Universe

19 pages, 349 KiB

Open AccessArticle

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

by Domagoj Kuić, Alemka Knapp and Diana Šaponja-Milutinović

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

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

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

t = 0

are as follows: (1) their Fourier transform

f (p)

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

p = 0

and possibly others with

ℑ (p) < 0

; and (2)

f (p) / p

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

tanh (η t)

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

t = 0

. In the limit of large

η

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

(This article belongs to the Section Field Theory)

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26 pages, 2204 KiB

Open AccessReview

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

by Avrajit Bandyopadhyay and Timothy C. Beers

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

Abstract

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

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

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

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22 pages, 388 KiB

Open AccessArticle

Gauge-Invariant Slavnov–Taylor Decomposition for Trilinear Vertices

by Andrea Quadri

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

Abstract

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

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

(This article belongs to the Section Field Theory)

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15 pages, 914 KiB

Open AccessArticle

Spectral and Photometric Studies of NGC 7469 in the Optical Range

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

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

Abstract

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

M_{⊙} {yr}^{- 1}

. [...] Read more.

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

M_{⊙} {yr}^{- 1}

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

λ

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

β

and [NII]/H

α

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

900 - 1000 {cm}^{- 3}

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

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

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21 pages, 1200 KiB

Open AccessArticle

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

by Alla V. Suvorova and Alexei V. Dmitriev

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

Abstract

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

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

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

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12 pages, 694 KiB

Open AccessArticle

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

by Adriano Cherchiglia

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

Abstract

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

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

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

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35 pages, 715 KiB

Open AccessArticle

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

by David E. Alvarez-Castillo

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

Viewed by 252

Abstract

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

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

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

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14 pages, 281 KiB

Open AccessArticle

Leading Logarithm Quantum Gravity

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

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

Viewed by 88

Abstract

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

G H^{2}

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

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

G H^{2}

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

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

18 pages, 6896 KiB

Open AccessArticle

Relationship Between Recurrent Magnetic Flux Rope and Moving Magnetic Features

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

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

Viewed by 174

Abstract

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

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

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

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14 pages, 796 KiB

Open AccessArticle

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

by Rashmi Uniyal

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

Viewed by 249

Abstract

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

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

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

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14 pages, 272 KiB

Open AccessArticle

Constant Density Models in Einstein–Gauss–Bonnet Gravity

by Sunil D. Maharaj, Shavani Naicker and Byron P. Brassel

Universe 2025, 11(7), 220; https://doi.org/10.3390/universe11070220 - 2 Jul 2025

Viewed by 183

Abstract

We investigate the influence of the higher-order curvature corrections on a static configuration with constant density in Einstein–Gauss–Bonnet (EGB) gravity. This analysis is applied to both neutral and charged fluid distributions in arbitrary spacetime dimensions. The EGB field equations are generated, and the [...] Read more.

We investigate the influence of the higher-order curvature corrections on a static configuration with constant density in Einstein–Gauss–Bonnet (EGB) gravity. This analysis is applied to both neutral and charged fluid distributions in arbitrary spacetime dimensions. The EGB field equations are generated, and the condition of pressure isotropy is shown to generalise the general relativity equation. The gravitational potentials are unique in all spacetime dimensions for neutral gravitating spheres. Charged gravitating spheres are not unique and depend on the form of the electric field. Our treatment is extended to the particular case of a charged fluid distribution with a constant energy density and constant electric field intensity. The charged EGB field equations are integrated to give exact models in terms of hypergeometric functions which can also be written as a series. Full article

(This article belongs to the Special Issue General Relativity, Modified Theories of Gravity and Their Applications in Astrophysics)

18 pages, 353 KiB

Open AccessArticle

Massive Graviton from Diffeomorphism Invariance

by João M. L. de Freitas and Iberê Kuntz

Universe 2025, 11(7), 219; https://doi.org/10.3390/universe11070219 - 2 Jul 2025

Viewed by 111

Abstract

In this work, we undertake a comprehensive study of the functional measure of gravitational path integrals within a general framework involving non-trivial configuration spaces. As in Riemannian geometry, the integration over non-trival configuration spaces requires a metric. We examine the interplay between the [...] Read more.

In this work, we undertake a comprehensive study of the functional measure of gravitational path integrals within a general framework involving non-trivial configuration spaces. As in Riemannian geometry, the integration over non-trival configuration spaces requires a metric. We examine the interplay between the functional measure and the dynamics of spacetime for general configuration-space metrics. The functional measure gives an exact contribution to the effective action at the one-loop level. We discuss the implications and phenomenological consequences of this correction, shedding light on the role of the functional measure in quantum gravity theories. In particular, we describe a mechanism in which the graviton acquires a mass from the functional measure without violating the diffeomorphism symmetry nor including Stückelberg fields. Since gauge invariance is not violated, the number of degrees of freedom goes as in general relativity. For the same reason, Boulware–Deser ghosts and the vDVZ discontinuity do not show up. The graviton thus becomes massive at the quantum level while avoiding the usual issues of massive gravity. Full article

(This article belongs to the Section Gravitation)

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22 pages, 1173 KiB

Open AccessArticle

Galactic Cosmic Ray Interaction with the Perseus Giant Molecular Cloud Using Geant4 Monte Carlo Simulation

by Luan Torres and Luiz Augusto Stuani Pereira

Universe 2025, 11(7), 218; https://doi.org/10.3390/universe11070218 - 2 Jul 2025

Viewed by 201

Abstract

Galactic cosmic rays (GCRs), composed of protons and atomic nuclei, are accelerated in sources such as supernova remnants and pulsar wind nebulae, reaching energies up to the PeV range. As they propagate through the interstellar medium, their interactions with dense regions like molecular [...] Read more.

Galactic cosmic rays (GCRs), composed of protons and atomic nuclei, are accelerated in sources such as supernova remnants and pulsar wind nebulae, reaching energies up to the PeV range. As they propagate through the interstellar medium, their interactions with dense regions like molecular clouds produce secondary particles, including gamma-rays and neutrinos. In this study, we use the Geant4 Monte Carlo toolkit to simulate secondary particle production from GCR interactions within the Perseus molecular cloud, a nearby star-forming region. Our model incorporates realistic cloud composition, a wide range of incidence angles, and both hadronic and electromagnetic processes across a broad energy spectrum. The results highlight molecular clouds as significant sites of multi-messenger emissions and contribute to understanding the propagation of GCRs and the origin of diffuse gamma-ray and neutrino backgrounds in the Galaxy. Full article

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

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8 pages, 236 KiB

Open AccessArticle

Power of the Radiative Friction Force for a Charged Particle Performing a Flyby Near a Rotating Black Hole

by Elizabeth P. Tito and Vadim I. Pavlov

Universe 2025, 11(7), 217; https://doi.org/10.3390/universe11070217 - 1 Jul 2025

Viewed by 162

Abstract

We analytically obtain a relativistic generalization of the classical Larmor formula for the power of the radiation friction force

P = m c^{3} r_{e} (- w_{i} w^{i})

for the case where a relativistic charged particle moves in the vicinity of a rotating Kerr black hole. Full article

(This article belongs to the Section Gravitation)

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

Viewed by 169

Abstract

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, 318 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

Viewed by 115

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

Viewed by 174

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

Viewed by 154

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

Viewed by 169

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

Viewed by 133

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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24 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

Viewed by 215

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

Viewed by 130

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

Viewed by 151

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

Viewed by 229

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

Viewed by 150

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

Viewed by 147

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

Viewed by 212

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

Viewed by 120

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

Viewed by 160

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

Viewed by 177

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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Universe, Volume 11, Issue 7 (July 2025) – 33 articles

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