Universe

36 pages, 527 KB

Open AccessArticle

The Most General Four-Derivative Unitary String Effective Action with Torsion and Stringy Running Vacuum Model Inflation: Old Ideas from a Modern Perspective

by Nick E. Mavromatos and George Panagopoulos

Universe 2026, 12(3), 90; https://doi.org/10.3390/universe12030090 - 22 Mar 2026

Viewed by 189

Abstract

The string-inspired running vacuum model (StRVM) of inflation is based on a Chern–Simons (CS) gravity effective action in which the only four-spacetime-derivative-order term is a gravitational anomalous CS–Pontryagin density coupled to an axion. In this work, we revisit curvature-squared string-inspired effective actions from [...] Read more.

The string-inspired running vacuum model (StRVM) of inflation is based on a Chern–Simons (CS) gravity effective action in which the only four-spacetime-derivative-order term is a gravitational anomalous CS–Pontryagin density coupled to an axion. In this work, we revisit curvature-squared string-inspired effective actions from the point of view of appropriate local field redefinitions, leaving the perturbative string scattering matrices invariant. We require simultaneously unitarity and torsion interpretation of the field strength of the Kalb–Ramond antisymmetric tensor, features characterizing the (3+1)-dimensional StRVM cosmology. Unlike the higher-dimensional case, the above features are possible in the context of (3+1)-dimensional spacetimes, obtained after string compactification. We demonstrate that the unitarity and torsion interpretation requirements lead to a single type of extra four-derivative terms in the effective gravitational action, not discussed in the previous literature on StRVM, which is, however, shown to be subleading by many orders of magnitude compared to the terms of the StRVM framework. Hence, its presence has no practical implications for the relevant inflationary (and, hence, postinflationary) physics of the StRVM. This demonstrates the phenomenological completeness of the StRVM cosmological scenario, which is thus fully embeddable in the UV-complete (quantum gravity-compatible) string theory framework. Full article

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

25 pages, 9187 KB

Open AccessArticle

Stereoscopic Observation of Recurrent Streamer Waves Driven by Successive Slow Coronal Mass Ejections

by Yuandeng Shen and Reetika Tiwari

Universe 2026, 12(3), 89; https://doi.org/10.3390/universe12030089 - 22 Mar 2026

Viewed by 273

Abstract

We report the stereoscopic observations of two recurrent streamer waves in a single streamer structure, utilizing coordinated observations from the SOHO, STEREO, and SDO missions. Contrary to the long-held view that fast coronal mass ejections (CMEs) are necessary drivers, we demonstrate that these [...] Read more.

We report the stereoscopic observations of two recurrent streamer waves in a single streamer structure, utilizing coordinated observations from the SOHO, STEREO, and SDO missions. Contrary to the long-held view that fast coronal mass ejections (CMEs) are necessary drivers, we demonstrate that these recurrent waves were excited by two consecutive slow CMEs (<500 km s⁻¹ accompanied by only modest flare activity. Three-dimensional reconstruction reveals that the first and second waves propagated with significant decelerations of −7.93 m s⁻² and −10.26 m s⁻², respectively. Their average amplitudes were

0.41 R_{⊙}

and

0.77 R_{⊙}

, wavelengths were

4.02 R_{⊙}

and

6.17 R_{⊙}

, and periods were 2.66 and 2.53 h, respectively. While the amplitude of the first wave declined with heliocentric distance (consistent with conventional energy convection), the second wave exhibited an intriguing increasing trend in amplitude. Both waves showed a linear increase in wavelength and period with distance, indicating a non-stationary and dispersive medium. Crucially, despite the disparity in driver energy and wave scales, the periods and their change rates remained nearly identical for both events. This provides compelling case-specific evidence that the streamer wave period is primarily determined by the inherent eigenmodes of the streamer plasma slab rather than the specific characteristics of the trigger. We conclude that the generation of observable streamer waves is a combined consequence of the streamer’s structural stability and the energy transfer efficiency of the triggering disturbance. Full article

(This article belongs to the Special Issue Oscillations and Instabilities of Solar Filaments)

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22 pages, 2970 KB

Open AccessArticle

K2 Photometry and Long-Term Hα Variability in Four Previously Unreported Be Stars

by Alan Wagner Pereira, Eduardo Janot-Pacheco, Jéssica Mayara Eidam, Bergerson Van Hallen Vieira da Silva, M. Cristina Rabello-Soares, Laerte Andrade and Marcelo Emilio

Universe 2026, 12(3), 88; https://doi.org/10.3390/universe12030088 - 20 Mar 2026

Viewed by 175

Abstract

Classical Be stars are key laboratories for investigating how rapid rotation, pulsations, and mass loss couple to the formation and evolution of circumstellar decretion disks. However, few studies have combined Kepler/K2 photometry with multi-epoch H

α

monitoring. Here we present four previously unclassified [...] Read more.

Classical Be stars are key laboratories for investigating how rapid rotation, pulsations, and mass loss couple to the formation and evolution of circumstellar decretion disks. However, few studies have combined Kepler/K2 photometry with multi-epoch H

α

monitoring. Here we present four previously unclassified Be-type variable stars observed by K2 (three in Campaign 11 and one in Campaign 15) and followed up with ground-based spectroscopy. We analyzed public PDC light curves and extracted variability frequencies using Lomb–Scargle periodograms and iterative prewhitening with a conservative detection threshold of S/N ≥ 5. Optical spectra obtained at the Observatório Pico dos Dias (Brazil) over a multi-year baseline (2017–2025) include repeated H

α

observations and blue-region spectra for photospheric characterization. All targets show detectable K2 variability on timescales from hours to days, with frequency spectra ranging from close multi-periodic components producing beating patterns to power dominated by low frequencies. Each star exhibits H

α

emission at multiple epochs, with long-term changes in line-profile morphology and equivalent width, indicating disk variability on year-long timescales. These results demonstrate that disk evolution can occur without conspicuous photometric outbursts over the time span of space-based observations, highlighting the diagnostic value of combining high-precision space photometry with long-term spectroscopy to characterize multiscale variability in Galactic Be stars. Full article

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

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8 pages, 362 KB

Open AccessArticle

Multiplicity Dependence of Υ(nS) Mean Transverse Momentum in Proton–Proton Collisions

by Luis Gabriel Gallegos Mariñez, Lizardo Valencia Palomo and Luis Cedillo Barrera

Universe 2026, 12(3), 87; https://doi.org/10.3390/universe12030087 - 20 Mar 2026

Viewed by 185

Abstract

A correct description of quarkonia production and kinematics is still one of the most challenging assignments for Quantum Chromodynamics. This document presents a study of the

Υ

(1S), (2S) and (3S) mean transverse momentum (

⟨ p_{T}^{Υ} ⟩

) as a [...] Read more.

A correct description of quarkonia production and kinematics is still one of the most challenging assignments for Quantum Chromodynamics. This document presents a study of the

Υ

(1S), (2S) and (3S) mean transverse momentum (

⟨ p_{T}^{Υ} ⟩

) as a function of the charged particle multiplicity (

N_{Track}

) in proton–proton collisions at

\sqrt{s}

= 7 TeV generated with Pythia 8.312 CUETP8M1 tune. The comparison to real data collected by the CMS experiment indicates that the agreement is much better for the excited states than for the ground state. The observed fast increase in the

⟨ p_{T}^{Υ} ⟩

at small values of

N_{Track}

is mainly due to the contribution from the away region. Furthermore, when computing the

⟨ p_{T}^{Υ} ⟩

from jetty and isotropic events, a clear

p_{T}

hardening is observed in jetty events. Finally, analyzing the fragmentation of jets containing an

Υ

(nS), a new method is proposed to test the new quarkonia shower present in the Monte Carlo event generator. Full article

(This article belongs to the Special Issue Exploring the Heavy Ion Collisions in Particle Physics)

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18 pages, 27032 KB

Open AccessArticle

Research on Ionospheric Scintillation Effects and Prediction Model in East Asia Based on COSMIC-1 Occultation Dataset

by Yuqiang Zhang, Ting Lan, Xiang Wang, Bo Chen and Yi Liu

Universe 2026, 12(3), 86; https://doi.org/10.3390/universe12030086 - 20 Mar 2026

Viewed by 190

Abstract

In this study, the temporal and spatial distribution characteristics of ionospheric scintillation in the East Asian sector are statistically analyzed based on S4 data provided by the COSMIC-1 occultation dataset and solar–terrestrial spatial environment parameters from 2007 to 2018. The results show that [...] Read more.

In this study, the temporal and spatial distribution characteristics of ionospheric scintillation in the East Asian sector are statistically analyzed based on S4 data provided by the COSMIC-1 occultation dataset and solar–terrestrial spatial environment parameters from 2007 to 2018. The results show that scintillation activity has an obvious distribution pattern with local time: the frequency gradually increases from 17:00 in the evening, with the peak concentrated at 22:00–01:00 at night; in terms of seasonal variation, scintillation activity is highest in spring and fall, followed by summer, and lowest in winter; and, regarding annual variation, it is highly correlated with the solar activity. Further analyses show that scintillation activity is strongly correlated with geomagnetic activity. On this basis, this study constructs a two-layer LSTM deep learning model based on weighted regression to realize S4 numerical forecasting for the next 1 h in the middle- and low-latitude regions of China, using F10.7, Kp, Dst, sunspot number, solar wind vertical velocity, and historical S4 values as inputs. The model demonstrates robust predictive performance on the validation dataset containing 8760 samples, with a mean squared error of 0.00546 and an absolute error that is distributed within the interval [−0.2, 0.2] 98% of the time, indicating strong accuracy and robustness. These results suggest that the proposed model provides a high-precision tool for ionospheric scintillation warning. Full article

(This article belongs to the Section Space Science)

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18 pages, 3555 KB

Open AccessReview

The Potential for Hadronic Particle Acceleration in Galactic Pulsar Wind Nebulae

by Alison M. W. Mitchell and Samuel T. Spencer

Universe 2026, 12(3), 85; https://doi.org/10.3390/universe12030085 - 18 Mar 2026

Viewed by 295

Abstract

Pulsar wind nebulae (PWNe), formed when the wind originating from a rapidly rotating neutron star flows out into its surroundings, have now been observed across the electromagnetic spectrum from the radio to the PeV gamma-ray regime. For most of these sources, leptonic processes, [...] Read more.

Pulsar wind nebulae (PWNe), formed when the wind originating from a rapidly rotating neutron star flows out into its surroundings, have now been observed across the electromagnetic spectrum from the radio to the PeV gamma-ray regime. For most of these sources, leptonic processes, where electrons interacting with background photon fields produce high-energy photons through inverse Compton scattering, are believed to be the origin of associated very-high-energy gamma-ray emission. As such, these objects cannot contribute significantly to the galactic hadronic cosmic ray flux at ∼TeV-PeV energies. However, in a handful of cases, the possibility for an energetically sub-dominant hadron population being accelerated and producing very to ultra-high energy gamma-rays through pion decay has not yet been comprehensively excluded. Such scenarios have received renewed attention in the light of recent results from the Large High Altitude Air Shower Observatory (LHAASO). In this review, we explore the theoretical background positing hadronic acceleration in galactic PWNe, considering cases where the hadrons escape from the pulsar surface and/or are accelerated in the wind, as well as potential ‘shock mixing’ scenarios. We also explore current and future possible constraints on a hadronic component to PWNe from observations. Full article

(This article belongs to the Special Issue Studying Astrophysics with High-Energy Cosmic Particles)

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14 pages, 400 KB

Open AccessReview

Towards a Quantum Erlangen Program

by Matthew J. Lake

Universe 2026, 12(3), 84; https://doi.org/10.3390/universe12030084 - 16 Mar 2026

Viewed by 231

Abstract

The classical Erlangen Program sought to classify metric spaces entirely in terms of their symmetries. In physical spacetimes, these symmetries define transformations between classical reference frames, yielding a one-to-one correspondence between frame transformations and the underlying geometry. More recently, the classical notion of [...] Read more.

The classical Erlangen Program sought to classify metric spaces entirely in terms of their symmetries. In physical spacetimes, these symmetries define transformations between classical reference frames, yielding a one-to-one correspondence between frame transformations and the underlying geometry. More recently, the classical notion of an ideal frame has been extended to the quantum regime, by considering observers as embodied physical systems, subject to the laws of quantum mechanics. Here, we build on this approach, but outline an alternative definition of the term ‘quantum reference frame’, which differs somewhat from the mainstream view. We then show how the new definition can be used to construct a simple model of Planck-scale spacetime, which makes contact with existing quantum gravity phenomenology. Finally, we show how classical spacetime symmetries can be ‘mathematically preserved but operationally broken’ using the new model, suggesting that quantum spacetime may be classified, at least locally, in terms of transformations between quantised frames of reference. This work is based on a talk given at the 13th Bolyai–Gauss–Lobachevsky Conference on Non-Euclidean Geometry in Oujda, Morocco, in May 2025. Full article

(This article belongs to the Special Issue BGL2025: The 13th Bolyai–Gauss–Lobachevsky Conference on Non-Euclidean Geometry and Its Applications)

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9 pages, 1093 KB

Open AccessArticle

Spin-Charge-Induced Scalarization of Kerr–Newman Black Holes in the Einstein–Maxwell-Scalar Theory with Scalar Potential

by Xiang Luo, Meng-Yun Lai, Yun Soo Myung, Yi-Bin Huang and De-Cheng Zou

Universe 2026, 12(3), 83; https://doi.org/10.3390/universe12030083 - 16 Mar 2026

Viewed by 220

Abstract

We investigate the spin-charge-induced scalarization of Kerr–Newman (KN) black holes in the Einstein–Maxwell-scalar (EMS) theory with a scalar potential and positive coupling parameter. In the linearized theory, there exists a bound of

0 < a < a_{o}

with onset spin

a_{c}

[...] Read more.

We investigate the spin-charge-induced scalarization of Kerr–Newman (KN) black holes in the Einstein–Maxwell-scalar (EMS) theory with a scalar potential and positive coupling parameter. In the linearized theory, there exists a bound of

0 < a < a_{o}

with onset spin

a_{c}

for the negative region signaling instability by analyzing the effective scalar mass term in the

θ

-direction. Solving the (2 + 1)-dimensional evolution equation numerically, we find the region where the KN black hole becomes unstable, giving rise to scalarized KN black holes. The threshold curve for representing the boundary between stable and unstable KN black holes depends on charge Q, scalar mass

m_{ϕ}

, coupling parameter

α

, and spin parameter a with upper bound

a^{2} \leq M^{2} - Q^{2}

. Full article

(This article belongs to the Special Issue Hairy Black Holes: Insights and Advances)

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31 pages, 19106 KB

Open AccessArticle

Next-Generation Gravitational Redshift Tests Simulated Using an Optical Link and a High-Precision Cesium Atomic Clock in Space

by Abdelrahim Ruby, Wenbin Shen, Ahmed Shaker, Pengfei Zhang, Kuangchao Wu, Mostafa Ashry and Ziyu Shen

Universe 2026, 12(3), 82; https://doi.org/10.3390/universe12030082 - 15 Mar 2026

Viewed by 434

Abstract

The Atomic Clock Ensemble in Space (ACES) mission, currently operating aboard the International Space Station (ISS), is designed to provide high-precision time and frequency measurements and to test fundamental aspects of relativistic physics. Gravitational redshift (GRS), a fundamental prediction of General Relativity (GR), [...] Read more.

The Atomic Clock Ensemble in Space (ACES) mission, currently operating aboard the International Space Station (ISS), is designed to provide high-precision time and frequency measurements and to test fundamental aspects of relativistic physics. Gravitational redshift (GRS), a fundamental prediction of General Relativity (GR), implies that clocks positioned at different gravitational potentials experience relative time dilation. Previous GRS experiments have focused primarily on microwave technologies, with negligible experimental coverage in the optical domain, particularly for ground-to-space links. Motivated by the European Laser Timing (ELT) experiment and the high-precision laser-cooled cesium clock aboard ACES, we introduce and evaluate an optical time-transfer method designed to achieve high-accuracy measurements of GRS. In the absence of actual ELT/ACES optical data, a high-fidelity numerical simulation framework was developed to assess the performance of this method. The framework incorporates representative ELT/ACES mission parameters, including the space-based cesium clock and the H-MASER clock located at the reference ground station, both providing frequency stability at the level of

10^{- 15}

for 1000 s averaging time. Applying a

\pm 1 σ

filtering criterion, we obtain a simulated dataset comprising 33 ELT/ACES passes, representing a total observation time of 4.38 h over a single week. Analysis of this high-fidelity dataset reveals a GRS deviation from GR of

(- 7.19 \pm 0.63) \times 10^{- 5}

, achieving a 3.4 orders of magnitude improvement over the best previous laser-ranging experiment conducted at the University of Maryland (UMD), USA, 51 years ago. These simulation results demonstrate that the optical time-transfer link constitutes a powerful tool for testing fundamental physics and, when combined with next-generation optical atomic clocks, enables unprecedented capabilities in space-based timekeeping and geoscience applications. Full article

(This article belongs to the Section Gravitation)

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24 pages, 2114 KB

Open AccessArticle

Modified Teleparallel f(T) Gravity, DESI BAO and the H₀ Tension

by Mariam Bouhmadi-López, Carlos G. Boiza, Maria Petronikolou and Emmanuel N. Saridakis

Universe 2026, 12(3), 81; https://doi.org/10.3390/universe12030081 - 14 Mar 2026

Viewed by 258

Abstract

We investigate whether late-time modifications of gravity in the teleparallel framework can impact the current tension in the Hubble constant

H_{0}

, focusing on

f (T)

cosmology as a minimal and well-controlled extension of General Relativity. We consider three representative [...] Read more.

We investigate whether late-time modifications of gravity in the teleparallel framework can impact the current tension in the Hubble constant

H_{0}

, focusing on

f (T)

cosmology as a minimal and well-controlled extension of General Relativity. We consider three representative

f (T)

parametrisations that recover the teleparallel equivalent of General Relativity at early times and deviate from it only in late epochs. The models are confronted with unanchored Pantheon+ Type Ia supernovae, DESI DR2 baryon acoustic oscillations, compressed Planck cosmic microwave background distance priors, and redshift-space distortion data, allowing us to jointly probe the background expansion and the growth of cosmic structures. Two of the three models partially shift the inferred value of

H_{0}

towards local measurements, while the third worsens the discrepancy. This behaviour is directly linked to the effective torsional dynamics, with phantom-like regimes favouring higher

H_{0}

values and quintessence-like regimes producing the opposite effect. A global statistical comparison shows that the minimal

f (T)

extensions considered here are not favoured over ΛCDM by the combined data. Nevertheless, our results demonstrate that late-time torsional modifications can non-trivially redistribute current cosmological tensions among the background and growth sectors. Full article

(This article belongs to the Special Issue Exploring and Constraining Alternative Theories of Gravity)

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22 pages, 2651 KB

Open AccessArticle

Study of 27 Polluted White Dwarfs Observed by LAMOST: Spectral Features, Ages, and Kinematics

by Yuhong Deng and Yangping Luo

Universe 2026, 12(3), 80; https://doi.org/10.3390/universe12030080 - 13 Mar 2026

Viewed by 212

Abstract

We present a comprehensive spectral and kinematic analysis of 27 polluted white dwarfs selected from a published catalog of polluted white dwarf candidates. Using LAMOST DR9 and Gaia DR3 data, we derive the effective temperature (

T_{eff}

), surface gravity ( [...] Read more.

We present a comprehensive spectral and kinematic analysis of 27 polluted white dwarfs selected from a published catalog of polluted white dwarf candidates. Using LAMOST DR9 and Gaia DR3 data, we derive the effective temperature (

T_{eff}

), surface gravity (

\log g

), and radial velocity (

R V

), and we measure the Ca II K line parameters, including equivalent width (

E W_{Ca II K}

) and radial velocity (

R V_{Ca II K}

). In addition, we estimate cooling ages and determine the three-dimensional Galactic kinematics and orbital parameters. Our results show that the majority of the targets lie above the pure-ISM expectation for the Ca II K line, suggesting that the line primarily originates from circumstellar material (CSM) rather than the interstellar medium (ISM). For DA-type white dwarfs in our sample, the Ca II K absorption is more prominent at lower effective temperatures and becomes significantly weaker toward higher temperatures, consistent with previous studies of metal-polluted white dwarfs. Additionally, DA stars show prominent

E W_{Ca II K}

values primarily in the cooling-age bin of

0.9

–

1.4 Gyr

, whereas DB stars are concentrated in the

τ_{cool} ≲ 0.5 Gyr

range, with a similar trend of first increasing and then decreasing

E W_{Ca II K}

with cooling age. Kinematic analysis reveals no significant differences between the Galactic populations of DA and DB white dwarfs. These findings indicate that metal pollution is common across different disk components of the Galaxy, with evidence for ongoing or recurrent evolution of white dwarf planetary systems within various Galactic structures. Full article

(This article belongs to the Section Compact Objects)

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36 pages, 417 KB

Open AccessArticle

A Dynamical Approach to General Relativity Based on Proper Time

by Jaume de Haro

Universe 2026, 12(3), 79; https://doi.org/10.3390/universe12030079 - 12 Mar 2026

Viewed by 385

Abstract

This work places the invariant

d s^{2}

at the center of the gravitational interaction, interpreting it not as a purely geometric object but as the differential of proper time, endowed with direct physical meaning. Starting from the extension of Fermat’s principle to [...] Read more.

This work places the invariant

d s^{2}

at the center of the gravitational interaction, interpreting it not as a purely geometric object but as the differential of proper time, endowed with direct physical meaning. Starting from the extension of Fermat’s principle to massive particles—namely, the requirement that freely falling bodies follow trajectories that extremize proper time, which for timelike motion corresponds to a local maximum—and invoking the universality of Galilean free fall, we derive the form of

d s^{2}

in a static gravitational field. Lorentz invariance then provides the natural framework to extend this result to systems involving moving matter. The invariant derived through this procedure matches the weak-field limit of General Relativity formulated in the harmonic gauge. Within this linearized regime, we show that the structure of the theory already contains the seeds of its nonlinear completion: any dynamically consistent extension to strong gravitational fields necessarily involves the Ricci tensor. From this viewpoint, Einstein’s field equations appear not as a postulated geometric law but as the unique covariant closure required to ensure energy–momentum conservation and the self-consistency of the gravitational interaction. Full article

(This article belongs to the Special Issue Theoretical Physics and Cosmology: A Themed Issue in Honor of Professor Emilio Elizalde on the Occasion of His 75th Birthday)

11 pages, 341 KB

Open AccessArticle

Dynamical Classification of Galactic Open Clusters Using Virial Theorem

by Chaolin Yu, Zhongmu Li, Jie Lan and Bingjie Qian

Universe 2026, 12(3), 78; https://doi.org/10.3390/universe12030078 - 12 Mar 2026

Viewed by 257

Abstract

Open clusters are important tracers for studying the structure and evolution of the Milky Way, but determining their dynamical states and gravitational binding properties remains a complex task. In this study, we systematically analysed the gravitational binding states of 4809 candidate clusters by [...] Read more.

Open clusters are important tracers for studying the structure and evolution of the Milky Way, but determining their dynamical states and gravitational binding properties remains a complex task. In this study, we systematically analysed the gravitational binding states of 4809 candidate clusters by calculating their observed velocity dispersions and comparing these with theoretical velocity dispersions. We identified 3897 objects as gravitationally bound. Relative to previous classification results, this work achieves 93.60% precision and 80.04% recall, with recall increasing to 83.55% for the high-quality open cluster subset. For objects with discrepant classifications, we analysed their dynamical and photometric properties, finding that this work preferentially retains clusters with cleaner colour–magnitude diagram morphologies. This study provides a more conservative sample for studies of Galactic open clusters. Full article

(This article belongs to the Section Galaxies and Clusters)

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21 pages, 19705 KB

Open AccessArticle

Magnetohydrodynamic Simulations of Transonic Accretion Flows

by Raj Kishor Joshi, Antonios Tsokaros, Sanjit Debnath, Indranil Chattopadhyay and Ramiz Aktar

Universe 2026, 12(3), 77; https://doi.org/10.3390/universe12030077 - 10 Mar 2026

Viewed by 333

Abstract

Theoretical studies of transonic accretion onto black holes reveal a wide range of possible solutions, broadly classified into smooth flows and flows featuring shocks. Accretion solutions that involve the formation of shocks are particularly intriguing, as they are expected to naturally produce observable [...] Read more.

Theoretical studies of transonic accretion onto black holes reveal a wide range of possible solutions, broadly classified into smooth flows and flows featuring shocks. Accretion solutions that involve the formation of shocks are particularly intriguing, as they are expected to naturally produce observable variability features. However, despite their theoretical significance, time-dependent studies exploring the stability and evolution of such shocked solutions remain relatively scarce. To address this gap, we perform simulations of transonic accretion flows around a black hole in an ideal magnetohydrodynamic framework. Our simulations are initialized using boundary conditions derived from semi-analytical hydrodynamical models, allowing us to explore the stability of these flows under varying magnetic field strengths. Our results indicate that mildly magnetized flows in a uniform vertical magnetic field alter the accretion dynamics through magnetic pressure, with the resulting force imbalance driving oscillations in the shock front. Variations in the emitted luminosity arising from shock oscillations appear as quasi-periodic oscillations (QPOs), a characteristic feature commonly observed in accreting black holes. We find that the QPO frequency is determined by the radial position of the shock front: oscillations occurring closer to the black hole produce frequencies of tens of hertz, whereas shocks located farther out yield sub-hertz frequencies. Full article

(This article belongs to the Special Issue Mechanisms Behind Black Holes and Relativistic Jets)

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12 pages, 924 KB

Open AccessArticle

Exploring Particle Production and Thermal-like Behavior in Relativistic Particle Collisions Through Quantum Entanglement

by Alek Hutson and Rene Bellwied

Universe 2026, 12(3), 76; https://doi.org/10.3390/universe12030076 - 10 Mar 2026

Viewed by 266

Abstract

Thermal-like features in hadron production are observed in small systems such as proton–proton interactions, where conventional kinetic equilibration on sub-fm/c time scales is challenging to justify. One proposed explanation is that quantum entanglement in the incoming hadron wave functions, together with coarse-graining [...] Read more.

Thermal-like features in hadron production are observed in small systems such as proton–proton interactions, where conventional kinetic equilibration on sub-fm/c time scales is challenging to justify. One proposed explanation is that quantum entanglement in the incoming hadron wave functions, together with coarse-graining over unobserved degrees of freedom, can generate an entropy-like signal without requiring extensive final-state rescattering. We test whether a final-state Shannon entropy extracted from the charged-particle multiplicity distributions measured by ALICE at

\sqrt{s} = 0.9

–8 TeV can be reproduced by an initial-state entanglement entropy computed from leading-order proton PDFs. In a low-x approximation where the reduced density matrix of the probed region is taken to be maximally mixed in an effective parton-number basis, the entanglement entropy reduces to

S_{E E} ≃ \ln N

, where N is obtained by integrating PDFs over an x-range mapped from the ALICE midrapidity acceptance. We include gluon and sea-quark contributions and apply correction factors accounting for the charged fraction and the limited set of measured degrees of freedom. Within the stated assumptions and PDF uncertainties, the initial- and final-state entropy become numerically compatible toward low x, supporting the interpretation that initial-state quantum entanglement can contribute to the apparent thermal-like behavior in small collision systems. Full article

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

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16 pages, 360 KB

Open AccessArticle

Perturbative Analysis of Singularity-Free Cosmological Solutions in Unimodular Kaluza–Klein Theory

by Júlio C. Fabris, Stéfani Faller and Richard Kerner

Universe 2026, 12(3), 75; https://doi.org/10.3390/universe12030075 - 9 Mar 2026

Viewed by 181

Abstract

The unimodular version of the Kaluza–Klein theory is briefly discussed, and its projection onto four-dimensional spacetime is constructed. Imposing the unimodularity condition on the five-dimensional Kaluza–Klein metric, det

g_{A B} = 1

is equivalent to introducing a cosmological term in Einstein’s equations [...] Read more.

The unimodular version of the Kaluza–Klein theory is briefly discussed, and its projection onto four-dimensional spacetime is constructed. Imposing the unimodularity condition on the five-dimensional Kaluza–Klein metric, det

g_{A B} = 1

is equivalent to introducing a cosmological term in Einstein’s equations in four dimensions with a scalar field of the Brans–Dicke type. Singularity-free cosmological solutions with scalar field and matter sources are constructed, and their basic properties are analyzed In the present paper, attention is focused on the perturbative analysis of cosmological solutions, providing insights into their stability against small fluctuations. Full article

(This article belongs to the Section Cosmology)

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33 pages, 2581 KB

Open AccessReview

Regulatory and Spectrum Challenges for Passive Space Weather Monitoring

by Valeria Leite, Tarcisio Bakaus, Mateus Cardoso, Marco Antonio Bockoski de Paula and Alison Moraes

Universe 2026, 12(3), 74; https://doi.org/10.3390/universe12030074 - 5 Mar 2026

Viewed by 261

Abstract

Space weather monitoring depends critically on passive sensor systems that detect and measure natural solar and geospace emissions without transmitting radio frequency energy. These include riometers, solar radio monitors, interplanetary scintillation detectors, GNSS-based ionospheric sensors, and broadband solar spectrographs that enable the provision [...] Read more.

Space weather monitoring depends critically on passive sensor systems that detect and measure natural solar and geospace emissions without transmitting radio frequency energy. These include riometers, solar radio monitors, interplanetary scintillation detectors, GNSS-based ionospheric sensors, and broadband solar spectrographs that enable the provision of critical data required to forecast geomagnetic storms, protect critical infrastructures, and support aviation services, satellite operations, and defense services. However, with the increasing proliferation of radiocommunication technologies such as 5G/6G networks, dense HF/VHF/UHF deployments, and large constellations of low-Earth-orbit (LEO) satellites, the interference threat to these exceptionally sensitive receivers has grown. Most of these operate near the thermal noise floor and thus require strict protection criteria to ensure continuity of data. This review and perspective article provides a cross-disciplinary synthesis of scientific requirements, documented RFI case studies, and ongoing regulatory developments related to spectrum protection for passive space weather sensors. It systematically integrates perspectives on physical, technical, and regulatory aspects that are typically addressed separately in the literature. The article reviews the operating principles of major sensor classes and analyzes documented RFI cases affecting GNSS, riometers, CALLISTO, BINGO, and systems impacted by LEO satellite emissions, drawing from existing reports and regulatory submissions. Building on this evidence base, the work comparatively evaluates regulatory methods under consideration for WRC-27 shows that hybrid approaches combining primary allocations in core observation bands with secondary status and coordination procedures in adjacent bands offer the most viable path forward. This synthesis contextualizes and analyzes how technical protection criteria can be integrated with existing and evolving regulatory instruments to inform spectrum governance. The study concludes that without coordinated international spectrum management incorporating explicit protection thresholds and registration procedures, the long-term viability of space weather monitoring infrastructure faces significant risk in an increasingly congested radio frequency environment. Full article

(This article belongs to the Topic Techniques and Science Exploitations for Earth Observation and Planetary Exploration-2nd Edition)

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18 pages, 708 KB

Open AccessReview

Numerical Tools for Electroweak Phase Transition

by Xinran Zeng and Yang Zhang

Universe 2026, 12(3), 73; https://doi.org/10.3390/universe12030073 - 5 Mar 2026

Viewed by 344

Abstract

The electroweak phase transition serves as a crucial portal to explore physics beyond the Standard Model, with profound implications for gravitational waves, baryogenesis, dark matter, and vacuum stability. We review the computational workflow for analyzing cosmological phase transitions, which includes constructing the finite-temperature [...] Read more.

The electroweak phase transition serves as a crucial portal to explore physics beyond the Standard Model, with profound implications for gravitational waves, baryogenesis, dark matter, and vacuum stability. We review the computational workflow for analyzing cosmological phase transitions, which includes constructing the finite-temperature effective potential, identifying possible phases, tracing transition history, calculating transition rates, milestone temperatures, and thermal parameters, as well as the numerical tools developed for each step. We compare the functionalities, strategies, and applicable scopes of these tools, aiming to provide a practical guide that helps researchers select the most appropriate computational resources for their studies. Full article

(This article belongs to the Special Issue Search for New Physics Through Combined Approaches)

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20 pages, 342 KB

Open AccessArticle

Gross–Pitaevskii–Poisson Equations from a ξRϕ⁴ Non-Minimal Scalar-Curvature Coupling

by Bryan Cordero-Patino, Álvaro Duenas-Vidal and Jorge Segovia

Universe 2026, 12(3), 72; https://doi.org/10.3390/universe12030072 - 4 Mar 2026

Viewed by 269

Abstract

In cosmological scenarios where the Peccei–Quinn symmetry is broken after inflation, small-scale axion field inhomogeneities can undergo gravitational collapse, leading to the formation of bound structures. The dynamics of these systems are commonly described using cosmological perturbation theory applied to the Einstein–Klein–Gordon equations. [...] Read more.

In cosmological scenarios where the Peccei–Quinn symmetry is broken after inflation, small-scale axion field inhomogeneities can undergo gravitational collapse, leading to the formation of bound structures. The dynamics of these systems are commonly described using cosmological perturbation theory applied to the Einstein–Klein–Gordon equations. In the non-relativistic regime, this description reduces to the Gross–Pitaevskii–Poisson or Schrödinger–Poisson equations, depending on whether axion self-interactions are included. In this work, we extend the axion’s relativistic action by introducing a non-minimal scalar-curvature coupling of the form

ξ R ϕ^{4}

, which effectively induces a gravitationally mediated pairwise interaction. By performing a perturbative expansion and subsequently taking the non-relativistic limit, we derive a modified set of evolution equations governing the early stages of axion structure formation. Full article

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

5 pages, 218 KB

Open AccessEditorial

Editorial for Special Issue “Quantum Field Theory, 2nd Edition”

by Ralf Hofmann

Universe 2026, 12(3), 71; https://doi.org/10.3390/universe12030071 - 4 Mar 2026

Viewed by 344

Abstract

This second Special Issue on Quantum Field Theory is a continuation of the first Special Issue on quantum field theory [...] Full article

(This article belongs to the Special Issue Quantum Field Theory, 2nd Edition)

15 pages, 774 KB

Open AccessArticle

The Impact of Recent LUNA Measurements of NeNa Reactions on ²⁶Al Stellar Nucleosynthesis

by Umberto Battino, Tommaso Gallo, Diego Vescovi, Sergio Cristallo, Andreas Best, Oscar Straniero, Eliana Masha, Erin R. Higgins and Raphael Hirschi

Universe 2026, 12(3), 70; https://doi.org/10.3390/universe12030070 - 2 Mar 2026

Viewed by 476

Abstract

Recent measurements performed by the LUNA(Laboratory for Underground Nuclear Astrophysics) collaboration between 2019 and 2024 have provided the most precise direct determinations to date of several key reaction rates in the NeNa cycle, specifically the ²⁰Ne(p,γ)²¹Na [...] Read more.

Recent measurements performed by the LUNA(Laboratory for Underground Nuclear Astrophysics) collaboration between 2019 and 2024 have provided the most precise direct determinations to date of several key reaction rates in the NeNa cycle, specifically the ²⁰Ne(p,γ)²¹Na and the ²²Ne(p,γ)²³Na reactions, as well as its bridge to the MgAl cycle, i.e., the ²³Na(p,γ)²⁴Mg reaction. Despite their improved accuracy, these updated rates are not yet consistently incorporated into widely used nuclear reaction network compilations. We explore the astrophysical impact of adopting the new LUNA rates by performing nucleosynthesis calculations, focusing on the case of ²⁶Al nucleosynthesis and considering four different stellar environments: low-mass AGB stars, massive stars, very massive stars and core-collapse supernovae. Our results show substantial sensitivity of ²⁶Al production to the revised rates. In the AGB model, the surface ²⁶Al abundance decreases by up to 30%, while in the massive star model, the ²⁶Al abundance in the C-burning shell increases by 51%. In contrast, the impact on both the ²⁶Al yields ejected by very massive stars and on the explosive nucleosynthesis in the supernova model is negligible. These findings have direct implications for galactic chemical evolution, the global budget of ²⁶Al, and theoretical predictions of the ⁶⁰Fe/²⁶Al ratio, which will be critically tested by forthcoming γ-ray observations from missions such as the Compton Spectrometer and Imager (COSI). Full article

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

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9 pages, 234 KB

Open AccessArticle

Representation Formalism and Quantum Mechanics in Curved Spacetime

by Théophile Caby

Universe 2026, 12(3), 69; https://doi.org/10.3390/universe12030069 - 2 Mar 2026

Viewed by 349

Abstract

We extend the representation frame formalism, previously introduced to account for key cosmological observations in the Einstein static universe, to non-relativistic quantum mechanics. In this framework, each inertial observer is associated with a flat representation referential

R_{o b s}

, defined [...] Read more.

We extend the representation frame formalism, previously introduced to account for key cosmological observations in the Einstein static universe, to non-relativistic quantum mechanics. In this framework, each inertial observer is associated with a flat representation referential

R_{o b s}

, defined as the tangent space to the spatial manifold at the observer’s position, in which all measurements are represented. The Euclidean structure of

R_{o b s}

allows quantum systems to be described using the standard Schrödinger formalism, avoiding the technical ambiguities that arise when quantising directly on curved manifolds. We derive the relation between the Hamiltonian governing quantum dynamics in

R_{o b s}

and its counterpart defined on the physical manifold U, and show that curvature effects enter as observer-dependent modifications of effective potentials. Although the resulting quantum description depends on the observer’s representation frame, we show that this does not lead to contradictions between observers: consistency of measurement outcomes follows from the standard structure of quantum correlations established by physical interactions. We illustrate the formalism with explicit applications, including the hydrogen atom in an Einstein static universe and quantum systems in the vicinity of a black hole, highlighting how spacetime curvature manifests itself in the observer’s quantum description. Full article

(This article belongs to the Section Cosmology)

21 pages, 2810 KB

Open AccessArticle

Stability of Circular Orbits Around Kerr Black Holes Immersed in a Dehnen-Type Dark Matter Halo

by Yu Wang, Meilin Liu and Haiguang Xu

Universe 2026, 12(3), 68; https://doi.org/10.3390/universe12030068 - 28 Feb 2026

Viewed by 328

Abstract

We investigate the dynamical stability of circular orbits around a Kerr black hole embedded in a Dehnen-type dark matter halo. The effective spacetime metric of the combined system is constructed using the Newman–Janis algorithm, and the effective potential for test-particle motion in the [...] Read more.

We investigate the dynamical stability of circular orbits around a Kerr black hole embedded in a Dehnen-type dark matter halo. The effective spacetime metric of the combined system is constructed using the Newman–Janis algorithm, and the effective potential for test-particle motion in the equatorial plane is derived. The stability of circular orbits is analyzed through the Hessian matrix of the effective potential, while the stability strength and restoring-force distribution are employed to quantify the orbital response to small perturbations. Our results show that the presence of the dark matter halo significantly alters the spatial structure of stable circular orbits, leading to non-continuous stable regions whose location and extent depend sensitively on the halo’s characteristic density, scale radius, and the black hole spin. The innermost stable circular orbit (ISCO) is shifted relative to the vacuum Kerr case, with its position determined by the combined effects of the spin and halo parameters. Two-dimensional heatmaps, parameter scans, and three-dimensional visualizations systematically illustrate how the black hole spin and dark matter halo properties influence the ISCO and the distribution of stable orbits. Finally, we analyze the influence of the dark matter halo on the structure of the black hole event horizon. These results provide a detailed theoretical investigation of orbital dynamics around rotating black holes in dark-matter-rich environments. Full article

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

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20 pages, 532 KB

Open AccessArticle

Empirical Determination of β-Law Wind Acceleration Profiles in High-Mass X-Ray Binaries

by Zhantay Muratkhan, Manas Khassanov, Aliya Taukenova and Saken Toktarbay

Universe 2026, 12(3), 67; https://doi.org/10.3390/universe12030067 - 28 Feb 2026

Viewed by 381

Abstract

Stellar winds in high-mass X-ray binaries (HMXBs) are strongly modified by the presence of an accreting neutron star, yet the impact of X-ray photoionisation on the wind–acceleration profile remains difficult to quantify observationally. In this work, we combine the measured wind velocities at [...] Read more.

Stellar winds in high-mass X-ray binaries (HMXBs) are strongly modified by the presence of an accreting neutron star, yet the impact of X-ray photoionisation on the wind–acceleration profile remains difficult to quantify observationally. In this work, we combine the measured wind velocities at the neutron star orbital radius with spectroscopic terminal velocities in order to infer empirical

β

-law parameters for six well-studied HMXBs. By inverting the

β

-law, we reconstruct the individual acceleration curves

v (r)

and obtain revised estimates of the wind-acceleration parameters b and

β

for each system. A suggestive trend emerges from the reconstructed profiles: systems with lower terminal velocities tend to exhibit systematically larger acceleration indices

β

, consistent with the interpretation that dense, slowly accelerating winds may be more strongly affected by X-ray photoionisation. A secondary, weaker pattern is suggested between the orbital separation

a / R_{*}

and

β

, although, for our small sample, it is not statistically significant, suggesting that compact systems experience a more pronounced suppression of wind acceleration in the vicinity of the neutron star. Taken together, these indicative relations provide a coherent observational picture linking the global wind–velocity scale to the local radiative environment. The resulting acceleration profiles and system-to-system correlations offer a practical empirical foundation for modelling wind-fed accretion in HMXBs. The parameter set derived here can be directly incorporated into studies of quasi-spherical accretion, torque evolution, and the dynamical influence of X-ray photoionisation in massive binaries. Full article

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

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23 pages, 1003 KB

Open AccessArticle

Easing the Hubble Tension in f(R,L_m) Gravity: A Bayesian MCMC Analysis with CC and Pantheon Plus & SH0ES Datasets

by Archana Dixit, Saurabh Verma, Anirudh Pradhan and M. S. Barak

Universe 2026, 12(3), 66; https://doi.org/10.3390/universe12030066 - 27 Feb 2026

Viewed by 398

Abstract

In this study, we explored the cosmological implications of the modified gravity framework

f (R, L_{m})

, taking the specific form

f (R, L_{m}) = \frac{R}{2} + L_{m}^{n},

where n denotes [...] Read more.

In this study, we explored the cosmological implications of the modified gravity framework

f (R, L_{m})

, taking the specific form

f (R, L_{m}) = \frac{R}{2} + L_{m}^{n},

where n denotes the model parameter. The analysis was carried out within a spatially flat FLRW background by adopting the Barboza–Alcaniz (BA) parametrization for the dark energy equation of state, expressed as

ω (z) = w_{0} + w_{1} \frac{z (1 + z)}{1 + z^{2}} .

Based on this setup, an expression for the Hubble parameter

H (z)

was derived. The parameters

(H_{0}, n, w_{0}, w_{1})

were estimated using a Bayesian Markov Chain Monte Carlo (MCMC) technique, implemented via the emcee package, with Cosmic Chronometers (CC), Pantheon Plus & SH0ES (PPS) and DESI BAO datasets. For the CC+PPS+DESI BAO combination, the best-fit Hubble constant was obtained as

H_{0} = {72.08}_{- 0.24}^{+ 0.30} km s^{- 1} {Mpc}^{- 1},

which shows better consistency with the local SH0ES measurement than with the Planck

Λ

CDM result, thereby reducing the Hubble tension. Furthermore, the dynamical evolution of the equation of state parameter

ω

, the deceleration parameter, the impact of various energy conditions, and the optimal model parameters were thoroughly examined. The study also investigated the behavior of the

(O_{m})

diagnostic and determined the present age of the universe predicted by this model. Full article

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

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49 pages, 2852 KB

Open AccessArticle

Color–Distance Relations in Cometary Comae: A 14-Comet, Multi-Epoch Statistical Study

by Alberto Silva Betzler, Ingrid dos Santos Delfino, Agábio Brasil dos Santos and Orahcio Felicio de Sousa

Universe 2026, 12(3), 65; https://doi.org/10.3390/universe12030065 - 27 Feb 2026

Viewed by 290

Abstract

Color–distance relations in the comae of 14 comets are analyzed using homogeneous broadband UBV/BVRI photometry. The sample includes several inner-Solar-System–reaching comets, including a subset from near-Earth orbits in the dynamical sense (perihelion distance

q < 1.3

au), so the results are directly relevant [...] Read more.

Color–distance relations in the comae of 14 comets are analyzed using homogeneous broadband UBV/BVRI photometry. The sample includes several inner-Solar-System–reaching comets, including a subset from near-Earth orbits in the dynamical sense (perihelion distance

q < 1.3

au), so the results are directly relevant to the near-Earth meteoroid environment. For each comet, we combine robust color statistics, rank-correlation tests, and simple activity laws to define two empirical diagnostics: an absolute color at 1 au and a differential heliocentric color index that measures color changes with distance. The ensemble does not follow a single universal trend; instead, we identify three empirical classes. One class of comets shows significant color gradients, usually confined to blue-sensitive indices and consistent with varying gas-to-dust ratios along the orbit. A second class exhibits colors that are persistently redder than the Sun and are statistically consistent with being constant both with heliocentric distance and across perihelion. A third class of “step comets” shows discrete changes in color level between pre- and post-perihelion branches, most often in red or red–near-IR indices, with little or no monotonic color–distance correlation within each branch. Several objects therefore defy the intuitive expectation of becoming bluer as they approach the Sun, emphasizing that heliocentric color evolution is highly object-dependent and that multi-epoch color monitoring is essential for interpreting cometary coma behavior. Full article

(This article belongs to the Special Issue Detection and Tracking of Near-Earth Asteroids)

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19 pages, 7743 KB

Open AccessArticle

SpecZoo: An AI-Powered Platform for Spectral Analysis and Visualization in Science and Education

by Yuanhao Pu, Guohong Lei, Yang Xu, Xunzhou Chen and Haijun Tian

Universe 2026, 12(3), 64; https://doi.org/10.3390/universe12030064 - 27 Feb 2026

Viewed by 331

Abstract

Astronomical spectra, which encode rich astrophysical and chemical information, are fundamental to understanding celestial objects and universal laws. The advent of large-scale spectroscopic surveys, generating tens of millions of spectra, presents significant challenges for efficient data processing and analysis. To address these challenges, [...] Read more.

Astronomical spectra, which encode rich astrophysical and chemical information, are fundamental to understanding celestial objects and universal laws. The advent of large-scale spectroscopic surveys, generating tens of millions of spectra, presents significant challenges for efficient data processing and analysis. To address these challenges, we develop an AI-powered platform (named “SpecZoo”) for spectral visualization and analysis. This platform integrates modern information technology and machine learning to lower the barrier to spectral data utilization and enhance research efficiency. Its core functionalities include interactive visualization, automated spectral classification, physical parameter measurement, spectral annotation, and multi-band/multi-modal data fusion, all supported by flexible user and data management systems. It has become an essential tool for the National Astronomical Data Center, directly supporting spectral data processing and research for major projects including LAMOST, SDSS, DESI, and so on. Furthermore, the platform demonstrates strong potential for science-education integration, providing a novel resource for cultivating talent in astronomy and data science. Full article

(This article belongs to the Special Issue Astroinformatics and Big Data in Astronomy)

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17 pages, 29417 KB

Open AccessArticle

The Magnetic Helicity Driven Solar-Type Dynamo

by Valery V. Pipin

Universe 2026, 12(3), 63; https://doi.org/10.3390/universe12030063 - 25 Feb 2026

Viewed by 274

Abstract

(1) Theoretical studies have shown that large-scale vorticity generates a divergent-type helicity flux associated with small-scale magnetic fluctuations. Similar to the

α

-effect, this mechanism breaks the equatorial reflection symmetry of magnetic fluctuations in stellar convection zones. This contribution has been termed the [...] Read more.

(1) Theoretical studies have shown that large-scale vorticity generates a divergent-type helicity flux associated with small-scale magnetic fluctuations. Similar to the

α

-effect, this mechanism breaks the equatorial reflection symmetry of magnetic fluctuations in stellar convection zones. This contribution has been termed the new Vishniac flux (hereafter NV flux). (2) Methods: We employ a mean-field dynamo model to investigate the influence of the NV flux on solar-type dynamos. (3) Results: We find that the NV flux leads to an enhancement of the dynamo efficiency for the turbulent generation of the large-scale poloidal magnetic field in the Sun. The dynamical impact of the NV flux on the evolution of the magnetic field results in a concentration of dynamo waves toward the equatorial region. Using numerical simulations of the mean-field dynamo, we compare the helicity production rates arising from different turbulent dynamo mechanisms, namely the

α

-effect and the NV flux. The model demonstrates that the new dynamo source associated with large-scale vorticity and small-scale dynamo action leads to an amplification of poloidal field generation in the polar regions near the top of the dynamo domain. (4) Conclusions: Any fluctuating magnetic activity arising within the differentially rotating stellar convection zone can serve as an additional source for the generation of the large-scale poloidal magnetic field of a star. 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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24 pages, 5297 KB

Open AccessArticle

TEPCat: The Transiting Extrasolar Planet Catalogue

by John Southworth

Universe 2026, 12(3), 62; https://doi.org/10.3390/universe12030062 - 25 Feb 2026

Viewed by 357

Abstract

Transiting extrasolar planets are extraordinarily valuable for understanding the characteristics and formation of planets, because they are the only exoplanets whose physical and orbital properties can be measured to high precision. Thousands are now known, and it is important to maintain a database [...] Read more.

Transiting extrasolar planets are extraordinarily valuable for understanding the characteristics and formation of planets, because they are the only exoplanets whose physical and orbital properties can be measured to high precision. Thousands are now known, and it is important to maintain a database of them for use by the scientific community. TEPCat performs this task: it is a critical compilation of the physical and observable properties of the known transiting planetary systems. This work introduces the motivation for TEPCat, its scope, contents, and implementation. Example plots of interesting quantities are constructed. The classification of planets and of the eclipse features in their light curves is discussed. TEPCat is maintained and freely available online. Full article

(This article belongs to the Special Issue The Royal Road: Eclipsing Binaries and Transiting Exoplanets, 2nd Edition)

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24 pages, 985 KB

Open AccessReview

Neutrino Production Mechanisms in Strongly Magnetized Quark Matter: Current Status and Open Questions

by Igor A. Shovkovy and Ritesh Ghosh

Universe 2026, 12(3), 61; https://doi.org/10.3390/universe12030061 - 25 Feb 2026

Viewed by 349

Abstract

We review the main neutrino emission mechanisms operating in dense quark matter under strong magnetic fields, with particular emphasis on conditions expected in the interiors of compact stars. We discuss the direct Urca and neutrino synchrotron processes in unpaired quark matter, incorporating the [...] Read more.

We review the main neutrino emission mechanisms operating in dense quark matter under strong magnetic fields, with particular emphasis on conditions expected in the interiors of compact stars. We discuss the direct Urca and neutrino synchrotron processes in unpaired quark matter, incorporating the effects of Landau-level quantization. For the direct Urca process, the quantization of the electron energy spectrum plays a critical role, whereas quark quantization can often be neglected at sufficiently high baryon densities. The resulting field-dependent neutrino emissivity is anisotropic and exhibits an oscillatory behavior as a function of magnetic-field strength. We explore the implications of these effects for magnetar cooling and for possible anisotropic neutrino emission that could contribute to pulsar kicks. In addition, we review the

ν \bar{ν}

synchrotron emission process, which, although subdominant, provides valuable insights into the interplay between magnetic fields and weak interactions in dense quark matter. Overall, our analysis highlights the nontrivial influence of strong magnetic fields on neutrino production in magnetized quark cores, with potential consequences for the thermal and dynamical evolution of compact stars. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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Universe, Volume 12, Issue 3 (March 2026) – 32 articles

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