures of spacetime. From the theoretical point of view, the possibility of their existence is challenging but cannot be ruled out. This article is a compact and non-exhaustive review of past and current efforts to search for astrophysical wormholes in the Universe. Full article
(This article belongs to the Special Issue Recent Advances in Wormhole Physics)
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29 pages, 4295 KiB  
Article
INTEGRAL View of TeV Sources: A Legacy for the CTA Project
by Angela Malizia, Mariateresa Fiocchi, Lorenzo Natalucci, Vito Sguera, John B. Stephen, Loredana Bassani, Angela Bazzano, Pietro Ubertini, Elena Pian and Antony J. Bird
Universe 2021, 7(5), 135; https://doi.org/10.3390/universe7050135 - 7 May 2021
Cited by 3 | Viewed by 3744
Abstract
Investigations that were carried out over the last two decades with novel and more sensitive instrumentation have dramatically improved our knowledge of the more violent physical processes taking place in galactic and extra-galactic Black-Holes, Neutron Stars, Supernova Remnants/Pulsar Wind Nebulae, and other regions [...] Read more.
Investigations that were carried out over the last two decades with novel and more sensitive instrumentation have dramatically improved our knowledge of the more violent physical processes taking place in galactic and extra-galactic Black-Holes, Neutron Stars, Supernova Remnants/Pulsar Wind Nebulae, and other regions of the Universe where relativistic acceleration processes are in place. In particular, simultaneous and/or combined observations with γ-ray satellites and ground based high-energy telescopes, have clarified the scenario of the mechanisms responsible for high energy photon emission by leptonic and hadronic accelerated particles in the presence of magnetic fields. Specifically, the European Space Agency INTEGRAL soft γ-ray observatory has detected more than 1000 sources in the soft γ-ray band, providing accurate positions, light curves and time resolved spectral data for them. Space observations with Fermi-LAT and observations that were carried out from the ground with H.E.S.S., MAGIC, VERITAS, and other telescopes sensitive in the GeV-TeV domain have, at the same time, provided evidence that a substantial fraction of the cosmic sources detected are emitting in the keV to TeV band via Synchrotron-Inverse Compton processes, in particular from stellar galactic BH systems as well as from distant black holes. In this work, employing a spatial cross correlation technique, we compare the INTEGRAL/IBIS and TeV all-sky data in search of secure or likely associations. Although this analysis is based on a subset of the INTEGRAL all-sky observations (1000 orbits), we find that there is a significant correlation: 39 objects (∼20% of the VHE γ-ray catalogue) show emission in both soft γ-ray and TeV wavebands. The full INTEGRAL database, now comprising almost 19 years of public data available, will represent an important legacy that will be useful for the Cherenkov Telescope Array (CTA) and other ground based large projects. Full article
(This article belongs to the Special Issue High-Energy Gamma-Ray Astronomy: Results on Fundamental Questions after 30 Years of Ground-Based Observations)
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34 pages, 942 KiB  
Article
The de Broglie–Bohm Quantum Theory and Its Application to Quantum Cosmology
by Nelson Pinto-Neto
Universe 2021, 7(5), 134; https://doi.org/10.3390/universe7050134 - 7 May 2021
Cited by 11 | Viewed by 2981
Abstract
We review the de Broglie–Bohm quantum theory. It is an alternative description of quantum phenomena in accordance with all the quantum experiments already performed. Essentially, it is a dynamical theory about objectively real trajectories in the configuration space of the physical system under [...] Read more.
We review the de Broglie–Bohm quantum theory. It is an alternative description of quantum phenomena in accordance with all the quantum experiments already performed. Essentially, it is a dynamical theory about objectively real trajectories in the configuration space of the physical system under investigation. Hence, it is not necessarily probabilistic, and it dispenses with the collapse postulate, making it suitable to be applied to cosmology. The emerging cosmological models are usually free of singularities, with a bounce connecting a contracting era with an expanding phase, which we are now observing. A theory of cosmological perturbations can also be constructed under this framework, which can be successfully confronted with current observations, and can complement inflation or even be an alternative to it. Full article
(This article belongs to the Special Issue Gravitational Singularities and Their Quantum Fates)
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8 pages, 222 KiB  
Communication
Axion Electrodynamics and the Axionic Casimir Effect
by Iver Brevik
Universe 2021, 7(5), 133; https://doi.org/10.3390/universe7050133 - 5 May 2021
Cited by 13 | Viewed by 2199
Abstract
A general scheme for axion electrodynamics is given, in which a surrounding medium of constant permittivity and permeability is assumed. Then, as an application, we provide simple numerical estimates for the electromagnetic current density produced by the electrically neutral time-dependent axions [...] Read more.
A general scheme for axion electrodynamics is given, in which a surrounding medium of constant permittivity and permeability is assumed. Then, as an application, we provide simple numerical estimates for the electromagnetic current density produced by the electrically neutral time-dependent axions a=a(t) in a strong magnetic field. As is known, the assumption a=a(t) is common under astrophysical conditions. In the third part of the paper, we consider the implications by instead assuming an axion amplitude a(z) depending on one coordinate z only. If such an axion field is contained within two large metal plates, one obtains an axion-generated splitting of the eigenmodes for the dispersion relation. These modes yield equal, though opposite, contributions to the pressure on the plates. We calculate the magnitude of the splitting effect in a simple one-dimensional model. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
28 pages, 3217 KiB  
Review
Properties of Fossil Groups of Galaxies
by J. Alfonso L. Aguerri and Stefano Zarattini
Universe 2021, 7(5), 132; https://doi.org/10.3390/universe7050132 - 4 May 2021
Cited by 15 | Viewed by 2359
Abstract
We review the formation and evolution of fossil groups and clusters from both the theoretical and the observational points of view. In the optical band, these systems are dominated by the light of the central galaxy. They were interpreted as old systems that [...] Read more.
We review the formation and evolution of fossil groups and clusters from both the theoretical and the observational points of view. In the optical band, these systems are dominated by the light of the central galaxy. They were interpreted as old systems that had enough time to merge all the M* galaxies within the central one. During the last two decades, many observational studies were performed to prove the old and relaxed state of fossil systems. The majority of these studies that spans a wide range of topics including halos global scaling relations, dynamical substructures, stellar populations, and galaxy luminosity functions seem to challenge this scenario. The general picture that can be obtained by reviewing all the observational works is that the fossil state could be transitional. Indeed, the formation of the large magnitude gap observed in fossil systems could be related to internal processes rather than an old formation. Full article
(This article belongs to the Special Issue The Physical Properties of the Groups of Galaxies)
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15 pages, 1759 KiB  
Article
A New Mass Model for Nuclear Astrophysics: Crossing 200 keV Accuracy
by Matthew Shelley and Alessandro Pastore
Universe 2021, 7(5), 131; https://doi.org/10.3390/universe7050131 - 4 May 2021
Cited by 24 | Viewed by 2375
Abstract
By using a machine learning algorithm, we present an improved nuclear mass table with a root mean square deviation of less than 200 keV. The model is equipped with statistical error bars in order to compare with available experimental data. We use the [...] Read more.
By using a machine learning algorithm, we present an improved nuclear mass table with a root mean square deviation of less than 200 keV. The model is equipped with statistical error bars in order to compare with available experimental data. We use the resulting model to predict the composition of the outer crust of a neutron star. By means of simple Monte Carlo methods, we propagate the statistical uncertainties of the mass model to the equation of state of the system. Full article
(This article belongs to the Special Issue Fundamental Processes in Neutron Stars and Supernovae)
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11 pages, 2402 KiB  
Article
Influence of Fermions on Vortices in SU(2)-QCD
by Zeinab Dehghan, Sedigheh Deldar, Manfried Faber, Rudolf Golubich and Roman Höllwieser
Universe 2021, 7(5), 130; https://doi.org/10.3390/universe7050130 - 4 May 2021
Cited by 4 | Viewed by 1706
Abstract
Gauge fields control the dynamics of fermions, and, in addition, a back reaction of fermions on the gauge field is expected. This back reaction is investigated within the vortex picture of the QCD vacuum. We show that the center vortex model reproduces the [...] Read more.
Gauge fields control the dynamics of fermions, and, in addition, a back reaction of fermions on the gauge field is expected. This back reaction is investigated within the vortex picture of the QCD vacuum. We show that the center vortex model reproduces the string tension of the full theory also in the presence of fermionic fields. Full article
(This article belongs to the Special Issue Modern Approaches to Non-Perturbative QCD and other Confining Gauge Theories)
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20 pages, 658 KiB  
Article
Casimir Interaction between a Plane and a Sphere: Correction to the Proximity-Force Approximation at Intermediate Temperatures
by Vinicius Henning, Benjamin Spreng, Paulo A. Maia Neto and Gert-Ludwig Ingold
Universe 2021, 7(5), 129; https://doi.org/10.3390/universe7050129 - 3 May 2021
Cited by 4 | Viewed by 2345
Abstract
We consider the Casimir interaction energy between a plane and a sphere of radius R at finite temperature T as a function of the distance of closest approach L. Typical experimental conditions are such that the thermal wavelength [...] Read more.
We consider the Casimir interaction energy between a plane and a sphere of radius R at finite temperature T as a function of the distance of closest approach L. Typical experimental conditions are such that the thermal wavelength λT=ℏc/kBT satisfies the condition L≪λT≪R. We derive the leading correction to the proximity-force approximation valid for such intermediate temperatures by developing the scattering formula in the plane-wave basis. Our analytical result captures the joint effect of the spherical geometry and temperature and is written as a sum of temperature-dependent logarithmic terms. Surprisingly, two of the logarithmic terms arise from the Matsubara zero-frequency contribution. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
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