Universe

28 pages, 1345 KB

Open AccessArticle

Thermal Quarks and Polyakov Loops in Two-Color Dense QCD

by Yugo Kurebayashi, Toru Kojo and Daiki Suenaga

Universe 2025, 11(11), 357; https://doi.org/10.3390/universe11110357 - 29 Oct 2025

We study confinement and deconfinement in dense two-color QCD by analyzing the dynamics of thermal quarks and gluons. The Polyakov loop is used as a probe of the relevant thermal excitations, distinguishing quark- and hadron-dominated regimes in dense matter. To describe the Polyakov [...] Read more.

We study confinement and deconfinement in dense two-color QCD by analyzing the dynamics of thermal quarks and gluons. The Polyakov loop is used as a probe of the relevant thermal excitations, distinguishing quark- and hadron-dominated regimes in dense matter. To describe the Polyakov loop, we adopt both lattice-informed phenomenological models and the massive Yang–Mills framework. After calibrating these models at zero density, we investigate in-medium modifications of the Polyakov loops and gluon propagators at finite temperature and density. Diquark gaps control the screening at zero temperature, whereas the screening due to thermal quarks is sensitive to the Polyakov loop. Inclusion of the Polyakov loop helps to reproduce lattice data at low temperature, suggesting that thermal excitations are predominantly hadronic rather than uncorrelated quarks. Full article

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

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16 pages, 630 KB

Open AccessArticle

Identifying Companions in Pulsar Binary Systems via Gaia Data

by Yueqi Song, Li Guo, Zhen Yan, Qiqi Wu, Guangli Wang and Ying Wang

Universe 2025, 11(11), 358; https://doi.org/10.3390/universe11110358 - 28 Oct 2025

Abstract

In the optical band, very few pulsars can be directly detected, but some of the pulsar binary companions can be observed. This study leverages high-precision astrometric data from Gaia Data Release 3 (DR3) to identify pulsar companions in binary systems. Cross-matching the Australia [...] Read more.

In the optical band, very few pulsars can be directly detected, but some of the pulsar binary companions can be observed. This study leverages high-precision astrometric data from Gaia Data Release 3 (DR3) to identify pulsar companions in binary systems. Cross-matching the Australia Telescope National Facility (ATNF) Pulsar Catalogue with Gaia DR3 yielded 58 astrometric pairs, including 9 newly confirmed companions—primarily in the southern hemisphere—expanding the known pulsar distribution there. Among newly confirmed companions, eight are redback pulsars, offering insights into millisecond pulsar evolution and companion composition. All 58 companions are classified as main-sequence stars, neutron stars, white dwarfs, or ultra-light companion stars, with ∼40% being spider pulsars. Gaia’s exceptional astrometric precision advances pulsar studies, enabling gravitational wave detection via Pulsar Timing Arrays (PTAs) and improved reference frame link. Future multi-wavelength research will benefit from Gaia DR4, International Pulsar Timing Array (IPTA) collaborations (including Five-hundred-meter Aperture Spherical radio Telescope (FAST)), and Very Long Baseline Interferometry (VLBI) networks like the Chinese VLBI Network (CVN). Full article

(This article belongs to the Section Compact Objects)

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29 pages, 419 KB

Open AccessReview

Modified Gravity with Nonminimal Curvature–Matter Couplings: A Framework for Gravitationally Induced Particle Creation

by Francisco S. N. Lobo, Tiberiu Harko and Miguel A. S. Pinto

Universe 2025, 11(11), 356; https://doi.org/10.3390/universe11110356 - 28 Oct 2025

Abstract

Modified gravity theories with a nonminimal coupling between curvature and matter offer a compelling alternative to dark energy and dark matter by introducing an explicit interaction between matter and curvature invariants. Two of the main consequences of such an interaction are the emergence [...] Read more.

Modified gravity theories with a nonminimal coupling between curvature and matter offer a compelling alternative to dark energy and dark matter by introducing an explicit interaction between matter and curvature invariants. Two of the main consequences of such an interaction are the emergence of an additional force and the non-conservation of the energy–momentum tensor, which can be interpreted as an energy exchange between matter and geometry. By adopting this interpretation, one can then take advantage of many different approaches in order to investigate the phenomenon of gravitationally induced particle creation. One of these approaches relies on the so-called irreversible thermodynamics of open systems formalism. By considering the scalar–tensor formulation of one of these theories, we derive the corresponding particle creation rate, creation pressure, and entropy production, demonstrating that irreversible particle creation can drive a late-time de Sitter acceleration through a negative creation pressure, providing a natural alternative to the cosmological constant. Furthermore, we demonstrate that the generalized second law of thermodynamics holds: the total entropy, from both the apparent horizon and enclosed matter, increases monotonically and saturates in the de Sitter phase, imposing constraints on the allowed particle production dynamics. Furthermore, we present brief reviews of other theoretical descriptions of matter creation processes. Specifically, we consider approaches based on the Boltzmann equation and quantum-based aspects and discuss the generalization of the Klein–Gordon equation, as well as the problem of its quantization in time-varying gravitational fields. Hence, gravitational theories with nonminimal curvature–matter couplings present a unified and testable framework, connecting high-energy gravitational physics with cosmological evolution and, possibly, quantum gravity, while remaining consistent with local tests through suitable coupling functions and screening mechanisms. Full article

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

26 pages, 1275 KB

Open AccessReview

Artificial Intelligence Revolutionizing Time-Domain Astronomy

by Ze-Ning Wang, Da-Chun Qiang and Sheng Yang

Universe 2025, 11(11), 355; https://doi.org/10.3390/universe11110355 - 28 Oct 2025

Abstract

Artificial intelligence (AI) applications have attracted widespread attention and have proven to be highly successful in understanding messages across various dimensions. These applications have the potential to assist astronomers in exploring the massive amounts of astronomical data. In fact, the integration of AI [...] Read more.

Artificial intelligence (AI) applications have attracted widespread attention and have proven to be highly successful in understanding messages across various dimensions. These applications have the potential to assist astronomers in exploring the massive amounts of astronomical data. In fact, the integration of AI techniques with astronomy began some time ago, significantly advancing our understanding of the universe by aiding in exoplanet discovery, galaxy morphology classification, gravitational wave event analysis, and more. In particular, AI is now recognized as a crucial component in time-domain astronomy, particularly given the rapid evolution of targeting transients and the increasing number of candidates detected by powerful surveys. A notable success is SN 2023tyk, the first transient discovered and spectroscopically classified without human inspection, an achievement made even more remarkable given that it was identified by the Zwicky Transient Facility, which detects millions of alert sources every night. There is no doubt that AI will play a crucial role in future astronomical observations across various messenger channels, aiding in transient discovery and classification, and helping, or even replacing, observers in making real-time decisions. This review paper examines several cases where AI is transforming contemporary astronomy, especially time-domain astronomy. We discuss the AI algorithms and methodologies employed to date, highlight significant discoveries enabled by AI, and outline future research directions in this rapidly evolving field. Full article

(This article belongs to the Special Issue Applications of Artificial Intelligence in Modern Astronomy)

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15 pages, 5813 KB

Open AccessReview

Strangeon Matter: From Stars to Nuggets

by Haoyang Qi and Renxin Xu

Universe 2025, 11(11), 354; https://doi.org/10.3390/universe11110354 - 25 Oct 2025

Abstract

The fact that strange sea quarks are abundant in the nucleons, but with zero net strangeness, is of great importance for understanding the nature of matter condensed by strong interaction, particularly in the context of the “gigantic nucleus” created after the gravitational collapse [...] Read more.

The fact that strange sea quarks are abundant in the nucleons, but with zero net strangeness, is of great importance for understanding the nature of matter condensed by strong interaction, particularly in the context of the “gigantic nucleus” created after the gravitational collapse of an evolved massive star. We hypothesize that the basic unit of bulk strong matter with the approximately light-flavored symmetry of valence quarks is “strangeon”, which is the counterpart of the nucleon found in atomic nuclei. In addition to strangeon stars (SnSs) with a large baryon number of

A \approx 10^{57}

, strange nuggets (SnNs) with

A ≳ 10^{10}

could also exist in the universe. Both the SnSs and the SnNs are explained, with particular focus on the evidence obtained from observation and detection. Full article

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

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31 pages, 2176 KB

Open AccessArticle

Models of Charged Gravastars in f(T)-Gravity

by Mohamed A. Bakry and Ali Eid

Universe 2025, 11(10), 353; https://doi.org/10.3390/universe11100353 - 21 Oct 2025

Abstract

This study investigates three distinct charged gravastar models within the framework of

f (T)

modified gravity, considering the functional forms

f (T) = T

,

f (T) = a + b T

, and

f (T) = T^{2}

. Inspired by the [...] Read more.

This study investigates three distinct charged gravastar models within the framework of

f (T)

modified gravity, considering the functional forms

f (T) = T

,

f (T) = a + b T

, and

f (T) = T^{2}

. Inspired by the Mazur–Mottola conjecture, we propose these models as singularity-free alternatives to black holes, each characterized by a three-region structure: (i) an interior de Sitter core, (ii) an intermediate thin shell composed of ultrarelativistic matter, and (iii) an exterior region described by the Reisner Nordstrom solution and other novel spherically symmetric vacuum solutions. We derive a complete set of exact, singularity-free solutions for the charged gravastar configuration, demonstrating their mathematical consistency and physical viability in the context of alternative gravity theories. Notably, the field equations governing the thin shell are solved using an innovative approach based on Killing vector symmetries, eliminating the need for approximations commonly employed in prior studies. Furthermore, we analyze key physical properties of the thin shell, including its proper length, entropy distribution, and energy content. A thorough examination of the energy conditions reveals the thermodynamic stability and viability of these models. Our results contribute to the growing body of work on exotic compact objects and provide new insights into the interplay between modified gravity, electromagnetism, and non-singular black hole alternatives. Full article

(This article belongs to the Section Gravitation)

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18 pages, 3404 KB

Open AccessArticle

Model-Independent Inference of Galaxy Star Formation Histories in the Local Volume

by Robin Eappen and Pavel Kroupa

Universe 2025, 11(10), 352; https://doi.org/10.3390/universe11100352 - 20 Oct 2025

Abstract

Understanding the diversity of star formation histories (SFHs) of galaxies is key to reconstructing their evolutionary paths. Traditional models often assume parametric forms such as delayed-

τ

or exponentially declining models, which may not reflect the actual variety of formation processes. We aim [...] Read more.

Understanding the diversity of star formation histories (SFHs) of galaxies is key to reconstructing their evolutionary paths. Traditional models often assume parametric forms such as delayed-

τ

or exponentially declining models, which may not reflect the actual variety of formation processes. We aim to assess what types of SFHs are consistent with the observed present-day star formation rates (

{SFR}_{0}

) and time-averaged star formation rates (

⟨ SFR ⟩

) of galaxies in the Local Volume, without assuming any fixed functional form. We construct a non-parametric framework by generating large ensembles of randomized SFHs for each galaxy in the sample. For each SFH, we compute its predicted stellar mass and present-day SFR and retain only those consistent with the observed values within a 20% tolerance. We then infer the statistical distribution of power-law slopes

η

(fitted as

SFR (t) \propto {(t - t_{start})}^{η}

) and 50% stellar mass formation times

t_{50}

. Across the full sample of 555 galaxies, we find that ≈70% have flat SFHs (

| η | \leq 0.01

), ≈24% are mildly declining (

η < - 0.01

), and ≈6% are rising (

η > 0.01

). In the low-mass bin (

M_{★} < 3 \times 10^{9} M_{⊙}

), rising SFHs slightly increase (≈7%) but remain a minority as the majority have flat SFHs. Both

η

and

t_{50}

correlate strongly with the SFR ratio (Spearman

ρ > 0.75

,

p ≪ 10^{- 16}

), indicating that the shape and timing of star formation are primarily governed by this ratio. The

t_{50}

distribution shows sharp spikes near 7.74 and 7.86 Gyr, which we attribute to grid discretization combined with filtering, rather than a physical bimodality. Our results confirm that strongly declining SFH templates are disfavored in the Local Volume: most systems are consistent with flat long-term SFHs, with only mild decline or occasional rising. Importantly, this is demonstrated through a fully model-independent, data-driven approach, with per-galaxy uncertainties quantified using the standard error of

η

and

t_{50}

from the ensemble of accepted SFHs. Full article

(This article belongs to the Section Galaxies and Clusters)

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24 pages, 679 KB

Open AccessArticle

Study of Singly Charmed Dibaryons in Quark Model

by Yuxuan Du, Yao Cui, Zhiyun Tan, Jin Tao, Hongxia Huang and Jialun Ping

Universe 2025, 11(10), 351; https://doi.org/10.3390/universe11100351 - 20 Oct 2025

Abstract

We perform a systematic investigation of low-lying singly charmed dibaryon systems with

J = 1

,

I = 0, \frac{1}{2}, 1, \frac{3}{2}, 2, \frac{5}{2}

and strangeness [...] Read more.

We perform a systematic investigation of low-lying singly charmed dibaryon systems with

J = 1

,

I = 0, \frac{1}{2}, 1, \frac{3}{2}, 2, \frac{5}{2}

and strangeness

S = - 1, - 2, - 3, - 4, - 5

in the chiral quark model. According to the analysis of effective potentials, dibaryon systems characterized by lower isospin and magnitude of strangeness exhibit stronger attractive interactions, which may enhance their tendency to form bound states. Experimental efforts may therefore prioritize the search for such configurations. The bound-state calculation results indicate that we have obtained some single-channel bound states, which are

Σ Σ_{c}

,

Σ Σ_{c}^{*}

,

Σ^{*} Σ_{c}

,

Σ^{*} Σ_{c}^{*}

with

I = 0, S = - 1

;

Σ_{c} Δ

,

Σ_{c}^{*} Δ

with

I = \frac{1}{2}, S = 0

;

Σ Σ_{c}

with

I = 1, S = - 1

;

Σ_{c} Δ

with

I = \frac{3}{2}, S = 0

; and

Ξ^{*} Σ_{c}^{*}

with

I = \frac{3}{2}, S = - 2

. However, these states can decay through open channels. We have listed both these single-channel bound states and their corresponding decay channels in this work for experimental reference and search. In the future, we need to study the scattering processes of the open channels further to confirm whether these states are resonance states. Full article

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

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26 pages, 406 KB

Open AccessArticle

A Quasigroup Approach for Conservation Laws in Asymptotically Flat Spacetimes

by Alfonso Zack Robles, Alexander I. Nesterov and Claudia Moreno

Universe 2025, 11(10), 350; https://doi.org/10.3390/universe11100350 - 20 Oct 2025

Abstract

In the framework of the quasigroup approach to conservation laws in general relativity, we show how the infinite-parametric Newman–Unti group of asymptotic symmetries can be reduced to the Poincaré quasigroup. We compute Noether’s charges associated with any element of the Poincaré quasialgebra. The [...] Read more.

In the framework of the quasigroup approach to conservation laws in general relativity, we show how the infinite-parametric Newman–Unti group of asymptotic symmetries can be reduced to the Poincaré quasigroup. We compute Noether’s charges associated with any element of the Poincaré quasialgebra. The integral conserved quantities of energy momentum and angular momentum, being linear on generators of the Poincaré quasigroup, are identically equal to zero in Minkowski spacetime. We present a definition of the angular momentum free of the supertranslation ambiguity. We provide an appropriate notion of intrinsic angular momentum and a description of the mass reference frame’s center at future null infinity. Finally, in the center of mass reference frame, the momentum and angular momentum are defined by the Komar expression. Full article

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

13 pages, 1519 KB

Open AccessArticle

Thermodynamic Assessment of Prebiotic Molecule Formation Pathways on Comets

by Luca Tonietti

Universe 2025, 11(10), 349; https://doi.org/10.3390/universe11100349 - 18 Oct 2025

Abstract

Comets are chemically rich and thermally extreme, spanning surface temperatures from ~50 K in the Oort Cloud to >1000 K for sungrazing bodies. These conditions may support key steps of prebiotic chemistry, including the synthesis of nucleic acid precursors. This study present a [...] Read more.

Comets are chemically rich and thermally extreme, spanning surface temperatures from ~50 K in the Oort Cloud to >1000 K for sungrazing bodies. These conditions may support key steps of prebiotic chemistry, including the synthesis of nucleic acid precursors. This study present a thermodynamic evaluation of seven candidate reactions, producing nitrogenous bases, sugars, nucleosides, and nucleotides, across the cometary temperature spectrum, 50–1000 K. Purine nucleobase synthesis, including adenine formation via aminoacetonitrile polymerization and HCN polymerization, is strongly exergonic at all temperatures. Sugar formation from formaldehyde is also exergonic, while intermediate pathways, e.g., 2-aminooxazole synthesis, become thermodynamically viable only above ~700 K. Nucleoside formation is thermodynamically neutral at low T but becomes favorable at elevated temperatures, whereas phosphorylation to AMP, i.e., adenosine-monophosphate, a nucleotide serving as a critical regulator of cellular energy status, remains highly endergonic under the entire T range studied. My analysis suggests that, under standard-state assumptions, comets can thermodynamically support formation routes of nitrogenous bases and simple sugars but not a complete nucleotide assembly. This supports a dual-phase origin scenario, where comets act as molecular reservoirs, with further polymerization and biological activation occurring post-delivery on planetary surfaces. Importantly, these findings represent purely thermodynamic assessments under standard-state assumptions and do not address kinetic barriers, catalytic influences, or adsorption effects on ice or mineral surfaces. The results should therefore be viewed as a baseline map of feasibility, subject to modifications in more complex chemical environments. Full article

(This article belongs to the Section Planetary Sciences)

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31 pages, 5922 KB

Open AccessArticle

The Iwasawa–Taniguchi Effect in Compton-thick Seyfert 2 Galaxies with Extended Hard X-Ray Emission

by Jia Chen, Huili Yi, Chengchao Liu, Shenfang Ye and Shuangxi Yi

Universe 2025, 11(10), 348; https://doi.org/10.3390/universe11100348 - 18 Oct 2025

Abstract

The anti-correlation between the equivalent width of the neutral narrow Fe K

α

line and the 2–10 keV luminosity (the Iwasawa–Taniguchi effect) in the nuclear regions of active galactic nuclei has been debated in recent years. With the high angular resolution of Chandra, [...] Read more.

The anti-correlation between the equivalent width of the neutral narrow Fe K

α

line and the 2–10 keV luminosity (the Iwasawa–Taniguchi effect) in the nuclear regions of active galactic nuclei has been debated in recent years. With the high angular resolution of Chandra, an increasing number of Compton-thick sources have been found to show extended narrow Fe K

α

emission on scales from tens of parsecs to kiloparsecs, attributed to reprocessing of nuclear radiation by surrounding Compton-thick material. We analyze eight Compton-thick sources with prominent extended Fe K

α

emission. We confirm the Iwasawa–Taniguchi effect in the extended component relative to the reflection spectrum, with a steeper slope, indicating reduced production efficiency of neutral Fe K

α

photon outside the nucleus. Both the reflection spectrum and Fe K

α

luminosities correlate positively with intrinsic AGN luminosity, suggesting that the nucleus drives the extended emission. Finally, we find a linear relationship between redshift and the equivalent width of the nuclear Fe K

α

line, with no such trend in the extended component. Full article

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

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10 pages, 330 KB

Open AccessArticle

Traversable Kaluza–Klein Wormholes?

by Christopher Simmonds and Matt Visser

Universe 2025, 11(10), 347; https://doi.org/10.3390/universe11100347 - 17 Oct 2025

Abstract

Various authors have suggested that Kaluza–Klein variants of traversable wormholes might to some extent ameliorate the defocussing properties (the curvature condition violations, and implied energy condition violations) inherent in positing the existence of a traversable wormhole throat. Unfortunately such a hope is ill-founded. [...] Read more.

Various authors have suggested that Kaluza–Klein variants of traversable wormholes might to some extent ameliorate the defocussing properties (the curvature condition violations, and implied energy condition violations) inherent in positing the existence of a traversable wormhole throat. Unfortunately such a hope is ill-founded. We shall show that in a traditional Kaluza–Klein context the price paid for completely eliminating the defocussing properties of the wormhole throat is extremely high—to completely eliminate curvature condition violations the 5th dimension has to become truly enormous (formally infinite) in the vicinity of the wormhole throat, in a manner that is fundamentally incompatible with the traditional Kaluza–Klein ansatz. At best, the extra dimensions allow one to move the curvature condition violations around, they cannot be eliminated except at prohibitive cost. While traversable Kaluza–Klein wormholes might be interesting for other reasons, it must be emphasized that adding a 5th dimension is not particularly useful in terms of ameliorating violations of the curvature conditions. Full article

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

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21 pages, 454 KB

Open AccessArticle

Cosmology at the End of Time

by Meir Shimon

Universe 2025, 11(10), 346; https://doi.org/10.3390/universe11100346 - 16 Oct 2025

Abstract

If the recent acceleration phase of the expanding Universe is driven by a cosmologicalconstant the Universe is future-eternal with a few far reaching ramifications and disturbing consequences. We demonstrate with a non-ΛCDM cosmological model, that is nearly identical to the standard cosmological model [...] Read more.

If the recent acceleration phase of the expanding Universe is driven by a cosmologicalconstant the Universe is future-eternal with a few far reaching ramifications and disturbing consequences. We demonstrate with a non-ΛCDM cosmological model, that is nearly identical to the standard cosmological model insofar observations of our past are concerned—but otherwise has an effective cosmic time cutoff—that under certain plausible assumptions observers will virtually always find themselves at or near the end of time (EOT) terminal point, where H₀η₀ = π, and H₀ and η₀ are the present day expansion rate and the conformal time, respectively. Assuming a locally flat ΛCDM model for concreteness (but with a global terminal point which is with an overwhelming probability the present time) such observers will invariably infer that the energy densities associated with the cosmological constant, Λ, and non-relativistic (NR) matter constitute 67.5% and 32.5%, respectively, of the cosmic energy budget at present, which lie well within the 2σ confidence level of the concordance 68.5% and 31.5% values. This addresses the Cosmic Coincidence Problem associated with Λ. Future high-precision cosmological probes will be able to break the observational degeneracy between the proposed model and flat ΛCDM. A few additional implications of the proposed model are discussed as well. Full article

(This article belongs to the Section Cosmology)

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18 pages, 905 KB

Open AccessArticle

Chiral Invariant Mass Constraints from HESS J1731–347 in an Extended Parity Doublet Model with Isovector Scalar Meson

by Yuk Kei Kong, Bikai Gao and Masayasu Harada

Universe 2025, 11(10), 345; https://doi.org/10.3390/universe11100345 - 16 Oct 2025

Abstract

The recent discovery of an extremely light and small central compact object (CCO) within the supernova remnant HESS J1731-347, with mass

0 . 77_{- 0.17}^{+ 0.20} M_{⊙}

and radius

10 . 4_{- 0.78}^{+ 0.86}

km, is challenging our [...] Read more.

The recent discovery of an extremely light and small central compact object (CCO) within the supernova remnant HESS J1731-347, with mass

0 . 77_{- 0.17}^{+ 0.20} M_{⊙}

and radius

10 . 4_{- 0.78}^{+ 0.86}

km, is challenging our understanding of neutron stars. In this article, we identify it as an ultra-light neutron star (NS) and constrain the chiral invariant mass of nucleon

m_{0}

from the observational data of NS using an extended parity doublet model with the isovector scalar meson

a_{0} (980)

. We show that the

1 σ

data from the HESS J1731-347 impose a very narrow constraint on the allowed values of

m_{0}

and

L_{0}

in the crossover model:

830 MeV ≲ m_{0} ≲ 900 MeV

for

L_{0} = 40

MeV, and

850 MeV ≲ m_{0} ≲ 890 MeV

for

L_{0} = 45

MeV. We also study the higher-order asymmertic matter properties such as the symmetry incompressibility

K_{s y m}

and the symmetry skewness

Q_{s y m}

in the presence of the

a_{0}

meson. We find that

K_{s y m}

and

Q_{s y m}

are very sensitive to

m_{0}

in the presence of the

a_{0}

meson. Full article

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

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19 pages, 809 KB

Open AccessArticle

The Mass Profile of NGC 3268 from Its Stellar Kinematics

by Juan Pablo Caso, Bruno Javier De Bórtoli and Tom Richtler

Universe 2025, 11(10), 344; https://doi.org/10.3390/universe11100344 - 16 Oct 2025

Abstract

The mass profile of the central galaxy of the Antlia cluster, NGC 3268, is studied through a spherical Jeans analysis, combined with a Bayesian approach. The prior distributions are derived from dark matter simulations. The observational dataset consists of Gemini/GMOS multi-object spectra observed [...] Read more.

The mass profile of the central galaxy of the Antlia cluster, NGC 3268, is studied through a spherical Jeans analysis, combined with a Bayesian approach. The prior distributions are derived from dark matter simulations. The observational dataset consists of Gemini/GMOS multi-object spectra observed from several programmes, supplemented with the kinematics of a small sample of globular clusters from the literature. An NFW mass profile and several options of constant anisotropy are considered. The analysis indicates a moderately massive halo, with a virial mass of (1.4 – 4.3)

\times 10^{13} M_{⊙}

, depending on the assumed anisotropy. A comparison with the kinematics of the galaxy population from the Antlia cluster suggests that a fraction of galaxies is not yet virialised and may currently be infalling into the cluster. Full article

(This article belongs to the Section Galaxies and Clusters)

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17 pages, 6775 KB

Open AccessArticle

Gravitational Wave Signatures of Warm Dark Matter in the Gauge Extensions of the Standard Model

by Lucia A. Popa

Universe 2025, 11(10), 343; https://doi.org/10.3390/universe11100343 - 15 Oct 2025

Abstract

We studied the left-right symmetric extension of the standard model (LRSM), featuring a TeV-scale, right-handed (RH) gauge boson

W_{R}

and three RH neutrinos. This setup naturally realizes the type-II seesaw mechanism for active neutrino masses. We identified the conditions that yield sufficient [...] Read more.

We studied the left-right symmetric extension of the standard model (LRSM), featuring a TeV-scale, right-handed (RH) gauge boson

W_{R}

and three RH neutrinos. This setup naturally realizes the type-II seesaw mechanism for active neutrino masses. We identified the conditions that yield sufficient entropy dilution to reconcile the keV sterile neutrino dark matter energy density with observations while inducing an early matter domination (EMD) phase. These constrained the lightest active neutrino mass to

8.59 \times 10^{- 10} eV < m_{ν_{1}} < 5.06 \times 10^{- 9} eV

. The resulting frequency-dependent suppression of the stochastic gravitational wave (GW) background was set by the mass and lifetime of the heavier RH neutrinos. Computing the signal-to-noise ratio (SNR) for future detectors, we found that a blue-tilted primordial tensor spectrum can boost the GW signal to detectable levels (SNR > 10) in experiments such as LISA, BBO, and DECIGO. 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, 3243 KB

Open AccessArticle

The 1 June 2025 Forbush Decrease Measured over a Range of Primary Cosmic Ray Energies

by Roger Clay

Universe 2025, 11(10), 342; https://doi.org/10.3390/universe11100342 - 15 Oct 2025

Abstract

The 1 June 2025 Forbush decrease in the terrestrial ground-level flux of cosmic ray secondaries was recorded by many cosmic ray systems. This was the deepest such decrease from the quiescent value of the flux, which has been observed over the past two [...] Read more.

The 1 June 2025 Forbush decrease in the terrestrial ground-level flux of cosmic ray secondaries was recorded by many cosmic ray systems. This was the deepest such decrease from the quiescent value of the flux, which has been observed over the past two decades. It resulted from a complex series of solar events, none of which on its own reached the most extreme level. The extreme depth of this decrease has enabled measurements of the flux reduction to be made, which would normally be severely limited by particle counting statistics. In particular, here we examine the decrease phenomenon over a primary cosmic ray energy range, which is rarely accessible due to the low flux of high energy cosmic rays. This work considers data mainly from a muon telescope system, which can respond to both unaccompanied muons and small cosmic ray air showers, providing data from GeV to mid-TeV energies, where the Forbush decrease ceases to be statistically observable. This paper examines the depth of the flux decrease, as a fraction below its quiescent value, over that primary energy range. The progressive development of internal time structure in the flux through the seven days of the phenomenon is also demonstrated. Full article

(This article belongs to the Section Space Science)

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12 pages, 544 KB

Open AccessArticle

Ringing of Reissner–Nordström Black Holes with a Non-Abelian Hair in Gravity’s Rainbow

by Mehrab Momennia

Universe 2025, 11(10), 341; https://doi.org/10.3390/universe11100341 - 15 Oct 2025

Abstract

In this paper, we consider massless scalar perturbations minimally coupled to gravity in the background spacetime of charged black holes in Yang–Mills theory with gravity’s rainbow modification. We calculate the corresponding quasinormal frequencies by employing the sixth-order Wentzel—Kramers—Brillouin (WKB) approximation for both asymptotically [...] Read more.

In this paper, we consider massless scalar perturbations minimally coupled to gravity in the background spacetime of charged black holes in Yang–Mills theory with gravity’s rainbow modification. We calculate the corresponding quasinormal frequencies by employing the sixth-order Wentzel—Kramers—Brillouin (WKB) approximation for both asymptotically flat and de Sitter (dS) spacetimes. We show that the Yang–Mills modification of the Reissner–Nordström black holes leads to an increase in the real and imaginary parts of frequencies. Furthermore, we find that the perturbations in asymptotically flat spacetime decay faster with more oscillations compared to dS spacetime, and we study the effects of the rainbow functions on the oscillations. Interestingly, we reveal a novel feature of this black hole case study and show that, unlike typical black hole solutions such as Schwarzschild, RN, and Kerr, the higher multipole numbers live longer than the lower ones in both asymptotically flat and dS spacetimes. Furthermore, the reflection and transmission coefficients are explored for Einstein–Maxwell–Yang–Mills black holes, and the results are compared for flat and dS asymptotes. Full article

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

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23 pages, 11431 KB

Open AccessArticle

Characterisation of Nearby Ultracool Dwarf Candidates with OSIRIS/GTC: First Detection of Balmer Line Emission from the Dwarf Carbon Star LSR J2105+2514

by Antoaneta Antonova, Peter Pessev, Valeri Golev and Dinko Dimitrov

Universe 2025, 11(10), 340; https://doi.org/10.3390/universe11100340 - 14 Oct 2025

Abstract

Based on low-resolution OSIRIS/GTC optical spectra, we assign spectral classes to 38 poorly studied ultracool/brown dwarf candidates from the 2MASS database. For almost all of the targets, this is the first optical spectral classification. For the dwarfs showing H

α

emission, we calculate [...] Read more.

Based on low-resolution OSIRIS/GTC optical spectra, we assign spectral classes to 38 poorly studied ultracool/brown dwarf candidates from the 2MASS database. For almost all of the targets, this is the first optical spectral classification. For the dwarfs showing H

α

emission, we calculate the ratio of H

α

to bolometric luminosity, which is the most common characteristic of magnetic activity in cool stars. For the others, we give 3

σ

upper limits. We also include estimates of the effective temperatures and log g and distances from Gaia based on a comparison with models. For one of our targets—LSR J2105+2514, previously classified as a dwarf carbon star—we confirm this classification and report H

α

and H

β

line emission in the spectrum for the first time. Dwarf carbon stars (dC) are low-mass main sequence stars that have undergone mass-transfer binary evolution. The Balmer line emission from these objects most likely indicates coronal activity of the dwarf, which in turn may be due to either intrinsic magnetic activity or spin-up from accretion or tidal locking. Full article

(This article belongs to the Special Issue Magnetic Fields and Activity in Stars: Origins and Evolution)

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