Universe

26 pages, 5554 KB

Open AccessReview

Lattice Results for the Equation of State in Dense QCD-like Theories

by Etsuko Itou

Universe 2025, 11(11), 380; https://doi.org/10.3390/universe11110380 - 19 Nov 2025

Cited by 3 | Viewed by 921

We review the recent progress in Monte Carlo simulations of dense two-color QCD (QC₂D), focusing on the phase diagram, the equation of state, and the sound velocity in the low-temperature regime. In three-color QCD at finite density, especially at low temperatures, [...] Read more.

We review the recent progress in Monte Carlo simulations of dense two-color QCD (QC₂D), focusing on the phase diagram, the equation of state, and the sound velocity in the low-temperature regime. In three-color QCD at finite density, especially at low temperatures, the notorious sign problem makes lattice Monte Carlo simulations intractable. In contrast, QC₂D is free from this issue due to the pseudoreality of the quark representation. Recent independent lattice studies have revealed unexpected phenomena through first-principles calculations of the phase structure and thermodynamics. A particularly notable finding is that the sound velocity exceeds the so-called conformal (holography) bound,

c_{s}^{2} / c^{2} \leq 1 / 3

, which had not been observed in QCD-like theories at finite temperature. In this review, we focus primarily on results from a series of works by our group, along with related studies in dense QC₂D and three-color QCD with isospin chemical potential. We discuss the possibility and physical implications of conformal bound violation even for three-color dense QCD, together with insights from effective model analyses and recent observations of neutron stars. Full article

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

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9 pages, 1621 KB

Open AccessArticle

Can Inflation Resolve the Hubble Tension?

by Ione A. Silva and Edésio M. Barboza, Jr.

Universe 2025, 11(11), 379; https://doi.org/10.3390/universe11110379 - 18 Nov 2025

Viewed by 600

Abstract

In this work, we investigate how a change in the sound horizon due to inflation may affect the value of

H_{0}

. Performing an analysis using the measurements of BAO, SN Ia,

H (z)

, CMB, and the local [...] Read more.

In this work, we investigate how a change in the sound horizon due to inflation may affect the value of

H_{0}

. Performing an analysis using the measurements of BAO, SN Ia,

H (z)

, CMB, and the local

H_{0}

, we show that the modification in the sound horizon brings the nearly 5

σ

tension in

H_{0}

estimates to approximately

1 σ

. The overall fit is also improved as compared with the standard calculation of

r_{s}

. These findings highlight the importance of early-universe physics in resolving late-time cosmological tensions. Full article

(This article belongs to the Section Cosmology)

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30 pages, 1867 KB

Open AccessReview

Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST): A Powerful Explorer of Exotic Pulsars

by Cheng-Min Zhang, Zhi-Yao Yang, Xiang-Han Cui, De-Hua Wang, Yi-Yan Yang, Xin-Ji Wu, Jian-Wei Zhang, Shu Ma, Yun-Gang Zhou and Lin-Yan Jiang

Universe 2025, 11(11), 378; https://doi.org/10.3390/universe11110378 - 14 Nov 2025

Viewed by 1668

Abstract

Located in southern China, the five-hundred-meter aperture spherical radio telescope (FAST) is the world’s most sensitive radio telescope, especially for pulsar observation. Since its commissioning in 2016 and full operation in 2020, it has detected over 1100 new pulsars—boosting the globally known various [...] Read more.

Located in southern China, the five-hundred-meter aperture spherical radio telescope (FAST) is the world’s most sensitive radio telescope, especially for pulsar observation. Since its commissioning in 2016 and full operation in 2020, it has detected over 1100 new pulsars—boosting the globally known various pulsars to over 4000. In this concise overview, we highlight how harnessing FAST’s unique advantages—exceptional precision and ultra-high sensitivity—is set to fuel future discoveries of specialized pulsar types and exotic astrophysical objects. Notable targets include double millisecond pulsar binaries (MSP-MSPs), pulsar/millisecond pulsar–black hole systems (PSR-BHs or MSP-BHs), sub-millisecond pulsars, ultra-long-period pulsars, white dwarf pulsars, and short-orbit double neutron star systems (DNSs)—with orbital periods under one hour. As anticipated, in the 2040s, the combined capabilities of the FAST, the Square Kilometre Array (SKA), and other cutting-edge astronomical instruments will enable over 10,000 pulsar samples, which will usher in a golden era for pulsar research: such breakthroughs will not only significantly broaden and deepen our understanding of the “pulsar paradise” but also drive substantial progress in the field of multi-messenger astronomy. Beyond pulsar-focused research, FAST is poised to play a pivotal role in testing general relativity, detecting nanohertz gravitational waves, studying fast radio bursts (FRBs), and investigating the micro-structure of pulsar emissions. These investigations will not only strengthen our understanding of fundamental physics but also unlock deeper insights into the universe’s profound mysteries. Full article

(This article belongs to the Section Compact Objects)

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14 pages, 670 KB

Open AccessArticle

Tycho Supernova Exploded Inside a Planetary Nebula (SNIP)

by Noam Soker

Universe 2025, 11(11), 377; https://doi.org/10.3390/universe11110377 - 13 Nov 2025

Cited by 1 | Viewed by 760

Abstract

I analyze recent X-ray data from the literature of the type Ia supernova remnant (SNR Ia) Tycho and conclude that Tycho is a SN Ia inside a planetary nebula (SNIP), strengthening such a previous suggestion from 1985. The observations reveal two opposite protrusions, [...] Read more.

I analyze recent X-ray data from the literature of the type Ia supernova remnant (SNR Ia) Tycho and conclude that Tycho is a SN Ia inside a planetary nebula (SNIP), strengthening such a previous suggestion from 1985. The observations reveal two opposite protrusions, termed ears, projected on the main shell of Tycho. The pair of ear structures qualitatively resembles that of the SNRs Ia Kepler, SNR G299-2.9, and SNR G1.9+0.3, which earlier studies considered as SNIPs. The requirement that the explosion occurs within hundreds of thousands of years after the formation of the planetary nebula (by the second star to evolve) makes the core-degenerate scenario the most likely for Tycho, with the double-degenerate with merger to explosion delay time scenario somewhat less likely. Several other possible scenarios lead to a SNIP, but they are unlikely for Tycho. The identification of Tycho as a SNIP leads to two general conclusions. (1) The fraction of SNIPs among normal SNe Ia is very large, ≈70–90%. Thus, the vast majority of normal SNe Ia are SNIPs. (2) To accommodate the large fraction of SNIPs, the delay time distribution of normal SNe Ia includes not only the stellar evolution timescale (as usually assumed), but also includes pockets of younger stellar populations in galaxies without ongoing star formation; the SNIPs come from the younger stellar populations in galaxies. Full article

(This article belongs to the Special Issue Exploring the Formation and Impact of Type Ia Supernovae)

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46 pages, 694 KB

Open AccessReview

The Two-Measure Theory and an Overview of Some of Its Manifestations

by Alexander B. Kaganovich

Universe 2025, 11(11), 376; https://doi.org/10.3390/universe11110376 - 13 Nov 2025

Cited by 2 | Viewed by 893

Abstract

The Two-Measure Theory (TMT) has been developing since 1998 and has yielded a number of highly interesting results, including those not realized in traditional field theory models. The most important advantage of TMT as an alternative theory is that, under the conditions under [...] Read more.

The Two-Measure Theory (TMT) has been developing since 1998 and has yielded a number of highly interesting results, including those not realized in traditional field theory models. The most important advantage of TMT as an alternative theory is that, under the conditions under which all classical tests of general relativity are performed, TMT models are able to accurately reproduce Einstein’s general relativity. Despite this, TMT is still often perceived as something too exotic to be relevant to reality. In fact, the fundamental idea underlying TMT seems undeniable: if we truly believe in the effectiveness of mathematics in studying nature, we must agree that there must be a correspondence between the fundamental laws of nature and the structure of the mathematical apparatus necessary to adequately describe them. It then turns out that there is no reason to ignore the volume measure existing on the differentiable manifold on which the theory of gravity and matter fields is built. This idea has far-reaching implications. The goals of this paper are (1) to provide a clear mathematical and conceptual justification for TMT and (2) to collect in a single article some of the main results of TMT obtained over the past 25 years. Full article

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

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31 pages, 827 KB

Open AccessArticle

Asymptotic Freedom and Vacuum Polarization Determine the Astrophysical End State of Relativistic Gravitational Collapse: Quark–Gluon Plasma Star Instead of Black Hole

by Herman J. Mosquera Cuesta, Fabián H. Zuluaga Giraldo, Wilmer D. Alfonso Pardo, Edgardo Marbello Santrich, Guillermo U. Avendaño Franco and Rafael Fragozo Larrazabal

Universe 2025, 11(11), 375; https://doi.org/10.3390/universe11110375 - 12 Nov 2025

Viewed by 1747

Abstract

A general relativistic model of an astrophysical hypermassive extremely magnetized ultra-compact self-bound quark–gluon plasma (QGP: ALICE/LHC) object that is supported against its ultimate gravitational implosion by the simultaneous action of the vacuum polarization driven by nonlinear electrodynamics (NLED: ATLAS/LHC: light-by-light scattering)—the vacuum “awakening”—and [...] Read more.

A general relativistic model of an astrophysical hypermassive extremely magnetized ultra-compact self-bound quark–gluon plasma (QGP: ALICE/LHC) object that is supported against its ultimate gravitational implosion by the simultaneous action of the vacuum polarization driven by nonlinear electrodynamics (NLED: ATLAS/LHC: light-by-light scattering)—the vacuum “awakening”—and the asymptotic freedom, a key feature of quantum chromodynamics (QCD), is presented. These QCD stars can be the final figures of the equilibrium of collapsing stellar cores permeated by magnetic fields with strengths well beyond the Schwinger threshold due to being self-bound, and for which post-supernova fallback material pushes the nascent remnant beyond its stability, forcing it to collapse into a hybrid hypermassive neutron star (HHMNS). Hypercritical accretion can drive its innermost core to spontaneously break away color confinement, powering a first-order hadron-to-quark phase transition to a sea of ever-freer quarks and gluons. This core is hydro-stabilized by the steady, endlessly compression-admitting asymptotic freedom state, possibly via gluon-mediated enduring exchange of color charge among bound states, e.g., the odderon: a glueball state of three gluons, or either quark-pairing (color superconductivity) or tetraquark/pentaquark states (LHCb Coll.). This fast—at the QGP speed of sound—but incremental quark–gluon deconfinement unbinds the HHMNS’s baryons so catastrophically that transforms it, turning it inside-out, into a neat self-bound QGP star. A solution to the nonlinear Tolman–Oppenheimer–Volkoff (TOV) equation is obtained—that clarifies the nonlinear effects of both NLED and QCD on the compact object’s structure—which clearly indicates the occurrence of hypermassive QGP/QCD stars with a wide mass spectrum (

0 ≲ M_{Star}^{QGP} ≲

7 M_⊙ and beyond), for star radii (

0 ≲ R_{Star}^{QGP} ≲ 24

km and beyond) with B-fields (

10^{14} \leq B_{Star}^{QGP} \leq 10^{16}

G and beyond). This unexpected feature is described by a novel mass vs. radius relation derived within this scenario. Hence, endowed with these physical and astrophysical characteristics, such QCD stars can definitively emulate what the true (theoretical) black holes are supposed to gravitationally do in most astrophysical settings. This color quark star could be found through a search for its eternal “yo-yo” state gravitational-wave emission, or via lensing phenomena like a gravitational rainbow (quantum mechanics and gravity interaction), as in this scenario, it is expected that the light deflection angle—directly influenced by the larger effective mass/radius (

M_{Star}^{QGP} (B)

,

R_{Star}^{QGP} (B)

) and magnetic field of the deflecting object—increases as the incidence angle decreases, in view of the lower values of the impact parameter. The gigantic—but not infinite—surface gravitational redshift, due to NLED photon acceleration, makes the object appear dark. Full article

(This article belongs to the Section Cosmology)

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15 pages, 909 KB

Open AccessArticle

Gravitational Lensing by Lemaître–Tolman–Bondi Wormholes in a Friedmann Universe

by Kirill A. Bronnikov, Valeria A. Ishkaeva and Sergey V. Sushkov

Universe 2025, 11(11), 374; https://doi.org/10.3390/universe11110374 - 12 Nov 2025

Viewed by 588

Abstract

The Lemaître–Tolman–Bondi (LTB) solution to the Einstein equations describes the dynamics of a self-gravitating spherically symmetric dust cloud with an arbitrary density profile and any distribution of initial velocities, encoded in three arbitrary functions

f (R)

,

F (R)

[...] Read more.

The Lemaître–Tolman–Bondi (LTB) solution to the Einstein equations describes the dynamics of a self-gravitating spherically symmetric dust cloud with an arbitrary density profile and any distribution of initial velocities, encoded in three arbitrary functions

f (R)

,

F (R)

, and

τ_{0} (R)

, where R is a radial coordinate in the comoving reference frame. A particular choice of these functions corresponds to a wormhole geometry with a throat defined as a sphere of minimum radius at a fixed time instant. In this paper we explore LTB wormholes and discuss their possible observable appearance, studying in detail the effects of gravitational lensing by such objects. For this aim, we study photon motion in wormhole space-time inscribed in a closed Friedmann dust-filled universe and find the wormhole shadow as it could be seen by a distant observer. Because the LTB wormhole is a dynamic object, we analyze the dependence of its shadow size on the observation time and on the initial size of the wormhole region. We reveal that the angular size of the shadow exhibits a non-monotonic dependence on the observation time. At early times, the shadow size decreases as photons with smaller angular momentum gradually reach the observer. At later times, the expansion of the Friedmann universe becomes a dominant factor that leads to an increase in the shadow size. Full article

(This article belongs to the Special Issue Astrophysics and Cosmology at High Z)

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22 pages, 3238 KB

Open AccessArticle

Chaos in 3D and 4D Thermodynamic Models

by Bo Wang, Xin Wu and Fuyao Liu

Universe 2025, 11(11), 373; https://doi.org/10.3390/universe11110373 - 10 Nov 2025

Viewed by 627

Abstract

Recently, Aydiner considered dark matter (DM) and dark energy (DE) as two open, non-equilibrium thermodynamic systems, which have heat changes and particle number changes but have no volume changes. These systems are described by nonlinear coupled equations for the description of mutual and [...] Read more.

Recently, Aydiner considered dark matter (DM) and dark energy (DE) as two open, non-equilibrium thermodynamic systems, which have heat changes and particle number changes but have no volume changes. These systems are described by nonlinear coupled equations for the description of mutual and self-interactions and satisfy the energy conservation of thermodynamics. Based on this idea, two three-dimensional (3D) models and a four-dimensional (4D) model are produced. Due to the conservation of the energy–momentum tensor of the sum of the DM and DE energy densities, the continuity equations of both energy densities are also included together in these 3D and 4D thermodynamic models. For the parameters satisfying some conditions, one of the 3D models has two marginal stable non-hyperbolic equilibrium points with a negative real root and a pair of conjugate purely imaginary roots. The marginal stability is highly sensitive to nonlinear terms and parameter noise. Another of the 3D models has unstable saddle-focus equilibrium points, which have a negative real root corresponding to a 1D stable manifold and two conjugate complex roots with positive real parts corresponding to a 2D manifold of unstable spiral. At these equilibria, no energy exchange occurs between the two energy densities, and both energy components reach equilibrium. When some perturbations from the nonlinear terms or parameter noise are given, the DM and DE energy densities are far from equilibrium and continue to exchange each other until they reach equilibrium. The energy exchanges between them may exhibit chaotic behavior like chaotic attractors. However, hyperchaos is not easily found. The 4D model also has unstable saddle-focus equilibrium points and can allow for the onset of chaotic attractors and hyperchaos. In fact, the chaotic dynamics of the 3D and 4D models are caused because of the coupled interactions of particle and thermodynamic systems between DM and DE. Under both the self-interactions and the mutual interactions, the energy exchanges are far from and close to the equilibrium. These interactions cause the energy exchanges to become random, irregular and unpredictable. Full article

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

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14 pages, 2918 KB

Open AccessArticle

A New Phase of Optical Activity of BL Lacertae in the Fall of 2024: Intra-Night Flux and Polarization Variations

by Rumen Bachev, Milen Minev, Anton Strigachev and Alexander Kurtenkov

Universe 2025, 11(11), 372; https://doi.org/10.3390/universe11110372 - 9 Nov 2025

Viewed by 477

Abstract

BL Lacertae is not only archetypical of an entire class of jet-dominated active galactic nuclei, blazars, but also one of the most active and rapidly changing objects in this class. In the fall of 2024 (September–November), BL Lacertae underwent another episode of strong [...] Read more.

BL Lacertae is not only archetypical of an entire class of jet-dominated active galactic nuclei, blazars, but also one of the most active and rapidly changing objects in this class. In the fall of 2024 (September–November), BL Lacertae underwent another episode of strong optical activity, reaching an R-band magnitude of about 12 and showing extremely rapid and large-amplitude inter- and intra-night flux and polarization variations. During this period, the object was monitored over 40 nights using telescopes with an aperture of up to 2 m at three observatories: Rozhen and Belogradchik in Bulgaria and Skinakas in Greece. The results from this study include some of the most spectacular intra-night variability episodes detected in a blazar. These rapid variations, combined with high photometric accuracy and high time resolution, allowed for confirmation of consistency between different optical bands with zero time delays, down to a minute scale. Unlike previous activity reports, polarization was relatively stable on these short time-scales. Possible connections between polarization, flux, and intra-night variability were explored in order to better model or constrain the physical processes and emission mechanisms in the relativistic jets. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

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9 pages, 612 KB

Open AccessArticle

A Test of Amati Relation Using HII Galaxy Distances

by Rikiya Okazaki and Shantanu Desai

Universe 2025, 11(11), 371; https://doi.org/10.3390/universe11110371 - 9 Nov 2025

Viewed by 513

Abstract

We use model-independent luminosity distances of 186 HII galaxy observations to address the circularity problem in the Amati relation for Gamma-ray Bursts (GRBs). For this purpose, we used Artificial Neural Network-based interpolation to reconstruct the luminosity distance corresponding to the GRB redshift. We [...] Read more.

We use model-independent luminosity distances of 186 HII galaxy observations to address the circularity problem in the Amati relation for Gamma-ray Bursts (GRBs). For this purpose, we used Artificial Neural Network-based interpolation to reconstruct the luminosity distance corresponding to the GRB redshift. We then use two independent GRB datasets to test the robustness of the Amati relation at redshifts below

z = 2.6

. Our best-fit Amati relation parameters are consistent for the same datasets to within

1 σ

. The intrinsic scatters which we obtain for the two datasets of about 28% and 35% are comparatively larger. This implies that the Amati relation using HII galaxies as distance anchors cannot be used as a probe of precision cosmology. Full article

(This article belongs to the Special Issue Gamma-Ray Bursts: Unveiling the Mysteries of the Universe’s Most Powerful Explosions)

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23 pages, 1021 KB

Open AccessArticle

Probing Jet Compositions with Extreme Mass Ratio Binary Black Holes

by Hung-Yi Pu

Universe 2025, 11(11), 370; https://doi.org/10.3390/universe11110370 - 7 Nov 2025

Viewed by 704

Abstract

Determining whether black hole jets are dominated by leptonic or baryonic matter remains an open question in high-energy astrophysics. We propose that extreme mass ratio binary (EMRB) black holes, where an intermediate mass secondary black hole (a “miniquasar”) periodically interacts with the accretion [...] Read more.

Determining whether black hole jets are dominated by leptonic or baryonic matter remains an open question in high-energy astrophysics. We propose that extreme mass ratio binary (EMRB) black holes, where an intermediate mass secondary black hole (a “miniquasar”) periodically interacts with the accretion flow of a supermassive black hole (SMBH), offer a natural laboratory to probe jet composition. In an EMRB, the miniquasar jet is launched episodically after each disk-crossing event, triggered by the onset of super-Eddington accretion. The resulting emissions exhibit temporal evolution as the jet interacts with the SMBH accretion disk. Depending on whether the jet is leptonic or hadronic in composition, the radiative signatures differ substantially. Notably, a baryonic jet produces a more pronounced gamma-ray output than a purely leptonic jet. By modeling the evolution of the multifrequency characteristic features, it is suggested that the gamma-ray-to-UV emissions may serve as a diagnostic tool capable of distinguishing between leptonic and baryonic scenarios. The resulting electromagnetic signals, when combined with multi-messenger observations, offer a powerful means to constrain the physical nature of relativistic jets from black holes. Full article

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

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25 pages, 770 KB

Open AccessArticle

Observational Tests of the Conformal Osculating Barthel–Kropina Cosmological Model

by Himanshu Chaudhary, Rattanasak Hama, Tiberiu Harko, Sorin V. Sabau and Shibesh Kumar Jas Pacif

Universe 2025, 11(11), 369; https://doi.org/10.3390/universe11110369 - 7 Nov 2025

Cited by 1 | Viewed by 1236

Abstract

We consider detailed cosmological tests of dark energy models obtained from the general conformal transformation of the Kropina metric, representing an

(α, β)

-type Finslerian geometry. In particular, we restrict our analysis to the osculating Barthel–Kropina geometry. The Kropina metric [...] Read more.

We consider detailed cosmological tests of dark energy models obtained from the general conformal transformation of the Kropina metric, representing an

(α, β)

-type Finslerian geometry. In particular, we restrict our analysis to the osculating Barthel–Kropina geometry. The Kropina metric function is defined as the ratio of the square of a Riemannian metric

α

and of the one-form

β

. In this framework, we also consider the role of the conformal transformations of the metric, which allows us to introduce a family of conformal Barthel–Kropina theories in an osculating geometry. The models obtained in this way are described by second-order field equations, in the presence of an effective scalar field induced by the conformal factor. The generalized Friedmann equations of the model are obtained by adopting for the Riemannian metric

α

the Friedmann–Lemaitre–Robertson–Walker representation. In order to close the cosmological field equations, we assume a specific relationship between the component of the one-form

β

and the conformal factor. With this assumption, the cosmological evolution is determined by the initial conditions of the scalar field and a single free parameter

γ

of the model. The conformal Barthel–Kropina cosmological models are compared against several observational datasets, including Cosmic Chronometers, Type Ia Supernovae, and Baryon Acoustic Oscillations, using a Markov Chain Monte Carlo (MCMC) analysis, which allows the determination of

γ

. A comparison with the predictions of standard

Λ

CDM model is also performed. Our results indicate that the conformal osculating Barthel–Kropina model can be considered as a successful, and simple, alternative to standard cosmological models. Full article

(This article belongs to the Special Issue Geometric Theories of Gravity)

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13 pages, 272 KB

Open AccessArticle

Tides and Energy Conditions in Einstein–Gauss–Bonnet Thin-Shell Wormholes

by Ernesto F. Eiroa, Emilio Rubín de Celis and Claudio Simeone

Universe 2025, 11(11), 368; https://doi.org/10.3390/universe11110368 - 7 Nov 2025

Viewed by 771

Abstract

In this article we study spherical thin-shell wormholes in five-dimensional Einstein–Gauss–Bonnet gravity. We show that configurations supported by non-exotic matter, that is matter satisfying the weak energy condition, are possible at the same time that traversability problems associated with strong radial tides at [...] Read more.

In this article we study spherical thin-shell wormholes in five-dimensional Einstein–Gauss–Bonnet gravity. We show that configurations supported by non-exotic matter, that is matter satisfying the weak energy condition, are possible at the same time that traversability problems associated with strong radial tides at the throat can be avoided when suitable values of the parameters are adopted. Our construction is performed in such a way that it also allows for the admissible behaviour of the geometry in the whole spacetime. Full article

(This article belongs to the Section Gravitation)

27 pages, 10026 KB

Open AccessEditor’s ChoiceArticle

Dynamical Friction Constraints on the Dark Matter Hypothesis Across Astronomical Scales

by Xavier Hernandez and Pavel Kroupa

Universe 2025, 11(11), 367; https://doi.org/10.3390/universe11110367 - 6 Nov 2025

Cited by 2 | Viewed by 3422

Abstract

Dynamical friction implies a consistency check on any system where dark matter particles are hypothesised to explain orbital dynamics requiring more mass under Newtonian gravity than is directly detectable. Introducing the assumption of a dominant dark matter halo will also imply a decay [...] Read more.

Dynamical friction implies a consistency check on any system where dark matter particles are hypothesised to explain orbital dynamics requiring more mass under Newtonian gravity than is directly detectable. Introducing the assumption of a dominant dark matter halo will also imply a decay timescale for the orbits in question. A self-consistency constraint hence arises, such that the resulting orbital decay timescales must be longer than the lifetimes of the systems in question. While such constraints are often trivially passed, the combined dependencies of dynamical friction timescales on the mass and orbital radius of the orbital tracer and on the density and velocity dispersion of the assumed dark matter particles leads to the existence of a number of astronomical systems where such a consistency test is failed. Here, we review cases from stars in ultrafaint dwarf galaxies, galactic bars, satellite galaxies, and, particularly, the multi-period mutual orbits of the Magellanic Clouds, as recently inferred from the star formation histories of these two galaxies, as well as the nearby M81 group of galaxies, where introducing enough dark matter to explain observed kinematics leads to dynamical friction orbital decay timescales shorter than the lifetimes of the systems in question. Taken together, these observations exclude dark matter halos made of particles as plausible explanations for the observed kinematics of these systems. Full article

(This article belongs to the Section Galaxies and Clusters)

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11 pages, 289 KB

Open AccessArticle

Directional Quantum Singularities in Curzon Spacetime

by Mert Mangut, Ozay Gurtug and Mustafa Halilsoy

Universe 2025, 11(11), 366; https://doi.org/10.3390/universe11110366 - 6 Nov 2025

Viewed by 502

Abstract

The scalar quantum probe method developed by Horowitz and Marolf is applied to the cylindrically symmetric Curzon solution. The main cause for choosing the Curzon solution is that it is the best-known example that exhibits directional singularity. Interestingly the singularity at [...] Read more.

The scalar quantum probe method developed by Horowitz and Marolf is applied to the cylindrically symmetric Curzon solution. The main cause for choosing the Curzon solution is that it is the best-known example that exhibits directional singularity. Interestingly the singularity at

r = 0

, for the uncharged Curzon spacetime, which is classically very strong with a divergence rate of the order

\frac{1}{r^{10}}

, becomes regular when examined using a scalar quantum field. The charged Curzon spacetime, however, due to the emergence of a second singularity off the

r = 0

singularity, does not regularize quantum mechanically. All three different charged versions, i.e., electric, magnetic, and dyonic, share the same feature. Full article

(This article belongs to the Section Gravitation)

10 pages, 875 KB

Open AccessArticle

Hidden Momentum and the Absence of the Gravitational Spin Hall Effect in a Uniform Field

by Andrzej Czarnecki and Ting Gao

Universe 2025, 11(11), 365; https://doi.org/10.3390/universe11110365 - 6 Nov 2025

Cited by 1 | Viewed by 623

Abstract

We re-examine the recent claim that a Dirac particle freely falling in a uniform gravitational field exhibits a spin-dependent transverse deflection (gravitational spin Hall effect). Using a circulating mass model, we show that hidden momentum arises in uniform fields when an object carries [...] Read more.

We re-examine the recent claim that a Dirac particle freely falling in a uniform gravitational field exhibits a spin-dependent transverse deflection (gravitational spin Hall effect). Using a circulating mass model, we show that hidden momentum arises in uniform fields when an object carries angular momentum. On the quantum side, we analyze the Dirac Hamiltonian in a uniform potential, construct its Foldy–Wouthuysen form, and evaluate the Heisenberg evolution of spin-polarized Gaussian packets. The state used previously, with

⟨ p ⟩ = 0

, is not at rest: because canonical and kinetic momenta differ, the packet carries a spin-dependent hidden momentum from

t = 0

. Imposing

⟨ x (0) ⟩ = ⟨ v (0) ⟩ = 0

requires a compensating spin-dependent

⟨ p (0) ⟩

; with this preparation

⟨ x (t) ⟩ = 0

to leading order in the gravitational acceleration g. Generalizing, an exact Foldy–Wouthuysen transformation (linear in g but to all orders in

1 / c

) shows that spin-dependent transverse motion begins no earlier than at

O (g^{2})

for a broad class of wave packets. We conclude that a uniform field does not produce a gravitational spin Hall effect at linear order; the previously reported drift stems from inconsistent initial states and misinterpreting canonical momentum. Full article

(This article belongs to the Special Issue Geometric Theories of Gravity)

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15 pages, 1323 KB

Open AccessArticle

Spin Tetrad Formalism of Circular Polarization States in Relativistic Jets

by Ronald Gamble, Jr.

Universe 2025, 11(11), 364; https://doi.org/10.3390/universe11110364 - 4 Nov 2025

Viewed by 1088

Abstract

Relativistic jets from active galactic nuclei (AGN) have been a topic of peak interest in the high-energy astrophysics community for their uniquely dynamic nature and incredible radiative power emanating from supermassive black holes and similarly accreting compact dense objects. An overall consensus on [...] Read more.

Relativistic jets from active galactic nuclei (AGN) have been a topic of peak interest in the high-energy astrophysics community for their uniquely dynamic nature and incredible radiative power emanating from supermassive black holes and similarly accreting compact dense objects. An overall consensus on relativistic jet formation states that accelerated outflow at high Lorentz factors are generated by a complex relationship between the accretion disk of the system and the frame-dragging effects of the rotating massive central object. This paper will provide a basis for which circular polarization states, defined using a spin tetrad formalism, contribute to a description for the angular momentum flux in the jet emanating from the central engine. A representation of the Kerr spacetime is used in formulating the spin tetrad forms. A discussion on unresolved problems in jet formation and how we can use multi-method observations with polarimetry of AGN to direct future theoretical descriptions will also be given. Full article

(This article belongs to the Section Compact Objects)

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45 pages, 4592 KB

Open AccessReview

Multiwavelength View of Circumstellar Interaction in Supernovae

by Poonam Chandra

Universe 2025, 11(11), 363; https://doi.org/10.3390/universe11110363 - 3 Nov 2025

Cited by 1 | Viewed by 1576

Abstract

The interaction of post-explosion supernova ejecta with the surrounding circumstellar medium creates emissions across the electromagnetic spectrum. Since the circumstellar medium is created by the mass lost from the progenitor star, it carries tell-tale signatures of the progenitor. Consequently, observations and modeling of [...] Read more.

The interaction of post-explosion supernova ejecta with the surrounding circumstellar medium creates emissions across the electromagnetic spectrum. Since the circumstellar medium is created by the mass lost from the progenitor star, it carries tell-tale signatures of the progenitor. Consequently, observations and modeling of radiation produced by the interaction in various types of supernovae have provided valuable insights into their progenitors. Detailed studies have shown that the interaction in supernovae begins and sustains over various timescales and lengthscales, with differing mass-loss rates in distinct sub-classes. This reveals diverse progenitor histories for these stellar explosions. This review paper summarizes various supernova subtypes, linking them to stellar death pathways, and presents an updated supernova classification diagram. We then present a multi-wavelength study of circumstellar interaction in different supernova classes. We also present unpublished X-ray as well as radio observations of a type IIn supernova, SN 2010jl, which allow us to extend its circumstellar interaction studies to about 7 years post-explosion. The new data indicates that the extreme mass-loss rate (∼0.1 M_⊙ yr⁻¹) in SN 2010jl, reported by Chandra et al. commenced within the last 300 years before the explosion. We summarize the current status of the field and argue that via detailed studies of the circumstellar interaction, a.k.a. “Time Machine” technique, one of the big mysteries of stellar evolution, i.e., mapping supernovae progenitors to their explosive outcomes can be solved. Full article

(This article belongs to the Special Issue A Multiwavelength View of Supernovae)

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14 pages, 412 KB

Open AccessArticle

Data-Driven Reconstruction of f (R, T) Gravity Using Genetic Algorithms

by Redouane El Ouardi, Dalale Mhamdi, Amine Bouali and Taoufik Ouali

Universe 2025, 11(11), 362; https://doi.org/10.3390/universe11110362 - 31 Oct 2025

Viewed by 604

Abstract

We investigate

f (R, T)

gravity, where R is the Ricci scalar and T the trace of the energy–momentum tensor, focusing on the subclass defined by [...] Read more.

We investigate

f (R, T)

gravity, where R is the Ricci scalar and T the trace of the energy–momentum tensor, focusing on the subclass defined by

f (R, T) = R + 2 f (T)

. Instead of assuming a parametric form, we adopt a non-parametric reconstruction based on genetic algorithms (GA), a machine learning technique that does not rely on predefined models. Using Hubble parameter measurements from cosmic chronometers, baryon acoustic oscillations, and the Dark Energy Spectroscopic Instrument (DESI) data, we reconstruct

H (z)

in a model-independent way. This reconstruction allows us to derive both numerical and analytical forms of

f (T)

through the modified Friedmann equations. The analytic expression derived via GA provides an excellent fit to the numerical reconstruction. Furthermore, we compare the evolution of the Hubble parameter predicted by our model with that of the standard ΛCDM scenario (Planck), finding a good agreement for

z ≲ 2

. These results highlight the robustness of GA-based reconstructions and suggest that the functional form of

f (R, T)

obtained here may serve as a promising tool for further applications in cosmology and astrophysics. Full article

(This article belongs to the Section Cosmology)

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35 pages, 2170 KB

Open AccessReview

Probing Supernova Diversity Through High-Cadence Optical Observations

by Kuntal Misra, Bhavya Ailawadhi, Raya Dastidar, Monalisa Dubey, Naveen Dukiya, Anjasha Gangopadhyay, Divyanshu Janghel, Kumar Pranshu and Mridweeka Singh

Universe 2025, 11(11), 361; https://doi.org/10.3390/universe11110361 - 31 Oct 2025

Viewed by 905

Abstract

Supernovae (SNe) are among the most energetic and transient events in the universe, offering crucial insights into stellar evolution, nucleosynthesis, and cosmic expansion. Optical observations have historically played a central role in the discovery, classification, and physical interpretation of SNe. In this review, [...] Read more.

Supernovae (SNe) are among the most energetic and transient events in the universe, offering crucial insights into stellar evolution, nucleosynthesis, and cosmic expansion. Optical observations have historically played a central role in the discovery, classification, and physical interpretation of SNe. In this review, we summarize recent progress in the optical study of SNe, with a focus on advancements in time-domain surveys and photometric and spectroscopic follow-up strategies. High-cadence optical monitoring is pivotal in capturing the diverse behaviors of SNe, from early-time emission to late-phase decline. Leveraging data from ARIES telescopes and national/international collaborations, we systematically investigate various SN types, including Type Iax, IIP/L, IIb, IIn/Ibn and Ib/c events. Our analysis includes light curve evolution and spectral diagnostics, providing insights into early emission signatures (e.g., shock breakout), progenitor systems, explosion mechanisms, and circumstellar medium (CSM) interactions. Through detailed case studies, we demonstrate the importance of both early-time and nebular-phase observations in constraining progenitor and CSM properties. This comprehensive approach underscores the importance of coordinated global efforts in time-domain astronomy to deepen our understanding of SN diversity. We conclude by discussing the challenges and opportunities for future optical studies in the era of wide-field observatories such as the Vera C. Rubin Observatory (hereafter Rubin), with an emphasis on detection strategies, automation, and rapid-response capabilities. Full article

(This article belongs to the Special Issue A Multiwavelength View of Supernovae)

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29 pages, 16778 KB

Open AccessArticle

Detecting Intermediate-Mass Black Holes out to 20 Mpc with ELT/HARMONI: The Case of FCC 119

by Hai N. Ngo, Dieu D. Nguyen, Tinh T. Q. Le, Tien H. T. Ho, Truong N. Nguyen and Trung H. Dang

Universe 2025, 11(11), 360; https://doi.org/10.3390/universe11110360 - 31 Oct 2025

Cited by 3 | Viewed by 1080

Abstract

Intermediate-mass black holes (IMBHs;

M_{BH} \approx 10^{3 - 5} M_{⊙}

) play a critical role in understanding the formation of supermassive black holes in the early universe. In this study, we expand on Nguyen et al.’s simulated measurements of [...] Read more.

Intermediate-mass black holes (IMBHs;

M_{BH} \approx 10^{3 - 5} M_{⊙}

) play a critical role in understanding the formation of supermassive black holes in the early universe. In this study, we expand on Nguyen et al.’s simulated measurements of IMBH masses using stellar kinematics, which will be observed with the High Angular Resolution Monolithic Optical and Near-infrared Integral (HARMONI) field spectrograph on the Extremely Large Telescope (ELT) up to a distance of 20 Mpc. Our sample focuses on both the Virgo Cluster in the northern sky and the Fornax Cluster in the southern sky. We begin by identifying dwarf galaxies hosting nuclear star clusters, which are thought to be nurseries for IMBHs in the local universe. As a case study, we conduct simulations for FCC 119, the second faintest dwarf galaxy in the Fornax Cluster at 20 Mpc, which is also fainter than most of the Virgo Cluster members. We use the galaxy’s surface brightness profile from Hubble Space Telescope (HST) imaging, combined with an assumed synthetic spectrum, to create mock observations with the HSIM simulator and Jeans Anisotropic Models (JAMs). These mock HARMONI data cubes are analyzed as if they were real observations, employing JAMs within a Bayesian framework to infer IMBH masses and their associated uncertainties. We find that ELT/HARMONI can detect the stellar kinematic signature of an IMBH and accurately measure its mass for

M_{BH} ≳ 10^{5} M_{⊙}

out to distances of ∼20 Mpc. Full article

(This article belongs to the Special Issue Supermassive Black Hole Mass Measurements)

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13 pages, 342 KB

Open AccessArticle

Evaluating Gaia Astrometric Quality and Distances for Galactic Hot Supergiants

by Nadezhda L. Vaidman, Shakhida T. Nurmakhametova, Aziza B. Umirova, Serik A. Khokhlov, Aldiyar T. Agishev and Berik S. Yermekbayev

Universe 2025, 11(11), 359; https://doi.org/10.3390/universe11110359 - 30 Oct 2025

Cited by 1 | Viewed by 1204

Abstract

Distances to Galactic BA supergiants are essential for determining their luminosities, radii, and positions on the Hertzsprung–Russell diagram, yet Gaia parallaxes for these bright, extended sources are often affected by systematics. We compiled a homogeneous sample of 132 B0–A5 supergiants and re-evaluated their [...] Read more.

Distances to Galactic BA supergiants are essential for determining their luminosities, radii, and positions on the Hertzsprung–Russell diagram, yet Gaia parallaxes for these bright, extended sources are often affected by systematics. We compiled a homogeneous sample of 132 B0–A5 supergiants and re-evaluated their distances using a consistent, quality-controlled approach. Parallaxes from Gaia DR3 and EDR3 were corrected for a magnitude–colour zero-point bias and adjusted for excess noise through RUWE-dependent uncertainty inflation. A Bayesian inference with an exponentially decreasing space–density prior was then applied, adopting the catalogue with the smallest penalised total uncertainty. Distances were accepted only when the corrected parallax signal-to-noise ratio exceeded 2.5, the relative uncertainty was below 40%, and key Gaia quality indicators were nominal. The resulting catalogue delivers robust, quality-vetted distances with realistic uncertainties for each star, providing a reliable foundation for deriving fundamental parameters and for future studies of the flux-weighted gravity–luminosity relation and the evolution of Galactic BA supergiants. Full article

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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

Cited by 1 | Viewed by 787

Abstract

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

Viewed by 865

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 AccessEditor’s ChoiceReview

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

Cited by 1 | Viewed by 2272

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

Cited by 1 | Viewed by 2263

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

Viewed by 966

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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