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Universe, Volume 7, Issue 11 (November 2021) – 63 articles

Cover Story (view full-size image): A three-dimensional multi-component MHD model of the hot jupiter envelope is presented. For the first time, this model accurately considers the presence of the planetary proper magnetic field and the stellar wind magnetic field. This model allows us to perform simulations of all types of hot jupiter envelopes in the super-Alfvén, sub-Alfvén, and even in the trans-Alfvén regime. The presented model gives an opportunity to obtain information about the chemical composition and atmospheric gas dynamics and will be able to serve as a key tool for observational data interpreting. View this paper
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Review
The Effect of Charge, Isospin, and Strangeness in the QCD Phase Diagram Critical End Point
Universe 2021, 7(11), 454; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110454 (registering DOI) - 20 Nov 2021
Viewed by 176
Abstract
In this work, we discuss the deconfinement phase transition to quark matter in hot/dense matter. We examine the effect that different charge fractions, isospin fractions, net strangeness, and chemical equilibrium with respect to leptons have on the position of the coexistence line between [...] Read more.
In this work, we discuss the deconfinement phase transition to quark matter in hot/dense matter. We examine the effect that different charge fractions, isospin fractions, net strangeness, and chemical equilibrium with respect to leptons have on the position of the coexistence line between different phases. In particular, we investigate how different sets of conditions that describe matter in neutron stars and their mergers, or matter created in heavy-ion collisions affect the position of the critical end point, namely where the first-order phase transition becomes a crossover. We also present an introduction to the topic of critical points, including a review of recent advances concerning QCD critical points. Full article
(This article belongs to the Special Issue Universe: Feature Papers − Compact Objects)
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Review
A Decade and a Half of Fast Radio Burst Observations
Universe 2021, 7(11), 453; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110453 (registering DOI) - 20 Nov 2021
Viewed by 232
Abstract
Fast radio bursts (FRBs) have a story which has been told and retold many times over the past few years as they have sparked excitement and controversy since their pioneering discovery in 2007. The FRB class encompasses a number of microsecond- to millisecond-duration [...] Read more.
Fast radio bursts (FRBs) have a story which has been told and retold many times over the past few years as they have sparked excitement and controversy since their pioneering discovery in 2007. The FRB class encompasses a number of microsecond- to millisecond-duration pulses occurring at Galactic to cosmological distances with energies spanning about 8 orders of magnitude. While most FRBs have been observed as singular events, a small fraction of them have been observed to repeat over various timescales leading to an apparent dichotomy in the population. ∼50 unique progenitor theories have been proposed, but no consensus has emerged for their origin(s). However, with the discovery of an FRB-like pulse from the Galactic magnetar SGR J1935+2154, magnetar engine models are the current leading theory. Overall, FRB pulses exhibit unique characteristics allowing us to probe line-of-sight magnetic field strengths, inhomogeneities in the intergalactic/interstellar media, and plasma turbulence through an assortment of extragalactic and cosmological propagation effects. Consequently, they are formidable tools to study the Universe. This review follows the progress of the field between 2007 and 2020 and presents the science highlights of the radio observations. Full article
(This article belongs to the Special Issue Fast Radio Bursts)
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Article
Wave Functional of the Universe and Time
Universe 2021, 7(11), 452; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110452 (registering DOI) - 19 Nov 2021
Viewed by 165
Abstract
A version of the quantum theory of gravity based on the concept of the wave functional of the universe is proposed. To determine the physical wave functional, the quantum principle of least action is formulated as a secular equation for the corresponding action [...] Read more.
A version of the quantum theory of gravity based on the concept of the wave functional of the universe is proposed. To determine the physical wave functional, the quantum principle of least action is formulated as a secular equation for the corresponding action operator. Its solution, the wave functional, is an invariant of general covariant transformations of spacetime. In the new formulation, the history of the evolution of the universe is described in terms of coordinate time together with arbitrary lapse and shift functions, which makes this description close to the formulation of the principle of general covariance in the classical theory of Einstein’s gravity. In the new formulation of quantum theory, an invariant parameter of the evolutionary time of the universe is defined, which is a generalization of the classical geodesic time measured by a standard clock along time-like geodesics. Full article
(This article belongs to the Special Issue Cosmological Models, Quantum Theories and Astrophysical Observations)
Communication
A Note on the Gravitoelectromagnetic Analogy
Universe 2021, 7(11), 451; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110451 (registering DOI) - 19 Nov 2021
Viewed by 179
Abstract
We discuss the linear gravitoelectromagnetic approach used to solve Einstein’s equations in the weak-field and slow-motion approximation, which is a powerful tool to explain, by analogy with electromagnetism, several gravitational effects in the solar system, where the approximation holds true. In particular, we [...] Read more.
We discuss the linear gravitoelectromagnetic approach used to solve Einstein’s equations in the weak-field and slow-motion approximation, which is a powerful tool to explain, by analogy with electromagnetism, several gravitational effects in the solar system, where the approximation holds true. In particular, we discuss the analogy, according to which Einstein’s equations can be written as Maxwell-like equations, and focus on the definition of the gravitoelectromagnetic fields in non-stationary conditions. Furthermore, we examine to what extent, starting from a given solution of Einstein’s equations, gravitoelectromagnetic fields can be used to describe the motion of test particles using a Lorentz-like force equation. Full article
(This article belongs to the Special Issue Frame-Dragging and Gravitomagnetism)
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Review
Particle-in-Cell Simulations of Astrophysical Relativistic Jets
Universe 2021, 7(11), 450; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110450 (registering DOI) - 19 Nov 2021
Viewed by 172
Abstract
Astrophysical relativistic jets in active galactic nuclei, gamma-ray bursts, and pulsars is the main key subject of study in the field of high-energy astrophysics, especially regarding the jet interaction with the interstellar or intergalactic environment. In this work, we review studies of particle-in-cell [...] Read more.
Astrophysical relativistic jets in active galactic nuclei, gamma-ray bursts, and pulsars is the main key subject of study in the field of high-energy astrophysics, especially regarding the jet interaction with the interstellar or intergalactic environment. In this work, we review studies of particle-in-cell simulations of relativistic electron–proton (ep+) and electron–positron (e±) jets, and we compare simulations that we have conducted with the relativistic 3D TRISTAN-MPI code for unmagnetized and magnetized jets. We focus on how the magnetic fields affect the evolution of relativistic jets of different compositions, how the jets interact with the ambient media, how the kinetic instabilities such as the Weibel instability, the kinetic Kelvin–Helmholtz instability and the mushroom instability develop, and we discuss possible particle acceleration mechanisms at reconnection sites. Full article
(This article belongs to the Special Issue Women Physicists in Astrophysics, Cosmology and Particle Physics)
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Article
1D Supergravity FLRW Model of Starobinsky
Universe 2021, 7(11), 449; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110449 - 19 Nov 2021
Viewed by 169
Abstract
We study two homogeneous supersymmetric extensions for the f(R) modified gravity model of Starobinsky with the FLRW metric. The actions are defined in terms of a superfield R that contains the FLRW scalar curvature. One model has N = 1 [...] Read more.
We study two homogeneous supersymmetric extensions for the f(R) modified gravity model of Starobinsky with the FLRW metric. The actions are defined in terms of a superfield R that contains the FLRW scalar curvature. One model has N = 1 local supersymmetry, and its bosonic sector is the Starobinsky action; the other action has N = 2, its bosonic sector contains, in additional to Starobinsky, a massive scalar field without self-interaction. As expected, the bosonic sectors of these models are consistent with cosmic inflation, as we show by solving numerically the classical dynamics. Inflation is driven by the R2 term during the large curvature regime. In the N = 2 case, the additional scalar field remains in a low energy state during inflation. Further, by means of an additional superfield, we write equivalent tensor-scalar-like actions from which we can give the Hamiltonian formulation. Full article
(This article belongs to the Section Field Theory)
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Review
The Crab Pulsar and Nebula as Seen in Gamma-Rays
Universe 2021, 7(11), 448; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110448 - 19 Nov 2021
Viewed by 205
Abstract
Slightly more than 30 years ago, Whipple detection of the Crab Nebula was the start of Very High Energy gamma-ray astronomy. Since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast [...] Read more.
Slightly more than 30 years ago, Whipple detection of the Crab Nebula was the start of Very High Energy gamma-ray astronomy. Since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast variability, pulsed emission up to unexpectedly high energy, and the very recent detection of photons with energy exceeding 1 PeV. In this article, we review the impact of gamma-ray observations on our understanding of this extraordinary accelerator. Full article
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Article
Quasi-Periodic Oscillatory Motion of Particles Orbiting a Distorted, Deformed Compact Object
Universe 2021, 7(11), 447; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110447 - 19 Nov 2021
Viewed by 187
Abstract
This work explores the dynamic properties of test particles surrounding a distorted, deformed compact object. The astrophysical motivation was to choose such a background as to constitute a more reasonable model of a real situation that arises in the vicinity of compact objects [...] Read more.
This work explores the dynamic properties of test particles surrounding a distorted, deformed compact object. The astrophysical motivation was to choose such a background as to constitute a more reasonable model of a real situation that arises in the vicinity of compact objects with the possibility of having parameters such as the extra physical degrees of freedom. This can facilitate associating observational data with astrophysical systems. This work’s main goal is to study the dynamic regime of motion and quasi-periodic oscillation in this background, depending on different parameters of the system. In addition, we exercise the resonant phenomena of the radial and vertical oscillations at their observed quasi-periodic oscillations frequency ratio 3:2 and show that the oscillatory frequencies of charged particles can be adequately related to the frequencies of the twin high-frequency quasi-periodic oscillations observed in some sources of the microquasar observational data. Full article
(This article belongs to the Special Issue Compact Astrophysical Objects)
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Review
The Missing Lead: Developments in the Lead (Pb) Discrepancy in Intrinsically s-Process Enriched Single Post-AGB Stars
Universe 2021, 7(11), 446; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110446 - 19 Nov 2021
Viewed by 223
Abstract
Lead (Pb) is predicted to have large over-abundances with respect to other s-process elements in Asymptotic Giant Branch (AGB) stars, especially of low metallicities. However, our previous abundance studies of s-process enriched post-Asymptotic Giant Branch (post-AGB) stars in the Galaxy and [...] Read more.
Lead (Pb) is predicted to have large over-abundances with respect to other s-process elements in Asymptotic Giant Branch (AGB) stars, especially of low metallicities. However, our previous abundance studies of s-process enriched post-Asymptotic Giant Branch (post-AGB) stars in the Galaxy and the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. For the subset of post-AGB stars with low metallicities the determined upper limits based on detailed chemical abundance studies are much lower than what is predicted. Recent theoretical studies have pointed to the occurrence of the i-process to explain the observed chemical patterns, especially of Pb. A major development, in the observational context, is the release of the GAIA EDR3 parallaxes of the post-AGBs in the Galaxy, which has opened the gateway to systematically studying the sample of stars as a function of current luminosities (which can be linked to their initial masses). In this paper, we succinctly review the Pb discrepancy in post-AGB stars and present the latest observational and theoretical developments in this research landscape. Full article
(This article belongs to the Special Issue AGB Stars: Element Forges of the Universe)
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Article
Global Dynamics of the Hořava–Lifshitz Cosmological Model in a Non-Flat Universe with Non-Zero Cosmological Constant
Universe 2021, 7(11), 445; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110445 (registering DOI) - 17 Nov 2021
Cited by 2 | Viewed by 217
Abstract
When the cosmological constant is non-zero, the dynamics of the cosmological model based on Hořava–Lifshitz gravity in a non-flat universe are characterized by using the qualitative theory of differential equations. Full article
(This article belongs to the Section Cosmology)
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Article
Real Valued Functions for the BFKL Eigenvalue
Universe 2021, 7(11), 444; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110444 - 17 Nov 2021
Viewed by 206
Abstract
We consider known expressions for the eigenvalue of the Balitsky-Fadin-Kuraev-Lipatov (BFKL) equation in N=4 super Yang-Mills theory as a real valued function of two variables. We define new real valued functions of two complex conjugate variables that have a definite complexity [...] Read more.
We consider known expressions for the eigenvalue of the Balitsky-Fadin-Kuraev-Lipatov (BFKL) equation in N=4 super Yang-Mills theory as a real valued function of two variables. We define new real valued functions of two complex conjugate variables that have a definite complexity analogous to the weight of the nested harmonic sums. We argue that those functions span a general space of functions for the BFKL eigenvalue at any order of the perturbation theory. Full article
(This article belongs to the Section High Energy Nuclear and Particle Physics)
Article
On the 2PN Periastron Precession of the Double Pulsar PSR J0737–3039A/B
Universe 2021, 7(11), 443; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110443 - 17 Nov 2021
Viewed by 232
Abstract
One of the post-Keplerian (PK) parameters determined in timing analyses of several binary pulsars is the fractional periastron advance per orbit kPK. Along with other PK parameters, it is used in testing general relativity once it is translated into the periastron [...] Read more.
One of the post-Keplerian (PK) parameters determined in timing analyses of several binary pulsars is the fractional periastron advance per orbit kPK. Along with other PK parameters, it is used in testing general relativity once it is translated into the periastron precession ω˙PK. It was recently remarked that the periastron ω of PSR J0737–3039A/B may be used to measure/constrain the moment of inertia of A through the extraction of the general relativistic Lense–Thirring precession ω˙LT,A0.00060yr1 from the experimentally determined periastron rate ω˙obs provided that the other post-Newtonian (PN) contributions to ω˙exp can be accurately modeled. Among them, the 2PN seems to be of the same order of magnitude of ω˙LT,A. An analytical expression of the total 2PN periastron precession ω˙2PN in terms of the osculating Keplerian orbital elements, valid not only for binary pulsars, is provided, thereby elucidating the subtleties implied in correctly calculating it from k1PN+k2PN and correcting some past errors by the present author. The formula for ω˙2PN is demonstrated to be equivalent to that obtainable from k1PN+k2PN by Damour and Schäfer expressed in the Damour–Deruelle (DD) parameterization. ω˙2PN actually depends on the initial orbital phase, hidden in the DD picture, so that 0.00080yr1ω˙2PN0.00045yr1. A recently released prediction of ω˙2PN for PSR J0737–3039A/B is discussed. Full article
(This article belongs to the Special Issue Current and Future Tests of General Relativity)
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Article
Coherent States for Fractional Powers of the Harmonic Oscillator Hamiltonian
Universe 2021, 7(11), 442; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110442 - 16 Nov 2021
Cited by 1 | Viewed by 305
Abstract
Inspired by special and general relativistic systems that can have Hamiltonians involving square roots or more general fractional powers, in this article, we address the question of how a suitable set of coherent states for such systems can be obtained. This becomes a [...] Read more.
Inspired by special and general relativistic systems that can have Hamiltonians involving square roots or more general fractional powers, in this article, we address the question of how a suitable set of coherent states for such systems can be obtained. This becomes a relevant topic if the semiclassical sector of a given quantum theory is to be analysed. As a simple setup, we consider the toy model of a deparametrised system with one constraint that involves a fractional power of the harmonic oscillator Hamiltonian operator, and we discuss two approaches to finding suitable coherent states for this system. In the first approach, we consider Dirac quantisation and group averaging, as have been used by Ashtekar et al., but only for integer powers of operators. Our generalisation to fractional powers yields in the case of the toy model a suitable set of coherent states. The second approach is inspired by coherent states based on a fractional Poisson distribution introduced by Laskin, which however turn out not to satisfy all properties to yield good semiclassical results for the operators considered here and in particular do not satisfy a resolution of identity as claimed. Therefore, we present a generalisation of the standard harmonic oscillator coherent states to states involving fractional labels, which approximate the fractional operators in our toy model semiclassically more accurately and satisfy a resolution of identity. In addition, motivated by the way the proof of the resolution of identity is performed, we consider these kind of coherent states also for the polymerised harmonic oscillator and discuss their semiclassical properties. Full article
(This article belongs to the Special Issue Cosmological Models, Quantum Theories and Astrophysical Observations)
Article
f(T, B) Cosmography for High Redshifts
Universe 2021, 7(11), 441; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110441 - 16 Nov 2021
Viewed by 214
Abstract
In light of the statistical performance of cosmological observations, in this work we present the cosmography in f(T,B) gravity. In this scenario we found a cosmological viable standard case that allows the reduction of the degeneracy between several [...] Read more.
In light of the statistical performance of cosmological observations, in this work we present the cosmography in f(T,B) gravity. In this scenario we found a cosmological viable standard case that allows the reduction of the degeneracy between several f(T,B) models already proposed in the literature. Furthermore, we constrain this model using Pantheon SNeIa compilation, cosmic chronometers and a newly GRB calibrated data sample. We found that with an appropriate strategy for including the cosmographic parameter, we do produce a viable cosmology with our model within f(T,B) gravity. Full article
(This article belongs to the Special Issue Beyond Riemannian Geometry in Classical and Quantum Gravity)
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Review
Stellar Chromospheric Variability
Universe 2021, 7(11), 440; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110440 - 15 Nov 2021
Viewed by 275
Abstract
Cool stars with convective envelopes of spectral types F and later tend to exhibit magnetic activity throughout their atmospheres. The presence of strong and variable magnetic fields is evidenced by photospheric starspots, chromospheric plages and coronal flares, as well as by strong Ca [...] Read more.
Cool stars with convective envelopes of spectral types F and later tend to exhibit magnetic activity throughout their atmospheres. The presence of strong and variable magnetic fields is evidenced by photospheric starspots, chromospheric plages and coronal flares, as well as by strong Ca ii H+K and Hα emission, combined with the presence of ultraviolet resonance lines. We review the drivers of stellar chromospheric activity and the resulting physical parameters implied by the observational diagnostics. At a basic level, we explore the importance of stellar dynamos and their activity cycles for a range of stellar types across the Hertzsprung–Russell diagram. We focus, in particular, on recent developments pertaining to stellar rotation properties, including the putative Vaughan–Preston gap. We also pay specific attention to magnetic variability associated with close binary systems, including RS Canum Venaticorum, BY Draconis, W Ursae Majoris and Algol binaries. At the present time, large-scale photometric and spectroscopic surveys are becoming generally available, thus leading to a resurgence of research into chromospheric activity. This opens up promising prospects to gain a much improved understanding of chromospheric physics and its wide-ranging impact. Full article
(This article belongs to the Special Issue Stellar Astrophysics)
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Article
Considerations on Quantum Gravity Phenomenology
Universe 2021, 7(11), 439; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110439 - 15 Nov 2021
Viewed by 298
Abstract
I describe two phenomenological windows on quantum gravity that seem promising to me. I argue that we already have important empirical inputs that should orient research in quantum gravity. Full article
(This article belongs to the Special Issue Quantum Gravity Phenomenology)
Article
Automatic Search of Cataclysmic Variables Based on LightGBM in LAMOST-DR7
Universe 2021, 7(11), 438; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110438 - 15 Nov 2021
Viewed by 187
Abstract
The search for special and rare celestial objects has always played an important role in astronomy. Cataclysmic Variables (CVs) are special and rare binary systems with accretion disks. Most CVs are in the quiescent period, and their spectra have the emission lines of [...] Read more.
The search for special and rare celestial objects has always played an important role in astronomy. Cataclysmic Variables (CVs) are special and rare binary systems with accretion disks. Most CVs are in the quiescent period, and their spectra have the emission lines of Balmer series, HeI, and HeII. A few CVs in the outburst period have the absorption lines of Balmer series. Owing to the scarcity of numbers, expanding the spectral data of CVs is of positive significance for studying the formation of accretion disks and the evolution of binary star system models. At present, the research for astronomical spectra has entered the era of Big Data. The Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST) has produced more than tens of millions of spectral data. the latest released LAMOST-DR7 includes 10.6 million low-resolution spectral data in 4926 sky regions, providing ideal data support for searching CV candidates. To process and analyze the massive amounts of spectral data, this study employed the Light Gradient Boosting Machine (LightGBM) algorithm, which is based on the ensemble tree model to automatically conduct the search in LAMOST-DR7. Finally, 225 CV candidates were found and four new CV candidates were verified by SIMBAD and published catalogs. This study also built the Gradient Boosting Decision Tree (GBDT), Adaptive Boosting (AdaBoost), and eXtreme Gradient Boosting (XGBoost) models and used Accuracy, Precision, Recall, the F1-score, and the ROC curve to compare the four models comprehensively. Experimental results showed that LightGBM is more efficient. The search for CVs based on LightGBM not only enriches the existing CV spectral library, but also provides a reference for the data mining of other rare celestial objects in massive spectral data. Full article
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Article
Cherenkov Gravitational Radiation during the Radiation Era
Universe 2021, 7(11), 437; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110437 - 15 Nov 2021
Viewed by 213
Abstract
Cherenkov radiation may occur whenever the source is moving faster than the waves it generates. In a radiation dominated universe, with equation-of-state w=1/3, we have recently shown that the Bardeen scalar-metric perturbations contribute to the linearized Weyl tensor [...] Read more.
Cherenkov radiation may occur whenever the source is moving faster than the waves it generates. In a radiation dominated universe, with equation-of-state w=1/3, we have recently shown that the Bardeen scalar-metric perturbations contribute to the linearized Weyl tensor in such a manner that its wavefront propagates at acoustic speed w=1/3. In this work, we explicitly compute the shape of the Bardeen Cherenkov cone and wedge generated respectively by a supersonic point mass (approximating a primordial black hole) and a straight Nambu-Goto wire (approximating a cosmic string) moving perpendicular to its length. When the black hole or cosmic string is moving at ultra-relativistic speeds, we also calculate explicitly the sudden surge of scalar-metric induced tidal forces on a pair of test particles due to the passing Cherenkov shock wave. These forces can stretch or compress, depending on the orientation of the masses relative to the shock front’s normal. Full article
(This article belongs to the Special Issue Gravitational Radiation in Cosmological Spacetimes)
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Article
A Fit to the Available e+eΛc+Λ¯c Cross Section Data Nearby Production Threshold by Means of a Strong Correction to the Coulomb Enhancement Factor
Universe 2021, 7(11), 436; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110436 - 14 Nov 2021
Viewed by 387
Abstract
There are two available sets of data on the e+eΛc+Λ¯c cross section at energies close to the production threshold, collected by the Belle and by the BESIII Collaborations. The measurement of the [...] Read more.
There are two available sets of data on the e+eΛc+Λ¯c cross section at energies close to the production threshold, collected by the Belle and by the BESIII Collaborations. The measurement of the former, performed by means of the initial state radiation technique, is compatible with the presence of a resonance, called ψ(4660), observed also in other final states. On the contrary, the latter is measured an almost flat and hence non-resonant cross section in the energy region just above the production threshold, but the data stop before the possible rise in the cross section for the resonant production. We propose an effective model to describe the behavior of the data near this threshold, which is based on a Coulomb-like enhancement factor due to the strong interaction among the final state particles. In the framework of this model, it is possible to describe both datasets. Full article
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Communication
Renormalizable and Unitary Lorentz Invariant Model of Quantum Gravity
Universe 2021, 7(11), 435; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110435 - 13 Nov 2021
Viewed by 221
Abstract
We analyze the R+R2 model of quantum gravity where terms quadratic in the curvature tensor are added to the General Relativity action. This model was recently proved to be a self-consistent quantum theory of gravitation, being both renormalizable and unitary. [...] Read more.
We analyze the R+R2 model of quantum gravity where terms quadratic in the curvature tensor are added to the General Relativity action. This model was recently proved to be a self-consistent quantum theory of gravitation, being both renormalizable and unitary. The model can be made practically indistinguishable from General Relativity at astrophysical and cosmological scales by the proper choice of parameters. Full article
Article
Topics in Supersymmetric and Noncommutative Quantum Cosmology
Universe 2021, 7(11), 434; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110434 - 12 Nov 2021
Viewed by 149
Abstract
In the present article we review the work carried out by us and collaborators on supersymmetric quantum cosmology, noncommutative quantum cosmology and the application of GUPs to quantum cosmology and black holes. The review represents our personal view on these subjects and it [...] Read more.
In the present article we review the work carried out by us and collaborators on supersymmetric quantum cosmology, noncommutative quantum cosmology and the application of GUPs to quantum cosmology and black holes. The review represents our personal view on these subjects and it is presented in chronological order. Full article
(This article belongs to the Special Issue Quantum Cosmology)
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Article
Spatio-Temporal Analysis of Dust Storm Activity in Chryse Planitia Using MGS-MOC Observations from Mars Years 24–28
Universe 2021, 7(11), 433; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110433 - 12 Nov 2021
Viewed by 224
Abstract
Dust storms, observed in all seasons, are among the most momentous of Mars’ atmospheric activities. The Entry–Descent–Landing (EDL) activity of a Martian landing mission is influenced by local atmospheric conditions, especially the probability of dust storm activity. Chryse Planitia, featuring many of the [...] Read more.
Dust storms, observed in all seasons, are among the most momentous of Mars’ atmospheric activities. The Entry–Descent–Landing (EDL) activity of a Martian landing mission is influenced by local atmospheric conditions, especially the probability of dust storm activity. Chryse Planitia, featuring many of the largest and most prominent outflow channels and possible mud volcanoes, is an important target site for current and future Mars landing missions. It is of great significance to understand that a Mars landing probe may encounter a dust storm situation during EDL season in the Chryse Planitia. In this study, based on four Martian years, Mars Orbiter Camera (MOC) Mars Daily Global Maps (MDGMs), 1172 dust storms were identified within Chryse’s 1600 km-radius ring. Secondly, the daily mean dust storm probability was calculated, binned by 1° of solar longitude in the Chryse landing area. The two active periods of dust storm activity are Ls = 177–239° and Ls = 288–4°, with an average daily mean dust storm probability of 9.5% and 4.1%. Dust storm activity frequency is closely interrelated with the seasonal ebb and flow of the north polar ice cap; consequently, most dust storms occur in either the cap’s growth or recession phase. We divided the Chryse landing area into square grids of 0.5° and computed the average probability of dust storm occurrence in each grid, which ranged from 0.19% to 2.42%, with an average of 1.22%. The dust storm activity probability in space was also inhomogeneous—low in the west and south but high in the east and north—which was mainly affected by the origin and the path of dust storm sequences. Based on empirical orthogonal function (EOF) analysis of storms in the Chryse area, 40.5% are cap-edge storms in the northern hemisphere. Finally, we concluded that the preferred time of a Mars landing mission is Ls = 18–65° in the Chryse Planitia, and three preferred landing areas were selected with low dust storm probability. Full article
(This article belongs to the Section Planetary Sciences)
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Article
35 Years of Ground-Based Gamma-ray Astronomy
Universe 2021, 7(11), 432; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110432 - 12 Nov 2021
Viewed by 186
Abstract
This paper provides a brief, personal account of the development of ground-based gamma-ray astronomy, primarily over the last 35 years, with some digressions into the earlier history of the field. Ideas related to the imaging of Cherenkov events and the potential for the [...] Read more.
This paper provides a brief, personal account of the development of ground-based gamma-ray astronomy, primarily over the last 35 years, with some digressions into the earlier history of the field. Ideas related to the imaging of Cherenkov events and the potential for the use of arrays were in existence for some time before the technical expertise required for their exploitation emerged. There has been occasional controversy, great creativity and some heroic determination—all of it part of establishing a new window into the universe. Full article
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Article
Three-Dimensional Simulation Study of the Interactions of Three Successive CMEs during 4–5 November 1998
Universe 2021, 7(11), 431; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110431 - 12 Nov 2021
Viewed by 215
Abstract
In this paper, using a 3D magnetohydrodynamics (MHD) numerical simulation, we investigate the propagation and interaction of the three halo CMEs originating from the same active region during 4–5 November 1998 from the Sun to Earth. Firstly, we try to reproduce the observed [...] Read more.
In this paper, using a 3D magnetohydrodynamics (MHD) numerical simulation, we investigate the propagation and interaction of the three halo CMEs originating from the same active region during 4–5 November 1998 from the Sun to Earth. Firstly, we try to reproduce the observed basic features near Earth by a simple spherical plasmoid model. We find that the first component of the compound stream at 1 AU is associated to the first CME of the three halo CMEs. During the propagation in the interplanetary space, the third CME overtakes the second one. The two CMEs merge to a new, larger entity with complex internal structure. The magnetic field of the first CME in the three successive CMEs event is compressed by the following complex ejecta. The interaction between the second and third CME results in the deceleration of the third CME and the enhancement of the density, total magnetic field and south component of the magnetic field. In addition we study the contribution of a single CME to the final simulation results, as well as the effect of the CME–CME interactions on the propagation of an isolated CME and multiple CMEs. This is achieved by analysing a single CME with or without the presence of the preceding CMEs. Our results show that the CME moves faster in a less dense, faster medium generated by the interaction of the preceding CME with the ambient medium. In addition, we show that the CME–CME interactions can greatly alter the kinematics and magnetic structures of the individual events. Full article
(This article belongs to the Special Issue Propagation of Coronal Mass Ejections)
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Review
First Observations of SN 1604 (Kepler’s Supernova)
Universe 2021, 7(11), 430; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110430 - 11 Nov 2021
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Abstract
A supernova close enough to the Earth is a spectacular event: it can appear as a “new star” as luminous as Venus, or even more, visible for several days. The rate of Galactic supernovae is expected to be of about one in 30 [...] Read more.
A supernova close enough to the Earth is a spectacular event: it can appear as a “new star” as luminous as Venus, or even more, visible for several days. The rate of Galactic supernovae is expected to be of about one in 30 years, with a fraction visible to the naked eye; however in all the history of human civilization only seven supernovae in the Milky Way have been reported, the last two (1572 and 1604) during Galilei’s life. The supernova of 1604, today called Kepler’s Supernova, was observed by Galilei, Kepler and other astronomers in Europe, Korea, China, Arabia. Like the supernova SN1572, today called Tycho’s supernova, it has been the subject of extensive studies, and inspired observational measurements and philosophical considerations on the nature of the heavens. The remnant of SN1604 has been indicated by recent X- and gamma-ray data to be a likely site of cosmic ray acceleration. The first recorded data of optical observations, together with new data, can still tell us a lot about the early evolution of this supernova. Full article
(This article belongs to the Section Compact Objects)
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Article
A Comprehensive Comparison of Period Extraction Algorithms for Asteroids with Long Term Observation
Universe 2021, 7(11), 429; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110429 - 10 Nov 2021
Viewed by 244
Abstract
The light curve period of an asteroid plays an important role in determining the rotation period, the collision evolution and the YORP effect. There are many period extraction algorithms used to find the light curve period of asteroids with long term observation, which [...] Read more.
The light curve period of an asteroid plays an important role in determining the rotation period, the collision evolution and the YORP effect. There are many period extraction algorithms used to find the light curve period of asteroids with long term observation, which are mainly based on the frequency, time and time–frequency domains. This paper presents a comprehensive and unparalleled comparison of the popular algorithms based on the DAMIT (Database of Asteroid Models from Inversion Techniques) data set to show the statistical results. Considering the quoted period, absolute magnitude, diameter, albedo, time span and number of observations, we analyze the accuracy of five popular methods using the light curve data of 2902 asteroids. We find that although the performance of all the algorithms varies little, Phase Dispersion Minimization (PDM) performs better, followed by Lomb-Scargle (LS), while Conditional Entropy (CE) is not better than the others under certain conditions. We also analyze the cases which are more suitable for searching by frequencies or by periods. Full article
(This article belongs to the Section Planetary Sciences)
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Article
The Sharma–Mittal Model’s Implications on FRW Universe in Chern–Simons Gravity
Universe 2021, 7(11), 428; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110428 - 10 Nov 2021
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Abstract
The Sharma–Mittal holographic dark energy model is investigated in this paper using the Chern–Simons modified gravity theory. We investigate several cosmic parameters, including the deceleration, equation of state, square of sound speed, and energy density. According to the deceleration parameter, the universe is [...] Read more.
The Sharma–Mittal holographic dark energy model is investigated in this paper using the Chern–Simons modified gravity theory. We investigate several cosmic parameters, including the deceleration, equation of state, square of sound speed, and energy density. According to the deceleration parameter, the universe is in an decelerating and expanding phase known as de Sitter expansion. The Sharma–Mittal HDE model supports a deceleration to acceleration transition that is compatible with the observational data. The EoS depicts the universe’s dominance era through a number of components, such as ω=0, 13, 1, which indicate that the universe is influenced by dust, radiation, and stiff fluid, while 1<ω<13, ω=1, and ω<1 are conditions for quintessence DE, ΛCDM, and Phantom era dominance. Our findings indicate that the universe is in an accelerated expansion phase, and this is similar to the observational data. Full article
(This article belongs to the Section Gravitation)
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Article
Puzzling Low-Temperature Behavior of the Van Der Waals Friction Force between Metallic Plates in Relative Motion
Universe 2021, 7(11), 427; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110427 - 09 Nov 2021
Viewed by 170
Abstract
This paper presents the results of calculating the van der Waals friction force (dissipative fluctuation-electromagnetic force) between metallic (Au) plates in relative motion at temperatures close to 1 K. The stopping tangential force arises between moving plates along with the usual Casimir force [...] Read more.
This paper presents the results of calculating the van der Waals friction force (dissipative fluctuation-electromagnetic force) between metallic (Au) plates in relative motion at temperatures close to 1 K. The stopping tangential force arises between moving plates along with the usual Casimir force of attraction, which has been routinely measured with high precision over the past two decades. At room temperatures, the former force is 10 orders of magnitude less than the latter, but at temperatures T<50 K, friction increases sharply. The calculations have been carried out in the framework of the Levin-Polevoi-Rytov fluctuation electromagnetic theory. For metallic plates with perfect crystal lattices and without defects, van der Waals friction force is shown to increase with decreasing temperature as T4. In the presence of residual resistance ρ0 of the metal, a plateau is formed on the temperature dependence of the friction force at T0 with a height proportional to ρ00.8. Another important finding is the weak force-distance dependence ~aq (with q<1). The absolute values of the friction forces are achievable for measurements in AFM-based experiments. Full article
(This article belongs to the Special Issue The Casimir Effect: From a Laboratory Table to the Universe)
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Review
Sterile Neutrinos with Neutrino Telescopes
Universe 2021, 7(11), 426; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110426 - 09 Nov 2021
Viewed by 188
Abstract
Searches for light sterile neutrinos are motivated by the unexpected observation of an electron neutrino appearance in short-baseline experiments, such as the Liquid Scintillator Neutrino Detector (LSND) and the Mini Booster Neutrino Experiment (MiniBooNE). In light of these unexpected results, a campaign using [...] Read more.
Searches for light sterile neutrinos are motivated by the unexpected observation of an electron neutrino appearance in short-baseline experiments, such as the Liquid Scintillator Neutrino Detector (LSND) and the Mini Booster Neutrino Experiment (MiniBooNE). In light of these unexpected results, a campaign using natural and anthropogenic sources to find the light (mass-squared-difference around 1 eV2) sterile neutrinos is underway. Among the natural sources, atmospheric neutrinos provide a unique gateway to search for sterile neutrinos due to the broad range of baseline-to-energy ratios, L/E, and the presence of significant matter effects. Since the atmospheric neutrino flux rapidly falls with energy, studying its highest energy component requires gigaton-scale neutrino detectors. These detectors—often known as neutrino telescopes since they are designed to observe tiny astrophysical neutrino fluxes—have been used to perform searches for light sterile neutrinos, and researchers have found no significant signal to date. This brief review summarizes the current status of searches for light sterile neutrinos with neutrino telescopes deployed in solid and liquid water. Full article
(This article belongs to the Special Issue Neutrinos from Astrophysical Sources)
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Article
Cosmological Perturbations via Quantum Corrections in M-Theory
Universe 2021, 7(11), 425; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7110425 - 07 Nov 2021
Viewed by 308
Abstract
In the early universe, it is important to take into account the quantum effect of gravity to explain the feature of inflation. In this paper, we consider the M-theory effective action which consists of 11-dimensional supergravity and (Weyl)4 terms. The equations of [...] Read more.
In the early universe, it is important to take into account the quantum effect of gravity to explain the feature of inflation. In this paper, we consider the M-theory effective action which consists of 11-dimensional supergravity and (Weyl)4 terms. The equations of motion are solved perturbatively, and the solution describes the inflation-like expansion in 4-dimensional spacetime. Equations of motion for tensor perturbations around this background are derived perturbatively. We also check that the equations of motion are obtained from the effective action up to the second order of the perturbations. Finally, we solve the equations of motion for the tensor perturbations perturbatively and obtain analytic expressions for them. Full article
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