Universe

23 pages, 11433 KB

Open AccessArticle

Characterisation of Nearby Ultracool Dwarf Candidates with OSIRIS/GTC: First Detection of Balmer Line Emission from the Dwarf Carbon Star LSR J21052514⁺

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

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

15 pages, 296 KB

Open AccessArticle

Symplectic Realization of Generalized Snyder–Poisson Algebra

by V. G. Kupriyanov and E. L. F. de Lima

Universe 2025, 11(10), 339; https://doi.org/10.3390/universe11100339 - 14 Oct 2025

Abstract

We investigate Snyder spacetime and its generalizations, including Yang and Snyder–de Sitter spaces, which constitute manifestly Lorentz-invariant noncommutative geometries. This work initiates a systematic study of gauge theory on such spaces in the semi-classical regime, formulated as Poisson gauge theory. As a first [...] Read more.

We investigate Snyder spacetime and its generalizations, including Yang and Snyder–de Sitter spaces, which constitute manifestly Lorentz-invariant noncommutative geometries. This work initiates a systematic study of gauge theory on such spaces in the semi-classical regime, formulated as Poisson gauge theory. As a first step, we construct the symplectic realizations of the relevant noncommutative spaces, a prerequisite for defining Poisson gauge transformations and field strengths. We present a general method for representing the Snyder algebra and its extensions in terms of canonical phase-space variables, enabling both the reproduction of known representations and the derivation of novel ones. These canonical constructions are employed to obtain explicit symplectic realizations for the Snyder–de Sitter space and to construct the deformed partial derivative which differentiates the underlying Poisson structure. Furthermore, we analyze the motion of freely falling particles in these backgrounds and comment on the geometry of the associated spaces. Full article

(This article belongs to the Section Field Theory)

25 pages, 812 KB

Open AccessArticle

Constructing Regular Lovelock Black Holes with Degenerate Vacuum and Λ < 0 Using the Gravitational Tension—Shadow Analysis

by Reginaldo Prado-Fuentes, Rodrigo Aros, Milko Estrada and Bastian Astudillo

Universe 2025, 11(10), 338; https://doi.org/10.3390/universe11100338 - 13 Oct 2025

Abstract

Recently, a link between gravitational tension (GT) and energy density via the Kretschmann scalar (KS) was proposed to construct regular black holes (RBHs) in pure Lovelock (PL) gravity. However, including a negative cosmological constant in PL gravity leads to a curvature singularity. Here, [...] Read more.

Recently, a link between gravitational tension (GT) and energy density via the Kretschmann scalar (KS) was proposed to construct regular black holes (RBHs) in pure Lovelock (PL) gravity. However, including a negative cosmological constant in PL gravity leads to a curvature singularity. Here, we choose the coupling constants such that the Lovelock equations admit an n-fold degenerate AdS vacuum (LnFDGS), allowing us to construct an RBH with

Λ < 0

, where the energy density is analogous to the previously mentioned model. To achieve this, we propose alternative definitions for both the KS and GT. We find that, for mass parameter values greater than the extremal value

M_{\min}

, our RBH solution becomes indistinguishable from the AdS vacuum black hole from inside the event horizon out to infinity. At small scales, quantum effects modify the geometry and thermodynamics, removing the singularity. Furthermore, due to the lack of analytical relationships between the event horizon, photon sphere, and shadow in LnFDGS, we propose a numerical method to represent these quantities. Full article

(This article belongs to the Section Gravitation)

► Show Figures

Figure 1

13 pages, 354 KB

Open AccessArticle

Holographic Thermodynamics of Higher-Dimensional AdS Black Holes with CFT Rescaling

by Yahya Ladghami and Taoufik Ouali

Universe 2025, 11(10), 337; https://doi.org/10.3390/universe11100337 - 10 Oct 2025

Viewed by 171

Abstract

In this paper, we study the thermodynamic behavior of charged AdS black holes in higher-dimensional spacetimes within the framework of conformal holographic extended thermodynamics. This formalism is based on a novel AdS/CFT dictionary in which the conformal rescaling factor of the boundary conformal [...] Read more.

In this paper, we study the thermodynamic behavior of charged AdS black holes in higher-dimensional spacetimes within the framework of conformal holographic extended thermodynamics. This formalism is based on a novel AdS/CFT dictionary in which the conformal rescaling factor of the boundary conformal field theory (CFT) is treated as a thermodynamic parameter, while Newton’s constant is held fixed and the AdS radius is allowed to vary. We explore how variations in the CFT state, represented by its central charge, influence the bulk thermodynamics, phase structure, and stability of black holes in five and six dimensions. Our analysis reveals the emergence of Van der Waals-like phase transitions, critical phenomena governed by the central charge. Additionally, we find that the thermodynamic behavior of AdS black holes is affected by the dimensionality of the bulk spacetime, as we compare higher-dimensional black holes to lower-dimensional ones, such as the BTZ black holes. These findings provide new insights into the role of boundary degrees of freedom in shaping the thermodynamics of gravitational systems via holography. Full article

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

► Show Figures

Figure 1

15 pages, 1215 KB

Open AccessCommunication

Agegraphic Dark Energy from Entropy of the Anti-de Sitter Black Hole

by Qihong Huang, Yang Liu and He Huang

Universe 2025, 11(10), 336; https://doi.org/10.3390/universe11100336 - 10 Oct 2025

Viewed by 212

Abstract

In this paper, we analyze the agegraphic dark energy from the entropy of the anti-de Sitter black hole using the age of the universe as the IR cutoff. We constrain its parameter with the Pantheon+ Type Ia supernova sample and observational Hubble parameter [...] Read more.

In this paper, we analyze the agegraphic dark energy from the entropy of the anti-de Sitter black hole using the age of the universe as the IR cutoff. We constrain its parameter with the Pantheon+ Type Ia supernova sample and observational Hubble parameter data, finding that the Akaike Information Criterion cannot effectively distinguish this model from the standard

Λ

CDM model. The present value of Hubble constant

H_{0}

and the model parameter

b^{2}

are constrained to

H_{0} = 67.7 \pm 1.8

and

b^{2} = 0 . 303_{- 0.024}^{+ 0.019}

. This model realizes the whole evolution of the universe, including the late-time accelerated expansion. Although it asymptotically approaches the standard

Λ

CDM model in the future, statefinder analysis shows that late-time deviations allow the two models to be distinguished. Full article

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

► Show Figures

Figure 1

15 pages, 596 KB

Open AccessArticle

Secular Evolution of a Two-Planet System of Three Bodies with Variable Masses

by Alexander Prokopenya, Mukhtar Minglibayev and Aiken Kosherbayeva

Universe 2025, 11(10), 335; https://doi.org/10.3390/universe11100335 - 10 Oct 2025

Viewed by 87

Abstract

A classical three-body problem with two planets moving around a central star of variable mass on quasi-periodic orbits is considered. The bodies are assumed to attract each other according to Newton’s law of universal gravitation. The star loses its mass anisotropically, and this [...] Read more.

A classical three-body problem with two planets moving around a central star of variable mass on quasi-periodic orbits is considered. The bodies are assumed to attract each other according to Newton’s law of universal gravitation. The star loses its mass anisotropically, and this leads to the appearance of reactive forces. The problem is analyzed in the framework of Newtonian’s formalism, and equations of motion are derived in terms of the osculating elements of aperiodic motion on quasi-conic sections. As equations of motion are not integrable, the perturbation theory is applied with the perturbing forces expanded into power series in terms of eccentricities and inclinations, which are assumed to be small. Averaging these equations over the mean longitudes of the planets in the absence of mean-motion resonances, we obtain the differential equations describing the long-term evolution of orbital elements. Numerical solutions to the evolution equations are obtained and analyzed for three different three-body systems. The obtained results demonstrate clearly that variability of masses may influence essentially the secular evolution of the orbital elements. All the relevant symbolic and numerical calculations are performed with the computer algebra system Wolfram Mathematica. Full article

(This article belongs to the Section Gravitation)

► Show Figures

Figure 1

12 pages, 384 KB

Open AccessArticle

QCD Sum Rule Study of Topped Mesons Within Heavy Quark Effective Theory

by Shu-Wei Zhang, Xuan Luo, Hui-Min Yang and Hua-Xing Chen

Universe 2025, 11(10), 334; https://doi.org/10.3390/universe11100334 - 9 Oct 2025

Viewed by 92

Abstract

Motivated by the recent CMS observation of a near-threshold enhancement in top quark pair production, we investigate a novel class of hadronic systems containing a single top quark: the topped mesons (

t \bar{q}

, with [...] Read more.

Motivated by the recent CMS observation of a near-threshold enhancement in top quark pair production, we investigate a novel class of hadronic systems containing a single top quark: the topped mesons (

t \bar{q}

, with

\bar{q} = \bar{u}, \bar{d}, \bar{s}

). In contrast to the extensively studied toponium (

t \bar{t}

) system—analyzed primarily within perturbative QCD—topped mesons offer a complementary nonperturbative probe of QCD dynamics in the heavy quark limit. These states are expected to exhibit longer lifetimes and narrower decay widths than toponium, as only a single top quark undergoes weak decay. We employ QCD sum rules within the framework of heavy quark effective theory to study the structure and mass spectrum of ground-state topped mesons. Our analysis predicts masses near 173.1 GeV, approximately 0.5–0.6 GeV above the top quark pole mass. Compared with singly topped baryons (

t q q

, with

q = u, d, s

), topped mesons have a simpler quark composition and more favorable decay channels (a topped meson is anticipated to decay weakly into a

Υ

meson and a charmed meson), enhancing their potential for both theoretical analysis and experimental discovery. Full article

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

► Show Figures

Figure 1

18 pages, 2086 KB

Open AccessReview

Jets in Low-Mass Protostars

by Somnath Dutta

Universe 2025, 11(10), 333; https://doi.org/10.3390/universe11100333 - 9 Oct 2025

Viewed by 171

Abstract

Jets and outflows are key components of low-mass star formation, regulating accretion and shaping the surrounding molecular clouds. These flows, traced by molecular species at (sub)millimeter wavelengths (e.g., CO, SiO, SO, H₂CO, and CH₃OH) and by atomic, ionized, and [...] Read more.

Jets and outflows are key components of low-mass star formation, regulating accretion and shaping the surrounding molecular clouds. These flows, traced by molecular species at (sub)millimeter wavelengths (e.g., CO, SiO, SO, H₂CO, and CH₃OH) and by atomic, ionized, and molecular lines in the infrared (e.g., H₂, [Fe II], [S I]), originate from protostellar accretion disks deeply embedded within dusty envelopes. Jets play a crucial role in removing angular momentum from the disk, thereby enabling continued mass accretion, while directly preserving a record of the protostar’s outflow history and potentially providing indirect insights into its accretion history. Recent advances in high-resolution, high-sensitivity observations, particularly with the James Webb Space Telescope (JWST) in the infrared and the Atacama Large Millimeter/submillimeter Array (ALMA) at (sub)millimeter wavelengths, have revolutionized studies of protostellar jets and outflows. These instruments provide complementary views of warm, shock-excited gas and cold molecular component of the jet–outflow system. In this review, we discuss the current status of observational studies that reveal detailed structures, kinematics, and chemical compositions of protostellar jets and outflows. Recent analyses of mass-loss rates, velocities, rotation, molecular abundances, and magnetic fields provide critical insights into jet launching mechanisms, disk evolution, and the potential formation of binary systems and planets. The synergy of JWST’s infrared sensitivity and ALMA’s high-resolution imaging is advancing our understanding of jets and outflows. Future large-scale, high-resolution surveys with these facilities are expected to drive major breakthroughs in outflow research. Full article

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

► Show Figures

Figure 1

14 pages, 5542 KB

Open AccessArticle

High-Resolution Infrared Spectroscopy of IRS 16CC and IRS 33N: Stellar Parameters and Implications for Star Formation Near Sgr A^*

by Shogo Nishiyama, Wakana Sato, Moeka Hotta, Momoka Ikarashi, Hiromi Saida, Yohsuke Takamori, Tetsuya Nagata, Hiroyuki Ikeda and Masaaki Takahashi

Universe 2025, 11(10), 332; https://doi.org/10.3390/universe11100332 - 5 Oct 2025

Viewed by 139

Abstract

IRS 16CC and IRS 33N are among more than 100 young, massive stars identified within 0.5 pc from the Galactic central supermassive black hole Sgr A*, where conventional star formation processes are expected to be strongly suppressed. A subset of these stars, including [...] Read more.

IRS 16CC and IRS 33N are among more than 100 young, massive stars identified within 0.5 pc from the Galactic central supermassive black hole Sgr A*, where conventional star formation processes are expected to be strongly suppressed. A subset of these stars, including IRS 16CC, has been confirmed to reside in a clockwise rotating stellar disk, and is thought to have formed in a massive, gaseous disk around Sgr A*. In contrast, other young massive stars, such as IRS 33N, exhibit dynamical behaviors that deviate significantly from those of the disk population, and their formation mechanism is still uncertain. To investigate their formation mechanism, we carried out near-infrared, high-resolution spectroscopic observations of IRS 16CC and IRS 33N using the Infrared Camera and Spectrograph on the Subaru telescope, equipped with an adaptive optics system. We compared the profiles of He I absorption lines with synthetic spectra generated from model atmospheres, and then compared derived stellar parameters with stellar evolutionary tracks to estimate their ages and initial masses. Our analysis yields their effective temperatures of ∼23,000 K, surface gravities of ∼2.8, and initial masses of

37 \pm 6 M_{⊙}

and

27_{- 3}^{+ 4} M_{⊙}

, consistent with spectral types of B0.5–1.5 supergiants. The ages of IRS 16CC and IRS 33N are estimated to be

4.4 \pm 0.7

Myr and

5 . 3_{- 0.7}^{+ 1.1}

Myr, respectively. These results suggest that, despite their different dynamical properties, the two stars are likely to share a common origin. Full article

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

► Show Figures

Figure 1

30 pages, 7520 KB

Open AccessReview

Probing the Sources of Ultra-High-Energy Cosmic Rays—Constraints from Cosmic-Ray Measurements

by Teresa Bister

Universe 2025, 11(10), 331; https://doi.org/10.3390/universe11100331 - 3 Oct 2025

Viewed by 207

Abstract

Ultra-high-energy cosmic rays (UHECRs) are the most energetic particles known—and yet their origin is still an open question. However, with the precision and accumulated statistics of the Pierre Auger Observatory and the Telescope Array, in combination with advancements in theory and modeling—e.g., of [...] Read more.

Ultra-high-energy cosmic rays (UHECRs) are the most energetic particles known—and yet their origin is still an open question. However, with the precision and accumulated statistics of the Pierre Auger Observatory and the Telescope Array, in combination with advancements in theory and modeling—e.g., of the Galactic magnetic field—it is now possible to set solid constraints on the sources of UHECRs. The spectrum and composition measurements above the ankle can be well described by a population of extragalactic, homogeneously distributed sources emitting mostly intermediate-mass nuclei. Additionally, using the observed anisotropy in the arrival directions, namely the large-scale dipole > 8 EeV, as well as smaller-scale warm spots at higher energies, even more powerful constraints on the density and distribution of sources can be placed. Yet, open questions remain—like the striking similarity of the sources that is necessary to describe the rather pure mass composition above the ankle, or the origin of the highest energy events whose tracked back directions point toward voids. The current findings and possible interpretation of UHECR data will be presented in this review. Full article

(This article belongs to the Special Issue Multi-Messenger Astrophysics: Synergies Between Cosmic Rays, Gravitational Waves, Neutrinos, and Electromagnetic Signals)

► Show Figures

Figure 1

16 pages, 1518 KB

Open AccessArticle

Shadow of a Collapsing Star in a Regular Spacetime

by Daniel Nuñez and Juan Carlos Degollado

Universe 2025, 11(10), 330; https://doi.org/10.3390/universe11100330 - 3 Oct 2025

Viewed by 123

Abstract

In this study, we describe the dynamical formation of the shadow of a collapsing star in Hayward spacetime from the points of view of an observer far away from the center and a free-falling observer. By solving the time-like and light-like radial geodesics, [...] Read more.

In this study, we describe the dynamical formation of the shadow of a collapsing star in Hayward spacetime from the points of view of an observer far away from the center and a free-falling observer. By solving the time-like and light-like radial geodesics, we determine the angular size of the shadow as a function of time. We find that the formation of the shadow is a finite process for both observers, and the size of the shadow is affected by the Hayward spacetime parameters. In this study, we consider several scenarios, from the Schwarzschild limit to an extreme Hayward black hole. Full article

(This article belongs to the Section Gravitation)

► Show Figures

Figure 1

27 pages, 1596 KB

Open AccessArticle

The Nuclear Astrophysics Program at the CERN n_TOF Facility: Results and Perspectives

by P. M. Milazzo, C. Lederer-Woods and A. Mengoni

Universe 2025, 11(10), 329; https://doi.org/10.3390/universe11100329 - 30 Sep 2025

Viewed by 120

Abstract

The CERN n_TOF facility is a research infrastructure specifically designed for studying neutron-induced nuclear reactions. Pulsed white neutron beams are delivered toward three experimental areas, two of them at different baselines to apply the time-of-flight technique, and another one very close to the [...] Read more.

The CERN n_TOF facility is a research infrastructure specifically designed for studying neutron-induced nuclear reactions. Pulsed white neutron beams are delivered toward three experimental areas, two of them at different baselines to apply the time-of-flight technique, and another one very close to the neutron source for activation studies. High intensity and high neutron energy resolution make n_TOF a unique facility. A major component of the physics program at n_TOF is dedicated to the measurement of key neutron induced reactions for nuclear astrophysics, relevant to nucleosynthesis in stars, the Big Bang primordial nucleosynthesis as well as Cosmochronology. A review of the relevant results obtained at the n_TOF facility is reported, together with details of challenging new measurements in preparation. Full article

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

► Show Figures

Figure 1

12 pages, 1328 KB

Open AccessArticle

Long-Term Variations in Background Bias and Magnetic Field Noise in HSOS/SMFT Observations

by Haiqing Xu, Hongqi Zhang, Suo Liu, Jiangtao Su, Yuanyong Deng, Shangbin Yang, Mei Zhang and Jiaben Lin

Universe 2025, 11(10), 328; https://doi.org/10.3390/universe11100328 - 28 Sep 2025

Viewed by 178

Abstract

The Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station (HSOS) has conducted continuous observations of solar vector magnetic fields for nearly four decades, and while the primary optical system remains unchanged, critical components—including filters, polarizers, and detectors—have undergone multiple upgrades and [...] Read more.

The Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station (HSOS) has conducted continuous observations of solar vector magnetic fields for nearly four decades, and while the primary optical system remains unchanged, critical components—including filters, polarizers, and detectors—have undergone multiple upgrades and replacements. Maintaining data consistency is essential for reliable long-term studies of magnetic field evolution and solar activity, as well as current helicity. In this study, we systematically analyze background bias and noise levels in SMFT observations from 1988 to 2019. Our dataset comprises 12,281 vector magnetograms of 1484 active regions. To quantify background bias, we computed mean values of Stokes

Q / I

,

U / I

and

V / I

over each entire magnetogram. The background bias of Stokes

V / I

is small for the whole dataset. The background biases of Stokes

Q / I

and

U / I

fluctuate around zero during 1988–2000. From 2001 to 2011, however, the fluctuations in the background bias of both

Q / I

and

U / I

become significantly larger, exhibiting mixed positive and negative values. Between 2012 and 2019, the background biases shift to predominantly positive values for both Stokes

Q / I

and

U / I

parameters. To address this issue, we propose a potential method for removing the background bias and further discuss its impact on the estimation of current helicity. For each magnetogram, we quantify measurement noise by calculating the standard deviation (

σ

) of the longitudinal (

B_{l}

) and transverse (

B_{t}

) magnetic field components within a quiet-Sun region. The noise levels for

B_{l}

and

B_{t}

components were approximately 15 Gauss (G) and 87 G, respectively, during 1988–2011. Since 2012, these values decreased significantly to ∼6 G for

B_{l}

and ∼55 G for

B_{t}

, likely due to the installation of a new filter. Full article

(This article belongs to the Special Issue Measurements, Observations and Theoretical Studies on the Solar Magnetic Field—Celebrating the 40th Anniversary of the Huairou Solar Observing Station)

► Show Figures

Figure 1

18 pages, 1611 KB

Open AccessReview

Blazars as Probes for Fundamental Physics

by Giorgio Galanti

Universe 2025, 11(10), 327; https://doi.org/10.3390/universe11100327 - 27 Sep 2025

Viewed by 152

Abstract

Blazars are a class of active galactic nuclei characterized by having one of their relativistic jets oriented close to our line of sight. Their broad emission spectrum makes them exceptional laboratories for probing fundamental physics. In this review, we explore the potential impact [...] Read more.

Blazars are a class of active galactic nuclei characterized by having one of their relativistic jets oriented close to our line of sight. Their broad emission spectrum makes them exceptional laboratories for probing fundamental physics. In this review, we explore the potential impact on blazar observations of three scenarios beyond the standard paradigm: (i) the hadron beam model, (ii) the interaction of photons with axion-like particles (ALPs), and (iii) Lorentz invariance violation. We focus on the very-high-energy spectral features these scenarios induce in the blazars Markarian 501 and 1ES 0229+200, making them ideal targets for testing such effects. Additionally, we examine ALP-induced effects on the polarization of UV-X-ray and high-energy photons from the blazar OJ 287. The unique signatures produced by these models are accessible to current and upcoming instruments—such as the ASTRI Mini Array, CTAO, LHAASO, IXPE, COSI, and AMEGO—offering new opportunities to probe and constrain fundamental physics through blazar observations. Full article

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

► Show Figures

Figure 1

11 pages, 210 KB

Open AccessCommunication

Toward a Reliability Scale for Assessing Reports of Unidentified Anomalous Phenomena (UAP)

by Dirk Schulze-Makuch and Tony Reichhardt

Universe 2025, 11(10), 326; https://doi.org/10.3390/universe11100326 - 25 Sep 2025

Viewed by 515

Abstract

Unidentified Anomalous Phenomena (UAP) observations have been reported from ancient times to today, but their true nature remains uncertain. In this paper we propose a rating scale designed to separate “signal” from “noise” in assessing UAP sighting reports. Our intention is that this [...] Read more.

Unidentified Anomalous Phenomena (UAP) observations have been reported from ancient times to today, but their true nature remains uncertain. In this paper we propose a rating scale designed to separate “signal” from “noise” in assessing UAP sighting reports. Our intention is that this will help professionals and laypeople alike distinguish cases that warrant further investigation from easily explainable false alarms. We categorize UAP sighting reports according to the quality of their evidence, considering such factors as number of observers, amount and quality of supporting evidence, especially physical evidence, and perhaps most importantly, whether UAP witnesses have made some effort to find an ordinary explanation for what they saw or experienced and whether the evidence has been subject to expert analysis. Full article

35 pages, 580 KB

Open AccessArticle

Quadrupole Perturbations of Slowly Spinning Ellis–Bronnikov Wormholes

by Bahareh Azad, Jose Luis Blázquez-Salcedo, Fech Scen Khoo, Jutta Kunz and Francisco Navarro-Lérida

Universe 2025, 11(10), 325; https://doi.org/10.3390/universe11100325 - 24 Sep 2025

Viewed by 198

Abstract

We study the axial and polar perturbations of slowly rotating Ellis–Bronnikov wormholes in General Relativity, applying a perturbative double expansion. In particular, we derive the equations for

l = 2

,

M_{z} = 2

perturbations of these objects, which are parametrized by [...] Read more.

We study the axial and polar perturbations of slowly rotating Ellis–Bronnikov wormholes in General Relativity, applying a perturbative double expansion. In particular, we derive the equations for

l = 2

,

M_{z} = 2

perturbations of these objects, which are parametrized by an asymmetry parameter. The equations constitute an astrophysically interesting sector of the perturbations that contribute dominantly to the gravitational wave radiation. Moreover, calculation of these modes may exhibit potential instabilities in the quadrupole sector. Full article

(This article belongs to the Special Issue Experimental and Observational Constraints on Wormhole Models)

► Show Figures

Figure 1

15 pages, 356 KB

Open AccessArticle

Energy–Momentum Squared Gravity Attached with Perfect Fluid Admitting Conformal Ricci Solitons

by Mohd Danish Siddiqi and Ibrahim Al-Dayel

Universe 2025, 11(10), 324; https://doi.org/10.3390/universe11100324 - 24 Sep 2025

Viewed by 277

Abstract

In the present research note, we explore the nature of the conformal Ricci solitons on the energy–momentum squared gravity model

F (R, T^{2})

that is a modification of general relativity. Furthermore, we deal with a subcase of the [...] Read more.

In the present research note, we explore the nature of the conformal Ricci solitons on the energy–momentum squared gravity model

F (R, T^{2})

that is a modification of general relativity. Furthermore, we deal with a subcase of the

F (R, T^{2}) = R + λ T^{2}

-gravity model coupled with a perfect fluid, which admits conformal Ricci solitons with a time-like concircular vector field. Using the steady conformal Ricci soliton, we derive the equation of state for the perfect fluid in the

F (R, T^{2})

-gravity model. In this series, we convey an indication of the pressure and density in the phantom barrier period and the stiff matter era, respectively. Finally, using a conformal Ricci soliton with a concircular vector field, we study the various energy constraints, black holes, and singularity circumstances for a perfect fluid coupled to

F (R, T^{2})

-gravity. Lastly, employing conformal Ricci solitons, we formulate the first law of thermodynamics, enthalpy, and the particle production rate in

F (R, T^{2})

-gravity and orthodox gravity. Full article

(This article belongs to the Section Gravitation)

32 pages, 1122 KB

Open AccessArticle

Distribution of Heavy-Element Abundances Generated by Decay from a Quasi-Equilibrium State

by Gerd Röpke, David Blaschke and Friedrich K. Röpke

Universe 2025, 11(10), 323; https://doi.org/10.3390/universe11100323 - 23 Sep 2025

Viewed by 204

Abstract

We present a freeze-out approach for describing the formation of heavy elements in expanding nuclear matter. Applying concepts used in modeling heavy-ion collisions or ternary fission, we determine the abundances of heavy elements taking into account in-medium effects such as Pauli blocking and [...] Read more.

We present a freeze-out approach for describing the formation of heavy elements in expanding nuclear matter. Applying concepts used in modeling heavy-ion collisions or ternary fission, we determine the abundances of heavy elements taking into account in-medium effects such as Pauli blocking and the Mott effect, which describes the dissolution of nuclei at high densities of nuclear matter. With this approach, we search for a universal initial distribution in a quasi-equilibrium state from which the coarse-grained pattern of the solar abundances of heavy elements freezes out and evolves by radioactive decay of the excited states. The universal initial state is characterized by the Lagrange parameters, which are related to temperature and chemical potentials of neutrons and protons. We show that such a state exists and determine a temperature of 5.266 MeV, a neutron chemical potential of 940.317 MeV and a proton chemical potential of 845.069 MeV, with a baryon number density of 0.013 fm⁻³ and a proton fraction of

0.13

. Heavy neutron-rich nuclei such as the hypothetical double-magic nucleus ³⁵⁸Sn appear in the initial distribution and contribute to the observed abundances after fission. We discuss astrophysical scenarios for the realization of this universal initial distribution for heavy-element nucleosynthesis, including supernova explosions, neutron star mergers and the inhomogeneous Big Bang. The latter scenario may be of interest in the light of early massive objects observed with the James Webb Space Telescope and opens new perspectives on the universality of the observed r-process patterns and the lack of observations of population III stars. Full article

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

► Show Figures

Figure 1

Journal Menu

Journal Browser

Universe, Volume 11, Issue 10 (October 2025) – 18 articles

Further Information

Guidelines

MDPI Initiatives

Follow MDPI