Universe

33 pages, 5916 KB

Open AccessArticle

Spectroscopic Observations and Emission-Line Diagnoses for H ii Regions in the Late-Type Spiral Galaxy NGC 2403

by Qi-Ming Wu, Ye-Wei Mao, Lin Lin, Hu Zou and Shu-Ting Wang

Universe 2025, 11(8), 280; https://doi.org/10.3390/universe11080280 - 21 Aug 2025

Viewed by 182

Being ionized nebulae where star formation events take place, H ii regions are not only natural laboratories for studying physical processes of star formation and photoionization but also signatures reflecting evolution of their internal stellar populations and hosting galaxies. In this paper, we [...] Read more.

Being ionized nebulae where star formation events take place, H ii regions are not only natural laboratories for studying physical processes of star formation and photoionization but also signatures reflecting evolution of their internal stellar populations and hosting galaxies. In this paper, we present a comprehensive analysis of spectral emission-line data for H ii regions in the nearby spiral galaxy NGC 2403, aimed at gaining deep insight into underlying properties and evolution for the H ii regions and the galaxy. The spectroscopic data are obtained through observations with the 2.16 m telescope at National Astronomical Observatories of China and a collection of published data in the literature. Photoionization modeling is combined in the analysis for diagnosing the spectral features and interpreting the observational data with certain physical mechanisms. Results of this work not only involve estimates of a set of parameters such as metallicity, the ionization parameter, etc., and evolution stages for the H ii regions in NGC 2403 but also reveal distinct characteristics of different spectral features and their sensitivities to specific parameters, which provides an instructive implication for proper usages of emission-line diagnostics for H ii regions or galaxies nearby and far away. Full article

(This article belongs to the Special Issue New Discoveries in Astronomical Data)

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20 pages, 594 KB

Open AccessArticle

Phantom Dark Energy Behavior in Weyl Type f(Q,T) Gravity Models with Observational Constraints

by Anirudh Pradhan, Mohammad Zeyauddin, Archana Dixit and Kamal Ghaderi

Universe 2025, 11(8), 279; https://doi.org/10.3390/universe11080279 - 20 Aug 2025

Viewed by 182

Abstract

This study explores the behavior of phantom dark energy within the framework of Weyl-type

f (Q, T)

gravity, considering a spatially flat FLRW universe under observational constraints. The field equations are analytically solved for a dust-like fluid source. To determine [...] Read more.

This study explores the behavior of phantom dark energy within the framework of Weyl-type

f (Q, T)

gravity, considering a spatially flat FLRW universe under observational constraints. The field equations are analytically solved for a dust-like fluid source. To determine the present values of the model parameters, we utilize observational data from the Hubble parameter measurements via cosmic chronometers (CC) and the apparent magnitude data from the Pantheon compilation of Type Ia supernovae (SNe Ia). With these obtained parameter values, we analyze the model’s physical characteristics by evaluating the effective and dark energy equation of state parameters

ω_{e f f}

and

ω_{d e}

, the deceleration parameter

q (z)

, and energy conditions. Additionally, we conduct the

O_{m}

diagnostic test for the model. We estimate the transition redshift

z_{t} \leq 0.5342, 0.6334

and the present age of the universe

t_{0} = 13.46, 13.49

Gyrs with

H_{0} = 67.4 \pm 3.6, 68.8 \pm 1.9

Km/s/Mpc,

Ω_{m 0} = {0.41}_{- 0.24}^{+ 0.13}, {0.299}_{- 0.077}^{+ 0.042}

, and

ω_{e f f} = - 0.6447, - 0.696

,

ω_{d e} = - 1.0347, - 1.0284

. We find a transit phase accelerating and physically acceptable phantom dark energy model of the universe. Full article

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15 pages, 542 KB

Open AccessFeature PaperArticle

The Effects of the Gravitational Coupling Variation on the Local H₀ Estimation

by Antonio Enea Romano

Universe 2025, 11(8), 278; https://doi.org/10.3390/universe11080278 - 19 Aug 2025

Viewed by 238

Abstract

We study the effects of the time evolution of the matter-gravity coupling on the luminosity distance, showing it can provide a natural explanation to the apparent Hubble tension. The gravitational coupling evolution induces a modification of the Friedman equation with respect to the [...] Read more.

We study the effects of the time evolution of the matter-gravity coupling on the luminosity distance, showing it can provide a natural explanation to the apparent Hubble tension. The gravitational coupling evolution induces a modification of the Friedman equation with respect to the

Λ

CDM model, which we study in both the Einstein and Jordan frame. We consider a phenomenological parametrization of the low redshift variation of the coupling in a narrow redshift shell, showing how it can affect the distance of the anchors used to calibrate supernovae (SNe), while higher redshift background observations are not affected. This effect is purely geometrical, and it is not related to any change of the intrinsic SNe physical properties. The effects of a time varying gravity coupling only manifest on sufficiently long time scales, such as in cosmological observations at different redshifts, and if ignored lead to apparent tensions in the values of cosmological parameters estimated with observations from different epochs of the Universe history. Full article

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16 pages, 332 KB

Open AccessArticle

The Influence of Zero-Point Fluctuations on the Photon Wave Packet Motion in a Vacuum

by S. L. Cherkas and V. L. Kalashnikov

Universe 2025, 11(8), 277; https://doi.org/10.3390/universe11080277 - 19 Aug 2025

Viewed by 256

Abstract

The influence of zero-point fluctuations on photon propagation in a vacuum is investigated without using normal ordering and renormalization procedures, but in a frame of the conformally unimodular metric for a description of the fluctuating gravitational field. The complete formula for decoherence time [...] Read more.

The influence of zero-point fluctuations on photon propagation in a vacuum is investigated without using normal ordering and renormalization procedures, but in a frame of the conformally unimodular metric for a description of the fluctuating gravitational field. The complete formula for decoherence time is presented. Full article

(This article belongs to the Special Issue Quantum Gravity Phenomenology: Insights and Advances)

39 pages, 713 KB

Open AccessTutorial

An Undergraduate Approach to the Quantum Hadrodynamics and Physics of Neutron Stars

by Luiz L. Lopes

Universe 2025, 11(8), 276; https://doi.org/10.3390/universe11080276 - 18 Aug 2025

Viewed by 262

Abstract

In this tutorial, I discuss how to model a neutron star from the Quantum Hadrodynamics microscopic approach. After a brief discussion about hydrostatic equilibrium, I discuss the role of each meson of the model and how to calculate the corresponding equation of state [...] Read more.

In this tutorial, I discuss how to model a neutron star from the Quantum Hadrodynamics microscopic approach. After a brief discussion about hydrostatic equilibrium, I discuss the role of each meson of the model and how to calculate the corresponding equation of state and the expected values. Each meson is introduced individually. Its effects are analyzed from both an analytical and a numerical point of view. To explicitly show the effects of a given meson, the coupling constant is varied in an arbitrary range before being fixed to reproduce well-known constraints. This work is intended for late undergraduate students as well as early graduate students. The equation of states is obtained from the statistical mechanics formalism, which is more familiar to students at this stage of their research career, instead of the traditional quantum field theory formalism. Full article

(This article belongs to the Section Compact Objects)

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19 pages, 989 KB

Open AccessArticle

The Efficiency of Mass Accretion and Disc Structure from the Stellar Wind Mass Transfer in Binary Systems

by Bushra Ata Ali, Hastyar Omar Mohammed and Peshwaz Abdulkareem Abdoul

Universe 2025, 11(8), 275; https://doi.org/10.3390/universe11080275 - 18 Aug 2025

Viewed by 248

Abstract

There have been many research works involving mass transfer in stellar binaries, all of which are limited to certain systems with specific binary parameters. In this work, we use three-dimensional smoothed particle (3D-SPH) simulations to explore the impact of binary mass ratio and [...] Read more.

There have been many research works involving mass transfer in stellar binaries, all of which are limited to certain systems with specific binary parameters. In this work, we use three-dimensional smoothed particle (3D-SPH) simulations to explore the impact of binary mass ratio and wind speed on the fraction of mass transferred to the accreting companion and the structure of accretion discs. We examine all possible cases of binary mass ratios as well as different conditions of wind speed in the vicinity of the accretor. We adhere to thermally driven winds, with sound speed being the main parameter, in which transonic stellar winds expand in the binary medium. We find that mass accretion fraction is close to unity for slow winds. However, fast winds lead to mass accretion fraction of thousandths which agree very well with the Bondi–Hoyle estimates. Mass accretion fraction is found to be the largest when the mass ratio is unity. Our results show that an increase in either sound speed or binary mass ratio leads to decrease in accretion disc size. In most cases, the disc shifts from being circular. These results would allow us to estimate the rate of mass accretion and the structure of accretion discs in any type of stellar binaries. Full article

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41 pages, 882 KB

Open AccessArticle

D-Branes, AdS/CFT, Dynamical Uhlmann Gauge, and Stabilisation of a Closed Causal Loop Geometry

by Andrei T. Patrascu

Universe 2025, 11(8), 274; https://doi.org/10.3390/universe11080274 - 17 Aug 2025

Viewed by 372

Abstract

I show here that if we construct D-branes not in the form of infinite superpositions of string modes, in order to satisfy the technical condition of coherence by means of eigenstates of annihilation operators, but instead insist on an approximate but much more [...] Read more.

I show here that if we construct D-branes not in the form of infinite superpositions of string modes, in order to satisfy the technical condition of coherence by means of eigenstates of annihilation operators, but instead insist on an approximate but much more physical and practical definition based on phase coherence, we obtain finite (and hence realistic) superpositions of string modes that would form realistic D-branes that would encode (at least as a semiclassical approximation) various quantum properties. Re-deriving the AdS/CFT duality by starting in the pre-Maldacena limit from such realistic D-branes would lead to quantum properties on the AdS side of the duality. Causal structures can be modified in various many-particle systems, including strings, D-branes, photons, or spins; however, there is a distinction between the emergence of an effective causal structure in the inner degrees of freedom of a material, in the form of a correlation-generated effective metric, for example, in a spin liquid system, and the emergence of a causal structure in an open propagating system by using classical light. I will show how an Uhlmann gauge construction would add stability to a modified causal structure that would retain the shape of a closed causal loop. Various other ideas related to the quantum origin of the string length are also discussed and an analogy of the emergence of string length from quantum correlations with the emergence of wavelength of an electromagnetic wave from coherence conditions of photon modes is presented. Full article

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56 pages, 4795 KB

Open AccessArticle

Bloch Waves, Magnetization and Domain Walls: The Case of the Gluon Propagator

by Attilio Cucchieri and Tereza Mendes

Universe 2025, 11(8), 273; https://doi.org/10.3390/universe11080273 - 17 Aug 2025

Viewed by 209

Abstract

We expand our previous study of replicated gauge configurations in lattice SU(

N_{c}

) Yang–Mills theory—employing Bloch’s theorem from condensed matter physics—to construct gauge-fixed field configurations on significantly larger lattices than the original, or primitive, one. We present a comprehensive discussion of [...] Read more.

We expand our previous study of replicated gauge configurations in lattice SU(

N_{c}

) Yang–Mills theory—employing Bloch’s theorem from condensed matter physics—to construct gauge-fixed field configurations on significantly larger lattices than the original, or primitive, one. We present a comprehensive discussion of the general gauge-fixing problem, identifying advantages of the replicated-lattice approach. In particular, the consideration of Bloch waves leads us to a visualization of the extended gauge-fixed configurations in terms of (color) magnetization domains. Moreover, we are able to explore features of the method to optimize the evaluation of gauge fields in momentum space, furthering our knowledge of the “allowed momenta”, an issue that has hindered wider applications of this approach up to now. Interestingly, our analysis yields both a better conceptual understanding of the problem and a more efficient way to compute the desired large-volume observables. Full article

(This article belongs to the Section Field Theory)

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27 pages, 1600 KB

Open AccessReview

A Primer on Spacetime Singularities I: Mathematical Framework

by Jean-Pierre Luminet

Universe 2025, 11(8), 272; https://doi.org/10.3390/universe11080272 - 17 Aug 2025

Viewed by 832

Abstract

This article presents a comprehensive and rigorous overview of spacetime singularities within the framework of classical General Relativity. Singularities are defined through the failure of geodesic completeness, reflecting the limits of predictability in spacetime evolution. This paper reviews the mathematical structures involved, including [...] Read more.

This article presents a comprehensive and rigorous overview of spacetime singularities within the framework of classical General Relativity. Singularities are defined through the failure of geodesic completeness, reflecting the limits of predictability in spacetime evolution. This paper reviews the mathematical structures involved, including differentiability classes of the metric, and explores key constructions such as Geroch’s and Schmidt’s formulations of singular boundaries. A detailed classification of singularities—quasi-regular, non-scalar, and scalar—is proposed, based on the behavior of curvature tensors along incomplete curves. The limitations of previous approaches, including the cosmic censorship conjecture and extensions beyond General Relativity, are critically examined. This work also surveys the major singularity theorems of Penrose and Hawking, emphasizing their implications for gravitational collapse and cosmology. By focusing exclusively on the classical regime, this article lays a solid foundation for the systematic study of singular structures in relativistic spacetimes. Full article

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35 pages, 1520 KB

Open AccessArticle

Dynamical System Analysis of Single-Axion Monodromy Inflation with Periodically Modulated Potentials

by Panagiotis Dorlis, Nick E. Mavromatos, Sotirios-Neilos Vlachos and Makarios Vyros

Universe 2025, 11(8), 271; https://doi.org/10.3390/universe11080271 - 17 Aug 2025

Viewed by 194

Abstract

In this work, we study field theoretic systems of a single axion-like field with linear potentials modulated by cosine terms, allegedly induced by non-perturbative instanton configurations. These systems are considered in expanding-Universe spacetime backgrounds (of Friedmann–Lema

\hat{i}

tre–Robertson–Walker-type). Using a dynamical system [...] Read more.

In this work, we study field theoretic systems of a single axion-like field with linear potentials modulated by cosine terms, allegedly induced by non-perturbative instanton configurations. These systems are considered in expanding-Universe spacetime backgrounds (of Friedmann–Lema

\hat{i}

tre–Robertson–Walker-type). Using a dynamical system approach, we classify the various de Sitter-like (inflationary) vacua from the point of view of their stability, which depend on the values of the model parameters. In this respect, bifurcation points are found to be present for the various models under consideration. Part of the parameter space of the systems under consideration includes the running-vacuum (approximately) linear axion monodromy potentials, considered in previous works by some of the authors, where inflation is induced by primordial gravitational wave condensates. A particularly interesting case, corresponding to another part of the parameter space of the models, includes a series of stable de Sitter vacua, which physically may correspond to a series of successive tunnelings of the system, via say non-perturbative effects, with a decreasing effective cosmological constant. Under certain values of the parameters, these successive tunnelings can reach a Minkowski spacetime, with zero value of the minimum of the axion potential. The situation is not dissimilar to the one of discrete inflation that arguably characterizes some minimal non-critical-string (Liouville) models of cosmology. Finally, for comparison, we also include in this article a dynamical system study of standard axion monodromy-modulated potentials characterizing some string/brane compactification models of inflation. Full article

(This article belongs to the Section Cosmology)

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24 pages, 1539 KB

Open AccessReview

Sculpting Spacetime: Thin Shells in Wormhole Physics

by Francisco S. N. Lobo

Universe 2025, 11(8), 270; https://doi.org/10.3390/universe11080270 - 15 Aug 2025

Viewed by 204

Abstract

In this work, we employ the Darmois–Israel thin-shell formalism to construct both static and dynamic thin-shell configurations surrounding traversable wormholes. Initially, using the cut-and-paste technique, we perform a linearized stability analysis in the presence of a general cosmological constant. Our results show that [...] Read more.

In this work, we employ the Darmois–Israel thin-shell formalism to construct both static and dynamic thin-shell configurations surrounding traversable wormholes. Initially, using the cut-and-paste technique, we perform a linearized stability analysis in the presence of a general cosmological constant. Our results show that for sufficiently large positive values of the cosmological constant—corresponding to the Schwarzschild–de Sitter geometry—the stability regions of the wormhole solutions are significantly enhanced compared to the Schwarzschild case. Subsequently, we construct static thin-shell solutions by matching an interior wormhole geometry to an exterior vacuum spacetime across a junction surface. In the spirit of minimizing the presence of exotic matter, we identify parameter domains in which the null and weak energy conditions are satisfied at the shell. We examine the surface stress-energy components in detail, determining regions where the tangential surface pressure is either positive or negative, interpreted, respectively, as the pressure or surface tension. Additionally, an expression describing the behavior of the radial pressure across the junction is derived. Finally, we determine key geometrical characteristics of the wormhole, including the throat radius and the junction interface radius, by imposing traversability conditions. Estimates for the traversal time and required velocity are also provided, further elucidating the physical viability of these configurations. Full article

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

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17 pages, 331 KB

Open AccessArticle

Extensive and Intensive Aspects of Astrophysical Systems and Fine-Tuning

by Meir Shimon

Universe 2025, 11(8), 269; https://doi.org/10.3390/universe11080269 - 15 Aug 2025

Viewed by 223

Abstract

Most astrophysical systems (except for very compact objects such as, e.g., black holes and neutron stars) in our Universe are characterized by shallow gravitational potentials, with dimensionless compactness

| Φ | \equiv r_{s} / R ≪ 1

, where

r_{s}

and [...] Read more.

Most astrophysical systems (except for very compact objects such as, e.g., black holes and neutron stars) in our Universe are characterized by shallow gravitational potentials, with dimensionless compactness

| Φ | \equiv r_{s} / R ≪ 1

, where

r_{s}

and R are their Schwarzschild radius and typical size, respectively. While the existence and characteristic scales of such virialized systems depend on gravity, we demonstrate that the value of

| Φ |

—and thus the non-relativistic nature of most astrophysical objects—arises from microphysical parameters, specifically the fine structure constant and the electron-to-proton mass ratio, and is fundamentally independent of the gravitational constant, G. In fact, the (generally extensive) gravitational potential becomes ‘locally’ intensive at the system boundary; the compactness parameter corresponds to the binding energy (or degeneracy energy, in the case of quantum degeneracy pressure-supported systems) per proton, representing the amount of work that needs to be done in order to allow proton extraction from the system. More generally, extensive properties of gravitating systems depend on G, whereas intensive properties do not. It then follows that peak rms values of large-scale astrophysical velocities and escape velocities associated with naturally formed astrophysical systems are determined by electromagnetic and atomic physics, not by gravitation, and that the compactness,

| Φ |

, is always set by microphysical scales—even for the most compact objects, such as neutron stars, where

| Φ |

is determined by quantities like the pion-to-proton mass ratio. This observation, largely overlooked in the literature, explains why the Universe is not dominated by relativistic, compact objects and connects the relatively low entropy of the observable Universe to underlying basic microphysics. Our results emphasize the central but underappreciated role played by dimensionless microphysical constants in shaping the macroscopic gravitational landscape of the Universe. In particular, we clarify that this independence of the compactness,

| Φ |

, from G applies specifically to entire, virialized, or degeneracy pressure-supported systems, naturally formed astrophysical systems—such as stars, galaxies, and planets—that have reached equilibrium between self-gravity and microphysical processes. In contrast, arbitrary subsystems (e.g., a piece cut from a planet) do not exhibit this property; well within/outside the gravitating object, the rms velocity is suppressed and G reappears. Finally, we point out that a clear distinction between intensive and extensive astrophysical/cosmological properties could potentially shed new light on the mass hierarchy and the cosmological constant problems; both may be related to the large complexity of our Universe. Full article

(This article belongs to the Section Gravitation)

13 pages, 793 KB

Open AccessArticle

Red Noise Suppression in Pulsar Timing Array Data Using Adaptive Splines

by Yi-Qian Qian, Yan Wang and Soumya D. Mohanty

Universe 2025, 11(8), 268; https://doi.org/10.3390/universe11080268 - 15 Aug 2025

Viewed by 235

Abstract

Noise in Pulsar Timing Array (PTA) data is commonly modeled as a mixture of white and red noise components. While the former is related to the receivers, and easily characterized by three parameters (EFAC, EQUAD and ECORR), the latter arises from a mix [...] Read more.

Noise in Pulsar Timing Array (PTA) data is commonly modeled as a mixture of white and red noise components. While the former is related to the receivers, and easily characterized by three parameters (EFAC, EQUAD and ECORR), the latter arises from a mix of hard to model sources and, potentially, a stochastic gravitational wave background (GWB). Since their frequency ranges overlap, GWB search methods must model the non-GWB red noise component in PTA data explicitly, typically as a set of mutually independent Gaussian stationary processes having power-law power spectral densities. However, in searches for continuous wave (CW) signals from resolvable sources, the red noise is simply a component that must be filtered out, either explicitly or implicitly (via the definition of the matched filtering inner product). Due to the technical difficulties associated with irregular sampling, CW searches have generally used implicit filtering with the same power law model as GWB searches. This creates the data analysis burden of fitting the power-law parameters, which increase in number with the size of the PTA and hamper the scaling up of CW searches to large PTAs. Here, we present an explicit filtering approach that overcomes the technical issues associated with irregular sampling. The method uses adaptive splines, where the spline knots are included in the fitted model. Besides illustrating its application on real data, the effectiveness of this approach is investigated on synthetic data that has the same red noise characteristics as the NANOGrav 15-year dataset and contains a single non-evolving CW signal. Full article

(This article belongs to the Special Issue Supermassive Black Hole Mass Measurements)

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22 pages, 374 KB

Open AccessArticle

Powerful Radio Sources as Probes of Black Hole Physics

by Ruth A. Daly

Universe 2025, 11(8), 267; https://doi.org/10.3390/universe11080267 - 14 Aug 2025

Viewed by 203

Abstract

Powerful jetted radio sources for which the luminosity in directed kinetic energy has been empirically determined, independent of assumptions, are considered. The total outflow lifetime of each source determined in the context of detailed cosmological studies was found to depend only upon the [...] Read more.

Powerful jetted radio sources for which the luminosity in directed kinetic energy has been empirically determined, independent of assumptions, are considered. The total outflow lifetime of each source determined in the context of detailed cosmological studies was found to depend only upon the luminosity in directed kinetic energy (L). The distributions of L, total outflow lifetime, and total outflow energy each have a broad range of values, as do the supermassive black hole masses. The total outflow energy relative to the black hole mass is a small number with a small dispersion. Three explanations of these remarkable results are considered. This could indicate (1) the efficiencies with which black hole irreducible mass is increased and spin mass energy is extracted during the outflow event, (2) that the merger of two supermassive black holes occurs over a timescale commensurate with the independently determined outflow lifetime and that these mergers lead to the production of the low-frequency gravitational wave background, or (3) that feedback shuts off black hole accretion due to energy injected into the ambient medium. Full article

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18 pages, 1462 KB

Open AccessArticle

From Gamma Rays to Cosmic Rays: Lepto-Hadronic Modeling of Blazar Sources as Candidates for Ultra-High-Energy Cosmic Rays

by Luiz Augusto Stuani Pereira and Samuel Victor Bernardo da Silva

Universe 2025, 11(8), 266; https://doi.org/10.3390/universe11080266 - 14 Aug 2025

Viewed by 270

Abstract

Ultra-high-energy cosmic rays (UHECRs) with energies exceeding

10^{19}

eV are believed to originate from extragalactic environments, potentially associated with relativistic jets in active galactic nuclei (AGN). Among AGNs, blazars, particularly those detected in very-high-energy (VHE) gamma rays, are promising candidates for UHECR [...] Read more.

Ultra-high-energy cosmic rays (UHECRs) with energies exceeding

10^{19}

eV are believed to originate from extragalactic environments, potentially associated with relativistic jets in active galactic nuclei (AGN). Among AGNs, blazars, particularly those detected in very-high-energy (VHE) gamma rays, are promising candidates for UHECR acceleration and high-energy neutrino production. In this work, we investigate three blazar sources, AP Librae, 1H 1914–194, and PKS 0735+178, using multiwavelength spectral energy distribution (SED) modeling. These sources span a range of synchrotron peak classes and redshifts, providing a diverse context to explore the physical conditions in relativistic jets. We employ both leptonic and lepto-hadronic models to describe their broadband emission from radio to TeV energies, aiming to constrain key jet parameters such as magnetic field strength, emission region size, and particle energy distributions. Particular attention is given to evaluating their potential as sources of UHECRs and high-energy neutrinos. Our results shed light on the complex interplay between particle acceleration mechanisms, radiative processes, and multi-messenger signatures in extreme astrophysical environments. Full article

(This article belongs to the Special Issue Ultra-High Energy Cosmic Rays: Past, Present and Future)

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24 pages, 14557 KB

Open AccessArticle

A Tailored Deep Learning Network with Embedded Space Physical Knowledge for Auroral Substorm Recognition: Validation Through Special Case Studies

by Yiyuan Han, Bing Han and Zejun Hu

Universe 2025, 11(8), 265; https://doi.org/10.3390/universe11080265 - 12 Aug 2025

Viewed by 309

Abstract

The dynamic morphological characteristics of the auroral oval serve as critical diagnostic indicators for auroral substorm recognition, with each pixel in ultraviolet imager (UVI) data carrying different physical implications. Existing deep learning approaches often overlook the physical properties of auroral images by directly [...] Read more.

The dynamic morphological characteristics of the auroral oval serve as critical diagnostic indicators for auroral substorm recognition, with each pixel in ultraviolet imager (UVI) data carrying different physical implications. Existing deep learning approaches often overlook the physical properties of auroral images by directly transplanting generic models into space physics applications without adaptation. In this study, we propose a visual–physical interactive deep learning model specifically designed and optimized for accurate auroral substorm recognition. The model leverages the significant variation in auroral morphology across different substorm phases to guide feature extraction. It integrates magnetospheric domain knowledge from space physics through magnetic local time (MLT) and magnetic latitude (MLAT) embeddings and incorporates cognitive features derived from expert eye-tracking data to enhance spatial attention. Experimental results on substorm sequences recognition demonstrate satisfactory performance, achieving an accuracy of 92.64%, precision of 90.29%, recall of 93%, and F1-score of 91.63%. Furthermore, several case studies are presented to illustrate how both visual and physical characteristics contribute to model performance, offering further insight into the spatiotemporal complexity of auroral substorm recognition. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2025—Space Science)

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7 pages, 229 KB

Open AccessCommunication

Modifications to the Entropy of a Rotating Bardeen Black Hole Due to Magnetic Charge

by Gu-Qiang Li

Universe 2025, 11(8), 264; https://doi.org/10.3390/universe11080264 - 11 Aug 2025

Viewed by 264

Abstract

Applying the Parikh–Wilczek method and based on the thermodynamics laws of black holes, we investigate the structure of the entropy of rotating Bardeen black holes. We find that entropy includes three terms and thus violates the area law. The first two terms depend [...] Read more.

Applying the Parikh–Wilczek method and based on the thermodynamics laws of black holes, we investigate the structure of the entropy of rotating Bardeen black holes. We find that entropy includes three terms and thus violates the area law. The first two terms depend on all of the black hole characteristics, while the third one is solely dependent on the charge of a magnetic monopole arising from nonlinear electrodynamics. The existence of the additional term means that the entropy of regular black holes has a different structure from that of classical ones, so it cannot be considered as a constant and disregarded, as was implemented in the previous literature. Full article

(This article belongs to the Collection Open Questions in Black Hole Physics)

17 pages, 1588 KB

Open AccessArticle

The Evolution of Radiating Stars Is Affected by Dimension

by Sunil D. Maharaj, Byron P. Brassel, Megandhren Govender and Keshlan S. Govinder

Universe 2025, 11(8), 263; https://doi.org/10.3390/universe11080263 - 9 Aug 2025

Viewed by 158

Abstract

The dynamics of a radiating star in general relativity are studied in higher dimensions for a specified shear-free metric. The temporal evolution of the radiating star depends on the spacetime dimension. In particular, we show explicitly that the gravitational potential changes with increasing [...] Read more.

The dynamics of a radiating star in general relativity are studied in higher dimensions for a specified shear-free metric. The temporal evolution of the radiating star depends on the spacetime dimension. In particular, we show explicitly that the gravitational potential changes with increasing spacetime dimension. A detailed analysis of the boundary condition is undertaken. We find new exact solutions and first integrals for the boundary condition equation. Known results in four dimensions are regained as special cases. A phase plane analysis indicates that the model asymptotically approaches a static end state or continues to radiate. The physical features are affected by dimension, and we indicate how the luminosity changes with increasing dimension. Full article

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

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14 pages, 299 KB

Open AccessArticle

Chern–Simons States in SO(1,n)Yang–Mills Gauge Theory of Quantum Gravity

by Zbigniew Haba

Universe 2025, 11(8), 262; https://doi.org/10.3390/universe11080262 - 7 Aug 2025

Viewed by 193

Abstract

We discuss a quantization of the Yang–Mills theory with an internal symmetry group

S O (1, n)

treated as a unified theory of all interactions. In one-loop calculations, we show that Einstein gravity can be considered as an approximation to [...] Read more.

We discuss a quantization of the Yang–Mills theory with an internal symmetry group

S O (1, n)

treated as a unified theory of all interactions. In one-loop calculations, we show that Einstein gravity can be considered as an approximation to gauge theory. We discuss the role of the Chern–Simons wave functions in the quantization. Full article

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

17 pages, 1500 KB

Open AccessArticle

A Study of the Origin of Two High-Speed R-Process-Enriched Stars by the Abundance Decomposition Approach

by Muhammad Zeshan Ashraf, Wenyuan Cui, Hongjie Li and Jianrong Shi

Universe 2025, 11(8), 261; https://doi.org/10.3390/universe11080261 - 7 Aug 2025

Viewed by 255

Abstract

TYC 622-742-1 and TYC 1193-1918-1 are evolved metal-poor (MP) high-speed stars with r-enhanced characteristics discovered in the Milky Way (MW) halo. The study of these halo stars is important for clarification of and knowledge about their origin. We employ the abundance decomposition method [...] Read more.

TYC 622-742-1 and TYC 1193-1918-1 are evolved metal-poor (MP) high-speed stars with r-enhanced characteristics discovered in the Milky Way (MW) halo. The study of these halo stars is important for clarification of and knowledge about their origin. We employ the abundance decomposition method to fit the observed abundances of 25 elements in TYC 622-742-1 and 24 elements in TYC 1193-1918-1, representing the largest number of elements fitted in the current observed dataset. We analyze the astrophysical formation sites of both sample stars by calculating their abundance ratios and component ratios. The calculation results suggest that both stars originated in a gas cloud that was contaminated by the ejecta of primary and main r-process materials such as those from a neutron star merger (NSM), which enriched their heavy neutron-capture elements (HNCEs), and the material from the massive stars (

M \geq 10 M_{⊙}

), which enriched their primary light, iron-group, and lighter neutron-capture elements (LNCEs). This implies that TYC 622-742-1 and TYC 1193-1918-1 are the main r-process-enhanced stars with strong primary-process contributions. We find that the component coefficients of the sample stars closely resemble those of metal-poor Galactic populations, indicating a probable origin within the MW. Furthermore, the

α

-enhanced abundance patterns and orbital trajectories suggest that both stars likely formed in the Galactic disk, possibly within a globular cluster (GC), and were subsequently ejected into the halo through dynamical processes. Full article

(This article belongs to the Section Solar and Stellar Physics)

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20 pages, 6555 KB

Open AccessArticle

Statistical Study of Whistler-Mode Waves in the Magnetospheric Magnetic Ducts

by Salman A. Nejad and Anatoly V. Streltsov

Universe 2025, 11(8), 260; https://doi.org/10.3390/universe11080260 - 6 Aug 2025

Viewed by 273

Abstract

This paper presents a comprehensive statistical analysis of extremely/very low-frequency (ELF/VLF) whistler-mode waves observed within magnetic ducts (B-ducts) using data from NASA’s Magnetospheric Multiscale (MMS) mission. A total of 687 events were analyzed, comprising 504 occurrences on the dawn-side flank of [...] Read more.

This paper presents a comprehensive statistical analysis of extremely/very low-frequency (ELF/VLF) whistler-mode waves observed within magnetic ducts (B-ducts) using data from NASA’s Magnetospheric Multiscale (MMS) mission. A total of 687 events were analyzed, comprising 504 occurrences on the dawn-side flank of the magnetosphere and 183 in the nightside magnetotail, to investigate the spatial distribution and underlying mechanisms of wave–particle interactions. We identify distinct differences between these regions by examining key parameters such as event width, frequency, plasma density, and magnetic field extrema within B-ducts. Using an independent two-sample t-test, we assess the statistical significance of variations in these parameters between different observation periods. This study provides valuable insights into the magnetospheric conditions influencing B-duct formation and wave propagation, offering a framework for understanding ELF/VLF wave dynamics in Earth’s space environment. Full article

(This article belongs to the Section Space Science)

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16 pages, 882 KB

Open AccessArticle

MatBYIB: A MATLAB-Based Toolkit for Parameter Estimation of Eccentric Gravitational Waves from EMRIs

by Genliang Li, Shujie Zhao, Huaike Guo, Jingyu Su and Zhenheng Lin

Universe 2025, 11(8), 259; https://doi.org/10.3390/universe11080259 - 6 Aug 2025

Viewed by 262

Abstract

Accurate parameter estimation is essential for gravitational wave data analysis. In extreme mass-ratio inspiral binary systems, orbital eccentricity is a critical parameter for parameter estimation. However, the current software for the parameter estimation of the gravitational wave often neglects the direct estimation of [...] Read more.

Accurate parameter estimation is essential for gravitational wave data analysis. In extreme mass-ratio inspiral binary systems, orbital eccentricity is a critical parameter for parameter estimation. However, the current software for the parameter estimation of the gravitational wave often neglects the direct estimation of orbital eccentricity. To fill this gap, we have developed the MatBYIB, a MATLAB-based software (Version 1.0) package for the parameter estimation of the gravitational wave with arbitrary eccentricity. The MatBYIB employs the Analytical Kludge waveform as a computationally efficient signal generator and computes parameter uncertainties via the Fisher Information Matrix and the Markov Chain Monte Carlo. For Bayesian inference, we implement the Metropolis–Hastings algorithm to derive posterior distributions. To guarantee convergence, the Gelman–Rubin convergence criterion (the Potential Scale Reduction Factor

\hat{R}

) is used to determine sampling adequacy, with MatBYIB dynamically increasing the sample size until

\hat{R} < 1.05

for all parameters. Our results demonstrate strong agreement between predictions based on the Fisher Information Matrix and full MCMC sampling. This program is user-friendly and allows for the estimation of the gravitational wave parameters with arbitrary eccentricity on standard personal computers. Full article

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32 pages, 606 KB

Open AccessArticle

Role of Thermal Fluctuations in Nucleation of Three-Flavor Quark Matter

by Mirco Guerrini, Giuseppe Pagliara, Andrea Lavagno and Alessandro Drago

Universe 2025, 11(8), 258; https://doi.org/10.3390/universe11080258 - 5 Aug 2025

Viewed by 227

Abstract

We present a framework that aims to investigate the role of thermal fluctuations in matter composition and color superconductivity in the nucleation of three-flavor deconfined quark matter in the typical conditions of high-energy astrophysical systems related to compact stars. It is usually assumed [...] Read more.

We present a framework that aims to investigate the role of thermal fluctuations in matter composition and color superconductivity in the nucleation of three-flavor deconfined quark matter in the typical conditions of high-energy astrophysical systems related to compact stars. It is usually assumed that the flavor composition is locally fixed during the formation of the first seed of deconfined quark matter, since a weak interaction acts too slowly to re-equilibrate flavors. However, the matter composition fluctuates around its average equilibrium values at the typical temperatures of high-energy astrophysical processes. Here, we extend our previous two-flavor nucleation formalism to a three-flavor case. We develop a thermodynamic framework incorporating finite-size effects and thermal fluctuations in the local composition to compute the nucleation probability as the product of droplet formation and composition fluctuation rates. Moreover, we discuss the role of color superconductivity in nucleation, arguing that it can play a role only in systems larger than the typical coherence length of diquark pairs. We found that thermal fluctuations in the matter composition led to lowering the potential barrier between the metastable hadronic phase and the stable quark phase. Moreover, the formation of diquark pairs reduced the critical radius and thus the potential barrier in the low baryon density and temperature regime. Full article

(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram 2024)

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11 pages, 317 KB

Open AccessArticle

Phenomenological Charged Extensions of the Quantum Oppenheimer–Snyder Collapse Model

by S. Habib Mazharimousavi

Universe 2025, 11(8), 257; https://doi.org/10.3390/universe11080257 - 4 Aug 2025

Viewed by 261

Abstract

This work presents a semi-classical, quantum-corrected model of gravitational collapse for a charged, spherically symmetric dust cloud, extending the classical Oppenheimer–Snyder (OS) framework through loop quantum gravity effects. Our goal is to study phenomenological quantum modifications to geometry, without necessarily embedding them within [...] Read more.

This work presents a semi-classical, quantum-corrected model of gravitational collapse for a charged, spherically symmetric dust cloud, extending the classical Oppenheimer–Snyder (OS) framework through loop quantum gravity effects. Our goal is to study phenomenological quantum modifications to geometry, without necessarily embedding them within full loop quantum gravity (LQG). Building upon the quantum Oppenheimer–Snyder (qOS) model, which replaces the classical singularity with a nonsingular bounce via a modified Friedmann equation, we introduce electric and magnetic charges concentrated on a massive thin shell at the boundary of the dust ball. The resulting exterior spacetime generalizes the Schwarzschild solution to a charged, regular black hole geometry akin to a quantum-corrected Reissner–Nordström metric. The Israel junction conditions are applied to match the interior APS (Ashtekar–Pawlowski–Singh) cosmological solution to the charged exterior, yielding constraints on the shell’s mass, pressure, and energy. Stability conditions are derived, including a minimum radius preventing full collapse and ensuring positivity of energy density. This study also examines the geodesic structure around the black hole, focusing on null circular orbits and effective potentials, with implications for the observational signatures of such quantum-corrected compact objects. 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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17 pages, 310 KB

Open AccessArticle

Statistical Entropy Based on the Generalized-Uncertainty-Principle-Induced Effective Metric

by Soon-Tae Hong, Yong-Wan Kim and Young-Jai Park

Universe 2025, 11(8), 256; https://doi.org/10.3390/universe11080256 - 2 Aug 2025

Viewed by 222

Abstract

We investigate the statistical entropy of black holes within the framework of the generalized uncertainty principle (GUP) by employing effective metrics that incorporate leading-order and all-order quantum gravitational corrections. We construct three distinct effective metrics induced by the GUP, which are derived from [...] Read more.

We investigate the statistical entropy of black holes within the framework of the generalized uncertainty principle (GUP) by employing effective metrics that incorporate leading-order and all-order quantum gravitational corrections. We construct three distinct effective metrics induced by the GUP, which are derived from the GUP-corrected temperature, entropy, and all-order GUP corrections, and analyze their impact on black hole entropy using ’t Hooft’s brick wall method. Our results show that, despite the differences in the effective metrics and the corresponding ultraviolet cutoffs, the statistical entropy consistently satisfies the Bekenstein–Hawking area law when expressed in terms of an invariant (coordinate-independent) distance near the horizon. Furthermore, we demonstrate that the GUP naturally regularizes the ultraviolet divergence in the density of states, eliminating the need for artificial cutoffs and yielding finite entropy even when counting quantum states only in the vicinity of the event horizon. These findings highlight the universality and robustness of the area law under GUP modifications and provide new insights into the interplay between quantum gravity effects and black hole thermodynamics. Full article

(This article belongs to the Collection Open Questions in Black Hole Physics)

19 pages, 694 KB

Open AccessArticle

Nuclear Matter and Finite Nuclei: Relativistic Thomas–Fermi Approximation Versus Relativistic Mean-Field Approach

by Shuying Li, Hong Shen and Jinniu Hu

Universe 2025, 11(8), 255; https://doi.org/10.3390/universe11080255 - 1 Aug 2025

Viewed by 325

Abstract

The Thomas–Fermi approximation is a powerful method that has been widely used to describe atomic structures, finite nuclei, and nonuniform matter in supernovae and neutron-star crusts. Nonuniform nuclear matter at subnuclear density is assumed to be composed of a lattice of heavy nuclei [...] Read more.

The Thomas–Fermi approximation is a powerful method that has been widely used to describe atomic structures, finite nuclei, and nonuniform matter in supernovae and neutron-star crusts. Nonuniform nuclear matter at subnuclear density is assumed to be composed of a lattice of heavy nuclei surrounded by dripped nucleons, and the Wigner–Seitz cell is commonly introduced to simplify the calculations. The self-consistent Thomas–Fermi approximation can be employed to study both a nucleus surrounded by nucleon gas in the Wigner–Seitz cell and an isolated nucleus in the nuclide chart. A detailed comparison is made between the self-consistent Thomas–Fermi approximation and the relativistic mean-field approach for the description of finite nuclei, based on the same nuclear interaction. These results are then examined using experimental data from the corresponding nuclei. Full article

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

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11 pages, 2887 KB

Open AccessArticle

INTEGRAL/ISGRI Post 2024-Periastron View of PSR B1259-63

by Aleksei Kuzin, Denys Malyshev, Maria Chernyakova, Brian van Soelen and Andrea Santangelo

Universe 2025, 11(8), 254; https://doi.org/10.3390/universe11080254 - 31 Jul 2025

Viewed by 203

Abstract

PSR B1259-63/LS 2883 is a well-studied gamma-ray binary hosting a pulsar in a 3.4-year eccentric orbit around a Be-type star. Its non-thermal emission spans from radio to TeV energies, exhibiting a significant increase near the periastron passage. This paper is dedicated to the [...] Read more.

PSR B1259-63/LS 2883 is a well-studied gamma-ray binary hosting a pulsar in a 3.4-year eccentric orbit around a Be-type star. Its non-thermal emission spans from radio to TeV energies, exhibiting a significant increase near the periastron passage. This paper is dedicated to the analysis of INTEGRAL observations of the system following its last periastron passage in June 2024. We aim to study the spectral evolution of this gamma-ray binary in the soft (0.3–10 keV) and hard (30–300 keV) X-ray energy bands. We performed a joint analysis of the data taken by INTEGRAL/ISGRI in July–August 2024 and quasi-simultaneous Swift/XRT observations. The spectrum of the system in the 0.3–300 keV band is well described by an absorbed power law with a photon index of

Γ = 1.42 \pm 0.03

. We place constraints on potential spectral curvature, limiting the break energy

E_{b} > 30

keV for

Δ Γ > 0.3

and cutoff energy

E_{cutoff} > 150

keV at a 95% confidence level. For one-zone leptonic emission models, these values correspond to electron distribution spectral parameters of

E_{b, e} > 0.8

TeV and

E_{cutoff, e} > 1.7

TeV, consistent with previous constraints derived by H.E.S.S. Full article

(This article belongs to the Section Compact Objects)

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34 pages, 3521 KB

Open AccessReview

Overview of Water-Ice in Asteroids—Targets of a Revolution by LSST and JWST

by Ákos Kereszturi, Mohamed Ramy El-Maarry, Anny-Chantal Levasseur-Regourd, Imre Tóth, Bernadett D. Pál and Csaba Kiss

Universe 2025, 11(8), 253; https://doi.org/10.3390/universe11080253 - 30 Jul 2025

Viewed by 585

Abstract

Water-ice occurs inside many minor bodies almost throughout the Solar System. To have an overview of the inventory of water-ice in asteroids, beside the general characteristics of their activity, examples are presented with details, including the Hilda zone and among the Trojans. There [...] Read more.

Water-ice occurs inside many minor bodies almost throughout the Solar System. To have an overview of the inventory of water-ice in asteroids, beside the general characteristics of their activity, examples are presented with details, including the Hilda zone and among the Trojans. There might be several extinct comets among the asteroids with only internal ice content, demonstrating the complex evolution of such bodies. To evaluate the formation of ice-hosting small objects, their migration and retention capacity by a surface covering dust layer are also overviewed to provide a complex picture of volatile occurrences. This review aims to support further work and search for sublimation-induced activity of asteroids by future missions and telescopic surveys. Based on the observed and hypothesized occurrence and characteristics of icy asteroids, future observation-related estimations were made regarding the low limiting magnitude future survey of LSST/Vera Rubin and also the infrared ice identification by the James Webb space telescope. According to these estimations, there is a high probability of mapping the distribution of ice in the asteroid belt over the next decade. Full article

(This article belongs to the Special Issue The Hidden Stories of Small Planetary Bodies)

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13 pages, 359 KB

Open AccessArticle

Toward the Alleviation of the H₀ Tension in Myrzakulov f(R,T) Gravity

by Mashael A. Aljohani, Emad E. Mahmoud, Koblandy Yerzhanov and Almira Sergazina

Universe 2025, 11(8), 252; https://doi.org/10.3390/universe11080252 - 29 Jul 2025

Cited by 1 | Viewed by 258

Abstract

In this work, we provide a promising way to alleviate the Hubble tension within the framework of Myrzakulov

f (R, T)

gravity. The latter incorporates both curvature and torsion under a non-special connection. We consider the [...] Read more.

In this work, we provide a promising way to alleviate the Hubble tension within the framework of Myrzakulov

f (R, T)

gravity. The latter incorporates both curvature and torsion under a non-special connection. We consider the

f (R, T) = R + α R^{2}

class, which leads to modified Friedmann equations and an effective dark energy sector. Within this class, we make specific connection choices in order to obtain a Hubble function that coincides with that of

Λ

CDM at early times while yielding higher

H_{0}

values at late times. The reason behind this behavior is that the dark energy equation of state exhibits phantom behavior, which is known to be a sufficient mechanism for alleviating the

H_{0}

tension. A full observational comparison with various datasets, including the Cosmic Microwave Background (CMB), is required to test the viability of this scenario. Strictly speaking, the present work does not provide a complete solution to the Hubble tension but rather proposes a promising way to alleviate it. Full article

(This article belongs to the Special Issue Gravity and Cosmology: Exploring the Mysteries of f(T) Gravity)

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4 pages, 210 KB

Open AccessOpinion

Pyknons: A Suggestion for Rebranding Black Holes

by Lorenzo Iorio

Universe 2025, 11(8), 251; https://doi.org/10.3390/universe11080251 - 29 Jul 2025

Viewed by 1369

Abstract

‘Black hole’ is the denomination of the most extreme prediction of the General Theory of Relativity made popular by J. A. Wheeler in the late sixties of the twentieth century, having now entered widely into the collective imagination. Nonetheless, the term is somewhat [...] Read more.

‘Black hole’ is the denomination of the most extreme prediction of the General Theory of Relativity made popular by J. A. Wheeler in the late sixties of the twentieth century, having now entered widely into the collective imagination. Nonetheless, the term is somewhat misleading since there is nothing that tears apart in black holes, which, furthermore, are not even black. Thus, the new name pyknons, from the ancient Greek word for ‘compact; constricted; close-packed’, is proposed for them since it captures a key distinctive feature of theirs. In deference to the objects thus renamed, it also has the merit of introducing a greater compactness in the terms denoting them. Full article

(This article belongs to the Section Gravitation)

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