Universe

16 pages, 10138 KiB

Open AccessArticle

Two Cases of Non-Radial Filament Eruption and Associated CME Deflection

by Kostadinka Koleva, Ramesh Chandra, Pooja Devi, Peter Duchlev and Momchil Dechev

Universe 2025, 11(7), 216; https://doi.org/10.3390/universe11070216 - 30 Jun 2025

The purpose of this paper is to analyze the multi-wavelength and multi-instrument observations of two quiescent filament eruptions as well as the deflection of associated CMEs from the radial direction. The events occurred on 18 October 2017 and 9 May 2021, respectively, in [...] Read more.

The purpose of this paper is to analyze the multi-wavelength and multi-instrument observations of two quiescent filament eruptions as well as the deflection of associated CMEs from the radial direction. The events occurred on 18 October 2017 and 9 May 2021, respectively, in the southern solar hemisphere. Both of them and associated flares were registered by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) and the Solar Terrestrial Relations Observatory–Ahead (STEREO A) Observatory in different EUV wavebands. Using data from STEREO A COR1 and COR2 instruments and the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO), we investigated morphology and kinematics of the eruptions and the latitudinal offset of the related CMEs with respect to the erupting filaments. Our observations provide the evidence that the two filament eruptions were highly non-radial. The observed deviations are attributed to the presence of low-latitude coronal holes. Full article

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

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17 pages, 312 KiB

Open AccessArticle

Black-Hole Thermodynamics from Gauge Freedom in Extended Iyer–Wald Formalism

by Thiago de L. Campos, Mario C. Baldiotti and C. Molina

Universe 2025, 11(7), 215; https://doi.org/10.3390/universe11070215 - 28 Jun 2025

Abstract

Thermodynamic systems admit multiple equivalent descriptions related by transformations that preserve their fundamental structure. This work focuses on exact isohomogeneous transformations (EITs), a class of mappings that keep fixed the set of independent variables of the thermodynamic potential, while preserving both the original [...] Read more.

Thermodynamic systems admit multiple equivalent descriptions related by transformations that preserve their fundamental structure. This work focuses on exact isohomogeneous transformations (EITs), a class of mappings that keep fixed the set of independent variables of the thermodynamic potential, while preserving both the original homogeneity and the validity of a first law. Our investigation explores EITs within the extended Iyer–Wald formalism for theories containing free parameters (e.g., the cosmological constant). EITs provide a unifying framework for reconciling the diverse formulations of Kerr-anti de Sitter (KadS) thermodynamics found in the literature. While the Iyer–Wald formalism is a powerful tool for deriving first laws for black holes, it typically yields a non-integrable mass variation that prevents its identification as a proper thermodynamic potential. To address this issue, we investigate an extended Iyer–Wald formalism where mass and thermodynamic volume become gauge dependent. Within this framework, we identify the gauge choices and Killing vector normalizations that are compatible with EITs, ensuring consistent first laws. As a key application, we demonstrate how conventional KadS thermodynamics emerges as a special case of our generalized approach. Full article

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

23 pages, 5059 KiB

Open AccessArticle

Outer Ionized Gas in Galaxy Group: Exchance Through Tidal Interaction or Accretion from Common Reservoirs?

by Olga Sil’chenko, Alexei Moiseev, Alexandrina Smirnova, Yael Kosareva and Dmitry Oparin

Universe 2025, 11(7), 214; https://doi.org/10.3390/universe11070214 - 27 Jun 2025

Abstract

To clarify the problem of outer cold gas accretion onto disk galaxies, we performed the panoramic spectroscopy of six compact galaxy groups to search for intergalactic gas flows. The groups selected are partly known to possess HI data obtained in the 21 cm [...] Read more.

To clarify the problem of outer cold gas accretion onto disk galaxies, we performed the panoramic spectroscopy of six compact galaxy groups to search for intergalactic gas flows. The groups selected are partly known to possess HI data obtained in the 21 cm line, and most of them contain a member galaxy revealing decoupled kinematics of gas and stars and thus having recently experienced a gas accretion event. Fabry-Perot scanning interferometry performed at the Russian 6 m telescope has provided us with the group maps at H

α

emission-line intensity and with ionized-gas velocity maps. We detected several intergalactic ionized-gas flows and some tidal outer ionized-gas structures; but none of them can be a source of gas accretion onto neighboring galaxies with decoupled gas–star kinematics. Only in a single case, that of NGC 7465, we can relate the inner inclined gaseous disk with the outer gas inflow; but the origin of this gas stream remains unknown—it does not originate from the neighboring NGC 7463 or NGC 7464. Full article

(This article belongs to the Section Galaxies and Clusters)

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12 pages, 1540 KiB

Open AccessReview

Gravitational Waves: Echoes of the Biggest Bangs Since the Big Bang and/or BSM Physics?

by John Ellis

Universe 2025, 11(7), 213; https://doi.org/10.3390/universe11070213 - 26 Jun 2025

Abstract

“If one could ever prove the existence of gravitational waves, the processes responsible for their generation would probably be much more curious and interesting than even the waves themselves.” (Gustav Mie, 1868–1957). The discovery of gravitational waves has opened new windows on [...] Read more.

“If one could ever prove the existence of gravitational waves, the processes responsible for their generation would probably be much more curious and interesting than even the waves themselves.” (Gustav Mie, 1868–1957). The discovery of gravitational waves has opened new windows on astrophysics, cosmology and physics beyond the Standard Model (BSM). Measurements by the LIGO, Virgo and KAGRA Collaborations of stellar–mass binaries and neutron star mergers have shown that gravitational waves travel at close to the velocity of light and constrain BSM possibilities, such as a graviton mass and Lorentz violation in gravitational wave propagation. Follow-up measurements of neutron star mergers have provided evidence for the production of heavy elements, possibly including some essential for human life. The gravitational waves in the nanoHz range observed by Pulsar Timing Arrays (PTAs) may have been emitted by supermassive black hole binaries, but might also have originated from BSM cosmological scenarios such as cosmic strings, or phase transitions in the early Universe. The answer to the question in the title may be provided by gravitational-wave detectors at higher frequencies, such as LISA and atom interferometers. KCL-PH-TH/2024-05. Full article

(This article belongs to the Special Issue Particle Physics and Cosmology: A Themed Issue in Honor of Professor Dimitri Nanopoulos)

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13 pages, 1632 KiB

Open AccessArticle

Cosmological Simulations with Massive Neutrinos: Efficiency and Accuracy

by Bing-Hang Chen, Jun-Jie Zhao, Hao-Ran Yu, Yu Liu, Jian-Hua He and Yipeng Jing

Universe 2025, 11(7), 212; https://doi.org/10.3390/universe11070212 - 26 Jun 2025

Abstract

Constraining neutrino mass through cosmological observations relies on precise simulations to calibrate their effects on large scale structure, while these simulations must overcome computational challenges like dealing with large velocity dispersions and small intrinsic neutrino perturbations. We present an efficient N-body implementation [...] Read more.

Constraining neutrino mass through cosmological observations relies on precise simulations to calibrate their effects on large scale structure, while these simulations must overcome computational challenges like dealing with large velocity dispersions and small intrinsic neutrino perturbations. We present an efficient N-body implementation with semi-linear neutrino mass response which gives accurate power spectra and halo statistics. We explore the necessity of correcting the expansion history caused by massive neutrinos and the transition between relativistic and non-relativistic components. The above method of including neutrino masses is built into the memory-, scalability-, and precision-optimized parallel N-body simulation code CUBE 2.0. Through a suite of neutrino simulations, we precisely quantify the neutrino mass effects on the nonlinear matter power spectra and halo statistics. Full article

(This article belongs to the Special Issue New Insights into High-Energy Astrophysics, Galaxies, and Cosmology—Celebrating the 10th Anniversary of the Re-establishment of the Department of Astronomy at Xiamen University (2012–2022))

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26 pages, 491 KiB

Open AccessArticle

Remarkable Scale Relation, Approximate SU(5), Fluctuating Lattice

by Holger B. Nielsen

Universe 2025, 11(7), 211; https://doi.org/10.3390/universe11070211 - 26 Jun 2025

Abstract

In this study, we discuss a series of eight energy scales, some of which are our own speculations, and fit the logarithms of these energies as a straight line versus a quantity related to the dimensionalities of action terms in a way to [...] Read more.

In this study, we discuss a series of eight energy scales, some of which are our own speculations, and fit the logarithms of these energies as a straight line versus a quantity related to the dimensionalities of action terms in a way to be defined in the article. These terms in the action are related to the energy scales in question. So, for example, the dimensionality of the Einstein–Hilbert action coefficient is one related to the Planck scale. In fact, we suppose that, in the cases described with quantum field theory, there is, for each of our energy scales, a pair of associated terms in the Lagrangian density, one “kinetic” and one “mass or current” term. To plot the energy scales, we use the ratio of the dimensionality of, say, the “non-kinetic” term to the dimensionality of the “kinetic” one. For an explanation of our phenomenological finding that the logarithm of the energies depends, as a straight line, on the dimensionality defined integer q, we give an ontological—i.e., it really exists in nature in our model—“fluctuating lattice” with a very broad distribution of, say, the link size a. We take the Gaussian in the logarithm,

ln (a)

. A fluctuating lattice is very natural in a theory with general relativity, since it corresponds to fluctuations in the gauge depth of the field of general relativity. The lowest on our energy scales are intriguing, as they are not described by quantum field theory like the others but by actions for a single particle or single string, respectively. The string scale fits well with hadronic strings, and the particle scale is presumably the mass scale of Standard Model group monopoles, the bound state of a couple of which might be the dimuon resonance (or statistical fluctuation) found in LHC with a mass of 28 GeV. Full article

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

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25 pages, 8519 KiB

Open AccessArticle

Probing Equatorial Ionospheric TEC at Sub-GHz Frequencies with Wide-Band (B4) uGMRT Interferometric Data

by Dipanjan Banerjee, Abhik Ghosh, Sushanta K. Mondal and Parimal Ghosh

Universe 2025, 11(7), 210; https://doi.org/10.3390/universe11070210 - 26 Jun 2025

Abstract

Phase stability at low radio frequencies is severely impacted by ionospheric propagation delays. Radio interferometers such as the giant metrewave radio telescope (GMRT) are capable of detecting changes in the ionosphere’s total electron content (TEC) over larger spatial scales and with greater sensitivity [...] Read more.

Phase stability at low radio frequencies is severely impacted by ionospheric propagation delays. Radio interferometers such as the giant metrewave radio telescope (GMRT) are capable of detecting changes in the ionosphere’s total electron content (TEC) over larger spatial scales and with greater sensitivity compared to conventional tools like the global navigation satellite system (GNSS). Thanks to its unique design, featuring both a dense central array and long outer arms, and its strategic location, the GMRT is particularly well-suited for studying the sensitive ionospheric region located between the northern peak of the equatorial ionization anomaly (EIA) and the magnetic equator. In this study, we observe the bright flux calibrator 3C48 for ten hours to characterize and study the low-latitude ionosphere with the upgraded GMRT (uGMRT). We outline the methods used for wideband data reduction and processing to accurately measure differential TEC (

δ TEC

) between antenna pairs, achieving a precision of< mTECU (1 mTECU =

10^{- 3}

TECU) for central square antennas and approximately mTECU for arm antennas. The measured

δ TEC

values are used to estimate the TEC gradient across GMRT arm antennas. We measure the ionospheric phase structure function and find a power-law slope of

β = 1.72 \pm 0.07

, indicating deviations from pure Kolmogorov turbulence. The inferred diffractive scale, the spatial separation over which the phase variance reaches

1 {rad}^{2}

, is ∼6.66 km. The small diffractive scale implies high phase variability across the field of view and reduced temporal coherence, which poses challenges for calibration and imaging. Full article

(This article belongs to the Section Planetary Sciences)

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44 pages, 683 KiB

Open AccessReview

Structural Stability and General Relativity

by Spiros Cotsakis

Universe 2025, 11(7), 209; https://doi.org/10.3390/universe11070209 - 26 Jun 2025

Abstract

We review recent developments in structural stability as applied to key topics in general relativity. For a nonlinear dynamical system arising from the Einstein equations by a symmetry reduction, bifurcation theory fully characterizes the set of all stable perturbations of the system, known [...] Read more.

We review recent developments in structural stability as applied to key topics in general relativity. For a nonlinear dynamical system arising from the Einstein equations by a symmetry reduction, bifurcation theory fully characterizes the set of all stable perturbations of the system, known as the ‘versal unfolding’. This construction yields a comprehensive classification of qualitatively distinct solutions and their metamorphoses into new topological forms, parametrized by the codimension of the bifurcation in each case. We illustrate these ideas through bifurcations in the simplest Friedmann models, the Oppenheimer-Snyder black hole, the evolution of causal geodesic congruences in cosmology and black hole spacetimes, crease flow on event horizons, and the Friedmann–Lemaître equations. Finally, we list open problems and briefly discuss emerging aspects such as partial differential equation stability of versal families, the general relativity landscape, and potential connections between gravitational versal unfoldings and those of the Maxwell, Dirac, and Schrödinger equations. Full article

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

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18 pages, 1269 KiB

Open AccessArticle

Many Phases in a Hairy Box in Three Dimensions

by Shoichiro Miyashita

Universe 2025, 11(7), 208; https://doi.org/10.3390/universe11070208 - 25 Jun 2025

Abstract

In this paper, I investigate gravitational thermodynamics of the Einstein–Maxwell–scalar system in three dimensions without a cosmological constant. In a previous work by Krishnan, Shekhar, and Bala Subramanian, it was argued that this system has no BH saddles, but has only empty (flat [...] Read more.

In this paper, I investigate gravitational thermodynamics of the Einstein–Maxwell–scalar system in three dimensions without a cosmological constant. In a previous work by Krishnan, Shekhar, and Bala Subramanian, it was argued that this system has no BH saddles, but has only empty (flat space) saddles and boson star saddles. It was then concluded that the structure of the thermodynamic phase space is much simpler than in the higher-dimensional cases. I will show that, in addition to the known boson star and empty saddles, three more types of saddles exist in this system: the BG saddle, its hairy generalization, and a novel configuration called the boson star-PL saddle. As a result, the structure is richer than one might naively expect and is very similar to the higher-dimensional ones. Full article

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

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17 pages, 898 KiB

Open AccessArticle

Extraction of Physical Parameters of RRab Variables Using Neural Network Based Interpolator

by Nitesh Kumar, Harinder P. Singh, Oleg Malkov, Santosh Joshi, Kefeng Tan, Philippe Prugniel and Anupam Bhardwaj

Universe 2025, 11(7), 207; https://doi.org/10.3390/universe11070207 - 24 Jun 2025

Abstract

Determining the physical parameters of pulsating variable stars such as RR Lyrae is essential for understanding their internal structure, pulsation mechanisms, and evolutionary state. In this study, we present a machine learning framework that uses feedforward artificial neural networks (ANNs) to infer stellar [...] Read more.

Determining the physical parameters of pulsating variable stars such as RR Lyrae is essential for understanding their internal structure, pulsation mechanisms, and evolutionary state. In this study, we present a machine learning framework that uses feedforward artificial neural networks (ANNs) to infer stellar parameters—mass (M), luminosity (log(

L / L_{⊙}

)), effective temperature (log(

T_{eff}

)), and metallicity (Z)—directly from Transiting Exoplanet Survey Satellite (TESS) light curves. The network is trained on a synthetic grid of RRab light curves generated from hydrodynamical pulsation models spanning a broad range of physical parameters. We validate the model using synthetic self-inversion tests and demonstrate that the ANN accurately recovers the input parameters with minimal bias. We then apply the trained model to RRab stars observed by the TESS. The observed light curves are phase-folded, corrected for extinction, and passed through the ANN to derive physical parameters. Based on these results, we construct an empirical period–luminosity–metallicity (PLZ) relation: log(

L / L_{⊙}

) = (1.458 ± 0.028) log(P/days) + (–0.068 ± 0.007) [Fe/H] + (2.040 ± 0.007). This work shows that ANN-based light-curve inversion offers an alternative method for extracting stellar parameters from single-band photometry. The approach can be extended to other classes of pulsators such as Cepheids and Miras. Full article

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

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8 pages, 2549 KiB

Open AccessCommunication

Blinkverse 2.0: Updated Host Galaxies for Fast Radio Bursts

by Jiaying Xu, Chao-Wei Tsai, Sean E. Lake, Yi Feng, Xiang-Lei Chen, Di Li, Han Wang, Xuerong Guo, Jingjing Hu and Xiaodong Ge

Universe 2025, 11(7), 206; https://doi.org/10.3390/universe11070206 - 24 Jun 2025

Abstract

Studying the host galaxies of fast radio bursts (FRBs) is critical to understanding the formation processes of their sources and, hence, the mechanisms by which they radiate. Toward this end, we have extended the Blinkverse database version 1.0, which already included burst information [...] Read more.

Studying the host galaxies of fast radio bursts (FRBs) is critical to understanding the formation processes of their sources and, hence, the mechanisms by which they radiate. Toward this end, we have extended the Blinkverse database version 1.0, which already included burst information about FRBs observed by various telescopes, by adding information about 92 published FRB host galaxies to make version 2.0. Each FRB host has 18 parameters describing it, including redshift, stellar mass, star-formation rate, emission line fluxes, etc. In particular, each FRB host includes images collated by FASTView, streamlining the process of looking for clues to understanding the origin of FRBs. FASTView is a tool and API for quickly exploring astronomical sources using archival imaging, photometric, and spectral data. This effort represents the first step in building Blinkverse into a comprehensive tool for facilitating source observation and analysis. Full article

(This article belongs to the Special Issue Planetary Radar Astronomy)

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26 pages, 391 KiB

Open AccessArticle

Primordial Magnetogenesis from Killing Vector Fields

by Nagabhushana Prabhu

Universe 2025, 11(7), 205; https://doi.org/10.3390/universe11070205 - 23 Jun 2025

Abstract

Papapetrou showed that the covariant derivative of a Killing vector field satisfies Maxwell’s equations in vacuum. Papapetrou’s result is extended, in this article, and it is shown that the covariant derivative of a Killing vector field satisfies Maxwell’s equations in non-vacuum backgrounds as [...] Read more.

Papapetrou showed that the covariant derivative of a Killing vector field satisfies Maxwell’s equations in vacuum. Papapetrou’s result is extended, in this article, and it is shown that the covariant derivative of a Killing vector field satisfies Maxwell’s equations in non-vacuum backgrounds as well if one allows electromagnetic currents of purely geometric origin. It is then postulated that every Killing vector field gives rise to a physical electromagnetic field and, in a non-vacuum background, a physical electromagnetic current—hereafter called Killing electromagnetic field and Killing electromagnetic current, respectively. It is shown that the Killing electromagnetic field of the flat FLRW (Friedmann–Lema

\hat{i}

tre–Robertson–Walker) universe comprises a Killing magnetic field and a rotational Killing electric field; an upper bound on the Killing magnetic field is derived, and it is found that the upper bound is consistent with the current observational bounds on the cosmic magnetic field. Next, the time-like Killing vector of the Schwarzschild spacetime is shown to give rise to a radial Killing electric field. It is also shown that in the weak field regime—and far from the matter distribution—the back reaction of the radial Killing electric field changes the Schwarzschild metric to the Reissner–Nordström metric, establishing a partial converse of Wald’s result. Drawing upon Rainich’s work on Rainich–Riemann manifolds, the etiological question of how a physical electromagnetic field can arise out of geometry is discussed; it is also argued that detection of the Killing electric field of flat FLRW spacetime may be within the current experimental reach. Finally, this article discusses the relevance of Killing electromagnetic currents and the aforementioned transmutation of Schwarzschild spacetime to Reissner–Nordstrom spacetime, to Misner and Wheeler’s program of realizing “charge without charge”. Full article

(This article belongs to the Section Cosmology)

19 pages, 2592 KiB

Open AccessArticle

Investigating the Variation and Periodicity of TXS 0506+056

by Xianglin Miao and Yunguo Jiang

Universe 2025, 11(7), 204; https://doi.org/10.3390/universe11070204 - 23 Jun 2025

Abstract

TXS 0506+056 is a blazar associated with neutrino events. The study on its variation mechanics and periodicity analysis is meaningful to understand other BL Lac objects. The local cross-correlation function (LCCF) analysis presents a 3

σ

correlation in both the

γ

-ray versus [...] Read more.

TXS 0506+056 is a blazar associated with neutrino events. The study on its variation mechanics and periodicity analysis is meaningful to understand other BL Lac objects. The local cross-correlation function (LCCF) analysis presents a 3

σ

correlation in both the

γ

-ray versus optical and optical versus radio light curves. The time lag analysis suggests that the optical and

γ

-ray band share the same emission region, located upstream of the radio band in the jet. We use both the weighted wavelet Z-transform and generalized Lomb–Scargle methods to analyze the periodicity. We find two plausible quasi-periodic oscillations (QPOs) at

506_{- 56}^{+ 133}

days and

175_{- 7}^{+ 15}

days for the light curve of the optical band. For the

γ

-ray band, we find that the spectrum varies with the softer when brighter (SWB) trend, which could be explained naturally if a stable very high energy component exists. For the optical band, TXS 0506+056 exhibits a harder when brighter (HWB) trend. We discover a trend transition from HWB to SWB in the X-ray band, which could be modeled by the shift in peak frequency assuming that the X-ray emission is composed of the synchrotron and the inverse Compton (IC) components. The flux correlations of

γ

-ray and optical bands behave anomalously during the period of neutrino events, indicating that there are possible other hadronic components associated with neutrino. Full article

(This article belongs to the Special Issue Blazar Bursts: Theory and Observation)

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25 pages, 2524 KiB

Open AccessArticle

α Effect and Magnetic Diffusivity β in Helical Plasma Under Turbulence Growth

by Kiwan Park

Universe 2025, 11(7), 203; https://doi.org/10.3390/universe11070203 - 22 Jun 2025

Abstract

We investigate the transport coefficients

α

and

β

in plasma systems with varying Reynolds numbers while maintaining a unit magnetic Prandtl number (

{Pr}_{M}

). The

α

and

β

tensors parameterize the turbulent electromotive force (EMF) in terms of the large-scale magnetic [...] Read more.

We investigate the transport coefficients

α

and

β

in plasma systems with varying Reynolds numbers while maintaining a unit magnetic Prandtl number (

{Pr}_{M}

). The

α

and

β

tensors parameterize the turbulent electromotive force (EMF) in terms of the large-scale magnetic field

\bar{B}

and current density as follows:

⟨ u \times b ⟩ = α \bar{B} - β \nabla \times \bar{B}

. In astrophysical plasmas, high fluid Reynolds numbers (

R e

) and magnetic Reynolds numbers (

R e_{M}

) drive turbulence, where

R e

governs flow dynamics and

R e_{M}

controls magnetic field evolution. The coefficients

α_{semi}

and

β_{semi}

are obtained from large-scale magnetic field data as estimates of the

α

and

β

tensors, while

β_{theo}

is derived from turbulent kinetic energy data. The reconstructed large-scale field

\bar{B}

agrees with simulations, confirming consistency among

α

,

β

, and

\bar{B}

in weakly nonlinear regimes. This highlights the need to incorporate magnetic effects under strong nonlinearity. To clarify

α

and

β

, we introduce a field structure model, identifying

α

as the electrodynamic induction effect and

β

as the fluid-like diffusion effect. The agreement between our method and direct simulations suggests that plasma turbulence and magnetic interactions can be analyzed using fundamental physical quantities. Moreover,

α_{semi}

and

β_{semi}

, which successfully reproduce the numerically obtained magnetic field, provide a benchmark for future theoretical studies. Full article

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

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16 pages, 483 KiB

Open AccessArticle

Dynamical Black Holes and Accretion-Induced Backreaction

by Thiago de L. Campos, C. Molina and Mario C. Baldiotti

Universe 2025, 11(7), 202; https://doi.org/10.3390/universe11070202 - 20 Jun 2025

Abstract

We investigate the evolution of future trapping horizons through the dynamics of the Misner–Sharp mass using ingoing Eddington–Finkelstein coordinates. Our analysis shows that an integral formulation of Hayward’s first law governs much of the evolution of general spherically symmetric spacetimes. To account for [...] Read more.

We investigate the evolution of future trapping horizons through the dynamics of the Misner–Sharp mass using ingoing Eddington–Finkelstein coordinates. Our analysis shows that an integral formulation of Hayward’s first law governs much of the evolution of general spherically symmetric spacetimes. To account for the accretion backreaction, we consider a near-horizon approximation, which yields first-order corrections of a Vaidya-dark energy form. We further propose a systematic perturbative scheme to study these effects for an arbitrary background. As an application, we analyze an accreting Reissner–Nordström black hole and demonstrate the horizon shifts produced. Finally, we compute accretion-induced corrections to an extremal configuration. It is shown that momentum influx and energy density produce distinct effects: the former forces the splitting of the extremal horizon, while the latter induces significant displacements in its position, computed up to first-order perturbative corrections. These results highlight how different components of the stress–energy tensor significantly affect horizon geometry, with potential implications for broader areas of research, including black-hole thermodynamics. Full article

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

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21 pages, 2916 KiB

Open AccessArticle

Reissner–Nordström and Kerr-like Solutions in Finsler–Randers Gravity

by Georgios Miliaresis, Konstantinos Topaloglou, Ioannis Ampazis, Nefeli Androulaki, Emmanuel Kapsabelis, Emmanuel N. Saridakis, Panayiotis C. Stavrinos and Alkiviadis Triantafyllopoulos

Universe 2025, 11(7), 201; https://doi.org/10.3390/universe11070201 - 20 Jun 2025

Abstract

In a previous study we investigated the spherically symmetric Schwarzschild and Schwarzschild–de Sitter solutions within a Finsler–Randers-type geometry. In this work, we extend our analysis to charged and rotating solutions, focusing on the Reissner–Nordström and Kerr-like metrics in the Finsler–Randers gravitational framework. In [...] Read more.

In a previous study we investigated the spherically symmetric Schwarzschild and Schwarzschild–de Sitter solutions within a Finsler–Randers-type geometry. In this work, we extend our analysis to charged and rotating solutions, focusing on the Reissner–Nordström and Kerr-like metrics in the Finsler–Randers gravitational framework. In particular, we extract the modified gravitational field equations and we examine the geodesic equations, analyzing particle trajectories and quantifying the deviations from their standard counterparts. Moreover, we compare the results with the predictions of general relativity, and we discuss how potential deviations from Riemannian geometry could be reached observationally. Full article

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

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12 pages, 690 KiB

Open AccessArticle

An Overview of the MUSES Calculation Engine and How It Can Be Used to Describe Neutron Stars

by Mateus Reinke Pelicer, Veronica Dexheimer and Joaquin Grefa

Universe 2025, 11(7), 200; https://doi.org/10.3390/universe11070200 - 20 Jun 2025

Abstract

For densities beyond nuclear saturation, there is still a large uncertainty in the equations of state (EoSs) of dense matter that translate into uncertainties in the internal structure of neutron stars. The MUSES Calculation Engine provides a free and open-source composable workflow management [...] Read more.

For densities beyond nuclear saturation, there is still a large uncertainty in the equations of state (EoSs) of dense matter that translate into uncertainties in the internal structure of neutron stars. The MUSES Calculation Engine provides a free and open-source composable workflow management system, which allows users to calculate the EoSs of dense and hot matter that can be used, e.g., to describe neutron stars. For this work, we make use of two MUSES EoS modules, i.e., Crust Density Functional Theory and Chiral Mean Field model, with beta-equilibrium with leptons enforced in the Lepton module, then connected by the Synthesis module using different functions: hyperbolic tangent, generalized Gaussian, bump, and smoothstep. We then calculate stellar structure using the QLIMR module and discuss how the different interpolating functions affect our results. Full article

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

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17 pages, 1029 KiB

Open AccessArticle

Hot Holographic 2-Flavor Quark Star

by Le-Feng Chen, Jing-Yi Wu, Hao Feng, Tian-Shun Chen and Kilar Zhang

Universe 2025, 11(7), 199; https://doi.org/10.3390/universe11070199 - 20 Jun 2025

Abstract

Applying the holographic 2-flavor Einstein–Maxwell-dilaton model, the parameters of which are fixed by lattice QCD, we extract the equations of state for hot quark–gluon plasma around the critical point at

T = 182

MeV, and have corresponding quark star cores constructed. By further [...] Read more.

Applying the holographic 2-flavor Einstein–Maxwell-dilaton model, the parameters of which are fixed by lattice QCD, we extract the equations of state for hot quark–gluon plasma around the critical point at

T = 182

MeV, and have corresponding quark star cores constructed. By further adding hadron shells, the mass range of the whole stars spans from 2 to 17 solar masses, with the maximum compactness around 0.22. This result allows them to be black hole mimickers and candidates for gap events. The I–Love–Q–C relations are also analyzed, which show consistency with the neutron star cases when the discontinuity at the quark–hadron interface is not large. Furthermore, we illustrate the full parameter maps of the energy density and pressure as functions of the temperature and chemical potential and discuss the constant thermal conductivity case supposing a heat source inside. Full article

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

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50 pages, 8738 KiB

Open AccessReview

From Barthel–Randers–Kropina Geometries to the Accelerating Universe: A Brief Review of Recent Advances in Finslerian Cosmology

by Amine Bouali, Himanshu Chaudhary, Lehel Csillag, Rattanasak Hama, Tiberiu Harko, Sorin V. Sabau and Shahab Shahidi

Universe 2025, 11(7), 198; https://doi.org/10.3390/universe11070198 - 20 Jun 2025

Abstract

We present a review of recent developments in cosmological models based on Finsler geometry, as well as geometric extensions of general relativity formulated within this framework. Finsler geometry generalizes Riemannian geometry by allowing the metric tensor to depend not only on position but [...] Read more.

We present a review of recent developments in cosmological models based on Finsler geometry, as well as geometric extensions of general relativity formulated within this framework. Finsler geometry generalizes Riemannian geometry by allowing the metric tensor to depend not only on position but also on an additional internal degree of freedom, typically represented by a vector field at each point of the spacetime manifold. We examine in detail the possibility that Finsler-type geometries can describe the physical properties of the gravitational interaction, as well as the cosmological dynamics. In particular, we present and review the implications of a particular implementation of Finsler geometry, based on the Barthel connection, and of the

(α, β)

geometries, where

α

is a Riemannian metric, and

β

is a one-form. For a specific construction of the deviation part

β

, in these classes of geometries, the Barthel connection coincides with the Levi–Civita connection of the associated Riemann metric. We review the properties of the gravitational field, and of the cosmological evolution in three types of geometries: the Barthel–Randers geometry, in which the Finsler metric function F is given by

F = α + β

, in the Barthel–Kropina geometry, with

F = α^{2} / β

, and in the conformally transformed Barthel–Kropina geometry, respectively. After a brief presentation of the mathematical foundations of the Finslerian-type modified gravity theories, the generalized Friedmann equations in these geometries are written down by considering that the background Riemannian metric in the Randers and Kropina line elements is of Friedmann–Lemaitre–Robertson–Walker type. The matter energy balance equations are also presented, and they are interpreted from the point of view of the thermodynamics of irreversible processes in the presence of particle creation. We investigate the cosmological properties of the Barthel–Randers and Barthel–Kropina cosmological models in detail. In these scenarios, the additional geometric terms arising from the Finslerian structure can be interpreted as an effective geometric dark energy component, capable of generating an effective cosmological constant. Several cosmological solutions—both analytical and numerical—are obtained and compared against observational datasets, including Cosmic Chronometers, Type Ia Supernovae, and Baryon Acoustic Oscillations, using a Markov Chain Monte Carlo (MCMC) analysis. A direct comparison with the standard

Λ

CDM model is also carried out. The results indicate that Finslerian cosmological models provide a satisfactory fit to the observational data, suggesting they represent a viable alternative to the standard cosmological model based on general relativity. Full article

(This article belongs to the Special Issue Cosmological Models of the Universe)

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