Universe

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

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)

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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

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)

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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

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

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)

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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

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

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)

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18 pages, 469 KB

Open AccessTechnical Note

pbhstat: A Python Package for Calculating the Primordial Black Hole Abundance

by Philippa S. Cole and Jacopo Fumagalli

Universe 2025, 11(9), 322; https://doi.org/10.3390/universe11090322 - 22 Sep 2025

Abstract

We present pbhstat, a publicly available Python package designed to compute the mass function and total abundance of primordial black holes (PBHs) from a given primordial power spectrum. The package offers a modular framework using multiple statistical approaches, including Press–Schechter theory, peaks theory, [...] Read more.

We present pbhstat, a publicly available Python package designed to compute the mass function and total abundance of primordial black holes (PBHs) from a given primordial power spectrum. The package offers a modular framework using multiple statistical approaches, including Press–Schechter theory, peaks theory, and formalisms based on the non-linear compaction function. Currently, the implementation is limited to scenarios with nearly Gaussian initial conditions. Full article

(This article belongs to the Special Issue Primordial Black Holes from Inflation)

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14 pages, 508 KB

Open AccessArticle

Comparing the Observational Properties of Rotation-Powered Binary Millisecond Pulsars with Various Companion Types

by De-Hua Wang, Cheng-Min Zhang and Shuang-Qiang Wang

Universe 2025, 11(9), 321; https://doi.org/10.3390/universe11090321 - 19 Sep 2025

Abstract

We compare the observational properties of rotation-powered binary millisecond pulsars (BMSPs) in the Galactic Field with various companion types. First, BMSPs with diverse companion types exhibit different properties in the relation of binary orbital period versus companion mass, and in the spin period [...] Read more.

We compare the observational properties of rotation-powered binary millisecond pulsars (BMSPs) in the Galactic Field with various companion types. First, BMSPs with diverse companion types exhibit different properties in the relation of binary orbital period versus companion mass, and in the spin period distribution of neutron stars (NSs), etc., implying multiple origins of BMSPs. Second, BMSPs with companions of CO/ONeMg white dwarfs (CO-BMSPs) show fewer sources than those with companions of Helium white dwarfs (He-BMSPs), which may result from the different evolutionary histories or accretion efficiencies in their progenitors. Third, BMSPs with main-sequence companions (MS-BMSPs) and ultra-light companions or planets (UL-BMSPs) are mostly eclipsing sources that are detected in both radio and

γ

-ray bands (i.e., radio+

γ

sources), implying that they may be younger systems and share a faster average spin period and higher average accretion rate than CO-BMSPs/He-BMSPs. We propose that the predecessors of MS-BMSPs may share a short binary orbital distance with low-mass companion stars of

M_{c} \sim 0.5

–

0.8 M_{⊙}

, which induces an efficient binary accretion process, and ultimately leaves a BMSP with a main-sequence companion due to the low efficiency of its hydrogen burning. Lastly, radio+

γ

He-BMSPs share a faster average spin period of NSs than radio-only He-BMSPs. Meanwhile, these two groups of sources share similar companion mass distributions, implying the

γ

-ray evaporation effect may not obviously strip the companion mass of He-BMSPs during ∼0.3 Gyr, which may be due to the strong gravitational potential energy of the white dwarf companions. Full article

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19 pages, 649 KB

Open AccessArticle

Study on the Gamma-Ray Radiation Properties of High-Redshift Blazars at z > 2.5

by Fan Wu, Li Zhang and Benzhong Dai

Universe 2025, 11(9), 320; https://doi.org/10.3390/universe11090320 - 18 Sep 2025

Abstract

The high-redshift blazars are important cosmological probes for exploring the early universe and unraveling the fundamental emission processes and the structure of the active galactic nuclei. The high-energy GeV gamma-ray emissions of 38 high-redshift blazars (z > 2.5) observed by Fermi-LAT were analyzed. [...] Read more.

The high-redshift blazars are important cosmological probes for exploring the early universe and unraveling the fundamental emission processes and the structure of the active galactic nuclei. The high-energy GeV gamma-ray emissions of 38 high-redshift blazars (z > 2.5) observed by Fermi-LAT were analyzed. Along with the Archive multiwavelength data, we employ one-zone leptonic external Compton (EC) models to reproduce the spectral energy distributions (SEDs) of 38 sources. Both the external photons from the molecular torus (MT) and the broad-line region (BLR) are considered. We obtained the best-fitting parameters for describing the characteristics of the jets and accretion disks. The results indicate that high-redshift blazars exhibit higher

γ

-ray luminosities, energy densities, jet powers, kinetic powers, accretion disk luminosities, black hole (BH) masses, radiation efficiencies, and mass accretion rates compared to low-redshift blazars. For high-redshift blazars, the influence of the accretion rate on jet power appears to weaken, and in most cases, the jet power exceeds the total accretion power. We speculate that for high-redshift blazars, rapid accretion may lead to magnetic field saturation, thereby reducing the effectiveness of the Blandford–Payne (BP) process. Consequently, the Blandford–Znajek (BZ) process is likely to play a more dominant role in powering jets in high-redshift blazars compared to low-redshift blazars. Naturally, we acknowledge that selection effects cannot be fully eliminated. Full article

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

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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

Abstract

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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21 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

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

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

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

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

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

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

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

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

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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