Universe

29 pages, 6197 KB

Open AccessReview

Selected Results on Variable Stars Observed by TESS

by Zsófia Bognár and Ádám Sódor

Universe 2025, 11(9), 319; https://doi.org/10.3390/universe11090319 - 17 Sep 2025

As we enter the final year of the second extended mission of the Transiting Exoplanet Survey Satellite (TESS), it is time to reflect on what the TESS mission has contributed to the advancement of astronomy. Thousands of papers based on TESS data have [...] Read more.

As we enter the final year of the second extended mission of the Transiting Exoplanet Survey Satellite (TESS), it is time to reflect on what the TESS mission has contributed to the advancement of astronomy. Thousands of papers based on TESS data have already been published, making it a challenge to select the ones we mention or summarise in this review. As the title suggests, this paper focuses on variable stars, that is, phenomena that causes a star’s brightness to change. We discuss all the major classes of extrinsic and intrinsic variables, from planetary transits to pulsating stars, excluding only the longest-period ones, which are not well suited for the typical time spans of TESS time-series observations. TESS has provided significant and interesting data and results for all these variable types. We hope that this selection successfully demonstrates the diverse applicability of TESS in variable star research. Full article

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

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20 pages, 528 KB

Open AccessArticle

Anisotropic Particle Creation from the T-Vacuum in the Radiation-Dominated Universe

by Dhamar S. Astilla, Sujoy K. Modak and Enrique Salazar

Universe 2025, 11(9), 318; https://doi.org/10.3390/universe11090318 - 13 Sep 2025

Viewed by 83

Abstract

We further investigate novel features of the T-vacuum state, originally defined in the context of quantum field theory in a (1+1)-dimensional radiation-dominated universe. Here, we extend the previous work to a realistic (3+1)-dimensional setup and show that the T-vacuum gives rise [...] Read more.

We further investigate novel features of the T-vacuum state, originally defined in the context of quantum field theory in a (1+1)-dimensional radiation-dominated universe. Here, we extend the previous work to a realistic (3+1)-dimensional setup and show that the T-vacuum gives rise to an anisotropic particle creation phenomenon in the radiation-dominated early universe. Unlike the Hawking or Unruh effect, where the particle content is thermal and asymptotically defined, here, it is non-thermal and instantaneous. This novel example of particle creation is interesting because these particles are detected in the frame of physical or cosmological observers, who envision the T-vacuum as a particle-excited state. Such results have the potential to be eventually compared with the observed anisotropies from the early universe and may provide new insights into cosmological particle creation. Full article

(This article belongs to the Section Gravitation)

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17 pages, 2190 KB

Open AccessArticle

Quasinormal Modes for Charged Lifshitz Black Holes with Scalar Hair

by Xufen Zhang, Shan Wu, Rui-Hong Yue, De-Cheng Zou and Ming Zhang

Universe 2025, 11(9), 317; https://doi.org/10.3390/universe11090317 - 13 Sep 2025

Viewed by 80

Abstract

In this paper, we investigate massive charged scalar perturbations in four-dimensional charged Lifshitz–AdS black holes with scalar hair within the framework of Einstein–Maxwell–Dilaton (EMD) gravity. Using the improved asymptotic iteration method (AIM), we compute the quasinormal modes (QNMs) and explore their dependence on [...] Read more.

In this paper, we investigate massive charged scalar perturbations in four-dimensional charged Lifshitz–AdS black holes with scalar hair within the framework of Einstein–Maxwell–Dilaton (EMD) gravity. Using the improved asymptotic iteration method (AIM), we compute the quasinormal modes (QNMs) and explore their dependence on key parameters, including the Lifshitz dynamical exponent z, the scalar field mass and charge, and the black hole charge, under various spatial curvature settings (

k = 0, \pm 1

). Our results reveal rich and sensitive behavior in both the real and imaginary parts of the QNMs. In particular, the decay rates can exhibit monotonic or non-monotonic dependence on the black hole charge, depending on the values of z,

m_{s}

, and

q_{s}

. These findings highlight the significant role of field and geometric parameters in governing the dynamical stability of Lifshitz black holes and offer insights into the perturbative properties of non-AdS holographic systems. Full article

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27 pages, 23938 KB

Open AccessArticle

Galaxy Clusters in Dark Matter Window: The Case of the Shapley Supercluster

by Maksym Stepanov, Lidiia Zadorozhna, Valentyna Babur, Olexandr Gugnin and Bohdan Hnatyk

Universe 2025, 11(9), 316; https://doi.org/10.3390/universe11090316 - 13 Sep 2025

Viewed by 70

Abstract

Dark matter dominates the matter content of the Universe, yet its particle nature remains elusive. Among the promising multi-messenger astronomy dark matter candidates are weakly interacting massive particles and superheavy dark matter, both of which may manifest themselves in cosmic ray,

γ

-ray, [...] Read more.

Dark matter dominates the matter content of the Universe, yet its particle nature remains elusive. Among the promising multi-messenger astronomy dark matter candidates are weakly interacting massive particles and superheavy dark matter, both of which may manifest themselves in cosmic ray,

γ

-ray, and neutrino signatures through annihilation or decay. Here, we explore potential multi-messenger signals from these candidates in galaxy clusters of the Shapley Supercluster—one of the most massive known structures in the local Universe (located at a distance of ∼200 Mpc and containing over

10^{16} M_{⊙}

of dark matter). Using the CLUMPY code, we model

γ

-ray and neutrino fluxes for weakly interacting massive particle masses between 0.1 and 100 TeV across various final states, comparing the predictions with the sensitivities of current and forthcoming observatories, including CTAO, IceCube, and KM3NeT. For superheavy dark matter scenarios with masses from

10^{19}

to

10^{28}

eV, we employ HDMSpectra code to compute ultra-high-energy cosmic ray proton and neutrino fluxes in the ranges available for observations using present (Pierre Auger Observatory, IceCube, KM3NeT) and future (GRAND, GCOS, etc.) instruments. Full article

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

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31 pages, 1533 KB

Open AccessArticle

Efficient Parallel Processing of Second-Generation TDI Data for Galactic Binaries in Space-Based Gravitational Wave Missions

by Xue-Hao Zhang, Soumya D. Mohanty, S. R. Valluri, Shao-Dong Zhao, Qun-Ying Xie and Yu-Xiao Liu

Universe 2025, 11(9), 313; https://doi.org/10.3390/universe11090313 - 13 Sep 2025

Viewed by 68

Abstract

Space-based gravitational wave missions such as LISA, Taiji, and Tianqin rely on the time-delay interferometry (TDI) technique to observe low-frequency signals such as Galactic binaries (GBs), massive black-hole binaries, and extreme-mass-ratio inspirals. Among these sources, resolving the large population of GBs poses a [...] Read more.

Space-based gravitational wave missions such as LISA, Taiji, and Tianqin rely on the time-delay interferometry (TDI) technique to observe low-frequency signals such as Galactic binaries (GBs), massive black-hole binaries, and extreme-mass-ratio inspirals. Among these sources, resolving the large population of GBs poses a central challenge for data analysis. In this work, we present GBSIEVER-C, a pipeline implemented in C and parallelized using OpenMP (Open Multi-Processing), along with a range of additional algorithmic optimizations, including a fast implementation of second-generation TDI response modeling. It builds upon the previous MATLAB-based pipeline that demonstrated competitive performance on LISA Data Challenge (LDC) data. To the best of our knowledge, GBSIEVER-C is the first pipeline to address the GB resolution problem using second-generation TDI data. We apply it to the GB dataset in Taiji Data Challenge (TDC) that contains 30 million GBs. Compared with our previous results on LDC data, it achieves improved source resolution, residual suppression, and parameter-estimation accuracy. These gains are consistent with the enhanced sensitivity expected from Taiji’s longer arm length. Although validated on Taiji data, the pipeline is fully compatible with LISA and similar mission configurations, and supports both single-detector and multi-detector network analyses. Full article

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28 pages, 587 KB

Open AccessArticle

The Lyra–Schwarzschild Spacetime

by M. C. Bertin, R. R. Cuzinatto, J. A. Paquiyauri and B. M. Pimentel

Universe 2025, 11(9), 315; https://doi.org/10.3390/universe11090315 - 12 Sep 2025

Viewed by 208

Abstract

In this paper, we provide a complete analysis of the most general spherical solution of the Lyra scalar-tensor (LyST) gravitational theory based on the proper definition of a Lyra manifold. Lyra’s geometry features the metric tensor and a scale function as fundamental fields, [...] Read more.

In this paper, we provide a complete analysis of the most general spherical solution of the Lyra scalar-tensor (LyST) gravitational theory based on the proper definition of a Lyra manifold. Lyra’s geometry features the metric tensor and a scale function as fundamental fields, resulting in generalizations of geometrical quantities such as the affine connection, curvature, torsion, and non-metricity. A proper action is defined considering the correct invariant volume element and the scalar curvature, obeying the symmetry of Lyra’s reference frame transformations and resulting in a generalization of the Einstein–Hilbert action. The LyST gravity assumes zero torsion in a four-dimensional metric-compatible spacetime. In this work, geometrical quantities are presented and solved via Cartan’s technique for a spherically symmetric line element. Birkhoff’s theorem is demonstrated so that the solution is proven to be static, resulting in the Lyra–Schwarzschild metric, which depends on both the geometrical mass (through a modified version of the Schwarzschild radius

r_{S}

) and an integration constant dubbed the Lyra radius

r_{L}

. We study particle and light motion in Lyra–Schwarzschild spacetime using the Hamilton–Jacobi method. The motion of massive particles includes the determination of the

r_{ISCO}

and the periastron shift. The study of massless particle motion shows the last photon’s unstable orbit. Gravitational redshift in Lyra–Schwarzschild spacetime is also reviewed. We find a coordinate transformation that casts Lyra–Schwarzschild spacetime in the form of the standard Schwarzschild metric; the physical consequences of this fact are discussed. Full article

(This article belongs to the Section Gravitation)

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13 pages, 283 KB

Open AccessArticle

Exact Kerr–Schild Spacetimes from Linearized Kinetic Gravity Braiding

by Bence Juhász and László Árpád Gergely

Universe 2025, 11(9), 314; https://doi.org/10.3390/universe11090314 - 12 Sep 2025

Viewed by 87

Abstract

We generalize our recent work on k-essence sourcing Kerr–Schild spacetimes to the kinetic gravity braiding scalar field. For k-essence, in order for a perturbative Kerr–Schild-type solution to become exact, the k-essence Lagrangian must either be linear in the kinetic term (with the Kerr–Schild [...] Read more.

We generalize our recent work on k-essence sourcing Kerr–Schild spacetimes to the kinetic gravity braiding scalar field. For k-essence, in order for a perturbative Kerr–Schild-type solution to become exact, the k-essence Lagrangian must either be linear in the kinetic term (with the Kerr–Schild congruence autoparallel) or unrestricted, provided the scalar gradient along the congruence vanishes. A similar reasoning for the pure kinetic braiding contribution leads to either a vanishing Lagrangian or a scalar that is constant along the congruence. From the scalar dynamics we also derive an accompanying constraint. Finally, we discuss pp-waves, an example of Kerr–Schild spacetime generated by a constant k-essence along the Kerr–Schild congruence with a vanishing Lagrangian. This allows for the construction of a Fock-type space, consisting of a tower of Kerr–Schild maps first yielding a vacuum pp-wave from flat spacetime; next a k-essence-generated pp-wave from the vacuum pp-wave; and finally an arbitrary number of k-essence pp-waves with different retarded time-dependent metric functions. Full article

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

12 pages, 366 KB

Open AccessArticle

Stability Analysis of Magnetized Quark Matter in Tsallis Statistics

by Jia Zhang and Xin-Jian Wen

Universe 2025, 11(9), 312; https://doi.org/10.3390/universe11090312 - 12 Sep 2025

Viewed by 163

Abstract

In this work, we employ the nonextensive Nambu–Jona-Lasinio model to analyze the thermodynamic properties of magnetized quark matter. The nonequilibrium state is described in Tsallis distribution by a dimensionless parameter q. We find that within a reasonable temperature range, the system undergoes [...] Read more.

In this work, we employ the nonextensive Nambu–Jona-Lasinio model to analyze the thermodynamic properties of magnetized quark matter. The nonequilibrium state is described in Tsallis distribution by a dimensionless parameter q. We find that within a reasonable temperature range, the system undergoes a crossover transition at the critical chemical potential, which is decreased by the increase of both the temperature and q value. In contrast to the enhanced stability by magnetic field in Boltzmann statistics, it is found that the stability of chiral restored matter in Tsallis statistics would be reduced by an increase of the magnetic field. Conversely, the increase of the q would enhance the stability of quark matter. Finally, we display the different magnetic effects on the stability in the chiral broken and restored regions. Full article

(This article belongs to the Special Issue Magnetized Dense Matter in Compact Stars: From Fundamental Properties to Astrophysical Observables)

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22 pages, 2087 KB

Open AccessArticle

Constraining the Primordial Black Hole Mass Function by the Lensing Events of Fast Radio Bursts

by Jing-Hao Li, Shi-Jie Wang, Xin-Yang Zhao and Nan Li

Universe 2025, 11(9), 311; https://doi.org/10.3390/universe11090311 - 11 Sep 2025

Viewed by 115

Abstract

Light from fast radio bursts (FRBs) can be deflected by the gravitational lensing effect of primordial black holes (PBHs), if they are distributed along the path from the FRBs to the observer. Consequently, the PBH mass function can be constrained by the lensing [...] Read more.

Light from fast radio bursts (FRBs) can be deflected by the gravitational lensing effect of primordial black holes (PBHs), if they are distributed along the path from the FRBs to the observer. Consequently, the PBH mass function can be constrained by the lensing events of FRBs. In this work, four different PBH mass functions are investigated (i.e., the monochromatic, log-normal, skew log-normal, and power-law distributions), and the constraints on the model parameters are obtained, if the PBH abundance

f_{PBH}

and the event rate of lensed FRBs

\bar{τ}

are given. We find that, if

\bar{τ} < 10^{- 4}

in future FRB experiments,

f_{PBH}

will be less than

10^{- 2.5}

in most of the PBH mass range from 1–

100 M_{⊙}

for the monochromatic mass function. Moreover, for the three extended mass functions,

\bar{τ}

increases when the PBH mass distributions spread to larger masses, setting more stringent constraints on

f_{PBH}

. Full article

(This article belongs to the Special Issue Primordial Black Holes: Observational Strategies)

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24 pages, 1135 KB

Open AccessArticle

Birth of an Isotropic and Homogeneous Universe with a Running Cosmological Constant

by A. Oliveira Castro Júnior, A. Corrêa Diniz, G. Oliveira-Neto and G. A. Monerat

Universe 2025, 11(9), 310; https://doi.org/10.3390/universe11090310 - 11 Sep 2025

Viewed by 168

Abstract

The present work discusses the birth of the Universe via quantum tunneling through a potential barrier, based on quantum cosmology, taking a running cosmological constant into account. We consider the Friedmann–Lemaître–Robertson–Walker (FLRW) metric with positively curved spatial sections (

k = 1

) [...] Read more.

The present work discusses the birth of the Universe via quantum tunneling through a potential barrier, based on quantum cosmology, taking a running cosmological constant into account. We consider the Friedmann–Lemaître–Robertson–Walker (FLRW) metric with positively curved spatial sections (

k = 1

) and the matter’s content is a dust perfect fluid. The model was quantized by the Dirac formalism, leading to a Wheeler–DeWitt equation. We solve that equation both numerically and using a WKB approximation. We study the behavior of tunneling probabilities

T P_{W K B}

and

T P_{i n t}

by varying the energy E of the dust perfect fluid, the phenomenological parameter

ν

, the present value of the Hubble function

H_{0}

, and the constant energy density

ρ_{Λ}^{0}

, with the last three parameters all being associated with the running cosmological constant. We observe that both tunneling probabilities,

T P_{W K B}

and

T P_{i n t}

, decrease as one increases

ν

. We also note that

T P_{W K B}

and

T P_{i n t}

grow as E increases, indicating that the Universe is more likely to be born with higher dust energy E values. The same is observed for the parameter

ρ_{Λ}^{0}

, that is,

T P_{W K B}

and

T P_{i n t}

are larger for higher values of

ρ_{Λ}^{0}

. Finally, the tunneling probabilities decrease as one increases the value of

H_{0}

. Therefore, the best conditions for the Universe to be born, in the present model, would be to have the highest possible values for E and

Λ

and the lowest possible values for

ν

and

H_{0}

. Full article

(This article belongs to the Section Cosmology)

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22 pages, 501 KB

Open AccessArticle

Initial Conditions for Tidal Synchronisation of a Planet by Its Moon

by Valeri V. Makarov and Michael Efroimsky

Universe 2025, 11(9), 309; https://doi.org/10.3390/universe11090309 - 10 Sep 2025

Viewed by 152

Abstract

Moons tidally interact with their host planets and stars. A close moon is quickly synchronised by the planet or becomes captured in a higher spin–orbit resonance. However, the planet requires much more time to significantly alter its rotation rate under the influence of [...] Read more.

Moons tidally interact with their host planets and stars. A close moon is quickly synchronised by the planet or becomes captured in a higher spin–orbit resonance. However, the planet requires much more time to significantly alter its rotation rate under the influence of moon-generated tides. The situation becomes more complex for close-in planets, as star-generated tides come into play and compete with moon-generated tides. The synchronisation of the planet by its moon changes the tidal dynamics of the entire star–planet–moon system and can lead to long-term stable configurations. In this paper, we demonstrate that a certain initial condition must be met for this to occur. Based on the angular momentum conservation, the derived condition is universal and bears no dependence upon the planet’s internal structure or tidal dissipation model. It is applicable to dwindling systems as well as to tidally expanding orbits and cases of initially retrograde motion. We present calculations for specific planet–moon systems (Earth and the Moon; Neptune and Triton; Venus and its hypothetical presently extinct moon Neith; Mars, Phobos, and Deimos; and Pluto and Charon) to constrain dynamically plausible formation and evolution scenarios. Among other things, our analysis prompts the question of whether Pluto and Charon evolved into their current state from an initially more compact configuration (as is commonly assumed) or from a wider orbit—a topic that will be discussed at length elsewhere. Our results are equally applicable to exoplanets. For example, if asynchronous close-in exoplanets are detected, the possibility of tidal synchronisation by an exomoon should be considered. Full article

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15 pages, 290 KB

Open AccessArticle

Non-Linear Equation of Motion for Page–Wootters Mechanism with Interaction and Quasi-Ideal Clocks

by Leandro R. S. Mendes, Frederico Brito and Diogo O. Soares-Pinto

Universe 2025, 11(9), 308; https://doi.org/10.3390/universe11090308 - 10 Sep 2025

Viewed by 227

Abstract

We explore a timeless approach to quantum theory, in the form of the Page–Wootters mechanism, in which a gravitational interaction is introduced between the system and a finite-dimensional clock. The clock model used is the recently proposed quasi-ideal clock, a construction that can [...] Read more.

We explore a timeless approach to quantum theory, in the form of the Page–Wootters mechanism, in which a gravitational interaction is introduced between the system and a finite-dimensional clock. The clock model used is the recently proposed quasi-ideal clock, a construction that can approximate the time–energy canonical commutation relation. We derive equations of motion for the case in which the system is in a pure and mixed state, obtaining a Schrödinger-like equation that leads to a non-linear equation exhibiting decoherence due to the non-ideal nature of the clock and gravitational coupling. A distinctive feature of this equation is that it exhibits dependence on the system’s initial conditions. Full article

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

17 pages, 749 KB

Open AccessArticle

Probing the Cosmic Distance Duality Relation via Non-Parametric Reconstruction for High Redshifts

by Felipe Avila, Fernanda Oliveira, Camila Franco, Maria Lopes, Rodrigo Holanda, Rafael C. Nunes and Armando Bernui

Universe 2025, 11(9), 307; https://doi.org/10.3390/universe11090307 - 9 Sep 2025

Viewed by 314

Abstract

We test the validity of the cosmic distance duality relation (CDDR) by combining angular diameter distance and luminosity distance measurements from recent cosmological observations. For the angular diameter distance, we use data from transverse baryon acoustic oscillations and galaxy clusters. On the other [...] Read more.

We test the validity of the cosmic distance duality relation (CDDR) by combining angular diameter distance and luminosity distance measurements from recent cosmological observations. For the angular diameter distance, we use data from transverse baryon acoustic oscillations and galaxy clusters. On the other hand, the luminosity distance is obtained from Type Ia supernovae in the Pantheon+ sample and from quasar catalogs. To reduce the large dispersion in quasar luminosity distances, we apply a selection criterion based on their deviation from the

Λ

CDM model and implement a binning procedure to suppress statistical noise. We reconstruct the CDDR using Gaussian Processes, a non-parametric supervised machine learning method. Our results show no significant deviation from the CDDR within the

2 σ

confidence level across the redshift range explored, supporting its validity even at high redshifts. Full article

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

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19 pages, 1517 KB

Open AccessArticle

Probing the Topology of the Early Universe Using CMB Temperature and Polarization Anisotropies

by Miguel-Angel Sanchis-Lozano

Universe 2025, 11(9), 306; https://doi.org/10.3390/universe11090306 - 9 Sep 2025

Viewed by 235

Abstract

The temperature and polarization anisotropies of the cosmic microwave background (CMB) as measured today can offer key insights into the topology of the early universe prior to inflation, for example by discriminating between flat and warped geometries. In this paper, we focus on [...] Read more.

The temperature and polarization anisotropies of the cosmic microwave background (CMB) as measured today can offer key insights into the topology of the early universe prior to inflation, for example by discriminating between flat and warped geometries. In this paper, we focus on a Kaluza–Klein model with an extra spatial dimension that compactifies at the Grand Unified Theory (GUT) epoch, subject to mixed Neumann/Dirichlet boundary conditions at fixed points. As a consequence, a set of Infrared (IR) cutoffs emerges in both the scalar and tensor spectra, leading to observable consequences in the CMB. We examine the possible signatures of such a topology in detail, particularly in relation to the even–odd parity imbalance already reported by the COBE, WMAP and Planck missions in the temperature angular correlations. Furthermore, we extend our analysis to the existing Planck E-mode polarization data and to the high-precision B-mode polarization measurements expected from the forthcoming LiteBIRD mission. Full article

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

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17 pages, 531 KB

Open AccessArticle

Black Hole Solution Free of Ghosts in f(R) Gravity Coupled with Two Scalar Fields

by G. G. L. Nashed and A. Eid

Universe 2025, 11(9), 305; https://doi.org/10.3390/universe11090305 - 9 Sep 2025

Viewed by 176

Abstract

One extension of general relativity, known as

f (R)

gravity, where

R

denotes the Ricci scalar, is regarded as a promising candidate for addressing the anomalies observed in conventional general relativity. In this work, we apply the field equations of [...] Read more.

One extension of general relativity, known as

f (R)

gravity, where

R

denotes the Ricci scalar, is regarded as a promising candidate for addressing the anomalies observed in conventional general relativity. In this work, we apply the field equations of

f (R)

gravity to a spacetime with spherical symmetry with distinct metric potentials, i.e.,

g_{t t} \neq g_{r r}

. By solving the resulting nonlinear differential equations, we derive a novel black hole solution without imposing constraints on the Ricci scalar or on the specific form of

f (R)

gravity. This solution does not reduce to the Schwarzschild solution of Einstein’s general relativity. This solution is notable because it includes a gravitational mass and extra terms that make the singularities in the curvature stronger than those in black holes from Einstein’s general relativity. We analyze these black holes within the framework of thermodynamics and demonstrate their consistency with standard thermodynamic quantities. Furthermore, we investigate the stability by examining odd-type perturbation modes and show that the resulting black hole is stable. Finally, we derive the coefficients of the two scalar fields and demonstrate that the black hole obtained in this study is free from ghosts. 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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24 pages, 10348 KB

Open AccessReview

The Variable Sky Through the OGLE Eye

by Patryk Iwanek

Universe 2025, 11(9), 304; https://doi.org/10.3390/universe11090304 - 8 Sep 2025

Viewed by 251

Abstract

The Optical Gravitational Lensing Experiment (OGLE) is one of the most productive and influential photometric sky surveys in the history of observational astronomy. Originally designed to detect dark matter through gravitational microlensing events, OGLE has evolved into a cornerstone of time-domain astrophysics, delivering [...] Read more.

The Optical Gravitational Lensing Experiment (OGLE) is one of the most productive and influential photometric sky surveys in the history of observational astronomy. Originally designed to detect dark matter through gravitational microlensing events, OGLE has evolved into a cornerstone of time-domain astrophysics, delivering three decades of two-band, high-cadence observations of approximately two billion stars across the Galactic bulge, disk, and Magellanic System. This review summarizes OGLE’s key contributions to variable star research, including the discovery, classification and characterization of pulsating stars, eclipsing, ellipsoidal, and rotating variables, or irregular and eruptive stars. Particular emphasis is placed on the OGLE Collection of Variable Stars (OCVS), a publicly available and systematically expanded dataset that has become a fundamental resource for studies of stellar variability and evolution, Milky Way and other galaxies structure, microlensing, compact objects, exoplanets and more. The synergy between OGLE and other major sky surveys, including ASAS, ASAS-SN, ATLAS, Gaia, KMTNet, MACHO, MOA, TESS, PLATO, or ZTF further amplifies its scientific reach. Full article

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

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19 pages, 1748 KB

Open AccessArticle

On the True Significance of the Hubble Tension: A Bayesian Error Decomposition Accounting for Information Loss

by Nathalia M. N. da Rocha, Andre L. B. Ribeiro and Francisco B. S. Oliveira

Universe 2025, 11(9), 303; https://doi.org/10.3390/universe11090303 - 6 Sep 2025

Viewed by 226

Abstract

The Hubble tension, a persistent discrepancy between early and late Universe measurements of

H_{0}

, poses a significant challenge to the standard cosmological model. In this work, we present a new Bayesian hierarchical framework designed to meticulously decompose this observed tension into [...] Read more.

The Hubble tension, a persistent discrepancy between early and late Universe measurements of

H_{0}

, poses a significant challenge to the standard cosmological model. In this work, we present a new Bayesian hierarchical framework designed to meticulously decompose this observed tension into its constituent parts: standard measurement errors, information loss arising from parameter-space projection, and genuine physical tension. Our approach, employing Fisher matrix analysis with MCMC-estimated loss coefficients and explicitly modeling information loss via variance inflation factors (

λ

), is particularly important in high-precision analysis where even seemingly small information losses can impact conclusions. We find that the real tension component (

T_{r e a l}

) has a mean value of 5.94 km/s/Mpc (95% CI: [3.32, 8.64] km/s/Mpc). Quantitatively, approximately 78% of the observed tension variance is attributed to real tension, 13% to measurement error, and 9% to information loss. Despite this, our decomposition indicates that the observed ∼

6.39 σ

discrepancy is predominantly a real physical phenomenon, with real tension contributing ∼

5.64 σ

. Our findings strongly suggest that the Hubble tension is robust and probably points toward new physics beyond the ΛCDM model. Full article

(This article belongs to the Special Issue Tensions, Anomalies, and Challenges in the Standard Cosmological Model)

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42 pages, 17899 KB

Open AccessArticle

A Systematic Search for New δ Scuti and γ Doradus Stars Using TESS Data

by Ai-Ying Zhou

Universe 2025, 11(9), 302; https://doi.org/10.3390/universe11090302 - 5 Sep 2025

Viewed by 170

Abstract

Focusing on the discovery of new

δ

Scuti and

γ

Doradus stars, we analyzed the Transiting Exoplanet Survey Satellite (TESS) light curves for 193,940 A-F stars selected from four legacy catalogs—the Henry Draper Catalogue (HD), the Smithsonian Astrophysical Observatory (SAO) Star [...] Read more.

Focusing on the discovery of new

δ

Scuti and

γ

Doradus stars, we analyzed the Transiting Exoplanet Survey Satellite (TESS) light curves for 193,940 A-F stars selected from four legacy catalogs—the Henry Draper Catalogue (HD), the Smithsonian Astrophysical Observatory (SAO) Star Catalog, the Positions and Proper Motions Catalog (PPM), and the Bonner Durchmusterung (BD, including its extensions). Through visual inspection of light curve morphologies and periodograms, combined with evaluation of stellar parameters, we identified over 51,850 previously unreported variable stars. These include 15,380

δ

Scuti, 18,560

γ

Doradus, 28 RR Lyrae stars, 260 heartbeat candidates, and 2645 eclipsing binaries, along with thousands of other variable types. Notably, over 4145 variables exhibit hybrid

δ

Scuti-

γ

Doradus pulsations, and more than 380 eclipsing binaries feature pulsating primary components. This study reveals a substantial population of bright, previously undetected variables, providing a valuable resource for ensemble asteroseismology, binary evolution studies, and Galactic structure research. Our results also highlight the surprising richness in variability still hidden within well-known stellar catalogs and the continued importance of high-precision, time-domain surveys such as TESS. Full article

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

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75 pages, 17108 KB

Open AccessArticle

A Catalog of 73 B-Type Stars and Their Brightness Variation from K2 Campaign 13–18

by Bergerson V. H. V. da Silva, Jéssica M. Eidam, Alan W. Pereira, M. Cristina Rabello-Soares, Eduardo Janot-Pacheco, Laerte Andrade and Marcelo Emilio

Universe 2025, 11(9), 301; https://doi.org/10.3390/universe11090301 - 3 Sep 2025

Viewed by 290

Abstract

The variability of B-type stars offers valuable insights into the interiors of stars and the processes that drive pulsation and rotation in massive stars. In this study, we present the classification of the variability of 197 B-type stars observed in various Kepler/K2 [...] Read more.

The variability of B-type stars offers valuable insights into the interiors of stars and the processes that drive pulsation and rotation in massive stars. In this study, we present the classification of the variability of 197 B-type stars observed in various Kepler/K2 campaigns, including 73 newly classified stars from Campaigns 13–18. For these stars, we derived atmospheric and evolutionary parameters using space-based photometry and ground-based spectroscopy. We obtained spectroscopic data for 34 targets with high-resolution instruments at OPD/LNA, which were supplemented by archival LAMOST spectra. After correcting for instrumental systematics, we analyzed the light curves using Fourier transforms and wavelet decomposition to identify both periodic and stochastic signals. The identified variability types included SPB stars,

β

Cephei/SPB hybrids, fast-rotating pulsators, stochastic low-frequency variables, eclipsing binaries, and rotational variables. We also revised classifications of misidentified stars using Gaia astrometry, confirming the main-sequence nature of objects once considered subdwarfs. Our results indicate that hot-star variability exists along a continuum shaped by mass, rotation, and internal mixing rather than distinct instability domains. This study enhances our understanding of B-type star variability and supports future asteroseismic modeling with missions like PLATO. Full article

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

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13 pages, 324 KB

Open AccessArticle

Gauge Symmetry and Radiatively Induced Terms in Dimension-5 Non-Minimal Lorentz-Violating QED

by A. P. B. Scarpelli and A. R. Vieira

Universe 2025, 11(9), 300; https://doi.org/10.3390/universe11090300 - 3 Sep 2025

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Abstract

In this work, we derive the conditions that ensure gauge invariance of non-minimal dimension-5 Lorentz-violating QED. The two- and three-point functions at one loop are computed. The gauge Ward identities are checked, as well as the conditions, to ensure that the gauge symmetry [...] Read more.

In this work, we derive the conditions that ensure gauge invariance of non-minimal dimension-5 Lorentz-violating QED. The two- and three-point functions at one loop are computed. The gauge Ward identities are checked, as well as the conditions, to ensure that the gauge symmetry of this non-minimal framework is found to be the same of the usual QED. Induced terms are also investigated, and it is shown that the non-minimal Lorentz-violating

a_{F}^{(5)}

-term of the fermion sector can radiatively induce a non-minimal Lorentz-violating term in the photon sector. Full article

(This article belongs to the Section Field Theory)

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4 pages, 154 KB

Open AccessEditorial

Editorial for the Special Issue “New Discoveries in Astronomical Data”

by Yanxia Zhang and A-Li Luo

Universe 2025, 11(9), 299; https://doi.org/10.3390/universe11090299 - 3 Sep 2025

Viewed by 243

Abstract

Over the past decade, astronomy has shifted from a data-starved to a data-drenched science [...] Full article

(This article belongs to the Special Issue New Discoveries in Astronomical Data)

1 pages, 159 KB

Open AccessCorrection

Correction: Kuić et al. Finite Time Path Field Theory and a New Type of Universal Quantum Spin Chain Quench Behavior. Universe 2025, 11, 230

by Domagoj Kuić, Alemka Knapp and Diana Šaponja-Milutinović

Universe 2025, 11(9), 298; https://doi.org/10.3390/universe11090298 - 2 Sep 2025

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Abstract

We would like to report an error in the published paper [...] Full article

9 pages, 1664 KB

Open AccessArticle

Quantized Nuclear Recoil in the Search for Sterile Neutrinos in Tritium Beta Decay with PTOLEMY

by Wonyong Chung, Mark Farino, Andi Tan, Christopher G. Tully and Shiran Zhang

Universe 2025, 11(9), 297; https://doi.org/10.3390/universe11090297 - 2 Sep 2025

Viewed by 355

Abstract

The search for keV-scale sterile neutrinos in tritium beta decay is made possible through the theoretically allowed small admixture of electron flavor in right-handed, singlet, massive neutrino states. A distinctive feature of keV-scale sterile-neutrino–induced threshold distortions in the tritium beta spectrum is the [...] Read more.

The search for keV-scale sterile neutrinos in tritium beta decay is made possible through the theoretically allowed small admixture of electron flavor in right-handed, singlet, massive neutrino states. A distinctive feature of keV-scale sterile-neutrino–induced threshold distortions in the tritium beta spectrum is the presence of quantized nuclear-recoil effects, as predicted for atomic tritium bound to two-dimension materials such as graphene. The sensitivities to the sterile neutrino mass and electron-flavor mixing are considered in the context of the PTOLEMY detector simulation with tritiated graphene substrates. The ability to scan the entire tritium energy spectrum with a narrow energy window, low backgrounds, and high-resolution differential energy measurements provides the opportunity to pinpoint the quantized nuclear-recoil effects. providing an additional tool for identifying the kinematics of the production of sterile neutrinos. Background suppression is achieved by transversely accelerating electrons into a high magnetic field, where semi-relativistic electron tagging can be performed with cyclotron resonance emission RF antennas followed by deceleration through the PTOLEMY filter into a high-resolution differential energy detector operating in a zero-magnetic-field region. The PTOLEMY-based approach to keV-scale searches for sterile neutrinos involves a novel precision apparatus utilizing two-dimensional materials to yield high-resolution, sub-eV mass determination for electron-flavor mixing fractions of

| U_{e 4} |^{2} ≃ 10^{- 5}

and smaller. Full article

(This article belongs to the Special Issue keV Warm Dark Matter (ΛWDM) in Agreement with Observations in Tribute to Héctor J. de Vega (2nd Edition))

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10 pages, 21975 KB

Open AccessArticle

A Comparison Study of Collisions at Relativistic Energies Involving Light Nuclei

by Hai-Cheng Wang, Song-Jie Li, Jun Xu and Zhong-Zhou Ren

Universe 2025, 11(9), 296; https://doi.org/10.3390/universe11090296 - 1 Sep 2025

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Abstract

We present extensive comparisons of ¹⁶O+¹⁶O collisions at a center-of-mass energy per nucleon pair

\sqrt{s_{N N}} = 200

GeV and ²⁰⁸Pb+¹⁶O collisions at

\sqrt{s_{N N}} = 68.5

GeV as well as ²⁰Ne+²⁰Ne [...] Read more.

We present extensive comparisons of ¹⁶O+¹⁶O collisions at a center-of-mass energy per nucleon pair

\sqrt{s_{N N}} = 200

GeV and ²⁰⁸Pb+¹⁶O collisions at

\sqrt{s_{N N}} = 68.5

GeV as well as ²⁰Ne+²⁰Ne collisions at

\sqrt{s_{N N}} = 200

GeV and ²⁰⁸Pb+²⁰Ne collisions at

\sqrt{s_{N N}} = 68.5

GeV based on a multiphase transport (AMPT) model. We recommend measuring the ratio of the elliptic flow to the triangular flow, which shows appreciable sensitivity to the structure of light nuclei, as also found in other studies. This is especially so if the observable is measured near the target rapidity in ²⁰⁸Pb+¹⁶O or ²⁰⁸Pb+²⁰Ne collisions, as originally found in the present study. Our study serves as a useful reference for understanding the effect of structure on observables in collisions involving light nuclei under analysis or on the schedule. Full article

(This article belongs to the Special Issue Relativistic Heavy-Ion Collisions: Theory and Observation)

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20 pages, 16141 KB

Open AccessArticle

Low-Latitude Ionospheric Anomalies During Geomagnetic Storm on 10–12 October 2024

by Plamen Mukhtarov and Rumiana Bojilova

Universe 2025, 11(9), 295; https://doi.org/10.3390/universe11090295 - 1 Sep 2025

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Abstract

This research examines in detail the behavior of the Equatorial Ionization Anomaly (EIA) during a severe geomagnetic storm that occurred on 10–11 October 2024. The global data of Total Electron Content (TEC) represented by relative deviation, giving information about the variations compared to [...] Read more.

This research examines in detail the behavior of the Equatorial Ionization Anomaly (EIA) during a severe geomagnetic storm that occurred on 10–11 October 2024. The global data of Total Electron Content (TEC) represented by relative deviation, giving information about the variations compared to quiet conditions, were used. The main attention is paid to the appearance of an additional “fountain effect” under the action of disturbed dynamo currents and the vertical drift of the ionospheric plasma caused by them. The results show that the area in which a positive response (increase) of TEC is observed occurs in an area corresponding to local time around 18–20 h (longitude around 60 °W) at magnetic latitudes ±30° and during the storm shifts westward to around 180 °W. The westward drift of the storm-induced “fountain effect” is moving at a speed much slower than the Earth’s rotation speed. As a result, the area of positive TEC response (vertical upward drift) and the area of negative response (vertical downward drift) are localized in both nighttime and daytime conditions. In this investigation, an example of a very similar geomagnetic storm registered on 25 September 1998 is given for comparison, in which a similar stationing of the storm-induced EIA was observed at longitudes around 180 °E. Full article

(This article belongs to the Section Space Science)

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11 pages, 711 KB

Open AccessCommunication

What Do Radio Emission Constraints Tell Us About Little Red Dots as Tidal Disruption Events?

by Krisztina Perger, Judit Fogasy and Sándor Frey

Universe 2025, 11(9), 294; https://doi.org/10.3390/universe11090294 - 1 Sep 2025

Viewed by 348

Abstract

The real nature of little red dots (LRDs), a class of very compact galaxies in the early Universe recently discovered by the James Webb Space Telescope, is still poorly understood. The most popular theories competing to interpret the phenomena include active galactic nuclei [...] Read more.

The real nature of little red dots (LRDs), a class of very compact galaxies in the early Universe recently discovered by the James Webb Space Telescope, is still poorly understood. The most popular theories competing to interpret the phenomena include active galactic nuclei and enhanced star formation in dusty galaxies. To date, however, neither model gives a completely satisfactory explanation to the population as a whole; thus, alternative theories have arisen, including tidal disruption events (TDEs). By considering observational constraints on the radio emission of LRDs, we discuss whether TDEs are adequate alternatives solving these high-redshift enigmas. We utilise radio flux density upper limits from LRD stacking analyses, TDE peak radio luminosities, and volumetric density estimates. We find that the characteristic values of flux densities and luminosities allow radio-quiet TDEs as the underlying process of LRDs in any case, while the less common radio-loud TDEs are compatible with the model under special constraints only. Considering other factors, such as volumetric density estimates, delayed and long-term radio flares of TDEs, and cosmological time dilation, TDEs appear to be a plausible explanation for LRDs from the radio point of view. Full article

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

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11 pages, 407 KB

Open AccessCommunication

On Restrictions of Current Warp Drive Spacetimes and Immediate Possibilities of Improvement

by Hamed Barzegar and Thomas Buchert

Universe 2025, 11(9), 293; https://doi.org/10.3390/universe11090293 - 1 Sep 2025

Viewed by 323

Abstract

Looking at current proposals of so-called ‘warp drive spacetimes’, they appear to employ General Relativity only at an elementary level. A number of strong restrictions are imposed such as flow-orthogonality of the spacetime foliation, vanishing spatial Ricci tensor, and dimensionally reduced and coordinate-dependent [...] Read more.

Looking at current proposals of so-called ‘warp drive spacetimes’, they appear to employ General Relativity only at an elementary level. A number of strong restrictions are imposed such as flow-orthogonality of the spacetime foliation, vanishing spatial Ricci tensor, and dimensionally reduced and coordinate-dependent velocity fields, to mention the main restrictions. We here provide a brief summary of our proposal of a general and covariant description of spatial motions within General Relativity, then discuss the restrictions that are employed in the majority of the current literature. That current warp drive models are discussed to be unphysical may not be surprising; they lack important ingredients such as covariantly non-vanishing spatial velocity, acceleration, vorticity together with curved space, and a warp mechanism. Full article

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

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15 pages, 425 KB

Open AccessArticle

Application of the Magnetar Engine to an Intermediate-Luminosity Gamma-Ray Burst Associated with the Supernova GRB 201015A/SN 201015A

by Xingling Li, Defeng Kong, Liangjun Chen, Xianggao Wang and Enwei Liang

Universe 2025, 11(9), 292; https://doi.org/10.3390/universe11090292 - 31 Aug 2025

Viewed by 300

Abstract

We present optical photometry for the afterglow of GRB 201015A, which can be classified as a medium-luminosity gamma-ray burst (

L_{γ, iso} \approx 2.55 \times 10^{49} erg s^{- 1}

) and the associated underlying supernova SN 201015A. A millisecond [...] Read more.

We present optical photometry for the afterglow of GRB 201015A, which can be classified as a medium-luminosity gamma-ray burst (

L_{γ, iso} \approx 2.55 \times 10^{49} erg s^{- 1}

) and the associated underlying supernova SN 201015A. A millisecond magnetar engine has been widely suggested to exist in gamma-ray burst (GRB) phenomena. In this paper, we study the effects of the magnetar engine on GRB 201015A/SN 201015A by light curve analysis. We use a smooth broken power-law plus magnetar spin-down model to fit the X-ray and optical light curves of GRB 201015A/SN 201015A. The best-fitting results reveal that the magnetar initial spin period and surface magnetic field at the pole are constrained to be

P_{0} = 16 . 80_{- 0.47}^{+ 0.24} ms

and

B_{p} = 0 . 80_{- 0.32}^{+ 0.34} \times 10^{15} G

, respectively, and the SN ejected a total mass of

M_{ej} = 2 . 55_{- 0.37}^{+ 1.12} M_{⊙}

and an ejecta velocity of

v_{ej}

=

{30,000}_{- 2500}^{+ 4800} km s^{- 1}

, inferring a kinetic energy of

E_{SN, K} \approx 1.37 \times 10^{52} erg

. From our analysis, we find that the central engine of GRB 201015A/SN 201015A may well be a magnetar, and the emission from a magnetar central engine can be solely responsible for powering SN 201015A. Full article

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9 pages, 406 KB

Open AccessArticle

A Proposed Test for the Gravitational Tunnel Effect

by Alfonso González Jiménez, Enderson Falcón Gómez, Isabel Carnoto Amat and Luis Enrique García Muñoz

Universe 2025, 11(9), 291; https://doi.org/10.3390/universe11090291 - 28 Aug 2025

Viewed by 330

Abstract

This article addresses the problem of the tunnel effect with a gravitational potential. Specifically, the quasiclassical formulation is used here, and the Wentzel–Kramers–Brillouin approximation is applied to the potential. This allows the problem to be solved for different configurations with different values (masses, [...] Read more.

This article addresses the problem of the tunnel effect with a gravitational potential. Specifically, the quasiclassical formulation is used here, and the Wentzel–Kramers–Brillouin approximation is applied to the potential. This allows the problem to be solved for different configurations with different values (masses, distances, etc.). The chosen values are intended to provide a clear comparison when varying the different parameters. Furthermore, feasible values are considered for practically replicating the experiment without resorting to astrophysical methods. Due to the low intensity of the gravitational force, these experiments will be difficult to replicate, but it is possible to improve some parameters at the expense of others. Full article

(This article belongs to the Topic Quantum Systems and Their Applications)

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16 pages, 820 KB

Open AccessArticle

Exploring the Impact of Self-Excited Alfvén Waves on Transonic Winds: Applications in Galactic Outflows

by Bilal Ramzan, Syed Nasrullah Ali Qazi and Chung-Ming Ko

Universe 2025, 11(9), 290; https://doi.org/10.3390/universe11090290 - 26 Aug 2025

Viewed by 389

Abstract

The impact of cosmic rays is crucial to understand the energetic plasma outflows coming out from the Galactic centers against the strong gravitational potential well. Cosmic rays can interact with thermal plasma via streaming instabilities and produce hydromagnetic waves/fluctuations. During the propagation of [...] Read more.

The impact of cosmic rays is crucial to understand the energetic plasma outflows coming out from the Galactic centers against the strong gravitational potential well. Cosmic rays can interact with thermal plasma via streaming instabilities and produce hydromagnetic waves/fluctuations. During the propagation of cosmic rays it can effectively diffuse and advect through the thermal plasma which results the excitation of Alfvén waves. We are treating thermal plasma, cosmic rays and self-excited Alfvén waves as fluids and our model is referred as multi-fluid model. We investigate steady-state transonic solutions for four-fluid systems (with forward as well as backward propagating self-excited Alfvén waves) with certain boundary conditions at the base of the potential well. As a reference model, a four-fluid model with cosmic-ray diffusion, wave damping and cooling can be studied together and solution topology can be analyzed with different set of boundary conditions available at the base of the gravitational potential well. We compare cases with enhancing the backward propagating self-excited Alfvén waves pressure and examining the shifting of the transonic point near or far away from the base. In conclusion we argue that the variation of the back-ward propagating self-excited Alfvén waves significantly alters the transonic solutions at the base. Full article

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

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