Universe

3 pages, 145 KiB

Open AccessEditorial

Editorial to the Special Issue “Space Missions to Small Bodies: Results and Future Activities”

by Andrea Longobardo

Universe 2024, 10(11), 425; https://doi.org/10.3390/universe10110425 - 14 Nov 2024

Small bodies (asteroids, comets, and satellites) are the most primitive bodies of our solar system and, for this reason, represent the key to understanding its origin and early evolution [...] Full article

(This article belongs to the Special Issue Space Missions to Small Bodies: Results and Future Activities)

30 pages, 11511 KiB

Open AccessArticle

Sources and Radiations of the Fermi Bubbles

by Vladimir A. Dogiel and Chung-Ming Ko

Universe 2024, 10(11), 424; https://doi.org/10.3390/universe10110424 - 12 Nov 2024

Abstract

Two enigmatic gamma-ray features in the galactic central region, known as Fermi Bubbles (FBs), were found from Fermi-LAT data. An energy release, (e.g., by tidal disruption events in the Galactic Center, GC), generates a cavity with a shock that expands into the local [...] Read more.

Two enigmatic gamma-ray features in the galactic central region, known as Fermi Bubbles (FBs), were found from Fermi-LAT data. An energy release, (e.g., by tidal disruption events in the Galactic Center, GC), generates a cavity with a shock that expands into the local ambient medium of the galactic halo. A decade or so ago, a phenomenological model of the FBs was suggested as a result of routine star disruptions by the supermassive black hole in the GC which might provide enough energy for large-scale structures, like the FBs. In 2020, analytical and numerical models of the FBs as a process of routine tidal disruption of stars near the GC were developed; these disruption events can provide enough cumulative energy to form and maintain large-scale structures like the FBs. The disruption events are expected to be

10^{- 4} \sim 10^{- 5} {yr}^{- 1}

, providing an average power of energy release from the GC into the halo of

\dot{E} \sim 3 \times 10^{41}

erg

s^{- 1}

, which is needed to support the FBs. Analysis of the evolution of superbubbles in exponentially stratified disks concluded that the FB envelope would be destroyed by the Rayleigh–Taylor (RT) instabilities at late stages. The shell is composed of swept-up gas of the bubble, whose thickness is much thinner in comparison to the size of the envelope. We assume that hydrodynamic turbulence is excited in the FB envelope by the RT instability. In this case, the universal energy spectrum of turbulence may be developed in the inertial range of wavenumbers of fluctuations (the Kolmogorov–Obukhov spectrum). From our model we suppose the power of the FBs is transformed partly into the energy of hydrodynamic turbulence in the envelope. If so, hydrodynamic turbulence may generate MHD fluctuations, which accelerate cosmic rays there and generate gamma-ray and radio emission from the FBs. We hope that this model may interpret the observed nonthermal emission from the bubbles. Full article

(This article belongs to the Special Issue Studying Astrophysics with High-Energy Cosmic Particles)

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16 pages, 1992 KiB

Open AccessArticle

Exploring γ-Ray Flares from High-Redshift Blazar B3 1343+451 at GeV Energies

by Xiongfei Geng, Yang Liu, Gang Cao, Jing Fan, Xiongbang Yang, Nan Ding, Minghu Gao, Yehui Yang and Zhijie Zhang

Universe 2024, 10(11), 423; https://doi.org/10.3390/universe10110423 - 11 Nov 2024

Abstract

We study the temporal and spectral variability properties of the high-redshift blazar B3 1343+451 utilizing Fermi-LAT data from 2008 to 2022 in the energy range of 0.1–300 GeV. We identify six major flares with many substructures and analyze their temporal and spectral properties [...] Read more.

We study the temporal and spectral variability properties of the high-redshift blazar B3 1343+451 utilizing Fermi-LAT data from 2008 to 2022 in the energy range of 0.1–300 GeV. We identify six major flares with many substructures and analyze their temporal and spectral properties in detail. The fastest rise and decay timescales are found to be 4.8 ± 0.48 h and 5.28 ± 0.72 h, respectively. The size of the emission region is constrained to be R ∼ 5.18 ×

10^{15}

–1.56 ×

10^{16}

cm with the typical Doppler factors of

δ

∼ 10–30. Most of the peaks from the flares exhibit a symmetric temporal profile within the error bars, implying that the rise and decay timescales are dominated by the disturbances caused by dense plasma blobs passing through the standing shock front in the jet region. We also find that four flares are better fitted with a log-parabolic distribution, while two flares are better fitted with a power-law distribution. Our results indicate that the emission regions vary from one flare to another, which is consistent with earlier results. Full article

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4 pages, 166 KiB

Open AccessEditorial

Solar Radio Emissions

by Baolin Tan and Jing Huang

Universe 2024, 10(11), 422; https://doi.org/10.3390/universe10110422 - 9 Nov 2024

Abstract

The radio emission from the Sun covers a very wide frequency band ranging from several hundreds of GHz (sub-millimeter wavelength) down to sub-MHz (kilometer wavelength) [...] Full article

(This article belongs to the Special Issue Solar Radio Emissions)

14 pages, 616 KiB

Open AccessArticle

Gravitational Lensing Effects from Models of Loop Quantum Gravity with Rigorous Quantum Parameters

by Haida Li and Xiangdong Zhang

Universe 2024, 10(11), 421; https://doi.org/10.3390/universe10110421 - 8 Nov 2024

Abstract

Many previous works have studied gravitational lensing effects from Loop Quantum Gravity. So far, gravitational lensing effects from Loop Quantum Gravity have only been studied by choosing large quantum parameters much larger than the Planck scale. However, by construction, the quantum parameters of [...] Read more.

Many previous works have studied gravitational lensing effects from Loop Quantum Gravity. So far, gravitational lensing effects from Loop Quantum Gravity have only been studied by choosing large quantum parameters much larger than the Planck scale. However, by construction, the quantum parameters of the effective models of Loop Quantum Gravity are usually related to the Planck length and, thus, are extremely small. In this work, by strictly imposing the quantum parameters as initially constructed, we study the true quantum corrections of gravitational lensing effects by five effective black hole models of Loop Quantum Gravity. Our study reveals several interesting results, including the different scales of quantum corrections displayed by each model and the connection between the quantum correction of deflection angles and the quantum correction of the metric. Observables related to the gravitational lensing effect are also obtained for all models in the case of SgrA* and M87*. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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41 pages, 549 KiB

Open AccessArticle

Comparing a Gauge-Invariant Formulation and a “Conventional Complete Gauge-Fixing Approach” for l=0,1-Mode Perturbations on the Schwarzschild Background Spacetime

by Kouji Nakamura

Universe 2024, 10(11), 420; https://doi.org/10.3390/universe10110420 - 7 Nov 2024

Abstract

This article provides a comparison of the gauge-invariant formulation for

l = 0, 1

-mode perturbations on the Schwarzschild background spacetime, proposed by the same author in 2021, and a “conventional complete gauge-fixing approach” where the spherical harmonic functions

Y_{l m}

[...] Read more.

This article provides a comparison of the gauge-invariant formulation for

l = 0, 1

-mode perturbations on the Schwarzschild background spacetime, proposed by the same author in 2021, and a “conventional complete gauge-fixing approach” where the spherical harmonic functions

Y_{l m}

as the scalar harmonics are used from the starting point. Although it is often stated that “gauge-invariant formulations in general-relativistic perturbations are equivalent to complete gauge-fixing approaches”, we conclude that, as a result of this comparison, the derived solutions through the proposed gauge-invariant formulation and those through a “conventional complete gauge-fixing approach” are different. It is pointed out that there is a case where the boundary conditions and initial conditions are restricted in a conventional complete gauge-fixing approach. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

41 pages, 1918 KiB

Open AccessReview

Semi-Symmetric Metric Gravity: A Brief Overview

by Himanshu Chaudhary, Lehel Csillag and Tiberiu Harko

Universe 2024, 10(11), 419; https://doi.org/10.3390/universe10110419 - 7 Nov 2024

Abstract

We present a review of the Semi-Symmetric Metric Gravity (SSMG) theory, representing a geometric extension of standard general relativity, based on a connection introduced by Friedmann and Schouten in 1924. The semi-symmetric connection is a connection that generalizes the Levi-Civita one by allowing [...] Read more.

We present a review of the Semi-Symmetric Metric Gravity (SSMG) theory, representing a geometric extension of standard general relativity, based on a connection introduced by Friedmann and Schouten in 1924. The semi-symmetric connection is a connection that generalizes the Levi-Civita one by allowing for the presence of a simple form of the torsion, described in terms of a torsion vector. The Einstein field equations are postulated to have the same form as in standard general relativity, thus relating the Einstein tensor constructed with the help of the semi-symmetric connection, with the energy–momentum tensor tensor. The inclusion of the torsion contributions in the field equations has intriguing cosmological implications, particularly during the late-time evolution of the Universe. Presumably, these effects also dominate under high-energy conditions, and thus SSMG could potentially address unresolved issues in general relativity and cosmology, such as the initial singularity, inflation, or the ⁷Li problem of the Big-Bang Nucleosynthesis. The explicit presence of torsion in the field equations leads to the non-conservation of the energy–momentum tensor tensor, which can be interpreted within the irreversible thermodynamics of open systems as describing particle creation processes. We also review in detail the cosmological applications of the theory, and investigate the statistical tests for several models, by constraining the model parameters via comparison with several observational datasets. Full article

(This article belongs to the Special Issue Dark Energy and Dark Matter)

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26 pages, 416 KiB

Open AccessPerspective

Foundational Issues in Dynamical Casimir Effect and Analogue Features in Cosmological Particle Creation

by Jen-Tsung Hsiang and Bei-Lok Hu

Universe 2024, 10(11), 418; https://doi.org/10.3390/universe10110418 - 7 Nov 2024

Abstract

Moving mirrors as analogue sources of Hawking radiation from black holes have been explored extensively but less so with cosmological particle creation (CPC), even though the analogy between the dynamical Casimir effect (DCE) and CPC based on the mechanism of the parametric amplification [...] Read more.

Moving mirrors as analogue sources of Hawking radiation from black holes have been explored extensively but less so with cosmological particle creation (CPC), even though the analogy between the dynamical Casimir effect (DCE) and CPC based on the mechanism of the parametric amplification of quantum field fluctuations has also been known for a long time. This ‘perspective’ essay intends to convey some of the rigor and thoroughness of quantum field theory in curved spacetime, which serves as the theoretical foundation of CPC, to DCE, which enjoys a variety of active experimental explorations. We have selected seven issues of relevance to address, starting from the naively simple ones, e.g., why one should be bothered with ‘curved’ spacetime when performing a laboratory experiment in ostensibly flat space, to foundational theoretical ones, such as the frequent appearance of nonlocal dissipation in the system dynamics induced by colored noises in its field environment, the existence of quantum Lenz law and fluctuation–dissipation relations in the backreaction effects of DCE emission on the moving atom/mirror or the source, and the construction of a microphysics model to account for the dynamical responses of a mirror or medium. The strengthening of the theoretical ground for DCE is not only useful for improving conceptual clarity but needed for the development of the proof-of-concept type of future experimental designs for DCE. The results from the DCE experiments in turn will enrich our understanding of quantum field effects in the early universe because they are, in the spirit of analogue gravity, our best hopes for the verification of these fundamental processes. Full article

(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)

24 pages, 6959 KiB

Open AccessArticle

Linking Turbulent Interplanetary Magnetic Field Fluctuations and Current Sheets

by Maria O. Riazantseva, Timofey V. Treves, Olga Khabarova, Liudmila S. Rakhmanova, Yuri I. Yermolaev and Alexander A. Khokhlachev

Universe 2024, 10(11), 417; https://doi.org/10.3390/universe10110417 - 7 Nov 2024

Abstract

The study aims to understand the role of solar wind current sheets (CSs) in shaping the spectrum of turbulent fluctuations and driving dissipation processes in space plasma. Local non-adiabatic heating and acceleration of charged particles in the solar wind is one of the [...] Read more.

The study aims to understand the role of solar wind current sheets (CSs) in shaping the spectrum of turbulent fluctuations and driving dissipation processes in space plasma. Local non-adiabatic heating and acceleration of charged particles in the solar wind is one of the most intriguing challenges in space physics. Leading theories attribute these effects to turbulent heating, often associated with magnetic reconnection at small-scale coherent structures in the solar wind, such as CSs and flux ropes. We identify CSs observed at 1 AU in different types of the solar wind around and within an interplanetary coronal mass ejection (ICME) and analyze the corresponding characteristics of the turbulent cascade. It is found that the spectra of fluctuations of the interplanetary magnetic field may be reshaped due to the CS impact potentially leading to local disruptions in energy transfer along the cascade of turbulent fluctuations. Case studies of the spectra behavior at the peak of the CS number show their steepening at MHD scales, flattening at kinetic scales, and merging of the spectra into a single form, with the break almost disappearing. In the broader vicinity of the CS number peak, the behavior of spectral parameters changes sharply, but not always following the same pattern. The statistical analysis shows a clear correlation between the break frequency and the CS number. These results are consistent with the picture of turbulent reconnection at CSs. The CS occurrence is found to be statistically linked with the increased temperature. In the ICME sheath, there are two CS populations observed in the hottest and coldest plasma. Full article

(This article belongs to the Section Space Science)

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14 pages, 592 KiB

Open AccessArticle

Profile Variation in PSR B0355+54 over a Narrow Frequency Range

by Shibo Jiang, Lin Li, Rai Yuen, Jianping Yuan, Jumei Yao, Xun Shi, Yonghua Xu, Jianling Chen and Zhigang Wen

Universe 2024, 10(11), 416; https://doi.org/10.3390/universe10110416 - 6 Nov 2024

Abstract

We investigate changes in the shape of the averaged pulse profile in PSR B0355+54 (PSR J0358+5413) based on data obtained at the center frequency of 1250 MHz using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our dataset consists of 12 non-consecutive observations, each [...] Read more.

We investigate changes in the shape of the averaged pulse profile in PSR B0355+54 (PSR J0358+5413) based on data obtained at the center frequency of 1250 MHz using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our dataset consists of 12 non-consecutive observations, each lasting between 1 and 2 h. Considerable variation is observed in the averaged profiles across the observations even though each is folded from thousands of single pulses. Changes in the profile are measured through the ratio (R) between the peak intensities of the leading and trailing components. We find that the averaged pulse profile exhibits significant variation across observations, but distinctive from typical profile mode-changing. By dividing the frequency bandwidth into eight sub-bands, we demonstrate that the shape of the averaged profile undergoes significant evolution with frequency. In general, the changes in R across the sub-bands are different in different observations, but its value is uniform at low frequencies implying a more consistent emission. We demonstrate that the profile stabilization timescale for this pulsar is much longer than commonly suggested for ordinary pulsars, which is likely due to non-uniform and varying arrangement of the emission sources in the emission region. Full article

(This article belongs to the Section Compact Objects)

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14 pages, 449 KiB

Open AccessArticle

Neutron Beta Decay and Exact Conservation of Charged Weak Hadronic Vector Current in the Standard Model

by Derar Altarawneh, Roman Höllwieser and Markus Wellenzohn

Universe 2024, 10(11), 415; https://doi.org/10.3390/universe10110415 - 6 Nov 2024

Abstract

We investigate the reliability of the hypothesis of exact conservation of the charged weak hadronic vector current in neutron

β^{-}

-decay with a polarized neutron and an unpolarized proton and electron. We calculate the contributions of the phenomenological term responsible for Exact [...] Read more.

We investigate the reliability of the hypothesis of exact conservation of the charged weak hadronic vector current in neutron

β^{-}

-decay with a polarized neutron and an unpolarized proton and electron. We calculate the contributions of the phenomenological term responsible for Exact Conservation of the charged weak hadronic Vector Current (or the ECVC effect) in neutron

β^{-}

-decay, even for different masses of the neutron and proton, to the correlation coefficients, together with a complete set of contributions of scalar and tensor interactions beyond the Standard Model (SM). We argue that if the total contributions of scalar and tensor interactions beyond the SM fail to reconcile the experimental data on the correlation coefficients with the contributions of the ECVC effect, one may conclude that the charged weak hadronic vector current is not conserved in the hadronic transitions of weak processes with different masses of incoming and outgoing hadrons. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

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9 pages, 225 KiB

Open AccessArticle

Estimation of the Chances to Find New Phenomena at the LHC in a Model-Agnostic Combinatorial Analysis

by Sergei V. Chekanov

Universe 2024, 10(11), 414; https://doi.org/10.3390/universe10110414 - 5 Nov 2024

Abstract

In this paper, we estimate the number of event topologies that have the potential to be produced in

p p

collisions at the Large Hadron Collider (LHC) without violating kinematic and other constraints. We use numerical calculations and combinatorics, guided by large-scale Monte [...] Read more.

In this paper, we estimate the number of event topologies that have the potential to be produced in

p p

collisions at the Large Hadron Collider (LHC) without violating kinematic and other constraints. We use numerical calculations and combinatorics, guided by large-scale Monte Carlo simulations of Standard Model (SM) processes. Then, we set the upper limit on the probability that new physics may escape detection, assuming a model-agnostic approach. The calculated probability is unexpectedly large, and the fact that LHC has not found new physics until now is not entirely surprising. Theoretical limitations and experimental challenges in observing new physics within the studied exclusive event classes are examined. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

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17 pages, 528 KiB

Open AccessArticle

Studying the Properties of Spacetime with an Improved Dynamical Model of the Inner Solar System

by Dmitry Pavlov and Ivan Dolgakov

Universe 2024, 10(11), 413; https://doi.org/10.3390/universe10110413 - 3 Nov 2024

Abstract

Physical properties of the Sun (orientation of rotation axis, oblateness coefficient

J_{2 ⊙}

, and change rate of the gravitational parameter

{\dot{μ}}_{⊙}

) are determined using a dynamical model describing the motion of the Sun, planets, the Moon, asteroids, and [...] Read more.

Physical properties of the Sun (orientation of rotation axis, oblateness coefficient

J_{2 ⊙}

, and change rate of the gravitational parameter

{\dot{μ}}_{⊙}

) are determined using a dynamical model describing the motion of the Sun, planets, the Moon, asteroids, and Trans-Neptunian objects (TNOs). Among the many kinds of observations used to determine the orbits and physical properties of the bodies, the most important for our study are precise interplanetary ranging data: Earth–Mercury ranges from MESSENGER spacecraft and Earth–Mars ranges from Odyssey and MRO. The findings allow us to improve the model of the Sun in modern planetary ephemerides. First, the dynamically determined direction of the Sun’s pole is ≈2° off the visible axis of rotation of the Sun’s surface, which is corroborated by present knowledge of the Sun’s interior. Second, the change rate of the Sun’s gravitational parameter is found to be smaller (in absolute value) than the nominal value derived from the estimate of mass loss through radiation and solar wind. Possible interpretations are discussed. Full article

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29 pages, 440 KiB

Open AccessArticle

Analogous Hawking Radiation in Dispersive Media

by Francesco Belgiorno, Sergio L. Cacciatori and Simone Trevisan

Universe 2024, 10(11), 412; https://doi.org/10.3390/universe10110412 - 2 Nov 2024

Abstract

In the framework of the analogous Hawking effect, we significantly improve our previous analysis of the master equation that encompasses very relevant physical systems, like Bose–Einstein condensates (BECs), dielectric media, and water. In particular, we are able to provide two significant improvements to [...] Read more.

In the framework of the analogous Hawking effect, we significantly improve our previous analysis of the master equation that encompasses very relevant physical systems, like Bose–Einstein condensates (BECs), dielectric media, and water. In particular, we are able to provide two significant improvements to the analysis. As our main result, we provide a complete set of connection formulas for both the subluminal and superluminal cases without resorting to suitable boundary conditions, first introduced by Corley, but simply on the grounds of a rigorous mathematical setting. Moreover, we provide an extension to the four-dimensional case, showing explicitly that, apart from obvious changes, adding transverse dimensions does not substantially modify the Hawking temperature in the dispersive case. Furthermore, an important class of exact solutions of the so-called reduced equation that governs the behavior of non-dispersive modes is also provided. Full article

(This article belongs to the Section Field Theory)

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16 pages, 4105 KiB

Open AccessArticle

Structure Formation Through Magnetohydrodynamical Instabilities in Primordial Disks

by Koichi Noguchi, Toshiki Tajima and Wendell Horton

Universe 2024, 10(11), 411; https://doi.org/10.3390/universe10110411 - 31 Oct 2024

Abstract

The shear flow instabilities under the presence of magnetic fields in the primordial disk can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the gravitational Jeans instability. We evaluate the effects of the Parker, magnetorotational [...] Read more.

The shear flow instabilities under the presence of magnetic fields in the primordial disk can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the gravitational Jeans instability. We evaluate the effects of the Parker, magnetorotational and kinematic dynamo instabilities by comparing the properties of these instabilities. We calculate the mass spectra of coagulated density structures by the above mechanism in the radial direction for an axisymmetric magnetohydrodynamic (MHD) torus equilibrium and power density profile models. Our local three-dimensional MHD simulation indicates that the coupling of the Parker and magnetorotational instability creates spiral arms and gas blobs in an accretion disk, reinforcing the theory and model. Such a mechanism for the early structure formation may be tested in a laboratory. The recent progress in experiments involving shear flows in rotating tokamak, field reversed configuration (FRC) and laser plasmas may become a key element to advance in nonlinear studies. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

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23 pages, 3287 KiB

Open AccessArticle

Relational Lorentzian Asymptotically Safe Quantum Gravity: Showcase Model

by Renata Ferrero and Thomas Thiemann

Universe 2024, 10(11), 410; https://doi.org/10.3390/universe10110410 - 31 Oct 2024

Abstract

In a recent contribution, we identified possible points of contact between the asymptotically safe and canonical approaches to quantum gravity. The idea is to start from the reduced phase space (often called relational) formulation of canonical quantum gravity, which provides a reduced (or [...] Read more.

In a recent contribution, we identified possible points of contact between the asymptotically safe and canonical approaches to quantum gravity. The idea is to start from the reduced phase space (often called relational) formulation of canonical quantum gravity, which provides a reduced (or physical) Hamiltonian for the true (observable) degrees of freedom. The resulting reduced phase space is then canonically quantized, and one can construct the generating functional of time-ordered Wightman (i.e., Feynman) or Schwinger distributions, respectively, from the corresponding time-translation unitary group or contraction semigroup, respectively, as a path integral. For the unitary choice, that path integral can be rewritten in terms of the Lorentzian Einstein–Hilbert action plus observable matter action and a ghost action. The ghost action depends on the Hilbert space representation chosen for the canonical quantization and a reduction term that encodes the reduction of the full phase space to the phase space of observables. This path integral can then be treated with the methods of asymptotically safe quantum gravity in its Lorentzian version. We also exemplified the procedure using a concrete, minimalistic example, namely Einstein–Klein–Gordon theory, with as many neutral and massless scalar fields as there are spacetime dimensions. However, no explicit calculations were performed. In this paper, we fill in the missing steps. Particular care is needed due to the necessary switch to Lorentzian signature, which has a strong impact on the convergence of “heat” kernel time integrals in the heat kernel expansion of the trace involved in the Wetterich equation and which requires different cut-off functions than in the Euclidian version. As usual we truncate at relatively low order and derive and solve the resulting flow equations in that approximation. Full article

(This article belongs to the Section Foundations of Quantum Mechanics and Quantum Gravity)

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14 pages, 334 KiB

Open AccessArticle

An Update of the Hypothetical X17 Particle

by Attila J. Krasznahorkay, Attila Krasznahorkay, Margit Csatlós, János Timár, Marcell Begala, Attila Krakó, István Rajta, István Vajda and Nándor J. Sas

Universe 2024, 10(11), 409; https://doi.org/10.3390/universe10110409 - 31 Oct 2024

Abstract

Recently, when examining the differential internal pair creation coefficients of ⁸Be, ⁴He and ¹²C nuclei, we observed peak-like anomalies in the angular correlation of the e⁺e⁻ pairs. This was interpreted as the creation and immediate decay of [...] Read more.

Recently, when examining the differential internal pair creation coefficients of ⁸Be, ⁴He and ¹²C nuclei, we observed peak-like anomalies in the angular correlation of the e⁺e⁻ pairs. This was interpreted as the creation and immediate decay of an intermediate bosonic particle with a mass of

m_{X} c^{2} \approx

17 MeV, receiving the name X17 in subsequent publications. In this paper, our latest results obtained for the X17 particle are presented by investigating the e⁺e⁻ pair correlations in the decay of the Giant Dipole Resonance (GDR) of ⁸Be. Our results initiated a significant number of new experiments all over the world to detect the X17 particle and determine its properties. In this paper, we will also conduct a mini-review of the experiments whose results are already published, as well as the ones closest to being published. Full article

(This article belongs to the Special Issue Multiparticle Dynamics)

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22 pages, 347 KiB

Open AccessArticle

Generalized Schwarzschild Spacetimes with a Linear Term and a Cosmological Constant

by Orchidea Maria Lecian

Universe 2024, 10(11), 408; https://doi.org/10.3390/universe10110408 - 30 Oct 2024

Abstract

Particular Kottler spacetimes are analytically investigated. The investigated spacetimes are spherically symmetric nonrotating spacetimes endowed with a Schwarzschild solid-angle element. SchwarzschildNairiai spacetimes, Schwarzschild spacetimes with a linear term, and Schwarzschild spacetimes with a linear term and a cosmological constant are studied. The infinite-redshift [...] Read more.

Particular Kottler spacetimes are analytically investigated. The investigated spacetimes are spherically symmetric nonrotating spacetimes endowed with a Schwarzschild solid-angle element. SchwarzschildNairiai spacetimes, Schwarzschild spacetimes with a linear term, and Schwarzschild spacetimes with a linear term and a cosmological constant are studied. The infinite-redshift surfaces are analytically written. To this aim, the parameter spaces of the models are analytically investigated, and the conditions for which the analytical radii are reconducted to the physical horizons are used to set and to constrain the parameter spaces. The coordinate-singularity-avoiding coordinate extensions are newly written. Schwarzschild spacetimes with a linear term and a cosmological constant termare analytically studied, and the new singularity-avoiding coordinate extensions are detailed. The new roles of the linear term and of the cosmological constant term in characterizing the Schwarzschild radius are traced. The generalized Schwarzschild–deSitter case and generalized Schwarzschild–anti-deSitter case are characterized in a different manner. The weak field limit is newly recalled. The embeddings are newly provided. The quantum implementation is newly envisaged. The geometrical objects are newly calculated. As a result, for the Einstein field equations, the presence of quintessence is newly excluded. The Birkhoff theorem is newly proven to be obeyed. Full article

(This article belongs to the Section Gravitation)

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34 pages, 5373 KiB

Open AccessReview

Ultra-Low Frequency Waves of Foreshock Origin Upstream and Inside of the Magnetospheres of Earth, Mercury, and Saturn Related to Solar Wind–Magnetosphere Coupling

by Zsofia Bebesi, Navin Kumar Dwivedi, Arpad Kis, Antal Juhász and Balazs Heilig

Universe 2024, 10(11), 407; https://doi.org/10.3390/universe10110407 - 30 Oct 2024

Abstract

This review examines ultra-low frequency (ULF) waves across different planetary environments, focusing on Earth, Mercury, and Saturn. Data from spacecraft missions (CHAMP, Swarm, and Oersted for Earth; MESSENGER for Mercury; and Cassini for Saturn) provide insights into ULF wave dynamics. At Earth, compressional [...] Read more.

This review examines ultra-low frequency (ULF) waves across different planetary environments, focusing on Earth, Mercury, and Saturn. Data from spacecraft missions (CHAMP, Swarm, and Oersted for Earth; MESSENGER for Mercury; and Cassini for Saturn) provide insights into ULF wave dynamics. At Earth, compressional ULF waves, particularly Pc3 waves, show significant power near the equator and peak around Magnetic Local Time (MLT) = 11. These waves interact complexly with Alfvén waves, impacting ionospheric responses and geomagnetic field line resonances. At Mercury, ULF waves transition from circular to linear polarization, indicating resonant interactions influenced by compressional components. MESSENGER data reveal a lower occurrence rate of ULF waves in Mercury’s foreshock compared to Earth’s, attributed to reduced backstreaming protons and lower solar wind Alfvénic Mach numbers, as ULF wave activity increases with heliocentric distance. Short Large-Amplitude Magnetic Structures (SLAMS) observed at Mercury and Saturn show distinct characteristics compared to those of Earth, including the presence of whistler precursos waves. However, due to the large differences in heliospheric distances, SLAMS (their temporal scale size correlate with the ULF wave frequency) at Mercury are significantly shorter in duration than at Earth or Saturn, since the ULF wave frequency primarily depends on the strength of the interplanetary magnetic field. This review highlights the variability of ULF waves and SLAMS across planetary environments, emphasizing Earth’s well-understood ionospheric interactions and the unique behaviours observed for Mercury and Saturn. These findings enhance our understanding of space plasma dynamics and underline the need for further research regarding planetary magnetospheres. Full article

(This article belongs to the Section Space Science)

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