Universe

16 pages, 22205 KB

Open AccessArticle

Properties of Heavy Higgs Bosons and Dark Matter Under Current Experimental Limits in the μNMSSM

by Zhaoxia Heng, Xingjuan Li and Liangliang Shang

Universe 2025, 11(3), 103; https://doi.org/10.3390/universe11030103 - 20 Mar 2025

Cited by 3 | Viewed by 287

Searches for new particles beyond the Standard Model (SM) are an important task for the Large Hadron Collider (LHC). In this paper, we investigate the properties of the heavy non-SM Higgs bosons in the

μ

-term extended Next-to-Minimal Supersymmetric Standard Model (

μ

[...] Read more.

Searches for new particles beyond the Standard Model (SM) are an important task for the Large Hadron Collider (LHC). In this paper, we investigate the properties of the heavy non-SM Higgs bosons in the

μ

-term extended Next-to-Minimal Supersymmetric Standard Model (

μ

NMSSM). We scan the parameter space of the

μ

NMSSM considering the basic constraints from Higgs data, dark matter (DM) relic density, and LHC searches for sparticles. And we also consider the constraints from the LZ2022 experiment and the muon anomaly constraint at the 2

σ

level. We find that the LZ2022 experiment has a strict constraint on the parameter space of the

μ

NMSSM, and the limits from the DM-nucleon spin-independent (SI) and spin-dependent (SD) cross-sections are complementary. Then, we discuss the exotic decay modes of heavy Higgs bosons decaying into SM-like Higgs bosons. We find that for doublet-dominated Higgs

h_{3}

and

A_{2}

, the main exotic decay channels are

h_{3} \to Z A_{1}

,

h_{3} \to h_{1} h_{2}

,

A_{2} \to A_{1} h_{1}

, and

A_{2} \to Z h_{2}

, and the branching ratio can reach to about 23%, 10%, 35%, and 10% respectively. Full article

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

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43 pages, 520 KB

Open AccessReview

Polynomial Affine Model of Gravity: After 10 Years

by Oscar Castillo-Felisola, Bastian Grez, Manuel Morocho-López, Jose Perdiguero, Aureliano Skirzewski, Jefferson Vaca-Santana and Nicolas Zambra-Gómez

Universe 2025, 11(3), 102; https://doi.org/10.3390/universe11030102 - 18 Mar 2025

Cited by 3 | Viewed by 569

Abstract

The polynomial affine model of gravity was proposed as an alternative to metric and metric-affine gravitational models. What, in the beginning, was thought to be a source of unpredictability—the presence of many terms in the action—turned out to be a milestone since it [...] Read more.

The polynomial affine model of gravity was proposed as an alternative to metric and metric-affine gravitational models. What, in the beginning, was thought to be a source of unpredictability—the presence of many terms in the action—turned out to be a milestone since it contains all possible combinations of the fields compatible with the covariance under diffeomorphisms. Here, we present a review of the advances in the analysis of the model after 10 years of its proposal and sketch the guidelines for our future perspectives. Full article

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

22 pages, 4452 KB

Open AccessArticle

Influence of Solar Wind Driving and Geomagnetic Activity on the Variability of Sub-Relativistic Electrons in the Inner Magnetosphere

by Evangelia Christodoulou, Christos Katsavrias, Panayotis Kordakis and Ioannis A. Daglis

Universe 2025, 11(3), 101; https://doi.org/10.3390/universe11030101 - 18 Mar 2025

Viewed by 585

Abstract

Motivated by the need for more accurate radiation environment modeling, this study focuses on identifying and analyzing the drivers behind the sub-relativistic electron flux variations in the inner magnetosphere. We utilize electron flux data between 1 and 500 keV from the Hope and [...] Read more.

Motivated by the need for more accurate radiation environment modeling, this study focuses on identifying and analyzing the drivers behind the sub-relativistic electron flux variations in the inner magnetosphere. We utilize electron flux data between 1 and 500 keV from the Hope and MagEIS instruments on board the RBSP satellites, as well as from the FEEPS instruments on board the MMS spacecrafts, along with solar wind parameters and geomagnetic indices obtained from the OmniWeb2 and SuperMag data services. We calculate the correlation coefficients between these parameters and electron flux. Our analysis shows that substorm activity is a crucial driver of the source electron population (10–100 keV), while also showing that seed electrons (100–400 keV) are not purely driven by substorm events but also from enhanced convection/inward diffusion. By introducing time lags, we observed a delayed response of electron flux to changes in geospace conditions, and we identified specific time lag periods where the correlation is maximum. This work contributes to our broader understanding of the outer belt sub-relativistic electron dynamics and forms the basis for future research. Full article

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

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23 pages, 1793 KB

Open AccessArticle

Restrictions on Regularized Fisher and Dilatonic Spacetimes Implied by High-Frequency Quasiperiodic Oscillations Observed in Microquasars and Active Galactic Nuclei

by Jaroslav Vrba and Zdeněk Stuchlík

Universe 2025, 11(3), 99; https://doi.org/10.3390/universe11030099 - 17 Mar 2025

Viewed by 433

Abstract

The Bronnikov generalization of the Fisher naked singularity and Dilatonic black hole spacetimes attracts high interest, as it combines two fundamental transitions of the solutions of Einstein equations. These are the black hole/wormhole “black bounce” transition of geometry, and the phantom/canonical transition of [...] Read more.

The Bronnikov generalization of the Fisher naked singularity and Dilatonic black hole spacetimes attracts high interest, as it combines two fundamental transitions of the solutions of Einstein equations. These are the black hole/wormhole “black bounce” transition of geometry, and the phantom/canonical transition of the scalar field, called trapped ghost scalar, combined with an electromagnetic field described by a non-linear electrodynamics. In the present paper, we put restrictions on the parameters of the Fisher (wormhole) and Dilatonic (black hole or wormhole) regularized spacetimes by using frequencies of the epicyclic orbital motion in the geodesic model for explanation of the high-frequency oscillations observed in microquasars or active galactic nuclei, where stellar mass or supermassive black holes are usually assumed. Full article

(This article belongs to the Special Issue Exotic Scenarios for Compact Astrophysical Objects)

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15 pages, 412 KB

Open AccessArticle

Gauss–Bonnet-Induced Symmetry Breaking/Restoration During Inflation

by Yermek Aldabergenov and Daulet Berkimbayev

Universe 2025, 11(3), 98; https://doi.org/10.3390/universe11030098 - 17 Mar 2025

Cited by 2 | Viewed by 498

Abstract

We propose a mechanism for symmetry breaking or restoration that can occur in the middle of inflation, due to the coupling of the Gauss–Bonnet term to a charged scalar. The Gauss–Bonnet coupling results in an inflaton-dependent effective squared mass of the charged scalar, [...] Read more.

We propose a mechanism for symmetry breaking or restoration that can occur in the middle of inflation, due to the coupling of the Gauss–Bonnet term to a charged scalar. The Gauss–Bonnet coupling results in an inflaton-dependent effective squared mass of the charged scalar, which can change its sign (around the symmetric point) during inflation. This can lead to spontaneous breaking of the symmetry, or to its restoration, if it is initially broken. We show the conditions under which the backreaction of the Gauss–Bonnet coupling on the inflationary background is negligible, such that the predictions of a given inflationary model are unaffected by the symmetry breaking/restoration process. 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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14 pages, 279 KB

Open AccessArticle

K-Essence Sources of Kerr–Schild Spacetimes

by Bence Juhász and László Árpád Gergely

Universe 2025, 11(3), 100; https://doi.org/10.3390/universe11030100 - 17 Mar 2025

Cited by 1 | Viewed by 876

Abstract

We extend a result by one of the authors, established for nonvacuum Einstein gravity, to minimally coupled k-essence scalar–tensor theories. First, we prove that in order to source a Kerr–Schild-type spacetime, the k-essence Lagrangian should be at most quadratic in the kinetic term. [...] Read more.

We extend a result by one of the authors, established for nonvacuum Einstein gravity, to minimally coupled k-essence scalar–tensor theories. First, we prove that in order to source a Kerr–Schild-type spacetime, the k-essence Lagrangian should be at most quadratic in the kinetic term. This is reduced to linear dependence when the Kerr–Schild null congruence is autoparallel. Finally, we show that solutions of the Einstein equations linearized in Kerr–Schild-type perturbations are also required to solve the full nonlinear system of Einstein equations, selecting once again k-essence scalar fields with linear Lagrangians in the kinetic term. The only other k-essence sharing the property of sourcing perturbative Kerr–Schild spacetimes, which are also exact, is the scalar field constant along the integral curves of the Kerr–Schild congruence, with the otherwise unrestricted Lagrangian. Full article

(This article belongs to the Section Gravitation)

17 pages, 315 KB

Open AccessArticle

Conclusions Not Yet Drawn from the Unsolved 4/3-Problem—How to Get a Stable Classical Electron

by Manfried Faber

Universe 2025, 11(3), 97; https://doi.org/10.3390/universe11030097 - 16 Mar 2025

Viewed by 541

Abstract

It has been known for over 100 years that there is a discrepancy between Maxwell’s electrodynamics and the idea of a classical electron as the “atom” of electricity. This incompatibility is known under the terms 4/3 problem of the classical electron and radiation [...] Read more.

It has been known for over 100 years that there is a discrepancy between Maxwell’s electrodynamics and the idea of a classical electron as the “atom” of electricity. This incompatibility is known under the terms 4/3 problem of the classical electron and radiation reaction force and was circumvented in the currently most successful theories, the quantum field theories, by limit value considerations, by the mutual subtraction of infinities, i.e., by purely mathematical methods that eliminate obvious contradictions but are not really based on an intuitive understanding of its physical causes. The actual origin of the problems mentioned lies in the instability of the classical electron. Stabilization cannot be achieved within the framework of Maxwell’s electrodynamics. This raises the question of what a minimal change to the fundamentals of electrodynamics should look like, which contains Maxwell’s theory as a limiting case. A detailed analysis of the 4/3 problem points to models that fulfill these requirements. Full article

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

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14 pages, 1434 KB

Open AccessArticle

From Known to Unknown: Cosmic Ray Transitions from the Sun, the Galaxy, and Extra-Galactic Systems

by Yuhua Yao, Yiqing Guo and Wei Liu

Universe 2025, 11(3), 96; https://doi.org/10.3390/universe11030096 - 14 Mar 2025

Cited by 1 | Viewed by 757

Abstract

The question of at which energy the transition from galactic to extra-galactic cosmic rays takes place has been a long-standing conundrum in cosmic ray physics. The sun stands out as the closest and clearest astrophysical accelerator of cosmic rays, while other objects within [...] Read more.

The question of at which energy the transition from galactic to extra-galactic cosmic rays takes place has been a long-standing conundrum in cosmic ray physics. The sun stands out as the closest and clearest astrophysical accelerator of cosmic rays, while other objects within and beyond the galaxy remain enigmatic. It is probable that the cosmic ray spectrum and mass components from these celestial sources share similarities, offering a novel approach to study their origin. In this study, we perform joint analysis of spectra and mass in the energy range from MeV to 10 EeV, and find the following: (1)

⟨lnA⟩

demonstrates three clear peaks, tagging component transition; (2) a critical variable

Δ

is adopted to define the location of the transition; (3) for protons, the knee is located at ∼1.8 PeV, and the boundary between the galaxy and extra-galaxy occurs at ∼60 PeV, marked by a spectral dip; and (4) the all-particle spectrum exhibits hardening at ∼60 PeV due to the contribution of nearby galaxies, and the extra-galaxy dominates ∼0.8 EeV. We hope the LHAASO experiment can perform spectral measurements of individual species to validate these specific observations. Full article

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

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8 pages, 326 KB

Open AccessArticle

The Impact of the Eccentricity on the Collapse of an Ellipsoid into a Black Hole

by Aisen G. Nikiforov, Anton N. Baushev and Maxim V. Barkov

Universe 2025, 11(3), 95; https://doi.org/10.3390/universe11030095 - 12 Mar 2025

Viewed by 546

Abstract

We consider the gravitational collapse of a homogeneous pressureless ellipsoid. We have shown that the minimal size r that the ellipsoid can reach during collapse depends on its initial eccentricity

e_{0}

as

r \propto e_{0}^{ν}

, where [...] Read more.

We consider the gravitational collapse of a homogeneous pressureless ellipsoid. We have shown that the minimal size r that the ellipsoid can reach during collapse depends on its initial eccentricity

e_{0}

as

r \propto e_{0}^{ν}

, where

ν \approx 15 / 8

, and this dependence is very universal. We have estimated the parameters (in particular, the initial eccentricity) of a homogeneous pressureless ellipsoid, whereat it collapses directly into a black hole. Our consideration is purely Newtonian, but we present a discussion suggesting that the results obtained within the framework of general relativity are unlikely to be significantly different. Full article

(This article belongs to the Section Cosmology)

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12 pages, 1514 KB

Open AccessCommunication

Predicting Ground-Level Enhancement Events and >500 MeV Proton Intensity Using Proton and Electron Observations

by Marlon Núñez

Universe 2025, 11(3), 94; https://doi.org/10.3390/universe11030094 - 12 Mar 2025

Viewed by 689

Abstract

Ground-Level Enhancements (GLEs) pose a potential hazard for crew and passengers on polar routes. The accurate estimation of the integral proton flux of Solar Energetic Particle (SEP) events is crucial for assessing the expected radiation dose. This paper describes a new approach that [...] Read more.

Ground-Level Enhancements (GLEs) pose a potential hazard for crew and passengers on polar routes. The accurate estimation of the integral proton flux of Solar Energetic Particle (SEP) events is crucial for assessing the expected radiation dose. This paper describes a new approach that predicts the occurrence of GLEs and the associated >500 MeV intensity using proton and electron data. The new approach utilizes the Geostationary Operational Environmental Satellites (GOESs) for proton observations and the Advanced Composition Explorer (ACE) satellite for electron observations. Núñez et al. proposed a GLE occurrence predictor called the High Energy Solar Particle Events foRecastIng and Analysis (HESPERIA) University of Málaga Solar particle Event Predictor (UMASEP-500), which did not include a model for predicting the >500 MeV integral proton intensity. This paper presents a comparison in terms of the GLE event occurrence between the HESPERIA UMASEP-500 and a new approach called UMASEP-500. Although the new approach shows a slightly better critical success index (CSI), which combines the probability of detection (POD) and false alarm ratio (FAR), the difference is not statistically significant. The main advantage of the new UMASEP-500 is its ability to predict the expected >500 MeV proton intensity. This study provides initial insight into a new era of electron and proton telescopes that will be available at L1 in the coming years. Full article

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24 pages, 352 KB

Open AccessArticle

Bounce Cosmology in a Locally Scale Invariant Physics with a U(1) Symmetry

by Meir Shimon

Universe 2025, 11(3), 93; https://doi.org/10.3390/universe11030093 - 9 Mar 2025

Cited by 1 | Viewed by 636

Abstract

An asymmetric non-singular bouncing cosmological model is proposed in the framework of a locally scale-invariant scalar-tensor version of the standard model of particle physics and gravitation. The scalar field

ϕ

is complex. In addition to local scale invariance, the theory is U(1)-symmetric and [...] Read more.

An asymmetric non-singular bouncing cosmological model is proposed in the framework of a locally scale-invariant scalar-tensor version of the standard model of particle physics and gravitation. The scalar field

ϕ

is complex. In addition to local scale invariance, the theory is U(1)-symmetric and has a conserved global charge associated with time variations of the phase of

ϕ

. An interplay between the positive energy density contributions of relativistic and non-relativistic matter and that of the negative kinetic energy associated with the phase of

ϕ

results in a classical non-singular stable bouncing dynamics deep in the radiation-dominated era. This encompasses the observed redshifting era, which is preceded by a blueshifting era. The proposed model potentially avoids the flatness and horizon problems, as well as allowing for the generation of a scale-invariant spectrum of metric perturbations of the scalar type during a matter-dominated-like pre-bounce phase, with no recourse to an inflationary era. Full article

17 pages, 2731 KB

Open AccessArticle

Assessing Galaxy Rotation Kinematics: Insights from Convolutional Neural Networks on Velocity Variations

by Amirmohammad Chegeni, Fatemeh Fazel Hesar, Mojtaba Raouf, Bernard Foing and Fons J. Verbeek

Universe 2025, 11(3), 92; https://doi.org/10.3390/universe11030092 - 8 Mar 2025

Viewed by 667

Abstract

Distinguishing galaxies as either fast or slow rotators plays a vital role in understanding the processes behind galaxy formation and evolution. Standard techniques, which are based on the

λ_{R}

spin parameter obtained from stellar kinematics, frequently face difficulties in classifying fast and [...] Read more.

Distinguishing galaxies as either fast or slow rotators plays a vital role in understanding the processes behind galaxy formation and evolution. Standard techniques, which are based on the

λ_{R}

spin parameter obtained from stellar kinematics, frequently face difficulties in classifying fast and slow rotators accurately. These challenges arise particularly in cases where galaxies have complex interaction histories or exhibit significant morphological diversity. In this paper, we evaluate the performance of a Convolutional Neural Network (CNN) in classifying galaxy rotation kinematics based on stellar kinematic maps from the SAMI survey. Our results show that the optimal CNN architecture achieves an accuracy and precision of approximately 91% and 95%, respectively, on the test dataset. Subsequently, we apply our trained model to classify previously unknown rotator galaxies for which traditional statistical tools have been unable to determine whether they exhibit fast or slow rotation, such as certain irregular galaxies or those in dense clusters. We also used Integrated Gradients (IGs) to reveal the crucial kinematic features that influenced the CNN’s classifications. This research highlights the power of CNNs to improve our comprehension of galaxy dynamics and emphasizes their potential to contribute to upcoming large-scale Integral Field Spectrograph (IFS) surveys. Full article

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

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28 pages, 13572 KB

Open AccessArticle

High-Redshift Quasars at z ≥ 3—III: Parsec-Scale Jet Properties from Very Long Baseline Interferometry Observations

by Shaoguang Guo, Tao An, Yuanqi Liu, Chuanzeng Liu, Zhijun Xu, Yulia Sotnikova, Timur Mufakharov and Ailing Wang

Universe 2025, 11(3), 91; https://doi.org/10.3390/universe11030091 - 8 Mar 2025

Cited by 1 | Viewed by 993

Abstract

High-redshift active galactic nuclei (AGN) provide key insights into early supermassive black hole growth and cosmic evolution. This study investigates the parsec-scale properties of 86 radio-loud quasars at z ≥ 3 using very long baseline interferometry (VLBI) observations. Our results show predominantly compact [...] Read more.

High-redshift active galactic nuclei (AGN) provide key insights into early supermassive black hole growth and cosmic evolution. This study investigates the parsec-scale properties of 86 radio-loud quasars at z ≥ 3 using very long baseline interferometry (VLBI) observations. Our results show predominantly compact core and core-jet morphologies, with 35% having unresolved cores, 59% with core–jet structures, and only 6% with core–double jet morphology. Brightness temperatures are generally lower than expected for highly radiative sources. The jets’ proper motions are surprisingly slow compared to those of lower-redshift samples. We observe a high fraction of young and/or confined peak-spectrum sources, providing insights into early AGN evolution in dense environments during early cosmic epochs. The observed trends may reflect genuine evolutionary changes in AGN structure over cosmic time, or selection effects favoring more compact sources at higher redshifts. These results stress the complexity of high-redshift radio-loud AGN populations and emphasize the need for multi-wavelength, high-resolution observations to fully characterize their properties and evolution through cosmic history. Full article

(This article belongs to the Special Issue Advances in Studies of Galaxies at High Redshift)

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16 pages, 355 KB

Open AccessArticle

Rough Estimates of Solar System Gravitomagnetic Effects in Post-Newtonian Gravity

by Soon-Tae Hong

Universe 2025, 11(3), 90; https://doi.org/10.3390/universe11030090 - 7 Mar 2025

Cited by 1 | Viewed by 703

Abstract

In order to properly describe the gravity interactions, including the mass currents, in gravitomagnetism, we construct four Maxwell-type gravitational equations that are shown to be analogs of the Maxwell equations in electromagnetism. Next, exploiting the Maxwell-type gravitational equations, we explicitly predict the mass [...] Read more.

In order to properly describe the gravity interactions, including the mass currents, in gravitomagnetism, we construct four Maxwell-type gravitational equations that are shown to be analogs of the Maxwell equations in electromagnetism. Next, exploiting the Maxwell-type gravitational equations, we explicitly predict the mass magnetic fields for both the isolated system of the spinning Moon orbiting the spinning Earth and that of the Sun and solar system planets orbiting the spinning Sun, whose phenomenological values have not been evaluated in the preceding Newtonian gravity formalisms. In gravitomagnetism, we also phenomenologically investigate the mass magnetic general relativity (GR) forces associated with the mass magnetic fields, finding that they are extremely small but non-vanishing compared to the corresponding mass electric Newtonian forces. Moreover, the directions of the mass magnetic GR forces for the solar system planets, except Venus and Uranus, are shown to be anti-parallel to those of their mass electric Newtonian forces. Next, we investigate the mass magnetic dipole moment related to the B ring of Saturn to evaluate

{\vec{m}}_{M} (R i n g) = - 1.141 \times 10^{4} m^{3} s^{- 1} \hat{ω}

, with

\hat{ω}

being the unit vector along the axis direction of the spinning B ring. The predicted value of

{\vec{m}}_{M} (R i n g)

is shown to be directly related to the Cassini data on the total mass of the rings of Saturn. Full article

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

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11 pages, 467 KB

Open AccessArticle

A Hubble Constant Determination Through Quasar Time Delays and Type Ia Supernovae

by Leonardo R. Colaço

Universe 2025, 11(3), 89; https://doi.org/10.3390/universe11030089 - 7 Mar 2025

Cited by 2 | Viewed by 741

Abstract

This paper presents a new model-independent constraint on the Hubble constant (

H_{0}

) by anchoring relative distances from Type Ia supernovae (SNe Ia) observations to absolute distance measurements from time-delay strong Gravitational Lensing (SGL) systems. The approach only uses the validity [...] Read more.

This paper presents a new model-independent constraint on the Hubble constant (

H_{0}

) by anchoring relative distances from Type Ia supernovae (SNe Ia) observations to absolute distance measurements from time-delay strong Gravitational Lensing (SGL) systems. The approach only uses the validity of the cosmic distance duality relation (CDDR) to derive constraints on

H_{0}

. By using Gaussian Process (GP) regression to reconstruct the unanchored luminosity distance from the Pantheon+ compilation to match the time-delay angular diameter distance at the redshift of the lenses, one yields a value of

H_{0} = 75.57 \pm 4.415

km/s/Mpc at a 68% confidence level. The result aligns well with the local estimate from Cepheid variables within the

1 σ

confidence region, indicating consistency with late-universe probes. Full article

(This article belongs to the Special Issue Current Status of the Hubble Tension)

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29 pages, 6369 KB

Open AccessArticle

Keplerian Ringed-Disk Viscous-Diffusive Evolution and Combined Independent General Relativistic Evolutions

by Daniela Pugliese, Zdenek Stuchlík and Vladimir Karas

Universe 2025, 11(3), 88; https://doi.org/10.3390/universe11030088 - 6 Mar 2025

Viewed by 731

Abstract

We investigate the evolution of a set of viscous rings, solving a diffusion-like evolution equation in the (Keplerian disk) Newtonian regime. The Lynden-Bell and Pringle approach for a single disk regime is applied to a disk with a ring profile mimicking a set [...] Read more.

We investigate the evolution of a set of viscous rings, solving a diffusion-like evolution equation in the (Keplerian disk) Newtonian regime. The Lynden-Bell and Pringle approach for a single disk regime is applied to a disk with a ring profile mimicking a set of orbiting viscous rings. We discuss the time evolution of the disk, adopting different initial wavy (ringed) density profiles. Four different stages of the ring-cluster evolution are distinguished. In the second part of this analysis, we also explore the general relativistic framework by investigating the time evolution of composed systems of general relativistic co-rotating and counter-rotating equatorial disks orbiting a central Kerr black hole for faster spinning and slowly spinning black holes. In the sideline of this analysis, we consider a modified viscosity prescription mimicking an effective viscosity in the general relativistic ring interspace acting in the early phases of the rings’ evolutions, exploring the double system dynamics. Each ring of the separate sequence spreads inside the cluster modifying its inner structure following the rings merging. As the original ringed structure disappears, a single disk appears. The final configuration has a (well-defined) density peak, and its evolution turns in the final stages are dominated by its activity at the inner edge. Full article

(This article belongs to the Section Gravitation)

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23 pages, 17262 KB

Open AccessReview

Research Progress on Solar Supergranulation: Observations, Theories, and Numerical Simulations

by Chong Huang and Rui Wang

Universe 2025, 11(3), 87; https://doi.org/10.3390/universe11030087 - 6 Mar 2025

Viewed by 1127

Abstract

Solar supergranulation is a large-scale convective structure on the solar surface, whose formation mechanism and dynamical properties are closely related to key physical processes such as solar magnetic field evolution, coronal heating, and solar wind acceleration. This paper reviews recent research progress on [...] Read more.

Solar supergranulation is a large-scale convective structure on the solar surface, whose formation mechanism and dynamical properties are closely related to key physical processes such as solar magnetic field evolution, coronal heating, and solar wind acceleration. This paper reviews recent research progress on solar supergranulation, focusing on the latest achievements in high-resolution observations, theoretical models, and numerical simulations. By analyzing the flow field structure, magnetic field distribution, and their relationship with the solar activity cycle, the crucial role of supergranulation in solar physics is revealed. Studies indicate that supergranulation is not only a crucial component of the solar convection zone but also drives coronal heating and solar wind acceleration through mechanisms such as magnetic reconnection and Alfvén wave propagation. Furthermore, the interaction between supergranulation and larger-scale convective patterns (e.g., giant cells) provides new insights into the dynamics of the solar interior. Despite significant progress in recent years, the formation mechanism and dynamical nature of supergranulation remain unresolved. Future research should combine high-resolution observations, theoretical modeling, and numerical simulations to further elucidate the complex dynamical processes and the central role of supergranulation in solar physics. 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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11 pages, 771 KB

Open AccessArticle

Period Variation Rates of Four Radial Single-Mode High-Amplitude Delta Scuti Stars

by Tian-Fang Ma, Jia-Shu Niu and Hui-Fang Xue

Universe 2025, 11(3), 86; https://doi.org/10.3390/universe11030086 - 6 Mar 2025

Cited by 1 | Viewed by 810

Abstract

In this work, we present a study on the long time-scale period variations of four single-mode high-amplitude delta Scuti stars (HADS) via the classical

O - C

analysis. The target HADS are (i) XX Cygni, (ii) YZ Bootis, (iii) GP Andromedae, and (iv) [...] Read more.

In this work, we present a study on the long time-scale period variations of four single-mode high-amplitude delta Scuti stars (HADS) via the classical

O - C

analysis. The target HADS are (i) XX Cygni, (ii) YZ Bootis, (iii) GP Andromedae, and (iv) ZZ Microscopii. The newly determined times of maximum light came from the Transiting Exoplanet Survey Satellite (TESS), American Association of Variable Star Observers (AAVSO), and Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (BAV) projects. Together with the times of maximum light obtained in the historical literature, the

O - C

analysis was performed on these HADS, in which we obtained the linear period variation rates

\dot{P} / P

as

(9.2 \pm 0.2) \times 10^{- 9} {yr}^{- 1}

,

(3.2 \pm 0.2) \times 10^{- 9} {yr}^{- 1}

,

(4.22 \pm 0.03) \times 10^{- 8} {yr}^{- 1}

, and

(- 2.06 \pm 0.02) \times 10^{- 8} {yr}^{- 1}

, respectively. Based on these results and some earlier research, we also discuss the evolutionary stages and the mechanisms of the period variation of these four HADS. Full article

(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)

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21 pages, 1314 KB

Open AccessArticle

Revisiting Holographic Dark Energy from the Perspective of Multi-Messenger Gravitational Wave Astronomy: Future Joint Observations with Short Gamma-Ray Bursts

by Tao Han, Ze Li, Jing-Fei Zhang and Xin Zhang

Universe 2025, 11(3), 85; https://doi.org/10.3390/universe11030085 - 6 Mar 2025

Cited by 5 | Viewed by 904

Abstract

The advent of third-generation (3G) gravitational-wave (GW) detectors opens new opportunities for multi-messenger observations of binary neutron star merger events, holding significant potential for probing the history of cosmic expansion. In this paper, we investigate the holographic dark energy (HDE) model by using [...] Read more.

The advent of third-generation (3G) gravitational-wave (GW) detectors opens new opportunities for multi-messenger observations of binary neutron star merger events, holding significant potential for probing the history of cosmic expansion. In this paper, we investigate the holographic dark energy (HDE) model by using the future GW standard siren data observed from the 3G GW detectors and the short

γ

-ray burst THESEUS-like detector joint observations. We find that GW data alone can achieve a relatively precise estimation of the Hubble constant, with precision of

0.2

–

0.6 %

, but its ability to constrain other cosmological parameters remains limited. Nonetheless, since the GW data can break parameter degeneracies generated by the mainstream EM observations, CMB + BAO + SN (CBS), GW standard sirens play a crucial role in enhancing the accuracy of parameter estimation. With the addition of GW data to CBS, the constraints on cosmological parameters

H_{0}

, c and

Ω_{m}

can be improved by 63–

88 %

, 27–

44 %

and 55–

70 %

. In summary, observations of GW standard sirens from 3G GW detectors could be pivotal in probing the fundamental nature of dark energy. 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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12 pages, 635 KB

Open AccessArticle

Simultaneous Optical-to-X-Ray Spectrum of OJ 287 During Lowest X-Ray State: Synchrotron-Soft Tail and Harder X-Ray Spectrum

by Pankaj Kushwaha

Universe 2025, 11(3), 84; https://doi.org/10.3390/universe11030084 - 5 Mar 2025

Cited by 2 | Viewed by 686

Abstract

The X-ray spectrum of OJ 287 has exhibited diverse variations with broadband spectral behavior representative of all the spectral classes of blazars. These changes have been explained either via new emission components or as the sum of the jet synchrotron and its inverse [...] Read more.

The X-ray spectrum of OJ 287 has exhibited diverse variations with broadband spectral behavior representative of all the spectral classes of blazars. These changes have been explained either via new emission components or as the sum of the jet synchrotron and its inverse Compton part. In the current work, we focus on the systematic spectral investigation of the lowest X-ray state recorded by the Swift facility to understand X-ray spectral changes. Considering optical-to-X-ray observations jointly, we found a power-law optical–UV spectrum with a photon spectrum of 2.71 ± 0.03 extending to X-ray energies. Accounting for this contribution in X-rays, we inferred a power-law photon X-ray spectrum of 1.22 ± 0.20 that improves to 1.29 ± 0.06 when considering other observations with similar X-ray spectra. An extended optical–UV spectrum with an associated low hard X-ray spectrum is further strengthened by the natural explanation of another optical–UV state of similar flux with a very different optical–UV-to-X-ray spectrum by its synchrotron and this hard X-ray spectrum. This is the hardest reported X-ray spectrum (0.3–10 keV), consistent with the Swift-BAT X-ray spectrum. We further found that this X-ray spectrum can reproduce most of the flat X-ray spectra when combined with the corresponding optical–UV continuum during the low and intermediate flux states, strengthening the synchrotron as the primary driver of most of the X-ray spectral changes in the LBL state of the source. Compared with the sharp steepening/cutoff of the optical–UV spectrum during bright phases, the inferred extended spectrum implies a comparatively larger emission region and could be associated with large-scale jet emission. The optical–UV spectrum implies a high-energy power-law particle spectrum of ∼4.4, while X-ray implies a hard low-energy particle spectrum of 1.3–1.6 that alternatively can also result from a higher lower-energy cutoff in the particle spectrum. Full article

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

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10 pages, 281 KB

Open AccessArticle

Solid Identification of Extragalactic Gamma-Ray Source Using High-Resolution Radio Interferometric Observation

by Krisztina Éva Gabányi, Sándor Frey, Krisztina Perger and Emma Kun

Universe 2025, 11(3), 83; https://doi.org/10.3390/universe11030083 - 1 Mar 2025

Cited by 2 | Viewed by 1429

Abstract

The dominant fraction of the extragalactic

γ

-ray sources are blazars, active galactic nuclei with jets inclined ata small angle to the line of sight. Apart from blazars, a few dozen narrow-line Seyfert 1 galaxies (NLS1) and a number of radio galaxies are [...] Read more.

The dominant fraction of the extragalactic

γ

-ray sources are blazars, active galactic nuclei with jets inclined ata small angle to the line of sight. Apart from blazars, a few dozen narrow-line Seyfert 1 galaxies (NLS1) and a number of radio galaxies are associated with

γ

-ray sources. The identification of

γ

-ray sources requires multiwavelength follow-up observations since several candidates could reside within the relatively large

γ

-ray localisation area. The

γ

-ray source 4FGL 0959.6+4606 was originally associated with a radio galaxy. However, follow-up multiwavelength work suggested a nearby NLS1 as the more probable origin of the

γ

-ray emission. We performed high-resolution very long baseline interferometry (VLBI) observation at 5 GHz of both proposed counterparts of 4FGL 0959.6+4606. We clearly detected the NLS1 source SDSS J095909.51+460014.3 with relativistically boosted jet emission. On the other hand, we did not detect milliarcsecond-scale compact emission in the radio galaxy 2MASX J09591976+4603515. Our VLBI imaging results suggest that the NLS1 object is the origin of the

γ

-ray emission in 4FGL 0959.6+4606. Full article

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17 pages, 1028 KB

Open AccessArticle

Unveiling the Dynamics in Galaxy Clusters: The Hidden Role of Low-Luminosity Galaxies in Coma

by Alisson P. Costa, André L. B. Ribeiro, Flavio R. de Morais Neto and Juarez dos Santos Junior

Universe 2025, 11(3), 82; https://doi.org/10.3390/universe11030082 - 1 Mar 2025

Cited by 1 | Viewed by 694

Abstract

In this work, we study the Coma cluster, one of the richest and most well-known systems at low redshifts, to explore the importance of low-flux objects in the identification of cluster substructures. In addition, we conduct a study of the infall flow around [...] Read more.

In this work, we study the Coma cluster, one of the richest and most well-known systems at low redshifts, to explore the importance of low-flux objects in the identification of cluster substructures. In addition, we conduct a study of the infall flow around Coma, considering the presence or absence of low-flux objects across the projected phase space of the cluster. Our results indicate that low-luminosity galaxies play a fundamental role in understanding the dynamical state of galaxy clusters. These galaxies, often overlooked because of their faint nature, serve as sensitive tracers of substructure dynamics and provide crucial insights into the cluster’s evolutionary history. We show that not considering the low-flux objects present in clusters can lead to significant underestimates of the numbers of substructures, both in most central parts, in the infall regions, and beyond, connecting to the large-scale structure up to a distance of ∼8

R_{200}

from the center of Coma. Full article

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

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16 pages, 1581 KB

Open AccessArticle

Searching for Gravitational-Wave Bursts from Cosmic String Cusps with the Parkes Pulsar Timing Array’s Third Data Release

by Yong Xia, Jingbo Wang, Sachiko Kuroyanagi, Wenming Yan, Yirong Wen, Agastya Kapur, Jing Zou, Yi Feng, Valentina Di Marco, Saurav Mishra, Christopher J. Russell, Shuangqiang Wang, De Zhao and Xingjiang Zhu

Universe 2025, 11(3), 81; https://doi.org/10.3390/universe11030081 - 1 Mar 2025

Viewed by 905

Abstract

Pulsar timing arrays (PTAs) are designed to detect nanohertz-frequency gravitational waves (GWs). Since GWs are anticipated from cosmic strings, PTAs offer a viable approach to testing their existence. We present the results of the first Bayesian search for gravitational-wave bursts from cosmic string [...] Read more.

Pulsar timing arrays (PTAs) are designed to detect nanohertz-frequency gravitational waves (GWs). Since GWs are anticipated from cosmic strings, PTAs offer a viable approach to testing their existence. We present the results of the first Bayesian search for gravitational-wave bursts from cosmic string cusps (GWCSs) using the third PPTA data release for 30 ms pulsars. In this data collection, we find no evidence for GWCS signals. We compare a model with a GWCS signal to one with only noise, including a common spatially uncorrelated red noise (CURN), and find that our data are more consistent with the noise-only model. We then establish upper limits on the strain amplitude of GWCSs at the pulsar term, based on the analysis of 30 ms pulsars, after finding no compelling evidence. We find the addition of a CURN with different spectral indices into the noise model has a negligible impact on the upper limits. And the upper limit range of the amplitude of the pulsar-term GWCSs is concentrated between

10^{- 12}

and

10^{- 11}

. Finally, we set upper limits on the amplitude of GWCS events, parametrized by width and event epoch, for a single-pulsar PSR J1857 + 0943. Moreover, we derive the upper limit on the cosmic string tension as a function of burst width and compare it with previous results. Full article

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

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8 pages, 385 KB

Open AccessArticle

Looking for New Strategies to Probe Low-Mass Axion-like Particles in Ultraperipheral Heavy-Ion Collisions at the LHC

by Pedro Nogarolli, Victor P. Gonçalves and Murilo S. Rangel

Universe 2025, 11(3), 80; https://doi.org/10.3390/universe11030080 - 1 Mar 2025

Viewed by 687

Abstract

The possibility to search for long-lived axion-like particles (ALPs) decaying into photons is investigated in ultraperipheral PbPb collisions at the Large Hadron Collider (LHC). We propose a search strategy for low-mass ALPs using the LHCb and ALICE experiments. The ALP identification is performed [...] Read more.

The possibility to search for long-lived axion-like particles (ALPs) decaying into photons is investigated in ultraperipheral PbPb collisions at the Large Hadron Collider (LHC). We propose a search strategy for low-mass ALPs using the LHCb and ALICE experiments. The ALP identification is performed by requiring the decay vertex be reconstructed outside the region where a primary vertex is expected, which strongly suppress the contribution associated with the decay of light mesons. We also use the fact that a fraction of the photons convert into electron–positron pairs, allowing the reconstruction of the particle decay position. We present the predictions for the pseudorapidity and transverse momentum distributions of the ALPs and photons. Moreover, predictions for the fiducial cross-sections, derived considering the characteristics of the ALICE and LHCb detectors, are presented for different values of the ALP mass and the ALP—photon coupling. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—High Energy Nuclear and Particle Physics)

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15 pages, 290 KB

Open AccessArticle

Quasi-Homogeneous Black Hole Thermodynamics in Non-Commutative Geometry

by Hernando Quevedo and María N. Quevedo

Universe 2025, 11(3), 79; https://doi.org/10.3390/universe11030079 - 27 Feb 2025

Cited by 1 | Viewed by 784

Abstract

We study the thermodynamic properties of a black hole that takes into account the effects of non-commutative geometry. To emphasize the role of new effects, we have chosen a specific modified Schwarzschild black hole inspired by non-commutative geometry. We show that, in order [...] Read more.

We study the thermodynamic properties of a black hole that takes into account the effects of non-commutative geometry. To emphasize the role of new effects, we have chosen a specific modified Schwarzschild black hole inspired by non-commutative geometry. We show that, in order to apply the laws of quasi-homogeneous thermodynamics using the formalism of geometrothermodynamics, it is necessary to consider the non-commutative parameter as an independent thermodynamic variable. As a result, the properties of the black hole change drastically, leading to phase transitions that are directly related to the value of the non-commutative parameter. We also prove that an unstable commutative black hole can become stable in non-commutative geometry for particular values of the non-commutative parameter. Full article

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

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17 pages, 744 KB

Open AccessArticle

Ultra-High-Energy Cosmic Rays from Active Galactic Nuclei Jets: The Role of Supermassive Black Hole Growth and Accretion States

by Olmo Piana and Hung-Yi Pu

Universe 2025, 11(3), 78; https://doi.org/10.3390/universe11030078 - 24 Feb 2025

Viewed by 908

Abstract

Jets emanating from active galactic nuclei (AGNs) represent some of the most formidable particle accelerators in the universe, thereby emerging as viable candidates responsible for the detection of ultra-high-energy cosmic rays (UHECRs). If AGN jets indeed serve as origins of UHECRs, then the [...] Read more.

Jets emanating from active galactic nuclei (AGNs) represent some of the most formidable particle accelerators in the universe, thereby emerging as viable candidates responsible for the detection of ultra-high-energy cosmic rays (UHECRs). If AGN jets indeed serve as origins of UHECRs, then the diffuse flux of these cosmic rays would be dependent on the power and duty cycle of these jets, which are inherently connected to the nature of black hole accretion flows. In this article, we present our cosmological semi-analytic framework, JET (Jets from Early Times), designed to trace the evolution of jetted AGN populations. This framework serves as a valuable tool for predictive analyses of cosmic ray energy density and, potentially, neutrino energy density. By using JET, we model the formation and evolution of galaxies and supermassive black holes (SMBHs) from

z = 20

to

z = 1

, incorporating jet formation and feedback mechanisms and distinguishing between various accretion states determined by the SMBH Eddington ratios. The implications of different SMBH growth models on predicting cosmic ray flux are investigated. We provide illustrative examples demonstrating how the associated diffuse UHECR fluxes at the source may vary in relation to the jet production efficiencies and the selected SMBH growth model, linking cosmological models of SMBH growth with astroparticle backgrounds. Full article

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

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40 pages, 1110 KB

Open AccessArticle

Time Scales of Slow-Roll Inflation in Asymptotically Safe Cosmology

by József Nagy, Sándor Nagy and Kornél Sailer

Universe 2025, 11(3), 77; https://doi.org/10.3390/universe11030077 - 21 Feb 2025

Viewed by 610

Abstract

Making use of the well-known renormalization group (RG) scale dependences of the gravitational couplings in the framework of the two-parameter Einstein–Hilbert (EH) theory of gravity, the single scalar field-driven cosmological inflation is discussed in a spatially homogeneous, isotropic, and flat model universe. The [...] Read more.

Making use of the well-known renormalization group (RG) scale dependences of the gravitational couplings in the framework of the two-parameter Einstein–Hilbert (EH) theory of gravity, the single scalar field-driven cosmological inflation is discussed in a spatially homogeneous, isotropic, and flat model universe. The inflaton field is represented by a one-component real, non-self-interacting, massive scalar field minimally coupled to gravity. Cases without and with the incorporation of the RG scaling of the inflaton mass are compared with each other and with the corresponding classical case. It is shown that the quantum improvement drastically alters the timing of the slow-roll inflation with the desirable number

\underset{,}{N} \approx 60

e-foldings, as compared with the classical case. Furthermore, accounting for the RG flow of the inflaton mass has an enormous effect on the timing of the desirable slow roll, too. Although providing the desirable slow-roll inflation, none of the versions of the investigated quantum-improved toy models provide a realistic value of the amplitude of the scalar perturbations. 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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20 pages, 909 KB

Open AccessArticle

Where to Search for Supermassive Binary Black Holes

by Paola Marziani, Edi Bon, Natasa Bon and Mauro D’Onofrio

Universe 2025, 11(3), 76; https://doi.org/10.3390/universe11030076 - 21 Feb 2025

Viewed by 801

Abstract

Supermassive binary black holes (SMBBHs) are the anticipated byproducts of galaxy mergers and play a pivotal role in shaping galaxy evolution, gravitational wave emissions, and accretion physics. Despite their theoretical prevalence, direct observational evidence for SMBBHs remains elusive, with only a handful of [...] Read more.

Supermassive binary black holes (SMBBHs) are the anticipated byproducts of galaxy mergers and play a pivotal role in shaping galaxy evolution, gravitational wave emissions, and accretion physics. Despite their theoretical prevalence, direct observational evidence for SMBBHs remains elusive, with only a handful of candidates identified to date. This paper explores optimal strategies and key environments for locating SMBBHs, focusing on observational signatures in the broad Balmer lines. We present a preliminary analysis on a flux-limited sample of sources belonging to an evolved spectral type along the quasar main sequence, and we discuss the spectroscopic clues indicative of binary activity and highlight the critical role of time-domain spectroscopic surveys in uncovering periodic variability linked to binary systems. Full article

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

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25 pages, 591 KB

Open AccessArticle

Starobinsky Inflation with T-Model Kähler Geometries

by Constantinos Pallis

Universe 2025, 11(3), 75; https://doi.org/10.3390/universe11030075 - 21 Feb 2025

Cited by 2 | Viewed by 543

Abstract

We present novel implementations of Starobisky-like inflation within supergravity adopting Kähler potentials for the inflaton which parameterizes hyperbolic geometries known from T-model inflation. The associated superpotentials are consistent with an R and a global or gauge

U {(1)}_{X}

symmetries. The [...] Read more.

We present novel implementations of Starobisky-like inflation within supergravity adopting Kähler potentials for the inflaton which parameterizes hyperbolic geometries known from T-model inflation. The associated superpotentials are consistent with an R and a global or gauge

U {(1)}_{X}

symmetries. The inflaton is represented by a gauge-singlet or non-singlet superfield and is accompanied by a gauge-singlet superfield successfully stabilized thanks to its compact contribution into the total Kähler potential. Keeping the Kähler manifold intact, a conveniently violated shift symmetry is introduced which allows for slight variation in the predictions of Starobinsky inflation: The (scalar) spectral index exhibits an upper bound which lies close to its central observational value whereas the constant scalar curvature of the inflaton-sector Kähler manifold increases with the tensor-to-scalar ratio. 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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41 pages, 5616 KB

Open AccessReview

Searching for New Physics in an Ultradense Environment: A Review on Dark Matter Admixed Neutron Stars

by Francesco Grippa, Gaetano Lambiase and Tanmay Kumar Poddar

Universe 2025, 11(3), 74; https://doi.org/10.3390/universe11030074 - 21 Feb 2025

Cited by 10 | Viewed by 1219

Abstract

Neutron stars (NSs), among the densest objects in the universe, are exceptional laboratories for investigating the properties of dark matter (DM). Recent theoretical and observational developments have heightened interest in exploring the impact of DM on NS structure, giving rise to the concept [...] Read more.

Neutron stars (NSs), among the densest objects in the universe, are exceptional laboratories for investigating the properties of dark matter (DM). Recent theoretical and observational developments have heightened interest in exploring the impact of DM on NS structure, giving rise to the concept of dark matter admixed neutron stars (DANSs). This review examines how NSs can accumulate DM over time, potentially altering their fundamental properties. We explore the leading models describing DM behavior within NSs, focusing on the effects of both bosonic and fermionic candidates on key features such as mass, radius, and tidal deformability. Additionally, we review how DM can modify the cooling and heating processes, trigger the formation of a black hole, and impact gravitational wave (GW) emissions from binary systems. By synthesizing recent research, this work highlights how DANSs might produce observable signatures, offering new opportunities to probe DM’s properties through astrophysical phenomena. Full article

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

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