Universe

34 pages, 11750 KiB

Open AccessArticle

Accelerated and Energy-Efficient Galaxy Detection: Integrating Deep Learning with Tensor Methods for Astronomical Imaging

by Humberto Farias, Guillermo Damke, Mauricio Solar and Marcelo Jaque Arancibia

Universe 2025, 11(2), 73; https://doi.org/10.3390/universe11020073 - 18 Feb 2025

Viewed by 402

Abstract

Addressing the astronomical challenges posed by the interplay of data volume, AI sophistication, and energy consumption is crucial for the future of astronomy. As astronomical surveys continue to produce vast amounts of data, the computational and energy demands for galaxy classification have escalated, [...] Read more.

Addressing the astronomical challenges posed by the interplay of data volume, AI sophistication, and energy consumption is crucial for the future of astronomy. As astronomical surveys continue to produce vast amounts of data, the computational and energy demands for galaxy classification have escalated, necessitating more efficient and sustainable approaches. This study presents a novel application of tensor factorization within the Faster R-CNN framework, resulting in the development of our model, T-Faster R-CNN, designed to enhance both the energy efficiency and computational performance of deep learning models used in galaxy classification. By integrating tensor factorization, our T-Faster R-CNN significantly reduces the model’s complexity, memory footprint, and CO₂ emissions, while maintaining, and in some cases even improving, the accuracy of morphological classification. The effectiveness of this optimized model is validated using data from the Galaxy Zoo DECaLS, where it demonstrates substantial improvements in computational efficiency without compromising classification precision. Furthermore, this research incorporates green code principles, emphasizing reductions in energy consumption and environmental impact in computational astronomy. The T-Faster R-CNN model offers a resource-efficient, sustainable methodology for analyzing large-scale astronomical data, addressing the critical need for greener computational practices in the era of big data. Full article

(This article belongs to the Section Astroinformatics and Astrostatistics)

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43 pages, 1224 KiB

Open AccessArticle

Regularized Stress Tensor of Vector Fields in de Sitter Space

by Yang Zhang and Xuan Ye

Universe 2025, 11(2), 72; https://doi.org/10.3390/universe11020072 - 18 Feb 2025

Viewed by 346

Abstract

We study the Stueckelberg field in de Sitter space, which is a massive vector field with the gauge fixing (GF) term

\frac{1}{2 ζ} {({A^{μ}}_{; μ})}^{2}

. We obtain the vacuum stress tensor, which consists of the transverse, [...] Read more.

We study the Stueckelberg field in de Sitter space, which is a massive vector field with the gauge fixing (GF) term

\frac{1}{2 ζ} {({A^{μ}}_{; μ})}^{2}

. We obtain the vacuum stress tensor, which consists of the transverse, longitudinal, temporal, and GF parts, and each contains various UV divergences. By the minimal subtraction rule, we regularize each part of the stress tensor to its pertinent adiabatic order. The transverse stress tensor is regularized to the 0th adiabatic order, while the longitudinal, temporal, and GF stress tensors are regularized to the 2nd adiabatic order. The resulting total regularized vacuum stress tensor is convergent and maximally symmetric, has a positive energy density, and respects the covariant conservation, and thus, it can be identified as the cosmological constant that drives the de Sitter inflation. Under the Lorenz condition

{A^{μ}}_{; μ} = 0

, the regularized Stueckelberg stress tensor reduces to the regularized Proca stress tensor that contains only the transverse and longitudinal modes. In the massless limit, the regularized Stueckelberg stress tensor becomes zero, and is the same as that of the Maxwell field with the GF term, and no trace anomaly exists. If the order of adiabatic regularization were lower than our prescription, some divergences would remain. If the order were higher, say, under the conventional 4th-order regularization, more terms than necessary would be subtracted off, leading to an unphysical negative energy density and the trace anomaly simultaneously. Full article

(This article belongs to the Special Issue Origins and Natures of Inflation, Dark Matter and Dark Energy, 2nd Edition)

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18 pages, 4981 KiB

Open AccessArticle

Exploring the Variability of Three Be Stars with TESS Observations

by Laerte Andrade, Alan W. Pereira, Marcelo Emilio and Eduardo Janot-Pacheco

Universe 2025, 11(2), 71; https://doi.org/10.3390/universe11020071 - 18 Feb 2025

Viewed by 366

Abstract

Be stars are rapidly rotating B-type stars surrounded by circumstellar disks formed from self-ejected material. Understanding the mechanisms driving mass ejection and disk formation, known as the Be phenomenon, requires a detailed investigation of their variability and underlying physical processes. In this study, [...] Read more.

Be stars are rapidly rotating B-type stars surrounded by circumstellar disks formed from self-ejected material. Understanding the mechanisms driving mass ejection and disk formation, known as the Be phenomenon, requires a detailed investigation of their variability and underlying physical processes. In this study, we analyze the photometric, spectroscopic, and seismic characteristics of three Be stars—HD 212044, 28 Cyg, and HD 174237—using high-cadence data from the TESS mission and spectral data from the BeSS database. Photometric variability was analyzed through iterative prewhitening and wavelet techniques, revealing distinct frequency groups associated with non-radial pulsations (NRPs). Spectral data provided equivalent width measurements of the H

α

line, which correlated with photometric changes, reflecting dynamic interactions between the stars and their disks. Seismic analysis identified core rotation rates and buoyancy travel times for HD 212044 and 28 Cyg, offering insights into internal stellar processes and angular momentum distribution. HD 212044 exhibits a strong negative correlation between photometric brightness and H

α

equivalent width, whereas this correlation is weaker in the case of 28 Cyg. The findings for these two stars highlight the interplay between NRPs, rapid rotation, and circumstellar disk dynamics. In contrast, the case of HD 174237 presents an example of how a binary system with mass transfer and a B-type component is revealed when observed simultaneously with space-based photometry and ground-based spectroscopy, demonstrating the importance of distinguishing classical Be stars from interacting binaries. Full article

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

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13 pages, 2913 KiB

Open AccessArticle

Low-Latitude Ionospheric and Geomagnetic Disturbances Caused by the X7.13 Solar Flare of 25 February 2014

by Zane Nikia C. Domingo, Ernest P. Macalalad and Akimasa Yoshikawa

Universe 2025, 11(2), 70; https://doi.org/10.3390/universe11020070 - 17 Feb 2025

Viewed by 440

Abstract

On 25 February 2014 at around 00:39 UT, a major solar flare (code: SOL2014-02-25T00:39) erupted at sunspot region AR11990. Using the updated science quality data of GOES-15, it has been classified as an X7.13 solar flare. This gave rise to the electron density [...] Read more.

On 25 February 2014 at around 00:39 UT, a major solar flare (code: SOL2014-02-25T00:39) erupted at sunspot region AR11990. Using the updated science quality data of GOES-15, it has been classified as an X7.13 solar flare. This gave rise to the electron density changes that affected the strengths of ionospheric electric currents. In this work, the difference in total electron content (TEC), between the TEC during a flare day and a quiet, fitted TEC,

Δ T E C

, and rate of change of TEC,

d T E C / d t

, are determined to observe electron density changes due to the solar flare over a low-latitude region. These stations are at Quezon City (PIMO) and Taguig City (PTAG). Also, responses in the geomagnetic field component,

Δ H

, are calculated along with the variations in the equatorial electrojet (EEJ) strength. These are observed at equatorial, Davao (DAV) and Cagayan de Oro (CDO), and off-equatorial, Muntinlupa (MUT) and Legazpi (LGZ), stations. The resulting

Δ T E C

values were 1.17–1.97 TECU while

d T E C / d t

maxima were 0.29–0.48 TECU/min. The

d T E C / d t

maxima were found to concur with the time the solar EUV reached peak intensity at 00:45 UT, 4 min before the flare (i.e., X-ray) peaked. Furthermore, the

Δ H

variations exhibited larger enhancements at the equatorial stations. These are mostly attributed to the EEJ contributing to the geomagnetic field variations. The amplification experienced by the EEJ due to the increased ionospheric conductivity is then reflected in the geomagnetic responses. For the CDO-LGZ stations, the EEJ strength reached ~37 nT, while for the DAV-MUT, this was ~60 nT. Full article

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

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48 pages, 3794 KiB

Open AccessReview

Super-Eddington Accretion in Quasars

by Paola Marziani, Karla Garnica Luna, Alberto Floris, Ascensión del Olmo, Alice Deconto-Machado, Tania M. Buendia-Rios, C. Alenka Negrete and Deborah Dultzin

Universe 2025, 11(2), 69; https://doi.org/10.3390/universe11020069 - 17 Feb 2025

Viewed by 453

Abstract

This review provides an observational perspective on the fundamental properties of super-Eddington accretion onto supermassive black holes in quasars. It begins by outlining the selection criteria, particularly focusing on optical and UV broad-line intensity ratios, used to identify a population of unobscured super-Eddington [...] Read more.

This review provides an observational perspective on the fundamental properties of super-Eddington accretion onto supermassive black holes in quasars. It begins by outlining the selection criteria, particularly focusing on optical and UV broad-line intensity ratios, used to identify a population of unobscured super-Eddington candidates. Several defining features place these candidates at the extreme end of the Population A in main sequence of quasars: among them are the highest observed singly-ionized iron emission, extreme outflow velocities in UV resonance lines, and unusually high metal abundances. These key properties reflect the coexistence of a virialized sub-system within the broad-line region alongside powerful outflows, with the observed gas enrichment likely driven by nuclear or circumnuclear star formation. The most compelling evidence for the occurrence of super-Eddington accretion onto supermassive black holes comes from recent observations of massive black holes at early cosmic epochs. These black holes require rapid growth rates that are only achievable through radiatively inefficient super-Eddington accretion. Furthermore, extreme Eddington ratios, close to or slightly exceeding unity, are consistent with the saturation of radiative output per unit mass predicted by accretion disk theory for super-Eddington accretion rates. The extreme properties of super-Eddington candidates suggest that these quasars could make them stable and well-defined cosmological distance indicators, leveraging the correlation between broad-line width and luminosity expected in virialized systems. Finally, several analogies with accretion processes around stellar-mass black holes, particularly in the high/soft state, are explored to provide additional insight into the mechanisms driving super-Eddington accretion. Full article

(This article belongs to the Special Issue A Multimessenger View of Supermassive Black Holes and the Quasar Main Sequence)

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16 pages, 808 KiB

Open AccessArticle

Modern Bayesian Sampling Methods for Cosmological Inference: A Comparative Study

by Denitsa Staicova

Universe 2025, 11(2), 68; https://doi.org/10.3390/universe11020068 - 17 Feb 2025

Cited by 1 | Viewed by 283

Abstract

We present a comprehensive comparison of different Markov chain Monte Carlo (MCMC) sampling methods, evaluating their performance on both standard test problems and cosmological parameter estimation. Our analysis includes traditional Metropolis–Hastings MCMC, Hamiltonian Monte Carlo (HMC), slice sampling, nested sampling as implemented in [...] Read more.

We present a comprehensive comparison of different Markov chain Monte Carlo (MCMC) sampling methods, evaluating their performance on both standard test problems and cosmological parameter estimation. Our analysis includes traditional Metropolis–Hastings MCMC, Hamiltonian Monte Carlo (HMC), slice sampling, nested sampling as implemented in dynesty, and PolyChord. We examine samplers through multiple metrics including runtime, memory usage, effective sample size, and parameter accuracy, testing their scaling with dimension and response to different probability distributions. While all samplers perform well with simple Gaussian distributions, we find that HMC and nested sampling show advantages for more complex distributions typical of cosmological problems. Traditional MCMC and slice sampling become less efficient in higher dimensions, while nested methods maintain accuracy but at higher computational cost. In cosmological applications using BAO data, we observe similar patterns, with particular challenges arising from parameter degeneracies and poorly constrained parameters. Full article

(This article belongs to the Section Cosmology)

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12 pages, 788 KiB

Open AccessArticle

Issues in the Investigations of the Dark Matter Phenomenon in Galaxies: Parcere Personis, Dicere de Vitiis

by Paolo Salucci

Universe 2025, 11(2), 67; https://doi.org/10.3390/universe11020067 - 17 Feb 2025

Viewed by 685

Abstract

It is always more evident that the kinematics of galaxies provide us with unique information on the Nature of the dark particles and on the properties of the galaxy Dark Matter (DM) halos. However, in investigating this topic, we have to be very [...] Read more.

It is always more evident that the kinematics of galaxies provide us with unique information on the Nature of the dark particles and on the properties of the galaxy Dark Matter (DM) halos. However, in investigating this topic, we have to be very careful about certain issues related to the assumptions that we take or to the practices that we follow. Here, we critically discuss such issues, that, today, result of fundamental importance, in that we have realized that the Nature of the DM will be not provided by “The Theory” but, has to be inferred by reverse engineering the observational scenario. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

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21 pages, 419 KiB

Open AccessArticle

The Impact of Electric Currents on Majorana Dark Matter at Freeze Out

by Lukas Karoly and David C. Latimer

Universe 2025, 11(2), 66; https://doi.org/10.3390/universe11020066 - 14 Feb 2025

Viewed by 412

Abstract

Thermal relics with masses in the GeV to TeV range remain possible candidates for the Universe’s dark matter (DM). These neutral particles are often assumed to have vanishing electric and magnetic dipole moments so that they do not interact with single real photons, [...] Read more.

Thermal relics with masses in the GeV to TeV range remain possible candidates for the Universe’s dark matter (DM). These neutral particles are often assumed to have vanishing electric and magnetic dipole moments so that they do not interact with single real photons, but the anapole moment, a static electromagnetic property whose features are akin to that of a classical toroidal solenoid, can still be non-zero, permitting interactions with single virtual photons. In some models, DM predominantly annihilates into charged standard model particles through a p-wave process mediated by the anapole moment. The anapole moment is also responsible for another interaction of interest. If a DM medium were subjected to an electric current, a DM particle whose anapole moment was aligned with the current would have lower energy than the state with an antialigned anapole moment. Given these interactions, if a collection of initially unpolarized DM particles were subjected to an electric current, then the DM medium would become partially polarized, according to the Boltzmann distribution. In such a polarized medium, DM annihilation into photons, a subdominant s-wave process realizable through higher order interactions, would be somewhat suppressed. If the local electric current existed during a time in which the DM begins to drop out of thermal equilibrium with the rest of the Universe, the suppressed annihilation could lead to a small local excess in the relic DM density relative to a current-free region. This mechanism by which the local DM density can be perturbed is novel. Using effective interactions to model a DM particle’s anapole moment and polarizabilities (responsible for s-wave annihilation into two photons), we compute the changes in the DM density produced by long- and short-lived currents around freeze out. If we employ the most stringent constraints on DM annihilation into two photons, we find that long-lived currents can result in a fractional change in the DM density on the order of

10^{- 17}

for DM masses around 100 GeV; for short-lived currents, this fractional change in local DM density is on the order of

10^{- 23}

for the same DM mass. Full article

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

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18 pages, 1457 KiB

Open AccessArticle

Intermediate Coupling Regime in Dilatonic f(R,T) Inflationary Universe

by Francisco A. Brito, Carlos H. A. B. Borges, Jose A. V. Campos and Francisco G. Costa

Universe 2025, 11(2), 65; https://doi.org/10.3390/universe11020065 - 13 Feb 2025

Viewed by 496

Abstract

In the present work, we study cosmology in dilatonic

f (R, T)

gravity to address the inflationary phase of the early universe. As usual, in dilatonic gravity, the scalar potential assumes the exponential form. However, this potential is not good [...] Read more.

In the present work, we study cosmology in dilatonic

f (R, T)

gravity to address the inflationary phase of the early universe. As usual, in dilatonic gravity, the scalar potential assumes the exponential form. However, this potential is not good enough to be in accordance with the Planck 2018 data. More strikingly, the generalized

β

-exponential cannot take this into account either. It is just only presence of the dilatonic sector, in the intermediate coupling regime, that can help the theory to be in full accordance with the observational data. Full article

(This article belongs to the Section Cosmology)

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19 pages, 288 KiB

Open AccessArticle

On the Perturbed Friedmann Equations in Newtonian Gauge

by Jaume de Haro, Emilio Elizalde and Supriya Pan

Universe 2025, 11(2), 64; https://doi.org/10.3390/universe11020064 - 13 Feb 2025

Viewed by 511

Abstract

Based on Newtonian mechanics, in this article, we present a heuristic derivation of the Friedmann equations, providing an intuitive foundation for these fundamental relations in cosmology. Additionally, using the first law of thermodynamics and Euler’s equation, we derive a set of equations that, [...] Read more.

Based on Newtonian mechanics, in this article, we present a heuristic derivation of the Friedmann equations, providing an intuitive foundation for these fundamental relations in cosmology. Additionally, using the first law of thermodynamics and Euler’s equation, we derive a set of equations that, at linear order, coincide with those obtained from the conservation of the stress-energy tensor in general relativity. This approach not only highlights the consistency between Newtonian and relativistic frameworks in certain limits, but also serves as a pedagogical bridge, offering insights into the physical principles underlying the dynamics of the universe. Full article

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

18 pages, 2698 KiB

Open AccessArticle

Fate of Mixmaster Chaos in a Deformed Algebra Framework

by Gabriele Barca and Eleonora Giovannetti

Universe 2025, 11(2), 63; https://doi.org/10.3390/universe11020063 - 12 Feb 2025

Cited by 1 | Viewed by 425

Abstract

We analyze the anisotropic Bianchi models, and in particular the Bianchi Type IX known as the Mixmaster universe, where the Misner anisotropic variables obey Deformed Commutation Relations inspired by Quantum Gravity theories. We consider three different deformations, two of which have been able [...] Read more.

We analyze the anisotropic Bianchi models, and in particular the Bianchi Type IX known as the Mixmaster universe, where the Misner anisotropic variables obey Deformed Commutation Relations inspired by Quantum Gravity theories. We consider three different deformations, two of which have been able to remove the initial singularity similarly to Loop Quantum Cosmology when implemented in the single-volume variable. Here, the two-dimensional algebras naturally implement a form of Non-Commutativity between the space variables that affects the dynamics of the anisotropies. In particular, we implement the modifications in their classical limit, where the Deformed Commutators become Deformed Poisson Brackets. We derive the modified Belinskii–Khalatnikov–Lifshitz map in all the three cases, and we study the fate of the chaotic behavior that the model classically presents. Depending on the sign of the deformation, the dynamics will either settle into oscillations between two almost-constant angles, or stop reflecting after a finite number of iterations and reach the singularity as one last simple Kasner solution. In either case, chaos is removed. Full article

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

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31 pages, 953 KiB

Open AccessArticle

Gravitational Wave Signatures Induced by Dark Fluid Accretion in Binary Systems

by Evangelos Achilleas Paraskevas and Leandros Perivolaropoulos

Universe 2025, 11(2), 62; https://doi.org/10.3390/universe11020062 - 11 Feb 2025

Viewed by 619

Abstract

We investigate the impact of dark fluid accretion on gravitational waveforms emitted by a compact binary system consisting of a supermassive black hole and a stellar-mass black hole. Using a Lagrangian framework with 1 PN and 2.5 PN corrections, we analyze the effects [...] Read more.

We investigate the impact of dark fluid accretion on gravitational waveforms emitted by a compact binary system consisting of a supermassive black hole and a stellar-mass black hole. Using a Lagrangian framework with 1 PN and 2.5 PN corrections, we analyze the effects of the spherically symmetric accretion of a fluid with steady-state flow, including those characterized by an equation of state parameter resembling dark energy, on the binary’s dynamics. We validate our approach by comparing it with previous studies in the common region of validity and extend the analysis to include both local effects, such as dynamical friction, and global gravitational interactions with the stellar-mass black hole, focusing on their dependence on the fluid’s properties. Our analysis reveals that these interactions induce de-phasing in gravitational waveforms, with the phase shift influenced by the fluid’s equation of state and energy density. We also extend the study to sudden cosmological singularities, finding that, although they can deform the binary’s orbit from initially circular to elliptical, their effect on de-phasing is negligible for cosmologically relevant energy densities. By incorporating both the local and global gravitational interactions of a fluid on a two-body system into the equations of motion, this preliminary study provides a framework for understanding the interplay between fluid dynamics and gravitational wave emissions in astrophysical systems. It further reinforces the potential for probing the properties of astrophysically relevant fluids through gravitational wave observations. Full article

(This article belongs to the Special Issue Dark Energy and Dark Matter)

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19 pages, 9632 KiB

Open AccessArticle

Comparison of Rain-Driven Erosion and Accumulation Modelling of Zafit Basin on Earth and Tinto-B Valley on Mars

by Vilmos Steinmann and Ákos Kereszturi

Universe 2025, 11(2), 61; https://doi.org/10.3390/universe11020061 - 11 Feb 2025

Viewed by 408

Abstract

While fluvial features are plentiful on Mars and offer valuable insights into past surface conditions, the climatic conditions inferred from these valleys, like precipitation and surface runoff discharges, remain the subject of debate. Model-based estimations have already been applied to several Martian valleys, [...] Read more.

While fluvial features are plentiful on Mars and offer valuable insights into past surface conditions, the climatic conditions inferred from these valleys, like precipitation and surface runoff discharges, remain the subject of debate. Model-based estimations have already been applied to several Martian valleys, but exploration of the related numerical estimations has been limited. This work applies an improved precipitation-based, steady-state erosion/accumulation model to a Martian valley and compares it to a terrestrial Mars analogue dessert catchment area. The simulations are based on a previously observed precipitation event and estimate the fluvial-related hydrological parameters, like flow depth, velocity, and erosion/accumulation processes in two different but morphologically similar watersheds. Moderate differences were observed in the erosion/accumulation results (0.13/−0.06 kg/m²/s for Zafit (Earth) and 0.01/−0.007 for Tinto B (Mars)). The difference is probably related to the lower areal ratio of surface on Mars where the shield factor is enough to trigger sediment movement, while in the Zafit basin, there is a larger area of undulating surface. The model could be applied to the whole surface of Mars. Using grain size estimation from the global THEMIS dataset, the grain size value artificially increased above that observed, and decreased hypothetic target rock density tests demonstrated that the model works according to theoretical expectations and is useful for further development. The findings of this work indicate the necessity of further testing of similar models on Mars and a better general analysis of the background geomorphological understanding of surface evolution regarding slope angles. Full article

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

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34 pages, 435 KiB

Open AccessReview

Black Hole Thermodynamics and Generalised Non-Extensive Entropy

by Emilio Elizalde, Shin’ichi Nojiri and Sergei D. Odintsov

Universe 2025, 11(2), 60; https://doi.org/10.3390/universe11020060 - 11 Feb 2025

Cited by 2 | Viewed by 499

Abstract

The first part of this work provides a review of recent research on generalised entropies and their origin, as well as its application to black hole thermodynamics. To start, it is shown that the Hawking temperature and the Bekenstein–Hawking entropy are, respectively, the [...] Read more.

The first part of this work provides a review of recent research on generalised entropies and their origin, as well as its application to black hole thermodynamics. To start, it is shown that the Hawking temperature and the Bekenstein–Hawking entropy are, respectively, the only possible thermodynamical temperature and entropy of the Schwarzschild black hole. Moreover, it is investigated if the other known generalised entropies, which include Rényi’s entropy, Tsallis entropy, and the four- and five-parameter generalised entropies, could correctly yield the Hawking temperature and the ADM mass. The possibility that generalised entropies could describe hairy black hole thermodynamics is also considered, both for the Reissner–Nordström black hole and for Einstein’s gravity coupled with two scalar fields. Two possibilities are investigated, namely, the case when the ADM mass does not yield the Bekenstein–Hawking entropy, and the case in which the effective mass expressing the energy inside the horizon does not yield the Hawking temperature. For the model with two scalar fields, the radii of the photon sphere and of the black hole shadow are calculated, which gives constraints on the BH parameters. These constraints are seen to be consistent, provided that the black hole is of the Schwarzschild type. Subsequently, the origin of the generalised entropies is investigated, by using their microscopic particle descriptions in the frameworks of a microcanonical ensemble and canonical ensemble, respectively. Finally, the McLaughlin expansion for the generalised entropies is used to derive, in each case, the microscopic interpretation of the generalised entropies, via the canonical and the grand canonical ensembles. Full article

(This article belongs to the Section Gravitation)

22 pages, 714 KiB

Open AccessArticle

Dark Energy and Cosmological Bounce Supported by an Unconventional Spinor Field

by Barna Fekecs and Zoltán Keresztes

Universe 2025, 11(2), 59; https://doi.org/10.3390/universe11020059 - 11 Feb 2025

Viewed by 473

Abstract

Alternative scenarios where the Big Bang singularity of the standard cosmological model is replaced by a bounce, or by an early almost static phase (known as emergent universe) have been frequently studied. We investigate the role of the spinor degrees of freedom in [...] Read more.

Alternative scenarios where the Big Bang singularity of the standard cosmological model is replaced by a bounce, or by an early almost static phase (known as emergent universe) have been frequently studied. We investigate the role of the spinor degrees of freedom in overcoming the initial singularity. We introduce a model which generalizes the Einstein–Cartan–Dirac theory, including local phase invariance of the spinor field supported by a gauge scalar field and certain couplings to the torsion. A natural gauge choice reduces the field equations to that of the Einstein–Dirac theory with a Dirac field potential that has polar and axial spinor currents. We identify a new potential term proportional to the square of the ratio of Dirac scalar and axial scalar, which provides a dark energy contribution dominating in the late-time Universe. In addition, the presence of spinor currents in the potential may induce the bounce of a contracting universe. Full article

(This article belongs to the Section Gravitation)

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20 pages, 293 KiB

Open AccessArticle

Scan Quantum Mechanics: Quantum Inertia Stops Superposition

by Beatriz Gato-Rivera

Universe 2025, 11(2), 58; https://doi.org/10.3390/universe11020058 - 11 Feb 2025

Viewed by 481

Abstract

Scan Quantum Mechanics (SQM) is a novel interpretation in which the superposition of states is only an approximate effective concept due to lack of time resolution. Quantum systems scan all possible states in the “apparent” superpositions and switch randomly and very rapidly among [...] Read more.

Scan Quantum Mechanics (SQM) is a novel interpretation in which the superposition of states is only an approximate effective concept due to lack of time resolution. Quantum systems scan all possible states in the “apparent” superpositions and switch randomly and very rapidly among them. A crucial property that we postulate is quantum inertia

I_{q}

, that increases whenever a constituent is added, or the system is perturbed with all kinds of interactions. Once the quantum inertia reaches a critical value

I_{c r}

for an observable, the switching among its eigenvalues stops and the corresponding superposition comes to an end. Consequently, increasing the quantum inertia of a quantum system by increasing its mass, its temperature, or the strength of the electric, magnetic and gravitational fields in its environment, can lead to the end of the superpositions for all the observables, the quantum system transmuting into a classical one, as a result. This process could be reversible, however, by decreasing the size of the system, its temperature, etc. SQM also implies a radiation mechanism from astrophysical objects with very strong gravitational fields that could contribute to neutron star formation. Future experiments might determine the critical quantum inertia

I_{c r}

corresponding to different observables, which translates into critical masses, critical temperatures and critical electric, magnetic and gravitational fields. Full article

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

14 pages, 494 KiB

Open AccessArticle

Consistency of Histories of Neutrino Oscillation in the Presence of Normal Matter and Continuous Non-Selective Measurement

by Fazeel Ahmed Khan and Jerzy Dajka

Universe 2025, 11(2), 57; https://doi.org/10.3390/universe11020057 - 10 Feb 2025

Viewed by 402

Abstract

Two-flavor neutrino oscillation is analyzed using the consistent histories approach. We identify the properties of neutrinos and the oscillation conditions, such as the presence of matter or the impact of measurement, that ensure the consistency of three-time neutrino histories. The connection between the [...] Read more.

Two-flavor neutrino oscillation is analyzed using the consistent histories approach. We identify the properties of neutrinos and the oscillation conditions, such as the presence of matter or the impact of measurement, that ensure the consistency of three-time neutrino histories. The connection between the consistency of these histories and the well-known CP violation explored in this study serves as a specific example. Full article

(This article belongs to the Special Issue Neutrino Insights: Peering into the Subatomic Universe)

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20 pages, 355 KiB

Open AccessArticle

On the Yang-Mills Propagator at Strong Coupling

by Yves Gabellini, Thierry Grandou and Ralf Hofmann

Universe 2025, 11(2), 56; https://doi.org/10.3390/universe11020056 - 10 Feb 2025

Viewed by 427

Abstract

About twelve years ago, the use of standard functional manipulations was demonstrated to imply an unexpected property satisfied by the fermionic Green’s functions of

Q C D

. This non-perturbative phenomenon has been dubbed an effective locality. In a much simpler way [...] Read more.

About twelve years ago, the use of standard functional manipulations was demonstrated to imply an unexpected property satisfied by the fermionic Green’s functions of

Q C D

. This non-perturbative phenomenon has been dubbed an effective locality. In a much simpler way than in

Q C D

, the most remarkable and intriguing aspects of effective locality have been presented in a recent publication on the Yang-Mills theory on Minkowski spacetime. While quickly recalled in the current paper, these results are used to calculate the problematic gluonic propagator in the Yang-Mills non-perturbative regime. This paper is dedicated to the memory of Professor Herbert M. Fried (1929–2023), whose inspiring manner, impressive command of functional methods in quantum field theories, enthusiasm for a broad range of topics in Theoretical Physics, and warm friendship are missed greatly by the authors. Full article

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

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17 pages, 306 KiB

Open AccessArticle

An Effective Description of the Instability of Coherent States of Gravitons in String Theory

by Cesar Damian and Oscar Loaiza-Brito

Universe 2025, 11(2), 55; https://doi.org/10.3390/universe11020055 - 8 Feb 2025

Viewed by 410

Abstract

We study the dynamics of a coherent state of closed type II string gravitons within the framework of the Steepest Entropy Ascent Quantum Thermodynamics, an effective model where the quantum evolution is driven by a maximal increase of entropy. We find that by [...] Read more.

We study the dynamics of a coherent state of closed type II string gravitons within the framework of the Steepest Entropy Ascent Quantum Thermodynamics, an effective model where the quantum evolution is driven by a maximal increase of entropy. We find that by perturbing the pure coherent state of gravitons by the presence of other states in the string spectrum, there exist conditions upon which the system undergoes decoherence by reaching thermodynamical equilibrium. This suggests the instability of the coherent state of gravitons. We identify the time scale it takes the system to reach equilibrium consisting of a mixed state of fields in the string spectrum and compare it with the quantum-break time. Also we find that in such final state the quantum-break time seems to be larger than the classical break-time. Full article

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

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17 pages, 476 KiB

Open AccessArticle

Linking Planetary Ephemeris Reference Frames to ICRF via Millisecond Pulsars

by Li Guo, Yueqi Song, Zhen Yan, Liang Li and Guangli Wang

Universe 2025, 11(2), 54; https://doi.org/10.3390/universe11020054 - 7 Feb 2025

Viewed by 536

Abstract

The positions of millisecond pulsars (MSPs) can be determined with sub-milliarcsecond (mas) accuracy using both Very Long Baseline Interferometry (VLBI) and timing, referenced to the International Celestial Reference Frame (ICRF) and planetary ephemerides frame, respectively, representing kinematic and dynamical reference frames. The two [...] Read more.

The positions of millisecond pulsars (MSPs) can be determined with sub-milliarcsecond (mas) accuracy using both Very Long Baseline Interferometry (VLBI) and timing, referenced to the International Celestial Reference Frame (ICRF) and planetary ephemerides frame, respectively, representing kinematic and dynamical reference frames. The two frames can be connected through observations of common celestial objects, MSPs observed with VLBI and timing. However, previous attempts to establish this connection were unreliable due to the limited number of MSPs observed by both techniques. Currently, 23 MSPs have been precisely measured using both multiple timing and VLBI networks. Among them, 17 MSPs are used to link the two reference frames, marking a significant three-fold increase in the number of common MSPs used for frame linking. Nevertheless, six MSPs located near the ecliptic plane are excluded from frame linkage due to positional differences exceeding 20 mas measured by VLBI and timing. This discrepancy is primarily attributed to errors introduced in fitting positions in timing methods. With astrometric parameters obtained via both VLBI and timing for these MSPs, the precision of linking DE436 and ICRF3 has surpassed 0.4 mas. Furthermore, thanks to the improved timing precision of MeerKAT, even with data from just 13 MSPs observed by both MeerKAT and VLBI, the precision of linking DE440 and ICRF3 can also exceed 0.4 mas. The reliability of this linkage depends on the precision of pulsar astrometric parameters, their spatial distribution, and discrepancies in pulsar positions obtained by the two techniques. Notably, proper motion differences identified by the two techniques are the most critical factors influencing the reference frame linking parameters. The core shift of the calibrators in VLBI pulsar observations is one of the factors causing proper motion discrepancies, and multi-wavelength observations are expected to solve it. With the improvement in timing accuracy and the application of new observation modes like multi-view and multi-band observations in VLBI, the linkage accuracy of the dynamical and kinematic reference frames is expected to reach 0.3 mas. Full article

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13 pages, 5350 KiB

Open AccessArticle

Cosmic Ray Spectra and Anisotropy in an Anisotropic Propagation Model with Spiral Galactic Sources

by Aifeng Li, Zhaodong Lv, Wei Liu, Yiqing Guo and Fangheng Zhang

Universe 2025, 11(2), 53; https://doi.org/10.3390/universe11020053 - 7 Feb 2025

Viewed by 506

Abstract

In our previous work, we investigated the spectra and anisotropy of galactic cosmic rays (GCRs) under the assumption of an axisymmetric distribution of galactic sources. Currently, much observational evidence indicates that the Milky Way is a typical spiral galaxy. In this work, we [...] Read more.

In our previous work, we investigated the spectra and anisotropy of galactic cosmic rays (GCRs) under the assumption of an axisymmetric distribution of galactic sources. Currently, much observational evidence indicates that the Milky Way is a typical spiral galaxy. In this work, we further utilize an anisotropic propagation model under the framework of spiral distribution sources to study spectra and anisotropy. During the calculation process, we adopt the spatial-dependent propagation (SDP) model, while incorporating the contribution from the nearby Geminga source and the anisotropic diffusion of cosmic rays (CRs) induced by the local regular magnetic field (LRMF). By comparing the results of background sources with spiral and axisymmetric distribution models, it is found that both of them can well reproduce the CR spectra and anisotropy. However, there exist differences in their propagation parameters. The diffusion coefficient with spiral distribution is larger than that with axisymmetric distribution, and its spectral indices are slightly harder. To investigate the effects of a nearby Geminga source and LRMF on anisotropy, two-dimensional (2D) anisotropy sky maps under various contributing factors are compared. Below 100 TeV, the anisotropy is predominantly influenced by both the nearby Geminga source and the LRMF, causing the phase to align with the direction of the LRMF. Above 100 TeV, the background sources become dominant, resulting in the phase pointing towards the Galactic Center (GC). Future high-precision measurements of CR anisotropy and spectra, such as the LHAASO experiment, will be crucial in evaluating the validity of our proposed model. Full article

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

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42 pages, 499 KiB

Open AccessArticle

Gauge-Invariant Perturbation Theory on the Schwarzschild Background Spacetime: Part III—Realization of Exact Solutions

by Kouji Nakamura

Universe 2025, 11(2), 52; https://doi.org/10.3390/universe11020052 - 6 Feb 2025

Cited by 6 | Viewed by 385

Abstract

This is the Part III paper of our series of papers on a gauge-invariant perturbation theory on the Schwarzschild background spacetime. After reviewing our general framework for the gauge-invariant perturbation theory and the proposal for gauge-invariant treatments of

l = 0, 1

[...] Read more.

This is the Part III paper of our series of papers on a gauge-invariant perturbation theory on the Schwarzschild background spacetime. After reviewing our general framework for the gauge-invariant perturbation theory and the proposal for gauge-invariant treatments of

l = 0, 1

mode perturbations on the Schwarzschild background spacetime in the Part I paper, we examine the problem of whether the

l = 0, 1

even-mode solutions derived in the Part II paper are physically reasonable. We consider the linearized versions of the Lemaître–Tolman–Bondi solution and the non-rotating C-metric. As a result, we show that our derived even-mode solutions to the linearized Einstein equations realize these two linearized solutions. This supports the conclusion that our derived solutions are physically reasonable, which implies that our proposal for gauge-invariant treatments of

l = 0, 1

mode perturbations is also physically reasonable. We also briefly summarize the conclusions of our series of papers. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

13 pages, 2538 KiB

Open AccessArticle

From Be X-Ray Binaries to Double Neutron Stars: Exploring the Spin and Orbital Evolution

by Yungang Zhou, Dehua Wang and Chengmin Zhang

Universe 2025, 11(2), 51; https://doi.org/10.3390/universe11020051 - 6 Feb 2025

Viewed by 532

Abstract

We explore the evolutionary link between Galactic Be X-ray binaries (BeXBs) and Galactic field double neutron stars (DNSs) based on the properties of the NS spin period—P and binary orbital period—

P_{orb}

. First, both BeXB and DNS sources show positive [...] Read more.

We explore the evolutionary link between Galactic Be X-ray binaries (BeXBs) and Galactic field double neutron stars (DNSs) based on the properties of the NS spin period—P and binary orbital period—

P_{orb}

. First, both BeXB and DNS sources show positive correlation trends in P versus

P_{orb}

relation (

P - P_{orb}

diagram), which may relate to the influence of the accretion evolution. Secondly, the two types of sources exhibit similar bi-modal P/

P_{orb}

distributions with the gaps of

P \sim 40

s/

P_{orb} \sim 60

days for BeXBs and

P \sim 50

ms/

P_{orb} \sim 1

day for DNSs, respectively. We propose a possibility that Galactic BeXBs may transfer the bi-modal P/

P_{orb}

classifications to Galactic field DNSs during the evolution process. Furthermore, based on the bi-modal gaps, we infer the contraction factors of P (by

\sim \frac{1}{800}

) and

P_{orb}

(by

\sim \frac{1}{60}

) valuesin the evolution from Galactic BeXBs to Galactic field DNSs. We find that the scaled P or

P_{orb}

values of Galactic BeXBs share the similar bi-modal distributions to that of Galactic field DNSs, and these results can offer a reference to trace the classification and evolution between the two types of sources. Full article

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13 pages, 503 KiB

Open AccessArticle

Electromagnetic Waves in Cosmological Space–Time II. Luminosity Distance

by Denitsa Staicova and Michail Stoilov

Universe 2025, 11(2), 50; https://doi.org/10.3390/universe11020050 - 5 Feb 2025

Cited by 1 | Viewed by 481

Abstract

In this article, we continue our investigation on how the electromagnetic waves propagate in the Friedman–Lemaître–Robertson–Walker spacetime. Unlike the standard approach, which relies on null geodesics and geometric optics approximation, we derive explicit solutions for electromagnetic waves in expanding spacetime and examine their [...] Read more.

In this article, we continue our investigation on how the electromagnetic waves propagate in the Friedman–Lemaître–Robertson–Walker spacetime. Unlike the standard approach, which relies on null geodesics and geometric optics approximation, we derive explicit solutions for electromagnetic waves in expanding spacetime and examine their implications for cosmological observations. In particular, our analysis reveals potential modifications to the standard luminosity distance formula. Its effect on other cosmological parameters, e.g., the amount of cold dust matter in the Universe, is considered and estimated from Type Ia supernovae data. We see that this alternative model is able to fit the supernova data, but it gives a qualitatively different Universe without a cosmological constant but with stiff or ultra-stiff matter. Full article

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

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16 pages, 334 KiB

Open AccessArticle

A Note on Gravitational Dark Matter Production

by Jaume de Haro and Supriya Pan

Universe 2025, 11(2), 49; https://doi.org/10.3390/universe11020049 - 4 Feb 2025

Viewed by 520

Abstract

Dark matter, one of the fundamental components of the universe, has remained mysterious in modern cosmology and particle physics, and hence, this field is of utmost importance at the present moment. One of the foundational questions in this direction is the origin of [...] Read more.

Dark matter, one of the fundamental components of the universe, has remained mysterious in modern cosmology and particle physics, and hence, this field is of utmost importance at the present moment. One of the foundational questions in this direction is the origin of dark matter, which directly links to its creation. In the present article, we study the gravitational production of dark matter in two distinct contexts: firstly, when reheating occurs through gravitational particle production, and secondly, when it is driven by decay of the inflaton field. We establish a connection between the reheating temperature and the mass of dark matter, and from the reheating bounds, we determine the range of viable dark matter mass values. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')

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12 pages, 296 KiB

Open AccessArticle

Elucidating the Dark Energy and Dark Matter Phenomena Within the Scale-Invariant Vacuum (SIV) Paradigm

by Vesselin G. Gueorguiev and Andre Maeder

Universe 2025, 11(2), 48; https://doi.org/10.3390/universe11020048 - 2 Feb 2025

Cited by 1 | Viewed by 1151

Abstract

The enigmatic phenomenon of dark energy (DE) is the elusive entity driving the accelerated expansion of our Universe. A plausible candidate for DE is the non-zero Einstein Cosmological Constant

Λ_{E}

manifested as a constant energy density of the vacuum, yet it seemingly [...] Read more.

The enigmatic phenomenon of dark energy (DE) is the elusive entity driving the accelerated expansion of our Universe. A plausible candidate for DE is the non-zero Einstein Cosmological Constant

Λ_{E}

manifested as a constant energy density of the vacuum, yet it seemingly defies gravitational effects. In this work, we interpret the non-zero

Λ_{E}

through the lens of scale-invariant cosmology. We revisit the conformal scale factor

λ

and its defining equations within the Scale-Invariant Vacuum (SIV) paradigm. Furthermore, we address the profound problem of the missing mass across galactic and extragalactic scales by deriving an MOND-like relation,

g \sim \sqrt{a_{0} g_{N}}

, within the SIV context. Remarkably, the values obtained for

Λ_{E}

and the MOND fundamental acceleration,

a_{0}

, align with observed magnitudes, specifically,

a_{0} \approx 10^{- 10} m s^{- 2}

and

Λ_{E} \approx 1.8 \times 10^{- 52} m^{- 2}

. Moreover, we propose a novel early dark energy term,

{\tilde{T}}_{μ ν} \sim κ H

, within the SIV paradigm, which holds potential relevance for addressing the Hubble tension. Full article

(This article belongs to the Special Issue Dark Energy and Dark Matter)

39 pages, 23123 KiB

Open AccessArticle

Core to Cosmic Edge: SIMBA-C’s New Take on Abundance Profiles in the Intragroup Medium at z = 0

by Aviv Padawer-Blatt, Zhiwei Shao, Renier T. Hough, Douglas Rennehan, Ruxin Barré, Vida Saeedzadeh, Arif Babul, Romeel Davé, Chiaki Kobayashi, Weiguang Cui, François Mernier and Ghassem Gozaliasl

Universe 2025, 11(2), 47; https://doi.org/10.3390/universe11020047 - 1 Feb 2025

Viewed by 879

Abstract

We employ the simba-c cosmological simulation to study the impact of its upgraded chemical enrichment model (Chem5) on the distribution of metals in the intragroup medium (IGrM). We investigate the projected X-ray emission-weighted abundance profiles of key elements over two decades in halo [...] Read more.

We employ the simba-c cosmological simulation to study the impact of its upgraded chemical enrichment model (Chem5) on the distribution of metals in the intragroup medium (IGrM). We investigate the projected X-ray emission-weighted abundance profiles of key elements over two decades in halo mass (

10^{13} \leq M_{500} / M_{⊙} \leq 10^{15}

). Typically, simba-c generates lower-amplitude abundance profiles than simba with flatter cores, in better agreement with observations. For low-mass groups, both simulations over-enrich the IGrM with Si, S, Ca, and Fe compared to observations, a trend likely related to inadequate modeling of metal dispersal and mixing. We analyze the 3D mass-weighted abundance profiles, concluding that the lower simba-c IGrM abundances are primarily a consequence of fewer metals in the IGrM, driven by reduced metal yields in Chem5, and the removal of the instantaneous recycling of metals approximation employed by simba. Additionally, an increased IGrM mass in low-mass simba-c groups is likely triggered by changes to the AGN and stellar feedback models. Our study suggests that a more realistic chemical enrichment model broadly improves agreement with observations, but physically motivated sub-grid models for other key processes, like AGN and stellar feedback and turbulent diffusion, are required to realistically reproduce observed group environments. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

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10 pages, 230 KiB

Open AccessArticle

A Small Cosmological Constant from a Large Number of Extra Dimensions

by Changjun Gao

Universe 2025, 11(2), 46; https://doi.org/10.3390/universe11020046 - 1 Feb 2025

Viewed by 617

Abstract

In this article, we consider the 4 + n dimensional spacetimes among which one is the four dimensional physical Universe and the other is an n-dimensional sphere with constant radius in the framework of Lanczos-Lovelock gravity. We find that the curvature of extra [...] Read more.

In this article, we consider the 4 + n dimensional spacetimes among which one is the four dimensional physical Universe and the other is an n-dimensional sphere with constant radius in the framework of Lanczos-Lovelock gravity. We find that the curvature of extra dimensional sphere contributes a huge but negative energy density provided that its radius is sufficiently small, such as the scale of Planck length. Therefore, the huge positive vacuum energy, i.e., the large positive cosmological constant is exactly cancelled out by the curvature of extra sphere. In the mean time the higher order of Lanczos-Lovelock term contributes an observations-allowed small cosmological constant if the number of extra dimensions is sufficiently large, such as n ≈ 69. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Energy Theories)

18 pages, 852 KiB

Open AccessArticle

Non-Keplerian Charged Accretion Disk Orbiting a Black Hole Pulsar

by Audrey Trova and Eva Hackmann

Universe 2025, 11(2), 45; https://doi.org/10.3390/universe11020045 - 1 Feb 2025

Viewed by 471

Abstract

Recent studies have focused on how spinning black holes (BHs) within a binary system containing a strongly magnetized neutron star, then immersed in external magnetic fields, can acquire charge through mechanisms like the Wald process and how this charge could power pulsar-like electromagnetic [...] Read more.

Recent studies have focused on how spinning black holes (BHs) within a binary system containing a strongly magnetized neutron star, then immersed in external magnetic fields, can acquire charge through mechanisms like the Wald process and how this charge could power pulsar-like electromagnetic radiation. Those objects called “Black hole pulsar” mimic the behaviour of a traditional pulsar, and they can generate electromagnetic fields, such as magnetic dipoles. Charged particles within an accretion disk around the black hole would then be influenced not only by the gravitational forces but also by electromagnetic forces, leading to different geometries and dynamics. In this context, we focus here on the interplay of the magnetic dipole and the accretion disk. We construct the equilibrium structures of non-conducting charged perfect fluids orbiting Kerr black holes under the influence of a dipole magnetic field aligned with the rotation axis of the BH. The dynamics of the accretion disk in such a system are shaped by a complex interplay between the non-uniform, non-Keplerian angular momentum distribution, the black hole’s induced magnetic dipole, and the fluid’s charge. We show how these factors jointly influence key properties of the disk, such as its geometry, aspect ratio, size, and rest mass density. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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28 pages, 400 KiB

Open AccessReview

The Flipped SU(5) × U(1) Model from Four-Dimensional Strings

by Ignatios Antoniadis and John Rizos

Universe 2025, 11(2), 44; https://doi.org/10.3390/universe11020044 - 29 Jan 2025

Viewed by 451

Abstract

We review the construction of a flipped

S U (5) \times U (1)

model in the context of the Free-Fermionic Formulation of four-dimensional strings and present its main phenomenological consequences in particle physics and cosmology. 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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Universe, Volume 11, Issue 2 (February 2025) – 45 articles

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