Universe

22 pages, 357 KiB

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

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

18 pages, 1462 KiB

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

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 KiB

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 28

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 KiB

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 170

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 KiB

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 83

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 KiB

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 112

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 KiB

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 175

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 KiB

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 197

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 KiB

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 164

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 KiB

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 147

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 KiB

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 183

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 KiB

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 160

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 KiB

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 220

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 KiB

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 154

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 KiB

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 247

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 KiB

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

Viewed by 175

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 KiB

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 864

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)

12 pages, 244 KiB

Open AccessArticle

Towards Relational Foundations for Spacetime Quantum Physics

by Pietro Dall’Olio and José A. Zapata

Universe 2025, 11(8), 250; https://doi.org/10.3390/universe11080250 - 29 Jul 2025

Viewed by 220

Abstract

Rovelli’s relational interpretation of quantum mechanics tells us that the description of a system in the formalism of quantum mechanics is not an absolute but is relative to the observer itself. The interpretation goes further and proposes a set of axioms. In standard [...] Read more.

Rovelli’s relational interpretation of quantum mechanics tells us that the description of a system in the formalism of quantum mechanics is not an absolute but is relative to the observer itself. The interpretation goes further and proposes a set of axioms. In standard non-relational language, one of them states that an observer can only retrieve a finite amount information from a system by means of measurement. Our contribution starts with the observation that quantum mechanics, i.e., quantum field theory (QFT) in dimension 1, radically differs from QFT in higher dimensions. In higher dimensions, boundary data (or initial data) cannot be characterized by finitely many measurements. This calls for a notion of measuring scale, which we provide. At a given measuring scale, the observer has partial information about the system. Our notion of measuring scale generalizes the one implicitly used in Wilsonian QFT. At each measuring scale, there are effective theories, which may be corrected, and if the theory turns out to be renormalizable, the mentioned corrections converge to determine a completely corrected (or renormalized) theory at the given measuring scale. The notion of a measuring scale is the cornerstone of Wilsonian QFT; this notion tells us that we are not describing a system from an absolute perspective. An effective theory at that scale describes the system with respect to the observer, which may retrieve information from the system by means of measurement in a specific way determined by our notion of measuring scale. We claim that a relational interpretation of quantum physics for spacetimes of dimensions greater than 1 is Wilsonian. Full article

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

9 pages, 2434 KiB

Open AccessArticle

Locally Generated Whistler-Mode Waves Before Dipolarization Fronts

by Boning Zhao, Chengming Liu, Jinbin Cao, Yangyang Liu and Xining Xing

Universe 2025, 11(8), 249; https://doi.org/10.3390/universe11080249 - 29 Jul 2025

Viewed by 227

Abstract

Whistler-mode waves, electromagnetic emissions with frequencies between the lower hybrid and electron cyclotron frequencies, are ubiquitous in planetary magnetotails. They are known to play a vital role in electron scattering and acceleration, originating primarily within strong magnetic field regions behind dipolarization fronts (DFs). [...] Read more.

Whistler-mode waves, electromagnetic emissions with frequencies between the lower hybrid and electron cyclotron frequencies, are ubiquitous in planetary magnetotails. They are known to play a vital role in electron scattering and acceleration, originating primarily within strong magnetic field regions behind dipolarization fronts (DFs). In contrast to this established knowledge, we present a comprehensive analysis of whistler-mode waves generated locally within weak magnetic field regions ahead of DFs, utilizing high-cadence measurements from the MMS mission. By resolving the wave dispersion relations, we demonstrate that these emissions arise from cyclotron resonance with local electrons exhibiting weak perpendicular temperature anisotropy (A_e < 1.2). We further propose that this anisotropy may develop due to magnetic mirror structures forming upstream of DFs. Our findings challenge the conventional view that whistler-mode generation requires strong magnetic fields near DFs, providing new insights into understanding wave excitation mechanisms in planetary magnetotails. Full article

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

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18 pages, 1371 KiB

Open AccessArticle

Estimating Galactic Structure Using Galactic Binaries Resolved by Space-Based Gravitational Wave Observatories

by Shao-Dong Zhao, Xue-Hao Zhang, Soumya D. Mohanty, Màrius Josep Fullana i Alfonso, Yu-Xiao Liu and Qun-Ying Xie

Universe 2025, 11(8), 248; https://doi.org/10.3390/universe11080248 - 28 Jul 2025

Viewed by 213

Abstract

Space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA) and Taiji, will observe GWs from

O (10^{8})

galactic binary systems, allowing a completely unobscured view of the Milky Way structure. While previous studies have established theoretical expectations [...] Read more.

Space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA) and Taiji, will observe GWs from

O (10^{8})

galactic binary systems, allowing a completely unobscured view of the Milky Way structure. While previous studies have established theoretical expectations based on idealized data-analysis methods that use the true catalog of sources, we present an end-to-end analysis pipeline for inferring galactic structure parameters based on the detector output alone. We employ the GBSIEVER algorithm to extract GB signals from LISA Data Challenge data and develop a maximum likelihood approach to estimate a bulge-disk galactic model using the resolved GBs. We introduce a two-tiered selection methodology, combining frequency derivative thresholding and proximity criteria, to address the systematic overestimation of frequency derivatives that compromises distance measurements. We quantify the performance of our pipeline in recovering key Galactic structure parameters and the potential biases introduced by neglecting the errors in estimating the parameters of individual GBs. Our methodology represents a step forward in developing practical techniques that bridge the gap between theoretical possibilities and observational implementation. Full article

(This article belongs to the Special Issue Exploring Low-Frequency Gravitational Wave Sources: Waveforms, Detection and Sciences)

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22 pages, 2856 KiB

Open AccessArticle

Impact of Loop Quantum Gravity on the Topological Classification of Quantum-Corrected Black Holes

by Saeed Noori Gashti, İzzet Sakallı, Hoda Farahani, Prabir Rudra and Behnam Pourhassan

Universe 2025, 11(8), 247; https://doi.org/10.3390/universe11080247 - 27 Jul 2025

Viewed by 285

Abstract

We investigated the thermodynamic topology of quantum-corrected AdS-Reissner-Nordström black holes in Kiselev spacetime using non-extensive entropy formulation derived from Loop Quantum Gravity (LQG). Through systematic analysis, we examined how the Tsallis parameter

λ

influences topological charge classification with respect to various equation of [...] Read more.

We investigated the thermodynamic topology of quantum-corrected AdS-Reissner-Nordström black holes in Kiselev spacetime using non-extensive entropy formulation derived from Loop Quantum Gravity (LQG). Through systematic analysis, we examined how the Tsallis parameter

λ

influences topological charge classification with respect to various equation of state parameters. Our findings revealed a consistent pattern of topological transitions: for

λ = 0.1

, the system exhibited a single topological charge

(ω = - 1)

with total charge

W = - 1

, as

λ

increased to 0.8, the system transitioned to a configuration with two topological charges

(ω = + 1, - 1)

and total charge

W = 0

. When

λ = 1

, corresponding to the Bekenstein–Hawking entropy limit, the system displayed a single topological charge

(ω = + 1)

with

W = + 1

, signifying thermodynamic stability. The persistence of this pattern across different fluid compositions—from exotic negative pressure environments to radiation—demonstrates the universal nature of quantum gravitational effects on black hole topology. 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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16 pages, 14991 KiB

Open AccessArticle

Re-Examining Super-Nyquist Frequencies of 68 δ Scuti Stars Utilizing the Kepler Long-Cadence Photometry

by Zilu Yang, Jianning Fu, Xuan Wang, Yanqi Mo and Weikai Zong

Universe 2025, 11(8), 246; https://doi.org/10.3390/universe11080246 - 25 Jul 2025

Viewed by 144

Abstract

The high-precision and long-duration photometry provided by the

K e p l e r

mission has greatly advanced frequency analyses of a large number of pulsating stars, a fundamental step in asteroseismology. For

δ

Scuti stars, analyses are typically confined to frequencies below [...] Read more.

The high-precision and long-duration photometry provided by the

K e p l e r

mission has greatly advanced frequency analyses of a large number of pulsating stars, a fundamental step in asteroseismology. For

δ

Scuti stars, analyses are typically confined to frequencies below the Nyquist frequency. However, signals above this limit can be reflected into the sub-Nyquist range, especially in long-cadence data, where they may overlap with genuine pulsation modes and lead to misinterpretation. To address this issue, a recently proposed method—the sliding Lomb–Scargle periodogram (sLSP)—can effectively distinguish real frequencies from aliased ones. In this study, we compiled a sample of 68

δ

Scuti stars whose frequency analyses were based on the

K e p l e r

photometry. Using the sLSP method, we systematically examined the 1406 reported frequencies in the literature. As a result, we identified six previously unrecognized reflected super-Nyquist frequencies in four stars: KIC 3440495, KIC 5709664, KIC 7368103, and KIC 9204718. We have once again demonstrated the ability of the sLSP method to detect and correct such artifacts. This technique improves the reliability of frequency selection, thereby enhancing the accuracy of asteroseismic interpretation and stellar modeling for pulsating stars. Full article

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

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13 pages, 1895 KiB

Open AccessArticle

Class-Dependent Solar Flare Effects on Mars’ Upper Atmosphere: MAVEN NGIMS Observations of X8.2 and M6.0 from September 2017

by Junaid Haleem and Shican Qiu

Universe 2025, 11(8), 245; https://doi.org/10.3390/universe11080245 - 25 Jul 2025

Viewed by 294

Abstract

Transient increments of X-ray radiation and extreme ultraviolet (EUV) during solar flares are strong drivers of thermospheric dynamics on Mars, yet their class-dependent impacts remain poorly measured. This work provides the first direct, side-by-side study of Martian thermospheric reactions to flares X8.2 on [...] Read more.

Transient increments of X-ray radiation and extreme ultraviolet (EUV) during solar flares are strong drivers of thermospheric dynamics on Mars, yet their class-dependent impacts remain poorly measured. This work provides the first direct, side-by-side study of Martian thermospheric reactions to flares X8.2 on 10 September 2017 and M6.0 on 17 September 2017. This study shows nonlinear, class-dependent effects, compositional changes, and recovery processes not recorded in previous investigations. Species-specific responses deviated significantly from irradiance proportionality, even though the soft X-ray flux in the X8.2 flare was 13 times greater. Argon (Ar) concentrations rose 3.28× (compared to 1.13× for M6.0), and radiative cooling led CO₂ heating to approach a halt at ΔT = +40 K (X8.2) against +19 K (M6.0) at exobase altitudes (196–259 km). N₂ showed the largest class difference, where temperatures rose by +126 K (X8.2) instead of +19 K (M6.0), therefore displaying flare-magnitude dependent thermal sensitivity. The 1.95× increase in O concentrations during X8.2 and the subsequent decrease following M6.0 (−39 K cooling) illustrate the contradiction between photochemical production and radiative loss. The O/CO₂ ratio at 225 km dropped 46% during X8.2, revealing compositional gradients boosted by flares. Recovery timeframes varied by class; CO₂ quickly re-equilibrated because of effective cooling, whereas inert species (Ar, N₂) stabilized within 1–2 orbits after M6.0 but needed >10 orbits of the MAVEN satellite after the X8.2 flare. The observations of the X8.2 flare came from the western limb of the Sun, but the M6.0 flare happened on the far side. The CME shock was the primary driver of Mars’ EUV reaction. These findings provide additional information on atmospheric loss and planetary habitability by indicating that Mars’ thermosphere has a saturation threshold where strong flares induce nonlinear energy partitioning that encourages the departure of lighter species. Full article

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13 pages, 867 KiB

Open AccessArticle

Gravitational Wave Detection with Angular Deviation of Electromagnetic Waves

by John Maher and Arundhati Dasgupta

Universe 2025, 11(8), 244; https://doi.org/10.3390/universe11080244 - 25 Jul 2025

Viewed by 195

Abstract

In this note, we discuss interesting aspects of the interaction of electromagnetic waves (EMW) with gravitational waves (GWs) and how we can use them for GW detection. We show that there is (i) a deviation from the original path of the EMW, as [...] Read more.

In this note, we discuss interesting aspects of the interaction of electromagnetic waves (EMW) with gravitational waves (GWs) and how we can use them for GW detection. We show that there is (i) a deviation from the original path of the EMW, as measured by an angle of the scattered EMW, as well as (ii) a change in frequency. We show that the angular deviation is dependent on the frequency of the initial EMW and GW and suggest the use of MASERS/RASERS instead of LASERS for GW detection. We also briefly examine the influence of the Earth’s rotation and revolution, which can be sources of noise in the measurement of the angular deviation of EMW. Full article

(This article belongs to the Collection Gravitational Waves as a New Probe for Astronomy and Fundamental Physics)

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15 pages, 1420 KiB

Open AccessArticle

Spectral Dimensionality of Spacetime Around a Radiating Schwarzschild Black-Hole

by Mauricio Bellini, Juan Ignacio Musmarra, Pablo Alejandro Sánchez and Alan Sebastián Morales

Universe 2025, 11(8), 243; https://doi.org/10.3390/universe11080243 - 24 Jul 2025

Viewed by 165

Abstract

In this work we study the spectral dimensionality of spacetime around a radiating Schwarzschild black hole using a recently introduced formalism of quantum gravity, where the alterations of the gravitational field produced by the radiation are represented on an extended manifold, and describe [...] Read more.

In this work we study the spectral dimensionality of spacetime around a radiating Schwarzschild black hole using a recently introduced formalism of quantum gravity, where the alterations of the gravitational field produced by the radiation are represented on an extended manifold, and describe a non-commutative and nonlinear quantum algebra. The relation between classical and quantum perturbations of spacetime can be measured by the parameter

z \geq 0

. In this work we have found that when

z = (1 + \sqrt{3}) / 2 ≃ 1.3660

, a relativistic observer approaching the Schwarzschild horizon perceives a spectral dimension

N (z) = 4 [θ (z) - 1] ≃ 2.8849

, which is related to quantum gravitational interference effects in the environment of the black hole. Under these conditions, all studied Schwarzschild black holes with masses ranging from the Planck mass to

10^{46}

times the Planck mass present the same stability configuration, which suggests the existence of a universal property of these objects under those particular conditions. The difference from the spectral dimension previously obtained at cosmological scales leads to the conclusion that the spacetime dimensionality is scale-dependent. Another important result presented here is the fundamental alteration of the effective gravitational potential near the horizon due to Hawking radiation. This quantum phenomenon prevents the potential from diverging to negative infinity as the observer approaches the Schwarzschild horizon. Full article

(This article belongs to the Special Issue Extensions of General Relativity and Applications to Cosmology and Gravitation)

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11 pages, 961 KiB

Open AccessArticle

Viscous Cosmology in f(Q,L_m) Gravity: Insights from CC, BAO, and GRB Data

by Dheeraj Singh Rana, Sai Swagat Mishra, Aaqid Bhat and Pradyumn Kumar Sahoo

Universe 2025, 11(8), 242; https://doi.org/10.3390/universe11080242 - 23 Jul 2025

Viewed by 268

Abstract

In this article, we investigate the influence of viscosity on the evolution of the cosmos within the framework of the newly proposed

f (Q, L_{m})

gravity. We have considered a linear functional form [...] Read more.

In this article, we investigate the influence of viscosity on the evolution of the cosmos within the framework of the newly proposed

f (Q, L_{m})

gravity. We have considered a linear functional form

f (Q, L_{m}) = α Q + β L_{m}

with a bulk viscous coefficient

ζ = ζ_{0} + ζ_{1} H

for our analysis and obtained exact solutions to the field equations associated with a flat FLRW metric. In addition, we utilized Cosmic Chronometers (CC), CC + BAO, CC + BAO + GRB, and GRB data samples to determine the constrained values of independent parameters in the derived exact solution. The likelihood function and the Markov Chain Monte Carlo (MCMC) sampling technique are combined to yield the posterior probability using Bayesian statistical methods. Furthermore, by comparing our results with the standard cosmological model, we found that our considered model supports the acceleration of the universe in late time. Full article

(This article belongs to the Special Issue The 10th Anniversary of Universe: Standard Cosmological Models, and Modified Gravity and Cosmological Tensions)

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13 pages, 793 KiB

Open AccessCommunication

Gamma-Ray Bursts Calibrated by Using Artificial Neural Networks from the Pantheon+ Sample

by Zhen Huang, Xin Luo, Bin Zhang, Jianchao Feng, Puxun Wu, Yu Liu and Nan Liang

Universe 2025, 11(8), 241; https://doi.org/10.3390/universe11080241 - 23 Jul 2025

Viewed by 156

Abstract

In this paper, we calibrate the luminosity relation of gamma−ray bursts (GRBs) by employing artificial neural networks (ANNs) to analyze the Pantheon+ sample of type Ia supernovae (SNe Ia) in a manner independent of cosmological assumptions. The A219 GRB dataset is used to [...] Read more.

In this paper, we calibrate the luminosity relation of gamma−ray bursts (GRBs) by employing artificial neural networks (ANNs) to analyze the Pantheon+ sample of type Ia supernovae (SNe Ia) in a manner independent of cosmological assumptions. The A219 GRB dataset is used to calibrate the Amati relation (

E_{p}

-

E_{iso}

) at low redshift with the ANN framework, facilitating the construction of the Hubble diagram at higher redshifts. Cosmological models are constrained with GRBs at high redshift and the latest observational Hubble data (OHD) via the Markov chain Monte Carlo numerical approach. For the Chevallier−Polarski−Linder (CPL) model within a flat universe, we obtain

Ω_{m} = {0.321}_{- 0.069}^{+ 0.078}

,

h = {0.654}_{- 0.071}^{+ 0.053}

,

w_{0} = {- 1.02}_{- 0.50}^{+ 0.67}

, and

w_{a} = {- 0.98}_{- 0.58}^{+ 0.58}

at the 1 −

σ

confidence level, which indicates a preference for dark energy with potential redshift evolution (

w_{a} \neq 0

). These findings using ANNs align closely with those derived from GRBs calibrated using Gaussian processes (GPs). Full article

(This article belongs to the Special Issue Gamma-Ray Bursts: Unveiling the Mysteries of the Universe’s Most Powerful Explosions)

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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

Viewed by 219

Abstract

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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