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Universe, Volume 8, Issue 7 (July 2022) – 49 articles

Cover Story (view full-size image): Solar wind turbulence is often found to be Alfvénic, especially in fast wind streams. However, recent observations performed in the inner heliosphere have proved that also slow streams show strong Alfvénic signatures. This study focuses on a comparative analysis between a fast stream and Alfvénic slow wind intervals performed at L1 where one would expect a degradation of the Alfvénicity in slow streams. The behavior of different parameters to characterize Alfvénic fluctuations is examined, supported by a spectral analysis to compare power spectra, also using the Elsässer variables to derive the normalized cross-helicity and residual energy and study their spectral behavior. Although this study further confirms similarities between fast and Alfvénic slow wind (e.g., similar Alfvénic frequency ranges), fast wind is found to be closer to a balance between kinetic and magnetic energy. View this paper
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18 pages, 1322 KiB  
Article
Assessment of Dark Matter Models Using Dark Matter Correlations across Dwarf Spheroidal Galaxies
by Ahmad Borzou
Universe 2022, 8(7), 386; https://doi.org/10.3390/universe8070386 - 21 Jul 2022
Cited by 1 | Viewed by 1201
Abstract
The predicted size of dark matter substructures in kilo-parsec scales is model-dependent. Therefore, if the correlations between dark matter mass densities as a function of the distances between them are measured via observations, we can scrutinize dark matter scenarios. In this paper, we [...] Read more.
The predicted size of dark matter substructures in kilo-parsec scales is model-dependent. Therefore, if the correlations between dark matter mass densities as a function of the distances between them are measured via observations, we can scrutinize dark matter scenarios. In this paper, we present an assessment procedure of dark matter scenarios. First, we use Gaia’s data to infer the single-body phase-space density of the stars in the Fornax dwarf spheroidal galaxy. The latter, together with the Jeans equation, after eliminating the gravitational potential using the Poisson equation, reveals the mass density of dark matter as a function of its position in the galaxy. We derive the correlations between dark matter mass densities as a function of distances between them. No statistically significant correlation is observed. Second, for the sake of comparison with the standard cold dark matter, we also compute the correlations between dark matter mass densities in a small halo of the Eagle hydrodynamics simulation. We show that the correlations from the simulation and from Gaia are in agreement. Third, we show that Gaia observations can be used to limit the parameter space of the Ginzburg–Landau statistical field theory of dark matter mass densities and subsequently shrink the parameter space of any dark matter model. As two examples, we show how to leave limitations on (i) a classic gas dark matter and (ii) a superfluid dark matter. Full article
(This article belongs to the Special Issue Focus on Dark Matter)
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21 pages, 388 KiB  
Article
Wheeler-DeWitt Equation and the Applicability of Crypto-Hermitian Interaction Representation in Quantum Cosmology
by Miloslav Znojil
Universe 2022, 8(7), 385; https://doi.org/10.3390/universe8070385 - 20 Jul 2022
Cited by 8 | Viewed by 1546
Abstract
In the broader methodical framework of the quantization of gravity, the crypto-Hermitian (or non-Hermitian) version of Dirac’s interaction picture is considered. The formalism is briefly outlined and shown to be well suited for an innovative treatment of certain cosmological models. In particular, it [...] Read more.
In the broader methodical framework of the quantization of gravity, the crypto-Hermitian (or non-Hermitian) version of Dirac’s interaction picture is considered. The formalism is briefly outlined and shown to be well suited for an innovative treatment of certain cosmological models. In particular, it is demonstrated that the Wheeler-DeWitt equation could be a promising candidate for the description of the evolution of the quantized Universe near its initial Big Bang singularity. Full article
(This article belongs to the Special Issue Selected Topics in Gravity, Field Theory and Quantum Mechanics)
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18 pages, 2374 KiB  
Article
Physical Properties of Radio Stars Based on LAMOST Spectral Survey
by Liyun Zhang, Yao Cheng, Xianming L. Han, Qingfeng Pi, Prabhakar Misra, Baoda Li and Zhongzhong Zhu
Universe 2022, 8(7), 384; https://doi.org/10.3390/universe8070384 - 19 Jul 2022
Cited by 1 | Viewed by 1236
Abstract
Radio emission has been detected for all types of stars in the Hertzsprung Russell diagram. Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low and medium-resolution spectroscopic surveys provide a good opportunity to obtain the spectroscopic properties of radio stars. We cross-matched big [...] Read more.
Radio emission has been detected for all types of stars in the Hertzsprung Russell diagram. Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low and medium-resolution spectroscopic surveys provide a good opportunity to obtain the spectroscopic properties of radio stars. We cross-matched big data from the LAMOST DR7 low resolution spectral survey with a catalogue of radio stars, and obtained 449 stellar spectra of 258 stars. We detected 185 spectra with Hα emission. There are a total of 108 objects with repeated low resolution spectral observations, of which 63 show variations in the Hα line over a long time, about several months. We cross-matched LAMOST DR7 medium resolution spectra with the radio star catalogue. We obtained 1319 LAMOST medium-resolution spectra of the 156 radio stars and then calculated their equivalent widths (EWs) of the Hα line. Among them, 93 radio stars with Hα emission were found from the LAMOST medium resolution spectra, and 63 objects showed short and long-term variabilities, especially on a short time scale of approximately 20 min. Finally, we estimated the minimal detectable radio flux of the FAST telescope and provided a scientific plan for studying radio stars. Full article
(This article belongs to the Special Issue Stellar Multi-Band Observational Studies in the Era of Big Data)
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18 pages, 528 KiB  
Article
Determination of the Physical Parameters of AGNs in Seyfert 1 Galaxies LEDA 3095839 and VII Zw 244 Based on Spectropolarimetric Observations
by Elena Shablovinskaya, Mikhail Piotrovich, Eugene Malygin, Stanislava Buliga and Tinatin Natsvlishvili
Universe 2022, 8(7), 383; https://doi.org/10.3390/universe8070383 - 19 Jul 2022
Cited by 1 | Viewed by 1105
Abstract
Here we present the detailed investigation of AGNs in two Seyfert 1 galaxies, LEDA 3095839 and VII Zw 244. Both of them were observed within the photometric reverberation mapping project in Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS), during [...] Read more.
Here we present the detailed investigation of AGNs in two Seyfert 1 galaxies, LEDA 3095839 and VII Zw 244. Both of them were observed within the photometric reverberation mapping project in Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS), during which we earlier obtained the SMBHs masses. After that, both galaxies were observed in spectropolarimetric and polarimetric modes on the BTA 6 m telescope of the SAO RAS with the focal reducer SCORPIO-2. The linear polarization of the continuum and broad Balmer lines has been measured. It was found that (i) there were no signs of equatorial scattering in the LEDA 3095839 galaxy in the broad Hα line, and we were able to estimate the value of SMBH spin and the magnetic field strength in the disk from the level of continuum polarization; (ii) for the galaxy VII Zw 244, the presence of equatorial scattering was shown, due to which the mass of the SMBH was independently measured, the inclination angle of the system was obtained, and the value of the spin was estimated. Full article
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11 pages, 741 KiB  
Article
Probing Spacetime Foam with Extragalactic Sources of High-Energy Photons
by Y. Jack Ng and Eric S. Perlman
Universe 2022, 8(7), 382; https://doi.org/10.3390/universe8070382 - 19 Jul 2022
Cited by 4 | Viewed by 1195
Abstract
Quantum fluctuations can endow spacetime with a foamy structure. In this review article, we discuss our various proposals to observationally constrain models of spacetime foam. One way is to examine if the light wave-front from a distant quasar or GRB can be noticeably [...] Read more.
Quantum fluctuations can endow spacetime with a foamy structure. In this review article, we discuss our various proposals to observationally constrain models of spacetime foam. One way is to examine if the light wave-front from a distant quasar or GRB can be noticeably distorted by spacetime-foam-induced phase incoherence. As the phase fluctuations are proportional to the distance to the source but inversely proportional to the wavelength, ultra-high energy photons (>1 TeV) from distant sources are particularly useful. We elaborate on several proposals, including the possibility of detecting spacetime foam by observing “seeing disks” in the images of distant quasars and active galactic nuclei. We also discuss the appropriate distance measure for calculating the expected angular broadening. In addition, we discuss our more recent work in which we investigate whether wave-front distortions on small scales (due to spacetime foam) can cause distant objects become undetectable because the phase fluctuations have accumulated to the point at which image formation is impossible. Another possibility that has recently become accessible is to use interferometers to observe cosmologically distant sources, thereby giving a large baseline perpendicular to the local wave vector over which the wave front could become corrugated and thus distorted, reducing or eliminating its fringe visibility. We argue that all these methods ultimately depend on the availability of ways (if any) to carry out proper averaging of contributions from different light paths from the source to the telescope. Full article
(This article belongs to the Special Issue Ultra High Energy Photons)
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34 pages, 8846 KiB  
Review
Formation of Comets
by Jürgen Blum, Dorothea Bischoff and Bastian Gundlach
Universe 2022, 8(7), 381; https://doi.org/10.3390/universe8070381 - 15 Jul 2022
Cited by 15 | Viewed by 1901
Abstract
Questions regarding how primordial or pristine the comets of the solar system are have been an ongoing controversy. In this review, we describe comets’ physical evolution from dust and ice grains in the solar nebula to the contemporary small bodies in the outer [...] Read more.
Questions regarding how primordial or pristine the comets of the solar system are have been an ongoing controversy. In this review, we describe comets’ physical evolution from dust and ice grains in the solar nebula to the contemporary small bodies in the outer solar system. This includes the phases of dust agglomeration, the formation of planetesimals, their thermal evolution and the outcomes of collisional processes. We use empirical evidence about comets, in particular from the Rosetta Mission to comet 67P/Churyumov–Gerasimenko, to draw conclusions about the possible thermal and collisional evolution of comets. Full article
(This article belongs to the Special Issue The Advances of Comets' Activity)
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18 pages, 3401 KiB  
Article
Low Density Neutron Star Matter with Quantum Molecular Dynamics: The Role of Isovector Interactions
by Parit Mehta, Rana Nandi, Rosana de Oliveira Gomes, Veronica Dexheimer and Jan Steinheimer
Universe 2022, 8(7), 380; https://doi.org/10.3390/universe8070380 - 13 Jul 2022
Cited by 2 | Viewed by 1190
Abstract
The effect of isospin-dependent nuclear forces on the inner crust of neutron stars is modeled within the framework of Quantum Molecular Dynamics (QMD). To successfully control the density dependence of the symmetry energy of neutron-star matter below nuclear saturation density, a mixed vector-isovector [...] Read more.
The effect of isospin-dependent nuclear forces on the inner crust of neutron stars is modeled within the framework of Quantum Molecular Dynamics (QMD). To successfully control the density dependence of the symmetry energy of neutron-star matter below nuclear saturation density, a mixed vector-isovector potential is introduced. This approach is inspired by the baryon density and isospin density-dependent repulsive Skyrme force of asymmetric nuclear matter. In isospin-asymmetric nuclear matter, the system shows nucleation, as nucleons are arranged into shapes resembling nuclear pasta. The dependence of clusterization in the system on the isospin properties is also explored by calculating two-point correlation functions. We show that, as compared to previous results that did not involve such mixed interaction terms, the energy symmetry slope L is successfully controlled by varying the corresponding coupling strength. Nevertheless, the effect of changing the slope of the nuclear symmetry energy L on the crust-core transition density does not seem significant. To the knowledge of the authors, this is the first implementation of such a coupling in a QMD model for isospin asymmetric matter, which is relevant to the inner crust of neutron and proto-neutron stars. Full article
(This article belongs to the Special Issue Properties and Dynamics of Neutron Stars and Proto-Neutron Stars)
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16 pages, 455 KiB  
Review
Cosmological Perturbations in Bouncing Cosmologies and the Case of the Pre-Big Bang Scenario
by Valerio Bozza
Universe 2022, 8(7), 379; https://doi.org/10.3390/universe8070379 - 13 Jul 2022
Cited by 1 | Viewed by 1145
Abstract
Pre-Big Bang cosmology inspired generations of cosmologists in attempts to cure the initial Big Bang singularity using a fundamental length scale as proposed by string theory. The existence of a phase of collapse/inflation with increasing curvature followed by a cosmic bounce has been [...] Read more.
Pre-Big Bang cosmology inspired generations of cosmologists in attempts to cure the initial Big Bang singularity using a fundamental length scale as proposed by string theory. The existence of a phase of collapse/inflation with increasing curvature followed by a cosmic bounce has been proposed as an alternative to standard inflation in the solution of the horizon and curvature problems. However, the generation of a nearly scale-invariant spectrum of perturbations is not an automatic prediction of such scenarios. In this paper, I review some general statements about the evolution of perturbations in bouncing cosmologies and some historically significant attempts to reconcile the predicted spectra with the observations. Bouncing cosmologies and, in particular, the pre-Big Bang scenario stand as viable, although more complicated, alternatives to inflation that may still help solve current theoretical and observational tensions. Full article
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12 pages, 6779 KiB  
Article
The Updated Version of the A.Ne.Mo.S. GLE Alert System: The Case of the Ground-Level Enhancement GLE73 on 28 October 2021
by Helen Mavromichalaki, Pavlos Paschalis, Maria Gerontidou, Maria-Christina Papailiou, Evangelos Paouris, Anastasia Tezari, Dimitra Lingri, Maria Livada, Argyris N. Stassinakis, Norma Crosby and Mark Dierckxsens
Universe 2022, 8(7), 378; https://doi.org/10.3390/universe8070378 - 11 Jul 2022
Cited by 4 | Viewed by 1428
Abstract
A ground-level enhancement (GLE) event is a sudden increase in cosmic ray intensity originated by solar sources and recorded by ground-based detectors. GLEs are invariably associated with large solar flares that can release and accelerate solar particles at high energies. The minimum kinetic [...] Read more.
A ground-level enhancement (GLE) event is a sudden increase in cosmic ray intensity originated by solar sources and recorded by ground-based detectors. GLEs are invariably associated with large solar flares that can release and accelerate solar particles at high energies. The minimum kinetic energy of particles reaching the Earth’s surface is >433 MeV at sea level and about 300 MeV/n at high-mountain altitude of about 3000 m a.s.l. Even though these abrupt events linked to solar activity are quite rare, they can have a great impact on technological systems and human health when recorded. Therefore, the accurate and effective prognosis of such events is of great importance. In this paper, an overview of the most recently recorded GLE event and the first of solar cycle 25, i.e., GLE73, as well as a post-event analysis is presented. GLE73 was detected on 28 October 2021 and was associated with the active region AR12887 on the central part of the solar disk, which produced an X1.0 solar flare. The event was registered by several stations of the worldwide ground-based neutron monitor network. An accurate alert was issued successfully by the ESA R-ESC federated product GLE Alert Plus, as well as the updated GLE Alert++ System of the Athens Neutron Monitor Station (A.Ne.Mo.S.). It should be emphasized that the GLE Alert++ signal by NKUA/A.Ne.Mo.S. was issued 45 min earlier than the one issued by GOES. A short description and the advantages of this last system are provided. Full article
(This article belongs to the Special Issue Solar Energetic Particles)
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4 pages, 223 KiB  
Communication
Using Neutrino Oscillations to Measure H0?
by Luis A. Anchordoqui
Universe 2022, 8(7), 377; https://doi.org/10.3390/universe8070377 - 11 Jul 2022
Viewed by 971
Abstract
Recently, the idea of using neutrino oscillations to measure the Hubble constant was introduced. We show that such a task is unfeasible because for typical energies of cosmic neutrinos, oscillations average out over cosmological distances and so the oscillation probability depends only on [...] Read more.
Recently, the idea of using neutrino oscillations to measure the Hubble constant was introduced. We show that such a task is unfeasible because for typical energies of cosmic neutrinos, oscillations average out over cosmological distances and so the oscillation probability depends only on the mixing angles. Full article
26 pages, 576 KiB  
Article
How Inflationary Gravitons Affect the Force of Gravity
by Lintao Tan, Nikolaos Christos Tsamis and Richard Paul Woodard
Universe 2022, 8(7), 376; https://doi.org/10.3390/universe8070376 - 8 Jul 2022
Cited by 12 | Viewed by 1063
Abstract
We employ an unregulated computation of the graviton self-energy from gravitons on the de Sitter background to infer the renormalized result. This is used to quantum-correct the linearized Einstein equation. We solve this equation for the potentials that represent the gravitational response to [...] Read more.
We employ an unregulated computation of the graviton self-energy from gravitons on the de Sitter background to infer the renormalized result. This is used to quantum-correct the linearized Einstein equation. We solve this equation for the potentials that represent the gravitational response to a static, point mass. We find large spatial and temporal logarithmic corrections to the Newtonian potential and to the gravitational shift. Although suppressed by a minuscule loop-counting parameter, these corrections cause perturbation theory to break down at large distances and late times. Another interesting fact is that gravitons induce up to three large logarithms, whereas a loop of massless, minimally coupled scalars produces only a single large logarithm. This is in line with corrections to the graviton mode function: a loop of gravitons induces two large logarithms, whereas a scalar loop gives none. Full article
(This article belongs to the Section Gravitation)
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16 pages, 3313 KiB  
Article
Periodic Variations of Solar Corona Index during 1939–2020
by Rui Tang, Yu Fei, Chun Li, Wen Liu, Xinan Tian and Zhongjie Wan
Universe 2022, 8(7), 375; https://doi.org/10.3390/universe8070375 - 8 Jul 2022
Cited by 2 | Viewed by 2057
Abstract
Periodic behaviors of solar magnetic indicators might provide a clue for the understanding of solar dynamic processes. Combining with a Lomb–Scargle periodogram, the concentration of frequency and time via a multitapered synchrosqueezed transform is applied to investigate the periodic variations of modified coronal [...] Read more.
Periodic behaviors of solar magnetic indicators might provide a clue for the understanding of solar dynamic processes. Combining with a Lomb–Scargle periodogram, the concentration of frequency and time via a multitapered synchrosqueezed transform is applied to investigate the periodic variations of modified coronal index for the time interval from 1 January 1939 to 31 August 2020. The main results are as follows: (1) During solar cycles 19 to 23, the Schwabe cycle of the modified coronal index is operating with its length variating between 10.5 and 11-yr, and the average value of length is 10.67-yr with standard deviation of 0.14-yr. (2) The Rieger-type periods are mainly distributed in a range from 120 to 200 days. In addition, the periods vary somewhat intermittently during cycles 18 to 24, which are operating with the highest power in cycles 21 and 22 while the power is much lower in cycles 23 and 24. (3) For rotation periods, the temporal variation exhibits a highly intermittent pattern as an asymmetrical distribution with its 25th, 50th, and 75th quantile of 26, 27.8, and 31-day, respectively. (4) Other mid-range periods are also detected with an average period length of 8.07, 5.44, 3.42, 2.3, and 1.01-yr. Full article
(This article belongs to the Section Solar System)
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23 pages, 4996 KiB  
Review
Overview and Status of the International Celestial Reference Frame as Realized by VLBI
by Aletha de Witt, Patrick Charlot, David Gordon and Christopher S. Jacobs
Universe 2022, 8(7), 374; https://doi.org/10.3390/universe8070374 - 7 Jul 2022
Cited by 5 | Viewed by 2717
Abstract
Accurate measurement of angular positions on the sky requires a well-defined system of reference that is realized with accessible objects. The purpose of this study is to review the international standard realization of such a system, the International Celestial Reference Frame (ICRF). The [...] Read more.
Accurate measurement of angular positions on the sky requires a well-defined system of reference that is realized with accessible objects. The purpose of this study is to review the international standard realization of such a system, the International Celestial Reference Frame (ICRF). The ICRF uses the Very Long Baseline Interferometry (VLBI) technique as it has the highest resolution of any current astrometric technique for reference frames in order to observe Active Galactic Nuclei (AGN) which are at such great distances (typical redshift ∼1) that there is currently no observed parallax or proper motion of these objects thus giving the frame excellent stability. We briefly review the history of the transition from the Fundamental Katalog 5 (FK5) optical frame to VLBI-based frames with attention to each of the three generations: ICRF-1, ICRF-2, and ICRF-3. We present some of the more prominent applications of the ICRF and outline the methods used to construct the ICRF. Next we discuss in more detail the current standard ICRF-3—which is the first frame to be realized at multiple wavelengths (S/X, K, X/Ka-bands)—including an estimate of its accuracy and limiting errors. We conclude with an overview of future plans for improving the ICRF. Full article
(This article belongs to the Special Issue VLBI Science Applications)
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15 pages, 433 KiB  
Article
A Multidimensional Multicomponent Gas Dynamic with the Neutrino Transfer in Gravitational Collapse
by Alexey G. Aksenov
Universe 2022, 8(7), 372; https://doi.org/10.3390/universe8070372 - 7 Jul 2022
Cited by 2 | Viewed by 1123
Abstract
The self-consistent problem of gravitational collapse is solved using 2D gas dynamics with taking into account the neutrino transfer in the flux-limited diffusion approximation. Neutrino are described by spectral energy density, and weak interaction includes a simplified physical model of neutrino interactions with [...] Read more.
The self-consistent problem of gravitational collapse is solved using 2D gas dynamics with taking into account the neutrino transfer in the flux-limited diffusion approximation. Neutrino are described by spectral energy density, and weak interaction includes a simplified physical model of neutrino interactions with nucleons. I investigate convection on the stage of the collapse and then in the center of the core, where the unstable entropy profile was probably formed. It is shown that convection has large scale. Convection appears only in the semitransparent region near the neutrinosphere due to non-equilibrium nonreversible neutronization. Convection increases the energy of emitted neutrino up to 15÷18 MeV. The obtained neutrino spectrum is important for the registration of low-energy neutrinos from a supernova. Full article
(This article belongs to the Special Issue Kinetic Processes in Relativistic Domain)
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51 pages, 2636 KiB  
Review
Key Space and Ground Facilities in GRB Science
by Anastasia Tsvetkova, Dmitry Svinkin, Sergey Karpov and Dmitry Frederiks
Universe 2022, 8(7), 373; https://doi.org/10.3390/universe8070373 - 6 Jul 2022
Cited by 6 | Viewed by 3011
Abstract
Gamma-ray bursts (GRBs) are short and intense flashes of γ-rays coming from deep space. GRBs were discovered more than a half century ago and now are observed across the whole electromagnetic spectrum from radio to very-high-energy gamma rays. They carry information about [...] Read more.
Gamma-ray bursts (GRBs) are short and intense flashes of γ-rays coming from deep space. GRBs were discovered more than a half century ago and now are observed across the whole electromagnetic spectrum from radio to very-high-energy gamma rays. They carry information about the powerful energy release during the final stage of stellar evolution, as well as properties of matter on the way to the observer. At present, space-based observatories detect on average approximately one GRB per day. In this review, we summarize key space and ground facilities that contribute to the GRB studies. Full article
(This article belongs to the Special Issue GRBs Phenomenology, Models and Applications: A Beginner Guide)
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17 pages, 4018 KiB  
Article
A Three-Order, Divergence-Free Scheme for the Simulation of Solar Wind
by Man Zhang and Xueshang Feng
Universe 2022, 8(7), 371; https://doi.org/10.3390/universe8070371 - 5 Jul 2022
Cited by 1 | Viewed by 1252
Abstract
In this paper, we present a three-order, divergence-free finite volume scheme to simulate the steady state solar wind ambient. The divergence-free condition of the magnetic field is preserved by the constrained transport (CT) method. The CT method can keep the magnetic fields divergence [...] Read more.
In this paper, we present a three-order, divergence-free finite volume scheme to simulate the steady state solar wind ambient. The divergence-free condition of the magnetic field is preserved by the constrained transport (CT) method. The CT method can keep the magnetic fields divergence free if the magnetic fields is divergence free initially. Thus, a least-squares reconstruction of magnetic field with the divergence free constraints is used to make the magnetic fields global solenoidality initially. High order spatial accuracy is obtained through a non-oscillatory hierarchical reconstruction, while a high order time discretization is achieved using a three-order Runge–Kutta scheme. This new model of three order in space and time is validated by numerical results for Carrington rotation 2207. The numerical results show its capability for producing stable reliable results for structured solar wind. The high-order, divergence-free properties of this method make it an ideal tool for the simulations of coronal mass ejection in future. Full article
(This article belongs to the Special Issue Propagation of Coronal Mass Ejections)
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25 pages, 14717 KiB  
Article
Incorporating a Radiative Hydrodynamics Scheme in the Numerical-Relativity Code BAM
by Henrique Gieg, Federico Schianchi, Tim Dietrich and Maximiliano Ujevic
Universe 2022, 8(7), 370; https://doi.org/10.3390/universe8070370 - 5 Jul 2022
Cited by 5 | Viewed by 1325
Abstract
To study binary neutron star systems and to interpret observational data such as gravitational-wave and kilonova signals, one needs an accurate description of the processes that take place during the final stages of the coalescence, for example, through numerical-relativity simulations. In this work, [...] Read more.
To study binary neutron star systems and to interpret observational data such as gravitational-wave and kilonova signals, one needs an accurate description of the processes that take place during the final stages of the coalescence, for example, through numerical-relativity simulations. In this work, we present an updated version of the numerical-relativity code BAM in order to incorporate nuclear-theory-based equations of state and a simple description of neutrino interactions through a neutrino leakage scheme. Different test simulations, for stars undergoing a neutrino-induced gravitational collapse and for binary neutron stars systems, validate our new implementation. For the binary neutron stars systems, we show that we can evolve stably and accurately distinct microphysical models employing the different equations of state: SFHo, DD2, and the hyperonic BHBΛϕ. Overall, our test simulations have good agreement with those reported in the literature. Full article
(This article belongs to the Special Issue Neutron Stars and Gravitational Wave Observations)
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16 pages, 4642 KiB  
Article
Chaos in a Magnetized Brane-World Spacetime Using Explicit Symplectic Integrators
by Airong Hu and Guoqing Huang
Universe 2022, 8(7), 369; https://doi.org/10.3390/universe8070369 - 4 Jul 2022
Cited by 2 | Viewed by 1347
Abstract
A brane-world metric with an external magnetic field is a modified theory of gravity. It is suitable for the description of compact sources on the brane such as stars and black holes. We design a class of explicit symplectic integrators for this spacetime [...] Read more.
A brane-world metric with an external magnetic field is a modified theory of gravity. It is suitable for the description of compact sources on the brane such as stars and black holes. We design a class of explicit symplectic integrators for this spacetime and use one of the integrators to investigate how variations of the parameters affect the motion of test particles. When the magnetic field does not vanish, the integrability of the system is destroyed. Thus, the onset of chaos can be allowed under some circumstances. Chaos easily occurs when the electromagnetic parameter becomes large enough. Dark matter acts as a gravitational force, so that chaotic motion can become more obvious as dark matter increases. The gravity of the black hole is weakened with an increasing positive cosmological parameter; therefore, the extent of chaos can be also strengthened. The proposed symplectic integrator is applied to a ray-tracing method and the study of such chaotic dynamics will be a possible reference for future studies of brane-world black hole shadows with chaotic patterns of self-similar fractal structures based on the Event Horizon Telescope data for M87* and Sagittarius A*. Full article
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23 pages, 1137 KiB  
Article
The Effect of Age on the Grouping of Open Clusters: II—Are There Old Binary Clusters?
by Juan Casado
Universe 2022, 8(7), 368; https://doi.org/10.3390/universe8070368 - 4 Jul 2022
Cited by 2 | Viewed by 1374
Abstract
In the present study, we continue testing the Primordial Group hypothesis (Casado 2022), which postulates that only sufficiently young open clusters can be binary or multiple, and old clusters are essentially single. To this end, we revisit all the remaining binary cluster candidates [...] Read more.
In the present study, we continue testing the Primordial Group hypothesis (Casado 2022), which postulates that only sufficiently young open clusters can be binary or multiple, and old clusters are essentially single. To this end, we revisit all the remaining binary cluster candidates in the Galaxy having at least one cluster older than 100 Myr through Gaia data and careful revision of the literature. We found no convincing case for an old binary system among the 120 pairs/groups revised. Most of the pairs are optical pairs or flyby encounters. However, we found three dubious pairs that could falsify the title hypothesis upon further research. We also found two possible primordial pairs older than expected. Our results confirm that the vast majority of binary/multiple OCs in the Galaxy, if not all, are of primordial origin and are not stable for a long time. This finding is in line with similar studies of the Magellanic Clouds and theoretical N-body simulations in the Galaxy. The pairs of OCs in these groups are generally not binary systems since they are not gravitationally bound. We also point out some inconsistencies in previous works and databases, such as false open clusters and duplicities. Full article
(This article belongs to the Special Issue Stellar Astrophysics)
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8 pages, 2353 KiB  
Article
Measuring the Modified Gravitational Wave Propagation beyond General Relativity from CMB Observations
by Jun Li
Universe 2022, 8(7), 367; https://doi.org/10.3390/universe8070367 - 3 Jul 2022
Cited by 2 | Viewed by 1265
Abstract
In modified gravity theories, gravitational wave propagations are presented in nonstandard ways. We consider a friction term different from GR and constrain the modified gravitational waves propagation from observations. The modified gravitational waves produce anisotropies and polarization, which generate measurable tensor power spectra. [...] Read more.
In modified gravity theories, gravitational wave propagations are presented in nonstandard ways. We consider a friction term different from GR and constrain the modified gravitational waves propagation from observations. The modified gravitational waves produce anisotropies and polarization, which generate measurable tensor power spectra. We explore the impact of the friction term on the power spectrum of B-modes and the impact on the constraints on the other parameters (e.g., r or At) when ν0 is allowed to vary in the Monte Carlo analyses from Planck+BK18 datasets. If we assume the result of the scalar perturbations is unchanged, the inflation consistency relation alters with the friction term. In the ΛCDM+r+ν0 model, the tensor-to-scalar ratio and the amplitude of the tensor spectrum are obviously influenced. Full article
(This article belongs to the Special Issue Cosmic Microwave Background)
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17 pages, 3187 KiB  
Article
Monte Carlo Models of Comet Dust Tails Observed from the Ground
by Fernando Moreno
Universe 2022, 8(7), 366; https://doi.org/10.3390/universe8070366 - 3 Jul 2022
Cited by 4 | Viewed by 1592
Abstract
Dust particles leaving the comet nucleus surface are entrained by the gas within the first few nuclear radius distances and are subjected to a complex hydrodynamical environment. From distances of about 20 nuclear radii outwards, the particles decouple from the accelerating gas and [...] Read more.
Dust particles leaving the comet nucleus surface are entrained by the gas within the first few nuclear radius distances and are subjected to a complex hydrodynamical environment. From distances of about 20 nuclear radii outwards, the particles decouple from the accelerating gas and are mainly affected by solar gravity and radiation pressure for small-sized nuclei. Their motion is then a function of their so-called β parameter, which is the ratio of the radiation pressure force to gravity force, and their velocity when the gas drag vanishes. At a given observation time, the position of those particles projected on the sky plane form the coma, tail and trail structures that can be observed from ground-based or space-borne instrumentation. Monte Carlo models, based on the computer simulation of the Keplerian trajectories of a large set of dust particles, provide the best possible approach to extract the dust environment parameters from the observed scattered solar light or thermal emission. In this paper, we describe the Monte Carlo code along with some successful applications of such technique to a number of targets. Full article
(This article belongs to the Special Issue The Advances of Comets' Activity)
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24 pages, 857 KiB  
Article
Dynamics of a Cosmological Model in f(R,T) Gravity: I. On Invariant Planes
by Jianwen Liu, Ruifang Wang and Fabao Gao
Universe 2022, 8(7), 365; https://doi.org/10.3390/universe8070365 - 3 Jul 2022
Cited by 4 | Viewed by 1159
Abstract
Under the background of perfect fluid and flat Friedmann–Lemaître–Robertson–Walker (FLRW) space-time, this paper mainly describes the dynamics of the cosmological model constructed in f(R,T) gravity on three invariant planes, by using the singularity theory and Poincaré compactification in differential equations. Full article
(This article belongs to the Section Cosmology)
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16 pages, 4000 KiB  
Article
Trajectory Analysis and Optimization of Hesperides Mission
by Giovanni Mengali and Alessandro A. Quarta
Universe 2022, 8(7), 364; https://doi.org/10.3390/universe8070364 - 1 Jul 2022
Cited by 4 | Viewed by 1914
Abstract
A challenging problem from a technological viewpoint is to send a spacecraft at a distance of about 600 au from the Sun, comparable with that of the Sun’s gravitational focus (that is, the general relativistic focusing of light rays, whose minimum solar distance [...] Read more.
A challenging problem from a technological viewpoint is to send a spacecraft at a distance of about 600 au from the Sun, comparable with that of the Sun’s gravitational focus (that is, the general relativistic focusing of light rays, whose minimum solar distance is obtained when the light rays are assumed to graze the Sun’s surface), and reach it in a time interval on the order of a human working lifetime. A suitably oriented telescope at that distance would be theoretically able to observe exoplanets tens of light years far away and possibly to discover new life forms. The transfer trajectory of this mission is rather complex and requires a close selection of a suitable propulsion system, which must be able to provide the probe with the necessary energy to cruise at a velocity greater than 10 au/year. An effective outline of the these concepts is given by the Hesperides mission, originally proposed by Matloff in 2014. An interesting aspect of this mission proposal is the combination of a nuclear electric propulsion system and a classical solar sail that are jointly exploited to reach the necessary solar system escape velocity. However, the trajectory analysis reported by Matloff is very simplified and is essentially concentrated on a rough estimate of the time required by the spacecraft to reach a distance of 600au. Starting from the Hesperides baseline mission proposal, including the vehicle mass distribution, the aim of this work is to give a detailed mission analysis in an optimal framework. In particular, the spacecraft minimum time trajectory is calculated with indirect methods and a parametric analysis is made to highlight the impact of the main design parameters on the total flight time. The simulations show a substantial reduction of the mission time when compared with the original study. Full article
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12 pages, 897 KiB  
Article
Near-Horizon Thermodynamics of Hairy Black Holes from Gravitational Decoupling
by Rogerio Teixeira Cavalcanti, Kelvin dos Santos Alves and Julio Marny Hoff da Silva
Universe 2022, 8(7), 363; https://doi.org/10.3390/universe8070363 - 1 Jul 2022
Cited by 13 | Viewed by 1387
Abstract
The horizon structure and thermodynamics of hairy spherically symmetric black holes generated by the gravitational decoupling method are carefully investigated. The temperature and heat capacity of the black hole is determined, as well as how the hairy parameters affect the thermodynamics. This allows [...] Read more.
The horizon structure and thermodynamics of hairy spherically symmetric black holes generated by the gravitational decoupling method are carefully investigated. The temperature and heat capacity of the black hole is determined, as well as how the hairy parameters affect the thermodynamics. This allows for an analysis of thermal stability and the possible existence of a remanent black hole. We also calculate the Hawking radiation corrected by the generalized uncertainty principle. We consider the emission of fermions and apply the tunneling method to the generalized Dirac equation. This shows that, despite the horizon location being the same as the Schwarzschild one for a suitable choice of parameters, the physical phenomena that occur near the horizon of both black holes are qualitatively different. Full article
(This article belongs to the Section Gravitation)
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18 pages, 4906 KiB  
Review
Slow Neutron-Capture Process: Low-Mass Asymptotic Giant Branch Stars and Presolar Silicon Carbide Grains
by Nan Liu, Sergio Cristallo and Diego Vescovi
Universe 2022, 8(7), 362; https://doi.org/10.3390/universe8070362 - 30 Jun 2022
Cited by 4 | Viewed by 1303
Abstract
Presolar grains are microscopic dust grains that formed in the stellar winds or explosions of ancient stars that died before the formation of the solar system. The majority (~90% in number) of presolar silicon carbide (SiC) grains, including types mainstream (MS), Y, and [...] Read more.
Presolar grains are microscopic dust grains that formed in the stellar winds or explosions of ancient stars that died before the formation of the solar system. The majority (~90% in number) of presolar silicon carbide (SiC) grains, including types mainstream (MS), Y, and Z, came from low-mass C-rich asymptotic giant branch (AGB) stars, which is supported by the ubiquitous presence of SiC dust observed in the circumstellar envelope of AGB stars and the signatures of slow neutron-capture process preserved in these grains. Here, we review the status of isotope studies of presolar AGB SiC grains with an emphasis on heavy element isotopes and highlight the importance of presolar grain studies for nuclear astrophysics. We discuss the sensitives of different types of nuclei to varying AGB stellar parameters and how their abundances in presolar AGB SiC grains can be used to provide independent, detailed constraints on stellar parameters, including 13C formation, stellar temperature, and nuclear reaction rates. Full article
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18 pages, 379 KiB  
Article
Stueckelberg and Higgs Mechanisms: Frames and Scales
by Alexander D. Popov
Universe 2022, 8(7), 361; https://doi.org/10.3390/universe8070361 - 29 Jun 2022
Cited by 3 | Viewed by 1353
Abstract
We consider Yang–Mills theory with a compact gauge group G on Minkowski space R3,1 and compare the introduction of masses of gauge bosons using the Stueckelberg and Higgs mechanisms. The Stueckelberg field ϕ is identified with a G-frame on [...] Read more.
We consider Yang–Mills theory with a compact gauge group G on Minkowski space R3,1 and compare the introduction of masses of gauge bosons using the Stueckelberg and Higgs mechanisms. The Stueckelberg field ϕ is identified with a G-frame on the gauge vector bundle E and the kinetic term for ϕ leads to the mass of the gauge bosons. The Stueckelberg mechanism is extended to the Higgs mechanism by adding to the game a scalar field describing rescaling of metric on fibres of E. Thus, we associate Higgs fields as well as running coupling parameters with conformal geometry on fibres of gauge bundles. In particular, a running coupling tending to zero or to infinity is equivalent to an unbounded expansion of G-fibres or its contraction to a point. We also discuss scale connection, space-time dependent Higgs vacua and compactly supported gauge and quark fields as an attribute of confinement. Full article
18 pages, 2089 KiB  
Article
Effect of Ephemeris on Pulsar Timing and Navigation Accuracy Based on X-ray Pulsar Navigation-I Data
by Yongtao Deng and Shuanggen Jin
Universe 2022, 8(7), 360; https://doi.org/10.3390/universe8070360 - 27 Jun 2022
Cited by 1 | Viewed by 1433
Abstract
Solar system ephemeris is very important for pulsar timing and navigation. In order to explore the effect of different precision ephemerides on X-ray pulsar timing and navigation, the differences between timing and navigation results with four JPL Development Ephemerides based on the data [...] Read more.
Solar system ephemeris is very important for pulsar timing and navigation. In order to explore the effect of different precision ephemerides on X-ray pulsar timing and navigation, the differences between timing and navigation results with four JPL Development Ephemerides based on the data of X-ray pulsar navigation-I (XPNAV-I) were compared and analyzed in this paper. For pulsar timing, the ephemeris has a systematic effect on time scale conversion (nanosecond difference), light-travel delay (millisecond difference) and timing residuals (microsecond difference), and the pulse profile phase can reflect the systematic deviation caused by different ephemerides in the timing calculation. The timing results show that it is necessary to compile the pulsar timing model based on the newer ephemeris. For navigation, based on the significant enhancement of pulse profile with orbit-dynamic (SEPO), the absolute error between simulation orbit and actual orbit is less than 2 km for each ephemeris, and the differences between simulation orbits are less than 1 km. The orbit position accuracy calculated by the ephemeris used in pulsar timing parameter calculation is the highest (DE200 in this paper), which explains the necessity of using a unified ephemeris in the calculation of timing and navigation with satisfying its internal self-consistency. Full article
(This article belongs to the Section Space Science)
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41 pages, 5183 KiB  
Review
Multiple Populations in Star Clusters
by Antonino P. Milone and Anna F. Marino
Universe 2022, 8(7), 359; https://doi.org/10.3390/universe8070359 - 27 Jun 2022
Cited by 55 | Viewed by 2286
Abstract
We review the multiple population (MP) phenomenon of globular clusters (GCs): i.e., the evidence that GCs typically host groups of stars with different elemental abundances and/or distinct sequences in photometric diagrams. Most Galactic and extragalactic clusters exhibit internal variations of He, C, N, [...] Read more.
We review the multiple population (MP) phenomenon of globular clusters (GCs): i.e., the evidence that GCs typically host groups of stars with different elemental abundances and/or distinct sequences in photometric diagrams. Most Galactic and extragalactic clusters exhibit internal variations of He, C, N, O, Na, and Al. They host two distinct stellar populations: the first population of stars, which resemble field stars with similar metallicities, and one or more second stellar populations that show the signature of high-temperature H-burning. In addition, a sub-sample of clusters hosts stellar populations with different heavy-element abundances. The MP origin remains one of the most puzzling, open issues of stellar astrophysics. We summarize the scenarios for the MP formation and depict the modern picture of GCs and their stellar populations along with the main evolutionary phases. We show that the MP behavior dramatically changes from one cluster to another and investigate their complexity to define common properties. We investigate relations with the host galaxy, the parameters of the host clusters (e.g., GC’s mass, age, orbit), and stellar mass. We summarize results on spatial distribution and internal kinematics of MPs. Finally, we review the relation between MPs and the so-called second-parameter problem of the horizontal-branch morphology of GCs and summarize the main findings on the extended main sequence phenomenon in young clusters. Full article
(This article belongs to the Special Issue Stellar Ecology)
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51 pages, 5543 KiB  
Article
Statistical Properties of X-ray Flares in Gamma-ray Bursts
by Yong-Rui Shi, Xiao-Kang Ding, Si-Yuan Zhu, Wan-Peng Sun and Fu-Wen Zhang
Universe 2022, 8(7), 358; https://doi.org/10.3390/universe8070358 - 27 Jun 2022
Cited by 1 | Viewed by 1443
Abstract
X-ray flares are frequently detected in the X-ray afterglow light curves and are highly correlated with the prompt emission of gamma-ray bursts (GRBs). We compile a comprehensive sample of X-ray flares up to 2021 April, comprising 697 flares. We classify the total sample [...] Read more.
X-ray flares are frequently detected in the X-ray afterglow light curves and are highly correlated with the prompt emission of gamma-ray bursts (GRBs). We compile a comprehensive sample of X-ray flares up to 2021 April, comprising 697 flares. We classify the total sample into four types: early flares (tp103 s), late flares (tp>103 s), long gamma-ray burst (LGRB) flares and short gamma-ray burst (SGRB) flares, and analyze the distributions and relationships of the flare parameters. It is found that the early flares have a higher frequency, shorter duration, and more asymmetrical structure. In addition, the distributions of the morphological parameters of the SGRB flares are similar to those of the LGRB flares. We also find that the durations and rising (decay) times of the early flares are positively correlated with the peak times, but the late flares follow the different dependent relations. There is a strong anti-correlation between the peak luminosities (LX,P) and the peak times of the flares, e.g., LX,ptp,z1.45 for the LGRB flares, and LX,ptp,z1.27 for the SGRB flares, respectively. Furthermore, the peak luminosity is highly dependent on the isotropic energy (EX,iso) for the early LGRB flares, the best fit is LX,pEX,iso1.06(r=0.89). We also find a tight three-parameter correlation, LX,ptp,z1.03EX,iso0.92(r=0.96). All the late flares fall into the 3σ confidence region defined by the early flares. In terms of the point of kinematic arguments, both the SGRB and LGRB flares support a common scheme of internal origin. The SGRB flares have similar properties to the LGRB flares, suggesting that both of them share a similar physical mechanism from the late-time activity of central engine. Full article
(This article belongs to the Section Compact Objects)
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8 pages, 243 KiB  
Review
Instrumentation for Detecting Sulphur Isotopes as Biosignatures on Europa and Ganymede by Forthcoming Missions
by Julian Chela-Flores
Universe 2022, 8(7), 357; https://doi.org/10.3390/universe8070357 - 27 Jun 2022
Cited by 1 | Viewed by 1357
Abstract
There has been remarkable progress in identifying a certain type of biosignature, both from the point of view of the payloads of forthcoming missions, and from the point of view of biogeochemistry. This progress has been due to the evolution of miniaturized mass [...] Read more.
There has been remarkable progress in identifying a certain type of biosignature, both from the point of view of the payloads of forthcoming missions, and from the point of view of biogeochemistry. This progress has been due to the evolution of miniaturized mass spectrometry that can be used, under certain circumstances and for certain samples, to distinguish between putatively abiotic and biotic sulphur isotopes. These specific types of biosignatures are discussed in the context of Europa and Ganymede. Such instruments are sufficiently precise to differentiate between abiotic and biotic signatures. We reflect on new possibilities that will be available during this decade for exploring the nearest ocean worlds: Europa and Ganymede. We review arguments that point out the presence of intriguing sulphur patches on Europa’s icy surface that were discovered by the Galileo mission. These patches lead to a “sulphur dilemma”, which suggests not to focus future measurements exclusively on organics. We comment on the possibility of measurements of sulphur isotopes, as one kind of biosignature, to be complemented with additional biosignatures, in order to fully test biogenicity. These suggestions are intended to point out the best use of the available spacecrafts’ payloads during the planning of the forthcoming Jovian missions. Full article
(This article belongs to the Special Issue Developments and Technology Trends in the Space-Borne Instrumentation)
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