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Is Planckian Discreteness Observable in Cosmology?
Nethotrons: Exploring the Possibility of Measuring Relativistic Spin Precessions, from Earth’s Satellites to the Galactic Centre
Supernovae, by Chandra and XMM-Newton
Bayesian Analysis of Hybrid Neutron Star EOS Constraints Within an Instantaneous Nonlocal Chiral Quark Matter Model

Congratulations to Prof. Roger Penrose, Advisory Board member of Universe, for receiving the Nobel Prize in Physics 2020.

Journal Description

Universe

Universe is a peer-reviewed, open access journal focused on theoretical, experimental, and observational progress in fundamental and applied physics, from circumterrestrial space to cosmological scenarios. Universe is published monthly online by MDPI.

Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: indexed within Scopus, SCIE (Web of Science), Astrophysics Data System, INSPIRE, CAPlus / SciFinder, Inspec, and other databases.
Journal Rank: JCR - Q2 (Astronomy and Astrophysics) / CiteScore - Q2 (General Physics and Astronomy)
Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 22.6 days after submission; acceptance to publication is undertaken in 3.4 days (median values for papers published in this journal in the first half of 2025).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.
Companion journal: Astronomy.

Impact Factor: 2.6 (2024); 5-Year Impact Factor: 2.6 (2024)

Imprint Information Journal Flyer Open Access ISSN: 2218-1997

Latest Articles

12 pages, 549 KiB

Open AccessArticle

Disruption of Planetary System Architectures by Stellar Flybys

by Robert Przyłuski, Hans Rickman, Paweł Wajer, Tomasz Wiśniowski, Diego Turrini, Danae Polychroni, Camilla Danielski, J. M. Diederik Kruijssen, Steven Longmore and Mélanie Chevance

Universe 2025, 11(8), 240; https://doi.org/10.3390/universe11080240 - 22 Jul 2025

We investigate the survivability of solar system-like planetary systems during close encounters in stellar associations using a suite of 1980 N-body simulations. Each system is based on one of the possible five-planet resonant configurations proposed to represent the initial solar system architecture and [...] Read more.

We investigate the survivability of solar system-like planetary systems during close encounters in stellar associations using a suite of 1980 N-body simulations. Each system is based on one of the possible five-planet resonant configurations proposed to represent the initial solar system architecture and is systematically scaled in both planetary mass and orbital compactness to explore the parameter space of observed exoplanetary architectures. Simulations explore a range of stellar encounter scenarios drawn from four distinct cluster environments. Our results show that system survival depends critically on the interplay between planetary mass and orbital scale: compact configurations are more resistant to external perturbations, while increased planetary mass improves resilience only up to a threshold, beyond which internal instabilities dominate. No system whose planets are twice as massive as the ones in the solar system survives stellar encounters. Systems that are at least an order of magnitude more compact than the solar system remain stable under typical encounter conditions. These findings place strong constraints on the initial architectures of planetary systems that can endure stellar-dense birth environments. Full article

(This article belongs to the Section Planetary Sciences)

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30 pages, 25153 KiB

Open AccessArticle

Prospects for Multimessenger Observations of the Shapley Supercluster

by Valentyna Babur, Olexandr Gugnin and Bohdan Hnatyk

Universe 2025, 11(7), 239; https://doi.org/10.3390/universe11070239 - 21 Jul 2025

The Shapley Supercluster, one of the largest and most massive structures in the nearby (redshift

z \leq 0.1

) Universe, located approximately 200 Mpc away, is a unique laboratory for high-energy astrophysics. Galaxy clusters that comprise it are promising targets for [...] Read more.

The Shapley Supercluster, one of the largest and most massive structures in the nearby (redshift

z \leq 0.1

) Universe, located approximately 200 Mpc away, is a unique laboratory for high-energy astrophysics. Galaxy clusters that comprise it are promising targets for multimessenger study due to the presence in the intracluster medium of the necessary conditions for the acceleration of cosmic rays up to ultra-high energies and the generation by them of non-thermal electromagnetic and neutrino emission. Using the Shapley Supercluster’s observational data from the recent eROSITA-DE Data Release, we recover the physical parameters of 45 X-ray luminous galaxy clusters and calculate the expected multiwavelength—from radio to very-high-energy

γ

-ray as well as neutrino emission, with a particular focus on hadronic interactions of accelerated cosmic ray nuclei with the nuclei of the intracluster medium. The results obtained allow verification of cluster models based on multimessenger observations of clusters, especially in

γ

-ray (Fermi-LAT, H.E.S.S., CTAO-South for the Shapley Supercluster case), and neutrino (Ice Cube, KM3NeT). We also estimate the ability of the Shapley Supercluster to manifest as cosmic Zevatrons and show that it can contribute to the PAO Hot Spot in the Cen A region at UHECR energies over 50 EeV. Full article

(This article belongs to the Special Issue Ultra-High-Energy Cosmic Rays)

10 pages, 489 KiB

Open AccessArticle

The Morphological Classification of Galaxy Clusters: Algorithms for Applying the Numerical Criteria

by Elena Panko

Universe 2025, 11(7), 238; https://doi.org/10.3390/universe11070238 - 21 Jul 2025

We summarize the experience of studying 2D features in the galaxy distribution of galaxy cluster fields. For the detailed study of the inner structure of galaxy clusters, algorithms were developed for detecting various types of regular substructures inside such objects automatically. Substructures in [...] Read more.

We summarize the experience of studying 2D features in the galaxy distribution of galaxy cluster fields. For the detailed study of the inner structure of galaxy clusters, algorithms were developed for detecting various types of regular substructures inside such objects automatically. Substructures in galaxy clusters arise from interactions as well as the evolution of the cosmic web, but cannot be described according to the schemes of morphological classification, both classical and modern, because some regular substructures are not present. Our algorithms are based on numerical criteria that permit the determination of classical morphological types, connected with parameters such as the degree of concentration to the cluster center and/or to a straight line, on a statistically significant level. Other types of substructures can also be detected with corresponding algorithms. As a result, we can analyze intracluster features, such as crosses, semi-crosses, complex crosses, and compact dense chains. All algorithms are realized in the “Cluster Cartography” tool and can be used with data taken from different catalogs. The algorithms and their realization in program code must simplify, standardize, and speed up the analysis of 2D distributions of galaxies in clusters. It is possible in future to adapt the algorithms for the 3D case. The results of statistically valid morphological classification are useful for studies of the evolution of galaxy clusters. Full article

(This article belongs to the Section Galaxies and Clusters)

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21 pages, 648 KiB

Open AccessArticle

Structural Implications of the Chameleon Mechanism on White Dwarfs

by Joan Bachs-Esteban, Ilídio Lopes and Javier Rubio

Universe 2025, 11(7), 237; https://doi.org/10.3390/universe11070237 - 20 Jul 2025

We study the behaviour of the chameleon mechanism around white dwarfs and its impact on their structure. Using a shooting method of our own design, we solve the corresponding scalar–tensor equilibrium equations for a Chandrasekhar equation of state, exploring various energy scales and [...] Read more.

We study the behaviour of the chameleon mechanism around white dwarfs and its impact on their structure. Using a shooting method of our own design, we solve the corresponding scalar–tensor equilibrium equations for a Chandrasekhar equation of state, exploring various energy scales and couplings of the chameleon field to matter. For the considered parameter ranges, we find the chameleon field to be in a thick-shell configuration, identifying for the first time in the literature a similarity relation of the theory for the radially normalised scalar field gradient. Our analysis reveals that the chameleon mechanism alters the pressure gradient of white dwarfs, leading to a reduction in the stellar radii and masses and shifting the mass–radius curves below those predicted by Newtonian gravity. This also lowers the specific heat of white dwarfs, accelerating their cooling process. Finally, we derive parametric expressions from our results to expedite future analyses of white dwarfs in scalar–tensor theories. Full article

(This article belongs to the Section Compact Objects)

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23 pages, 556 KiB

Open AccessReview

Evolving Wormholes in a Cosmological Background

by Mahdi Kord Zangeneh and Francisco S. N. Lobo

Universe 2025, 11(7), 236; https://doi.org/10.3390/universe11070236 - 19 Jul 2025

Wormholes are non-trivial topological structures that arise as exact solutions to Einstein’s field equations, theoretically connecting distinct regions of spacetime via a throat-like geometry. While static traversable wormholes necessarily require exotic matter that violates the classical energy conditions, subsequent studies have sought to [...] Read more.

Wormholes are non-trivial topological structures that arise as exact solutions to Einstein’s field equations, theoretically connecting distinct regions of spacetime via a throat-like geometry. While static traversable wormholes necessarily require exotic matter that violates the classical energy conditions, subsequent studies have sought to minimize such violations by introducing time-dependent geometries embedded within cosmological backgrounds. This review provides a comprehensive survey of evolving wormhole solutions, emphasizing their formulation within both general relativity and alternative theories of gravity. We explore key developments in the construction of non-static wormhole spacetimes, including those conformally related to static solutions, as well as dynamically evolving geometries influenced by scalar fields. Particular attention is given to the wormholes embedded into Friedmann–Lemaître–Robertson–Walker (FLRW) universes and de Sitter backgrounds, where the interplay between the cosmic expansion and wormhole dynamics is analyzed. We also examine the role of modified gravity theories, especially in hybrid metric–Palatini gravity, which enable the realization of traversable wormholes supported by effective stress–energy tensors that do not violate the null or weak energy conditions. By systematically analyzing a wide range of time-dependent wormhole solutions, this review identifies the specific geometric and physical conditions under which wormholes can evolve consistently with null and weak energy conditions. These findings clarify how such configurations can be naturally integrated into cosmological models governed by general relativity or modified gravity, thereby contributing to a deeper theoretical understanding of localized spacetime structures in an expanding universe. Full article

(This article belongs to the Special Issue Experimental and Observational Constraints on Wormhole Models)

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20 pages, 2059 KiB

Open AccessArticle

Deep Learning Spinfoam Vertex Amplitudes: The Euclidean Barrett–Crane Model

by Hanno Sahlmann and Waleed Sherif

Universe 2025, 11(7), 235; https://doi.org/10.3390/universe11070235 - 19 Jul 2025

Spinfoam theories propose a well-defined path-integral formulation for quantum gravity, and it is hoped that they will provide the dynamics of loop quantum gravity. However, it is computationally hard to calculate spinfoam amplitudes. The well-studied Euclidean Barrett–Crane model provides an excellent setting for [...] Read more.

Spinfoam theories propose a well-defined path-integral formulation for quantum gravity, and it is hoped that they will provide the dynamics of loop quantum gravity. However, it is computationally hard to calculate spinfoam amplitudes. The well-studied Euclidean Barrett–Crane model provides an excellent setting for testing analytical and numerical tools to probe spinfoam models. We explore a data-driven approach to accelerating spinfoam computations by showing that the vertex amplitude is an object that can be learned from data using deep learning. We divide the learning process into a classification and a regression task: Two networks are independently engineered to decide whether the amplitude is zero or not and to predict the precise numerical value, respectively. The trained networks are tested with several accuracy measures. The classifier in particular demonstrates robust generalisation far outside the training domain, while the regressor demonstrates high predictive accuracy in the domain it is trained on. We discuss limitations, possible improvements, and implications for future work. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, Second Edition)

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12 pages, 5751 KiB

Open AccessArticle

Chaos of Charged Particles in Quadrupole Magnetic Fields Under Schwarzschild Backgrounds

by Qihan Zhang and Xin Wu

Universe 2025, 11(7), 234; https://doi.org/10.3390/universe11070234 - 16 Jul 2025

A four-vector potential of an external test electromagnetic field in a Schwarzschild background is described in terms of a combination of dipole and quadrupole magnetic fields. This combination is an interior solution of the source-free Maxwell equations. Such external test magnetic fields cause [...] Read more.

A four-vector potential of an external test electromagnetic field in a Schwarzschild background is described in terms of a combination of dipole and quadrupole magnetic fields. This combination is an interior solution of the source-free Maxwell equations. Such external test magnetic fields cause the dynamics of charged particles around the black hole to be nonintegrable, and are mainly responsible for chaotic dynamics of charged particles. In addition to the external magnetic fields, some circumstances should be required for the onset of chaos. The effect of the magnetic fields on chaos is shown clearly through an explicit symplectic integrator and a fast Lyapunov indicator. The inclusion of the quadrupole magnetic fields easily induces chaos, compared with that of the dipole magnetic fields. This result is because the Lorentz forces from the quadrupole magnetic fields are larger than those from the dipole magnetic fields. In addition, the Lorentz forces act as attractive forces, which are helpful for bringing the occurrence of chaos in the nonintegrable case. Full article

(This article belongs to the Special Issue Celebrating the 110th Anniversary of General Relativity: Advances, Challenges and Perspectives)

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10 pages, 244 KiB

Open AccessTechnical Note

Dirac Brackets and Time-Dependent Constraints

by Nuno Barros e Sá

Universe 2025, 11(7), 233; https://doi.org/10.3390/universe11070233 - 16 Jul 2025

We provide a simultaneous derivation of the Dirac bracket and of the equations of motion for second-class constrained systems when the constraints are time-dependent. The necessity of time-dependent gauge-fixing conditions is shown in the example of parameterized mechanics and is illustrated geometrically. Full article

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

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22 pages, 332 KiB

Open AccessEssay

On the Metric Lorentz Invariant Newtonian Cosmology

by Jaume de Haro

Universe 2025, 11(7), 232; https://doi.org/10.3390/universe11070232 - 15 Jul 2025

We review a metric theory of gravitation that combines Newtonian gravity with Lorentz invariance. Beginning with a conformastatic metric justified by the Weak Equivalence Principle. We describe, within the Newtonian approximation, the spacetime geometry generated by a static distribution of dust matter. To [...] Read more.

We review a metric theory of gravitation that combines Newtonian gravity with Lorentz invariance. Beginning with a conformastatic metric justified by the Weak Equivalence Principle. We describe, within the Newtonian approximation, the spacetime geometry generated by a static distribution of dust matter. To extend this description to moving sources, we apply a Lorentz transformation to the static metric. This procedure yields, again within the Newtonian approximation, the metric associated with moving bodies. In doing so, we construct a gravitational framework that captures key relativistic features—such as covariance under Lorentz transformations—while remaining rooted in Newtonian dynamics. This approach offers an alternative route to describing weak-field gravitational interactions, without relying directly on Einstein’s field equations. Full article

(This article belongs to the Section Gravitation)

16 pages, 4520 KiB

Open AccessReview

SN 2023ixf: The Closest Supernova of the Decade

by Wynn Jacobson-Galán

Universe 2025, 11(7), 231; https://doi.org/10.3390/universe11070231 - 15 Jul 2025

Supernova 2023ixf occurred on 18 May 2023 in the nearby galaxy Messier 101 (

D \approx 6.85

Mpc), making it the closest supernova in the last decade. Following its discovery, astronomers around the world rushed to observe the explosion across the electromagnetic spectrum [...] Read more.

Supernova 2023ixf occurred on 18 May 2023 in the nearby galaxy Messier 101 (

D \approx 6.85

Mpc), making it the closest supernova in the last decade. Following its discovery, astronomers around the world rushed to observe the explosion across the electromagnetic spectrum in order to uncover its early-time properties. Based on multi-wavelength analysis during its first year after explosion, Supernova 2023ixf is a type II supernova that interacted with dense, confined circumstellar material in its local environment—this material being lost from its red supergiant progenitor in the final years before explosion. In this article, we will review the findings of >80 studies already published on this incredible event and explore how the synthesis of SN 2023ixf observations across the electromagnetic spectrum can be used to constrain type II supernova explosion physics in addition to the uncertain mass loss histories of red supergiant stars in their final years. Full article

(This article belongs to the Special Issue A Multiwavelength View of Supernovae)

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19 pages, 349 KiB

Open AccessArticle

Finite Time Path Field Theory and a New Type of Universal Quantum Spin Chain Quench Behavior

by Domagoj Kuić, Alemka Knapp and Diana Šaponja-Milutinović

Universe 2025, 11(7), 230; https://doi.org/10.3390/universe11070230 - 11 Jul 2025

We discuss different quench protocols for Ising and XY spin chains in a transverse magnetic field. With a sudden local magnetic field quench as a starting point, we generalize our approach to a large class of local non-sudden quenches. Using finite time path [...] Read more.

We discuss different quench protocols for Ising and XY spin chains in a transverse magnetic field. With a sudden local magnetic field quench as a starting point, we generalize our approach to a large class of local non-sudden quenches. Using finite time path field theory (FTPFT) perturbative methods, we show that the difference between the sudden quench and a class of quenches with non-sudden switching on the perturbation vanishes exponentially with time, apart from non-substantial modifications that are systematically accounted for. As the consequence of causality and analytic properties of functions describing the discussed class of quenches, this is true at any order of perturbation expansion and thus for the resummed perturbation series. The only requirements on functions describing the perturbation strength switched on at a finite time

t = 0

are as follows: (1) their Fourier transform

f (p)

is a function that is analytic everywhere in the lower complex semiplane, except at the simple pole at

p = 0

and possibly others with

ℑ (p) < 0

; and (2)

f (p) / p

converges to zero at infinity in the lower complex semiplane. A prototypical function of this class is

tanh (η t)

, to which the perturbation strength is proportional after the switching at time

t = 0

. In the limit of large

η

, such a perturbation approaches the case of a sudden quench. It is shown that, because of this new type of universal behavior of Loschmidt echo (LE) that emerges in an exponentially short time scale, our previous results for the sudden local magnetic field quench of Ising and XY chains, obtained by the resummation of the perturbative expansion, extend in the long-time limit to all non-sudden quench protocols in this class, with non-substantial modifications systematically taken into account. We also show that analogous universal behavior exists in disorder quenches, and ultimately global ones. LE is directly connected to the work probability distribution, and the described universal behavior is therefore appropriate in potential concepts of quantum technology related to spin chains. Full article

(This article belongs to the Section Field Theory)

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26 pages, 2204 KiB

Open AccessReview

Recent Advances in Understanding R-Process Nucleosynthesis in Metal-Poor Stars and Stellar Systems

by Avrajit Bandyopadhyay and Timothy C. Beers

Universe 2025, 11(7), 229; https://doi.org/10.3390/universe11070229 - 11 Jul 2025

The rapid neutron-capture process (r-process) is responsible for the creation of roughly half of the elements heavier than iron, including precious metals like silver, gold, and platinum, as well as radioactive elements such as thorium and uranium. Despite its importance, the [...] Read more.

The rapid neutron-capture process (r-process) is responsible for the creation of roughly half of the elements heavier than iron, including precious metals like silver, gold, and platinum, as well as radioactive elements such as thorium and uranium. Despite its importance, the nature of the astrophysical sites where the r-process occurs, and the detailed mechanisms of its formation, remain elusive. The key to resolving these mysteries lies in the study of chemical signatures preserved in ancient, metal-poor stars. These stars, which formed in the early Universe, retain the chemical fingerprints of early nucleosynthetic events and offer a unique opportunity to trace the origins of r-process elements in the early Galaxy. In this review, we explore the state-of-the-art understanding of r-process nucleosynthesis, focusing on the sites, progenitors, and formation mechanisms. We discuss the role of potential astrophysical sites such as neutron star mergers, core-collapse supernovae, magneto-rotational supernovae, and collapsars, that can play a key role in producing the heavy elements. We also highlight the importance of studying these signatures through high-resolution spectroscopic surveys, stellar archaeology, and multi-messenger astronomy. Recent advancements, such as the gravitational wave event GW170817 and detection of the r-process in the ejecta of its associated kilonovae, have established neutron star mergers as one of the confirmed sites. However, questions remain regarding whether they are the only sites that could have contributed in early epochs or if additional sources are needed to explain the signatures of r-process found in the oldest stars. Additionally, there are strong indications pointing towards additional sources of r-process-rich nuclei in the context of Galactic evolutionary timescales. These are several of the outstanding questions that led to the formation of collaborative efforts such as the R-Process Alliance, which aims to consolidate observational data, modeling techniques, and theoretical frameworks to derive better constraints on deciphering the astrophysical sites and timescales of r-process enrichment in the Galaxy. This review summarizes what has been learned so far, the challenges that remain, and the exciting prospects for future discoveries. The increasing synergy between observational facilities, computational models, and large-scale surveys is poised to transform our understanding of r-process nucleosynthesis in the coming years. Full article

(This article belongs to the Special Issue Advances in Nuclear Astrophysics)

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22 pages, 388 KiB

Open AccessArticle

Gauge-Invariant Slavnov–Taylor Decomposition for Trilinear Vertices

by Andrea Quadri

Universe 2025, 11(7), 228; https://doi.org/10.3390/universe11070228 - 11 Jul 2025

We continue the analysis of the gauge-invariant decomposition of amplitudes in spontaneously broken massive gauge theories by performing the characterization of separately gauge-invariant subsectors for amplitudes involving trilinear interaction vertices for an Abelian theory with chiral fermions. We show that the use of [...] Read more.

We continue the analysis of the gauge-invariant decomposition of amplitudes in spontaneously broken massive gauge theories by performing the characterization of separately gauge-invariant subsectors for amplitudes involving trilinear interaction vertices for an Abelian theory with chiral fermions. We show that the use of Frohlich–Morchio–Strocchi gauge-invariant dynamical (i.e., propagating inside loops) fields yields a very powerful handle on the cancellations among unphysical degrees of freedom (the longitudinal mode of the massive gauge field, the Goldstone scalar and the ghosts). The resulting cancellations are encoded into separate Slavnov–Taylor invariant sectors for 1-PI amplitudes. The construction works to all orders in perturbation theory. This decomposition suggests a novel strategy for the determination of finite counter-terms required to restore the Slavnov–Taylor identities in chiral theories in the absence of an invariant regularization scheme. Full article

(This article belongs to the Section Field Theory)

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15 pages, 914 KiB

Open AccessArticle

Spectral and Photometric Studies of NGC 7469 in the Optical Range

by Saule Shomshekova, Inna Reva, Ludmila Kondratyeva, Nazim Huseynov, Vitaliy Kim and Laura Aktay

Universe 2025, 11(7), 227; https://doi.org/10.3390/universe11070227 - 10 Jul 2025

The galaxy NGC 7469 is a bright infrared source with an active galactic nucleus (AGN) and an intense star-forming region with a radius of approximately 500 parsecs, where the star formation rate is estimated to be 20–50

M_{⊙} {yr}^{- 1}

. [...] Read more.

The galaxy NGC 7469 is a bright infrared source with an active galactic nucleus (AGN) and an intense star-forming region with a radius of approximately 500 parsecs, where the star formation rate is estimated to be 20–50

M_{⊙} {yr}^{- 1}

. This study presents the results of spectral and photometric observations carried out during the period from 2020 to 2024 at the Fesenkov Astrophysical Institute (Almaty, Kazakhstan) and the Nasreddin Tusi Shamakhy Astrophysical Observatory (Shamakhy, Azerbaijan). Photometric data were obtained using B, V, and Rc filters, while spectroscopic observations covered the wavelength range of

λ

4000–7000 Å. Data reduction was performed using the IRAF and MaxIm DL Pro6 software packages. An analysis of the light curves revealed that after the 2019–2020 outburst, the luminosity level of NGC 7469 remained relatively stable until the end of 2024. In November–December 2024, an increase in brightness (∼0.3–0.5 magnitudes) was recorded. Spectral data show variations in the Ha fluxes and an enhancement of them at the end of 2024. On BPT diagrams, the emission line flux ratios [OIII]/H

β

and [NII]/H

α

place NGC 7469 on the boundary between regions dominated by different ionization sources: AGN and star-forming regions. The electron density of the gas, estimated from the intensity ratios of the [SII] 6717, 6731 Ålines, is about

900 - 1000 {cm}^{- 3}

. Continued observations will help to determine whether the trend of increasing brightness and emission line fluxes recorded at the end of 2024 will persist. Full article

(This article belongs to the Special Issue 10th Anniversary of Universe: Galaxies and Their Black Holes)

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21 pages, 1200 KiB

Open AccessArticle

On the Role of Abrupt Solar Wind Pressure Changes in Forbidden Energetic Electron Enhancements

by Alla V. Suvorova and Alexei V. Dmitriev

Universe 2025, 11(7), 226; https://doi.org/10.3390/universe11070226 - 9 Jul 2025

The sudden increase of fluxes of quasi-trapped energetic electrons under the Earth’s radiation belt (ERB) has remained a puzzling phenomenon for decades. It is known as enhancements of forbidden energetic electrons (FEEs). The FEE enhancements are occasionally observed by low-Earth orbit NOAA/POES satellites. [...] Read more.

The sudden increase of fluxes of quasi-trapped energetic electrons under the Earth’s radiation belt (ERB) has remained a puzzling phenomenon for decades. It is known as enhancements of forbidden energetic electrons (FEEs). The FEE enhancements are occasionally observed by low-Earth orbit NOAA/POES satellites. Previously, no strong correlation was established between FEEs and geomagnetic activity, while external control of FEE occurrence by solar activity and interplanetary parameters was revealed on a long time-scale. Two important questions are still open: (1) key parameters of the mechanism and (2) solar wind drivers or triggers. In the present study we conducted detailed analysis of three FEE events that occurred during the greatest geomagnetic storms, which dramatically affected space weather. The FEE enhancements occurred under northward IMF and, thus, Bz and convection electric fields could have been neither driver nor trigger. We found that an abrupt and significant change in solar wind pressure is a key solar wind driver of the FEE enhancements observed. The characteristic time of FEE injection from the inner edge of the ERB at L-shell 1.2 to the forbidden zone at L < 1.1 was estimated to be 10–20 min. In the mechanism of ExB drift, this characteristic time corresponds to the radial inward transport of electrons caused by a transient electric field with the magnitude ~10 mV/m. Full article

(This article belongs to the Special Issue Measurements, Observations and Theoretical Studies on the Solar Magnetic Field—Celebrating the 40th Anniversary of the Huairou Solar Observing Station)

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12 pages, 694 KiB

Open AccessArticle

Unveiling New Physics Models Through Meson Decays and Their Impact on Neutrino Experiments

by Adriano Cherchiglia

Universe 2025, 11(7), 225; https://doi.org/10.3390/universe11070225 - 9 Jul 2025

As neutrino experiments enter the precision era, it is desirable to identify any deviation between data and theoretical predictions and to provide possible models as explanation. Particularly useful is the description in terms of non-standard interactions (NSIs), which can be related to neutral [...] Read more.

As neutrino experiments enter the precision era, it is desirable to identify any deviation between data and theoretical predictions and to provide possible models as explanation. Particularly useful is the description in terms of non-standard interactions (NSIs), which can be related to neutral (NC-NSI) or charged (CC-NSI) currents. Previously, we have developed the code eft-neutrino that connects NSI with generic ultraviolet (UV) models at tree-level matching. In this work, we integrate our code with other tools, increasing the matching between the UV and infrared (IR) theories to a one-loop level. As a working example, we consider the pion and kaon decay, the main production mechanisms in accelerator neutrino experiments. We provide up-to-date allowed regions on a set of Wilson coefficients related to pion and kaon decay. We also illustrate how our chain of codes can be applied to particular UV models, showing that a seemingly large allowed CC-NSI value can be significantly reduced when considering a specific UV model. Full article

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

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35 pages, 715 KiB

Open AccessArticle

Properties of the Object HESS J1731-347 as a Twin Compact Star

by David E. Alvarez-Castillo

Universe 2025, 11(7), 224; https://doi.org/10.3390/universe11070224 - 5 Jul 2025

By consideration of the compact object HESS J1731-347 as a hybrid twin compact star, i.e., a more compact star than its hadronic twin of the same mass, its stellar properties are derived. In addition to showing that the properties of compact stars in [...] Read more.

By consideration of the compact object HESS J1731-347 as a hybrid twin compact star, i.e., a more compact star than its hadronic twin of the same mass, its stellar properties are derived. In addition to showing that the properties of compact stars in this work are in good agreement with state-of-the-art constraints both from measurements carried out in laboratory experiments as well as by multi-messenger astronomy observations, the realization of an early strong hadron–quark first-order phase transition as implied by the twins is discussed. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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14 pages, 281 KiB

Open AccessArticle

Leading Logarithm Quantum Gravity

by S. P. Miao, N. C. Tsamis and R. P. Woodard

Universe 2025, 11(7), 223; https://doi.org/10.3390/universe11070223 - 4 Jul 2025

The continual production of long wavelength gravitons during primordial inflation endows graviton loop corrections with secular growth factors. During a prolonged period of inflation, these factors eventually overwhelm the small loop-counting parameter of

G H^{2}

, causing perturbation theory to break down. [...] Read more.

The continual production of long wavelength gravitons during primordial inflation endows graviton loop corrections with secular growth factors. During a prolonged period of inflation, these factors eventually overwhelm the small loop-counting parameter of

G H^{2}

, causing perturbation theory to break down. A technique was recently developed for summing the leading secular effects at each order in non-linear sigma models, which possess the same kind of derivative interactions as gravity. This technique combines a variant of Starobinsky’s stochastic formalism with a variant of the renormalization group. Generalizing the technique to quantum gravity is a two-step process, the first of which is the determination of the gauge fixing condition that will allow this summation to be realized; this is the subject of this paper. Moreover, we briefly discuss the second step, which shall obtain the Langevin equation, in which secular changes in gravitational phenomena are driven by stochastic fluctuations of the graviton field. Full article

(This article belongs to the Special Issue Loop Quantum Gravity and Non-Perturbative Approaches to Quantum Cosmology, Second Edition)

18 pages, 6896 KiB

Open AccessArticle

Relationship Between Recurrent Magnetic Flux Rope and Moving Magnetic Features

by Yin Zhang, Jihong Liu, Quan Wang, Suo Liu, Jing Huang, Jie Chen and Baolin Tan

Universe 2025, 11(7), 222; https://doi.org/10.3390/universe11070222 - 3 Jul 2025

Large-scale magnetic flux ropes (MFRs) usually become visible during an eruption and are the core structures of coronal mass ejections, but the nature of MFRs is still a mystery. Here, we identify a large transequatorial MFR that spans across NOAA 13373 (in the [...] Read more.

Large-scale magnetic flux ropes (MFRs) usually become visible during an eruption and are the core structures of coronal mass ejections, but the nature of MFRs is still a mystery. Here, we identify a large transequatorial MFR that spans across NOAA 13373 (in the Northern Hemisphere) and NOAA 13374 (in the Southern Hemisphere). Here, NOAA 13373 is a growing, newly emerging active region with a leading sunspot moving rapidly to the southwest, and it is surrounded by a highly dynamic moving magnetic feature (MMF), while NOAA 13374 is a decaying active region with a tiny leading negative sunspot and a large fading area. Recurrent reconnection, which occurs under the MFRs around the leading sunspot of NOAA 13373, results in local energy release, appearing as local EUV brightening, and it is related to the appearance of a transequatorial MFR. The appearance of this MFR involves several stages: EUV brightening, the slow rising and expansion of the MFR and its hosted filament, and, eventually, fading and shrinking. These observations demonstrate that a large-scale MFR can exist for a long-term period and that MMFs play a key role in building up free energy and triggering small-scale reconnections in the lower atmosphere. The energy released by these reconnection events is insufficient for triggering the eruption of an MFR but results in local disturbances. Full article

(This article belongs to the Special Issue Measurements, Observations and Theoretical Studies on the Solar Magnetic Field—Celebrating the 40th Anniversary of the Huairou Solar Observing Station)

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Open AccessArticle

Tidal Forces Around Black-Bounce-Reissner–Nordström Black Hole

by Rashmi Uniyal

Universe 2025, 11(7), 221; https://doi.org/10.3390/universe11070221 - 2 Jul 2025

The central singularity present in black hole (BH) spacetimes arising in the general theory of relativity (GR) can be avoided by using various methods. In the present work we have investigated the gravitational effect of one of such spacetime known as a black-bounce-Reissner–Nordström [...] Read more.

The central singularity present in black hole (BH) spacetimes arising in the general theory of relativity (GR) can be avoided by using various methods. In the present work we have investigated the gravitational effect of one of such spacetime known as a black-bounce-Reissner–Nordström spacetime. We revisited its horizon structure along with first integrals of its geodesic equations. We derived the expressions for Newtonian radial acceleration for freely infalling neutral test particles. For the description of tidal effects, the geodesic deviation equations are derived and solved analytically as well as numerically. To be specific, in the numerical approach, we have opted for two initial conditions to elaborate on the evolution of geodesic deviation vectors in radial and angular directions. The corresponding nature of geodesic deviation vectors in radial and angular directions is then compared with the standard results such as Schwarzschild and Reissner–Nordström BHs in order to figure out the differences. Full article

(This article belongs to the Special Issue Recent Advances in Gravitational Lensing and Galactic Dynamics)

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