Universe

14 pages, 612 KiB

Open AccessArticle

Lower Dimensional Black Holes in Nonlinear Electrodynamics: Causal Structure and Scalar Perturbations

by Rodrigo Dal Bosco Fontana

Universe 2025, 11(6), 197; https://doi.org/10.3390/universe11060197 - 19 Jun 2025

Viewed by 223

We study the charged black-hole solutions of a 2 + 1 nonlinear electrodynamical theory with a cosmological constant. Considered as a one-parameter group of theories (the exponent of the squared Maxwell tensor), the causal structure of all possible black holes is scrutinized. We [...] Read more.

We study the charged black-hole solutions of a 2 + 1 nonlinear electrodynamical theory with a cosmological constant. Considered as a one-parameter group of theories (the exponent of the squared Maxwell tensor), the causal structure of all possible black holes is scrutinized. We analyze the singularity character that each theory delivers, together with their horizons and the plausible limitations in black-hole charges. The investigation demonstrates a rich structure of three different groups of theories according to the qualitative behavior of the singularity, horizons and limitations in the geometric charges. For such groups, we study the effect of a scalar field propagating in the spacetime of fixed black holes. All analyzed geometries are stable to such linear perturbations, evolving as usual quasinormal spectra of the black holes calculated for the different cases. Full article

(This article belongs to the Section Compact Objects)

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13 pages, 333 KiB

Open AccessArticle

Reframing Classical Mechanics: An AKSZ Sigma Model Perspective

by Thomas Basile, Nicolas Boulanger and Arghya Chattopadhyay

Universe 2025, 11(6), 196; https://doi.org/10.3390/universe11060196 - 19 Jun 2025

Viewed by 197

Abstract

The path-integral re-formulation due to E. Gozzi, M. Regini, M. Reuter, and W. D. Thacker of Koopman and von Neumann’s original operator formulation of a classical Hamiltonian system on a symplectic manifold M is identified as a gauge slice of a one-dimensional Alexandrov–Kontsevich–Schwarz–Zaboronsky [...] Read more.

The path-integral re-formulation due to E. Gozzi, M. Regini, M. Reuter, and W. D. Thacker of Koopman and von Neumann’s original operator formulation of a classical Hamiltonian system on a symplectic manifold M is identified as a gauge slice of a one-dimensional Alexandrov–Kontsevich–Schwarz–Zaboronsky sigma model with target

T^{*} (T [1] M \times R [1])

. Full article

(This article belongs to the Section Field Theory)

29 pages, 22860 KiB

Open AccessArticle

Laboratory Magnetoplasmas as Stellar-like Environment for ⁷Be β-Decay Investigations Within the PANDORA Project

by Eugenia Naselli, Bharat Mishra, Angelo Pidatella, Alessio Galatà, Giorgio S. Mauro, Domenico Santonocito, Giuseppe Torrisi and David Mascali

Universe 2025, 11(6), 195; https://doi.org/10.3390/universe11060195 - 18 Jun 2025

Viewed by 313

Abstract

Laboratory magnetoplasmas can become an intriguing experimental environment for fundamental studies relevant to nuclear astrophysics processes. Theoretical predictions indicate that the ionization state of isotopes within the plasma can significantly alter their lifetimes, potentially due to nuclear and atomic mechanisms such as bound-state [...] Read more.

Laboratory magnetoplasmas can become an intriguing experimental environment for fundamental studies relevant to nuclear astrophysics processes. Theoretical predictions indicate that the ionization state of isotopes within the plasma can significantly alter their lifetimes, potentially due to nuclear and atomic mechanisms such as bound-state β-decay. However, only limited experimental evidence on this phenomenon has been collected. PANDORA (Plasmas for Astrophysics, Nuclear Decay Observations, and Radiation for Archaeometry) is a novel facility which proposes to investigate nuclear decays in high-energy-density plasmas mimicking some properties of stellar nucleosynthesis sites (Big Bang Nucleosynthesis, s-process nucleosynthesis, role of CosmoChronometers, etc.). This paper focuses on the case of ⁷Be electron capture (EC) decay into ⁷Li, since its in-plasma decay rate has garnered considerable attention, particularly concerning the unresolved Cosmological Lithium Problem and solar neutrino physics. Numerical simulations were conducted to assess the feasibility of this possible lifetime measurement in the plasma of PANDORA. Both the ionization and atomic excitation of the ⁷Be isotopes in a He buffer Electron Cyclotron Resonance (ECR) plasma within PANDORA were explored via numerical modelling in a kind of “virtual experiment” providing the expected in-plasma EC decay rate. Since the decay of ⁷Be provides γ-rays at 477.6 keV from the ⁷Li excited state, Monte-Carlo GEANT4 simulations were performed to determine the γ-detection efficiency by the HPGe detectors array of the PANDORA setup. Finally, the sensitivity of the measurement was evaluated through a virtual experimental run, starting from the simulated plasma-dependent γ-rate maps. These results indicate that laboratory ECR plasmas in compact traps provide suitable environments for β-decay studies of ⁷Be, with the estimated duration of experimental runs required to reach 3σ significance level being few hours, which prospectively makes PANDORA a powerful tool to investigate the decay rate under different thermodynamic conditions and related charge state distributions. Full article

(This article belongs to the Special Issue Recent Outcomes and Future Challenges in Nuclear Astrophysics)

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15 pages, 1152 KiB

Open AccessArticle

A Novel Logarithmic Approach to General Relativistic Hydrodynamics in Dynamical Spacetimes

by Mario Imbrogno, Rita Megale, Luca Del Zanna and Sergio Servidio

Universe 2025, 11(6), 194; https://doi.org/10.3390/universe11060194 - 18 Jun 2025

Viewed by 158

Abstract

We introduce a novel logarithmic approach within the Baumgarte–Shapiro–Shibata–Nakamura (BSSN) formalism for self-consistently solving the equations of general relativistic hydrodynamics (GRHD) in evolving curved spacetimes. This method employs a “3 + 1” decomposition of spacetime, complemented by the “1 + log” slicing condition [...] Read more.

We introduce a novel logarithmic approach within the Baumgarte–Shapiro–Shibata–Nakamura (BSSN) formalism for self-consistently solving the equations of general relativistic hydrodynamics (GRHD) in evolving curved spacetimes. This method employs a “3 + 1” decomposition of spacetime, complemented by the “1 + log” slicing condition and Gamma-driver shift conditions, which have been shown to improve numerical stability in spacetime evolution. A key innovation of our work is the logarithmic transformation applied to critical variables such as rest-mass density, energy density, and pressure, thus preserving physical positivity and mitigating numerical issues associated with extreme variations. Our formulation is fully compatible with advanced numerical techniques, including spectral methods and Fourier-based algorithms, and it is particularly suited for simulating highly nonlinear regimes in which gravitational fields play a significant role. This approach aims to provide a solid foundation for future numerical implementations and investigations of relativistic hydrodynamics, offering promising new perspectives for modeling complex astrophysical phenomena in strong gravitational fields, including matter evolution around compact objects like neutron stars and black holes, turbulent flows in the early universe, and the nonlinear evolution of cosmic structures. Full article

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

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18 pages, 823 KiB

Open AccessArticle

Charged Scalar Boson in Melvin Universe

by Leonardo G. Barbosa, Luis C. N. Santos, João V. Zamperlini, Franciele M. da Silva and Celso C. Barros, Jr.

Universe 2025, 11(6), 193; https://doi.org/10.3390/universe11060193 - 18 Jun 2025

Viewed by 230

Abstract

This work investigates the dynamics of a charged scalar boson in the Melvin universe by solving the Klein–Gordon equation with minimal coupling in both inertial and non-inertial frames. Non-inertial effects are introduced through a rotating reference frame, resulting in a modified spacetime geometry [...] Read more.

This work investigates the dynamics of a charged scalar boson in the Melvin universe by solving the Klein–Gordon equation with minimal coupling in both inertial and non-inertial frames. Non-inertial effects are introduced through a rotating reference frame, resulting in a modified spacetime geometry and the appearance of a critical radius that limits the radial domain of the field. Analytical solutions are obtained under appropriate approximations, and the corresponding energy spectra are derived. The results indicate that both the magnetic field and non-inertial effects modify the energy levels, with additional contributions depending on the coupling between the rotation parameter and the quantum numbers. A numerical analysis is also presented, illustrating the behavior of the solutions for two characteristic magnetic field scales: one that may be considered extreme, of the order of the ones proposed to be produced in heavy-ion collisions, and another near the Planck scale. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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14 pages, 1816 KiB

Open AccessArticle

On Optimally Selecting Candidate Detectors with High Predicted Radio Signals from Energetic Cosmic Ray-Induced Extensive Air Showers

by Tudor Alexandru Calafeteanu, Paula Gina Isar and Emil Ioan Slușanschi

Universe 2025, 11(6), 192; https://doi.org/10.3390/universe11060192 - 18 Jun 2025

Viewed by 210

Abstract

Monte Carlo simulations of induced extensive air showers (EASs) by ultra-high-energy cosmic rays are widely used in comparison with measured events at experiments to estimate the main cosmic ray characteristics, such as mass, energy, and arrival direction. However, these simulations are computationally expensive, [...] Read more.

Monte Carlo simulations of induced extensive air showers (EASs) by ultra-high-energy cosmic rays are widely used in comparison with measured events at experiments to estimate the main cosmic ray characteristics, such as mass, energy, and arrival direction. However, these simulations are computationally expensive, with running time scaling proportionally with the number of radio antennas included. The AugerPrime upgrade of the Pierre Auger Observatory will feature an array of 1660 radio antennas. As a result, simulating a single EAS using the full detector array will take weeks on a single CPU thread. To reduce the simulation time, detectors are commonly pre-selected based on their proximity to the shower core, using a selection ellipse based on the Cherenkov radiation footprint scaled by a fixed constant factor. While effective, this approach often includes many noisy antennas at high zenith angles, reducing computational efficiency. In this paper, we introduce an optimal method for selecting candidate detectors with high predicted signal-to-noise ratio for proton and iron primary cosmic rays, replacing the constant scaling factor with a function of the zenith angle. This approach significantly reduces simulation time—by more than 50% per CPU thread for the heaviest, most inclined showers—without compromising signal quality. Full article

(This article belongs to the Special Issue Ultra-High-Energy Cosmic Rays)

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18 pages, 963 KiB

Open AccessArticle

Accuracy of Analytic Potentials for Orbits of Satellites Around a Milky Way-like Galaxy: Comparison with N-Body Simulations

by Rubens E. G. Machado, Giovanni C. Tauil and Nicholas Schweder-Souza

Universe 2025, 11(6), 191; https://doi.org/10.3390/universe11060191 - 17 Jun 2025

Viewed by 244

Abstract

To study the orbits of satellites, a galaxy can be modeled either by means of a static gravitational potential or by live N-body particles. Analytic potentials allow for fast calculations but are idealized and non-responsive. On the other hand, N-body simulations [...] Read more.

To study the orbits of satellites, a galaxy can be modeled either by means of a static gravitational potential or by live N-body particles. Analytic potentials allow for fast calculations but are idealized and non-responsive. On the other hand, N-body simulations are more realistic but demand higher computational cost. Our goal is to characterize the regimes in which analytic potentials provide a sufficient approximation and those where N-bodies are necessary. We perform two sets of simulations, using both Gala and Gadget, in order to closely compare the orbital evolution of satellites around a Milky Way-like galaxy. Focusing on the periods when the satellite has not yet been severely disrupted by tidal forces, we find that the orbits of satellites up to

10^{8} M_{⊙}

can be reliably computed with analytic potentials to within 5% error if they are circular or moderately eccentric. If the satellite is as massive as

10^{9} M_{⊙}

then errors of 9% are to be expected. However, if the orbital radius is smaller than 30 kpc then the results may not be relied upon with the same accuracy beyond 1–2 Gyr. Full article

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Graphical abstract

9 pages, 235 KiB

Open AccessArticle

Gravitational Redshift as a Measure of Rapid Mass Increase

by David L. Berkahn, James M. Chappell and Derek Abbott

Universe 2025, 11(6), 190; https://doi.org/10.3390/universe11060190 - 14 Jun 2025

Viewed by 275

Abstract

We begin by reviewing the special relativistic properties of a rotating system of coordinates. These were considered by Einstein in the spinning disk thought experiment during his initial considerations of time and length changes in gravity. Using a novel extension of this approach, [...] Read more.

We begin by reviewing the special relativistic properties of a rotating system of coordinates. These were considered by Einstein in the spinning disk thought experiment during his initial considerations of time and length changes in gravity. Using a novel extension of this approach, we identify a variation in an object’s internal energy within a gravitational field, through employing the equivalence principle. We then find an additional gravitational lensing and redshift prediction over and above current theory from the Schwarzschild metric. Implications for rapid clumping in the early universe and ultramassive blackhole formation are also considered. Correlations to recent James Webb findings are also discussed. Experiments to test this principle in the terrestrial domain are also proposed. 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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20 pages, 1555 KiB

Open AccessArticle

Nethotrons: Exploring the Possibility of Measuring Relativistic Spin Precessions, from Earth’s Satellites to the Galactic Centre

by Lorenzo Iorio

Universe 2025, 11(6), 189; https://doi.org/10.3390/universe11060189 - 11 Jun 2025

Viewed by 273

Abstract

By “nethotron”, from the ancient Greek verb for “to spin”, it is meant here a natural or artificial rotating object, like a pulsar or an artificial satellite, whose rotational axis is cumulatively displaced by the post-Newtonian static (gravitoelectric) and stationary (gravitomagnetic) components of [...] Read more.

By “nethotron”, from the ancient Greek verb for “to spin”, it is meant here a natural or artificial rotating object, like a pulsar or an artificial satellite, whose rotational axis is cumulatively displaced by the post-Newtonian static (gravitoelectric) and stationary (gravitomagnetic) components of the gravitational field of some massive body around which it freely moves. Until now, both relativistic effects have been measured only by the dedicated space-based mission Gravity Probe B in the terrestrial environment. It detected the gravitoelectric de Sitter and gravitomagnetic Pugh–Schiff spin precessions of four superconducting gyroscopes accumulated within a year after about 50 years from conception to completion of data analysis at a cost of 750 million US dollars to

0.3

and 19 percent accuracy, respectively. The perspectives to measure them with Earth’s long-lived laser-ranged geodetic satellites, like those of the LAGEOS family or possibly one or more of them to be built specifically from scratch, and pulsars orbiting the supermassive black hole in the Galactic Centre, yet to be discovered, are preliminarily investigated. The double pulsar PSR J0737-3039A/B is examined as well. Full article

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

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12 pages, 261 KiB

Open AccessReview

Supernovae, by Chandra and XMM-Newton

by Eric M. Schlegel

Universe 2025, 11(6), 188; https://doi.org/10.3390/universe11060188 - 11 Jun 2025

Viewed by 674

Abstract

X-ray emission from supernovae can arise from multiple interactions during their evolution. The immediate explosion is sufficiently energetic to generate X-rays; so, too, is the impact of the shock as it runs into circumstellar matter from earlier mass loss phases. A considerable range [...] Read more.

X-ray emission from supernovae can arise from multiple interactions during their evolution. The immediate explosion is sufficiently energetic to generate X-rays; so, too, is the impact of the shock as it runs into circumstellar matter from earlier mass loss phases. A considerable range of physics is on display during the evolution of such X-ray emission. This paper reviews some of the results of observing supernovae obtained by XMM-Newton and Chandra over the past 25 years. Each satellite has contributed significantly to the collection of observations and to our increased understanding of supernovae. Full article

(This article belongs to the Special Issue X-ray Astronomy Steps into the 21st Century: Chandra and XMM-Newton at 25)

31 pages, 926 KiB

Open AccessReview

A Comprehensive Guide to Interpretable AI-Powered Discoveries in Astronomy

by Maggie Lieu

Universe 2025, 11(6), 187; https://doi.org/10.3390/universe11060187 - 11 Jun 2025

Viewed by 1611

Abstract

The exponential growth of astronomical data necessitates the adoption of artificial intelligence (AI) and machine learning for timely and efficient scientific discovery. While AI techniques have achieved significant successes across diverse astronomical domains, their inherent complexity often obscures the reasoning behind their predictions, [...] Read more.

The exponential growth of astronomical data necessitates the adoption of artificial intelligence (AI) and machine learning for timely and efficient scientific discovery. While AI techniques have achieved significant successes across diverse astronomical domains, their inherent complexity often obscures the reasoning behind their predictions, hindering scientific trust and verification. This review addresses the crucial need for interpretability in AI-powered astronomy. We survey key applications where AI is making significant impacts and review the foundational concepts of transparency, interpretability, and explainability. A comprehensive overview of various interpretable machine learning methods is presented, detailing their mechanisms, applications in astronomy, and associated challenges. Given that no single method offers a complete understanding, we emphasize the importance of employing a suite of techniques to build robust interpretations. We argue that prioritizing interpretability is essential for validating results, guarding against biases, understanding model limitations, and ultimately enhancing the scientific value of AI in astronomy. Building trustworthy AI through explainable methods is fundamental to advancing our understanding of the universe. Full article

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

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15 pages, 3722 KiB

Open AccessArticle

Solar Astrometry in Rome at the End of the Maunder Minimum

by Costantino Sigismondi, Andrea Brucato and Giulia Andreasi Bassi

Universe 2025, 11(6), 186; https://doi.org/10.3390/universe11060186 - 10 Jun 2025

Viewed by 450

Abstract

With the great Clementine Gnomon in St. Maria degli Angeli, a 45 m pinhole meridian line, built in 1700–1702 upon the will of Pope Clemens XI, Francesco Bianchini inaugurated the Roman tradition of solar astrometry. We analyze two thousand dedicated observations at the [...] Read more.

With the great Clementine Gnomon in St. Maria degli Angeli, a 45 m pinhole meridian line, built in 1700–1702 upon the will of Pope Clemens XI, Francesco Bianchini inaugurated the Roman tradition of solar astrometry. We analyze two thousand dedicated observations at the Clementine Gnomon between 2018 and 2025, with solar altitudes from 20° to 71° and in various meteorological conditions, in order to assess the observational uncertainties on the solar diameter and their causes. We compare the meridian diameters measured by Bianchini near the winter solstices of 1701–1702 with the ones measured by Sigismondi in 2018–2025, reporting the observational errorbars per single measure and the systematic diminutions of the observed diameters with respect to the ephemerides, due to the turbulence and image contrast loss. Simulated datasets based on our measured uncertainties show that pinhole meridian lines cannot resolve solar diameter variations smaller than 1″ over 80 years. These limitations prevent tighter constraints on solar evolution across centuries using such instruments. Full article

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

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13 pages, 666 KiB

Open AccessArticle

Frozen Coherence in an Emergent Universe with Anisotropy

by Helder A. S. Costa and Paulo R. S. Carvalho

Universe 2025, 11(6), 185; https://doi.org/10.3390/universe11060185 - 8 Jun 2025

Viewed by 694

Abstract

We investigate the dynamics of quantum coherence in an anisotropically expanding emergent universe, modeled by a Bianchi type I spacetime. In particular, our findings suggest that the presence of small anisotropic perturbations introduces a directional dependence in the behavior of quantum coherence. Notably, [...] Read more.

We investigate the dynamics of quantum coherence in an anisotropically expanding emergent universe, modeled by a Bianchi type I spacetime. In particular, our findings suggest that the presence of small anisotropic perturbations introduces a directional dependence in the behavior of quantum coherence. Notably, we identify the emergence of frozen coherence regimes when the

{a_{0}, H_{0}, m, k}

parameters lie within particular ranges. The physical origin of these frozen regimes can be attributed to the suppression of mode mixing, which consequently leads to reduced particle creation. Full article

(This article belongs to the Section Cosmology)

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20 pages, 402 KiB

Open AccessArticle

Thermodynamics of Fluid Elements in the Context of Turbulent Isothermal Self-Gravitating Molecular Clouds

by Sava Donkov, Ivan Zh. Stefanov and Valentin Kopchev

Universe 2025, 11(6), 184; https://doi.org/10.3390/universe11060184 - 6 Jun 2025

Viewed by 484

Abstract

In the present work, we suggest a new approach for studying the equilibrium states of an hydrodynamic isothermal turbulent self-gravitating system as a statistical model for a molecular cloud. The main hypothesis is that the local turbulent motion of the fluid elements is [...] Read more.

In the present work, we suggest a new approach for studying the equilibrium states of an hydrodynamic isothermal turbulent self-gravitating system as a statistical model for a molecular cloud. The main hypothesis is that the local turbulent motion of the fluid elements is purely chaotic and can be regarded as a perfect gas. Then, the turbulent kinetic energy per fluid element can be substituted for the temperature of the chaotic motion of the fluid elements. Using this, we write down effective formulae for the internal and total the energy and for the first principal of thermodynamics. Then, we obtain expressions for the entropy, the free energy, and the Gibbs potential. Searching for equilibrium states, we explore two possible systems: the canonical ensemble and the grand canonical ensemble. Studying the former, we conclude that there is no extrema for the free energy. Through the latter system, we obtain a minimum of the Gibbs potential when the macro-temperature and pressure of the cloud are equal to those of the surrounding medium. This minimum corresponds to a possible stable local equilibrium state of our system. Full article

(This article belongs to the Section Galaxies and Clusters)

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9 pages, 453 KiB

Open AccessArticle

Constraints on Lorentz Invariance Violation from Gamma-Ray Burst Rest-Frame Spectral Lags Using Profile Likelihood

by Vyaas Ramakrishnan and Shantanu Desai

Universe 2025, 11(6), 183; https://doi.org/10.3390/universe11060183 - 6 Jun 2025

Cited by 1 | Viewed by 484

Abstract

We reanalyze the spectral lag data for 56 Gamma-Ray Bursts (GRBs) in the cosmological rest frame to search for Lorentz Invariance Violation (LIV) using frequentist inference. For this purpose, we use the technique of profile likelihood to deal with the nuisance parameters, corresponding [...] Read more.

We reanalyze the spectral lag data for 56 Gamma-Ray Bursts (GRBs) in the cosmological rest frame to search for Lorentz Invariance Violation (LIV) using frequentist inference. For this purpose, we use the technique of profile likelihood to deal with the nuisance parameters, corresponding to a constant time lag in the GRB rest frame and an unknown intrinsic scatter, while the parameter of interest is the energy scale for LIV (

E_{Q G}

). With this method, we do not obtain a global minimum for

χ^{2}

as a function of

E_{Q G}

up to the Planck scale. Thus, we can obtain one-sided lower limits on

E_{Q G}

in a seamless manner. Therefore, the 95% c.l. lower limits which we thus obtain on

E_{Q G}

are then given by

E_{Q G} \geq 2.07 \times 10^{14}

GeV and

E_{Q G} \geq 3.71 \times 10^{5}

GeV, for linear and quadratic LIV, respectively. Full article

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

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8 pages, 207 KiB

Open AccessEditorial

Editorial: Modified Theories of Gravity and Cosmological Applications—Topical Collection

by Panayiotis Stavrinos and Emmanuel N. Saridakis

Universe 2025, 11(6), 182; https://doi.org/10.3390/universe11060182 - 6 Jun 2025

Viewed by 714

Abstract

General relativity (GR) has been remarkably successful in describing the gravitational interaction through the curvature of spacetime [...] Full article

(This article belongs to the Collection Modified Theories of Gravity and Cosmological Applications)

7 pages, 462 KiB

Open AccessCommunication

Strength Ratios of Diffuse Interstellar Bands in Slightly Reddened Objects

by Jacek Krełowski and Arkadii Bondar

Universe 2025, 11(6), 181; https://doi.org/10.3390/universe11060181 - 6 Jun 2025

Viewed by 417

Abstract

The disk of the Milky Way fills the interstellar medium in the form of discrete clouds, many (∼30) light-years across. The average density of this medium is 1 hydrogen atom per cm³ (Oort limit), in the clouds—several dozen atoms, and between the [...] Read more.

The disk of the Milky Way fills the interstellar medium in the form of discrete clouds, many (∼30) light-years across. The average density of this medium is 1 hydrogen atom per cm³ (Oort limit), in the clouds—several dozen atoms, and between the clouds about 0.01 atoms per cm³. It is well documented that physical properties of individual interstellar clouds are evidently different using high-resolution spectroscopic observations of slightly reddened stars. We prove here that the 5780/5797 strength ratio is nearly constant for all slightly reddened targets. The reason for this phenomenon remains unknown. All optically thin clouds are apparently of σ-type. The question of at which value of color excess one may expect a ζ-type cloud remains unanswered. For some (unknown) reason ζ-type clouds are always relatively opaque and contain a lot of molecular species. In all slightly reddened objects we always observe σ-type intervening clouds, almost free of simple molecules. Full article

(This article belongs to the Section Galaxies and Clusters)

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12 pages, 2754 KiB

Open AccessArticle

μPPET: Investigating the Muon Puzzle with J-PET Detectors

by Alessio Porcelli, Kavya Valsan Eliyan, Gabriel Moskal, Nousaba Nasrin Protiti, Diana Laura Sirghi, Ermias Yitayew Beyene, Neha Chug, Catalina Curceanu, Eryk Czerwiński, Manish Das, Marek Gorgol, Jakub Hajduga, Sharareh Jalali, Bożena Jasińska, Krzysztof Kacprzak, Tevfik Kaplanoglu, Łukasz Kapłon, Kamila Kasperska, Aleksander Khreptak, Grzegorz Korcyl, Tomasz Kozik, Deepak Kumar, Karol Kubat, Edward Lisowski, Filip Lisowski, Justyna Mędrala-Sowa, Wiktor Mryka, Simbarashe Moyo, Szymon Niedźwiecki, Szymon Parzych, Piyush Pandey, Elena Perez del Rio, Bartłomiej Rachwał, Martin Rädler, Sushil Sharma, Magdalena Skurzok, Ewa Łucja Stȩpień, Tomasz Szumlak, Pooja Tanty, Keyvan Tayefi Ardebili, Satyam Tiwari and Paweł Moskal Show full author list Hide full author list

Universe 2025, 11(6), 180; https://doi.org/10.3390/universe11060180 - 2 Jun 2025

Viewed by 874

Abstract

The

μ

PPET [mu(

μ

)on Probe with J-PET] project aims to investigate the “Muon Puzzle” seen in cosmic ray air showers. This puzzle arises from the observation of a significantly larger number of muons on Earth’s surface than that predicted by the [...] Read more.

The

μ

PPET [mu(

μ

)on Probe with J-PET] project aims to investigate the “Muon Puzzle” seen in cosmic ray air showers. This puzzle arises from the observation of a significantly larger number of muons on Earth’s surface than that predicted by the current theoretical models. The investigated hypothesis is based on recently observed asymmetries in the parameters for the strong interaction cross-section and trajectory of an outgoing particle due to projectile–target polarization. The measurements require detailed information about muons at the ground level, including their track and charge distributions. To achieve this, the two PET scanners developed at the Jagiellonian University in Krakow (Poland), the J-PET detectors, will be employed, taking advantage of their well-known resolution and convenient location for detecting muons that reach long depths in the atmosphere. One station will be used as a muon tracker, while the second will reconstruct the core of the air shower. In parallel, the existing hadronic interaction models will be modified and fine-tuned based on the experimental results. In this work, we present the conceptualization and preliminary designs of

μ

PPET. Full article

(This article belongs to the Special Issue Ultra-High-Energy Cosmic Rays)

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24 pages, 1839 KiB

Open AccessArticle

Relic Gravitational Waves in the Noncommutative Foliated Riemannian Quantum Gravity

by César A. Zen Vasconcellos, Peter O. Hess, José A. de Freitas Pacheco, Fridolin Weber, Remo Ruffini, Dimiter Hadjimichef, Moisés Razeira, Benno August Ludwig Bodmann, Marcelo Netz-Marzola, Geovane Naysinger, Rodrigo Fraga da Silva and João G. G. Gimenez

Universe 2025, 11(6), 179; https://doi.org/10.3390/universe11060179 - 31 May 2025

Viewed by 845

Abstract

We present a study of relic gravitational waves based on a foliated gauge field theory defined over a spacetime endowed with a noncommutative algebraic–geometric structure. As an ontological extension of general relativity—concerning manifolds, metrics, and fiber bundles—the conventional space and time coordinates, typically [...] Read more.

We present a study of relic gravitational waves based on a foliated gauge field theory defined over a spacetime endowed with a noncommutative algebraic–geometric structure. As an ontological extension of general relativity—concerning manifolds, metrics, and fiber bundles—the conventional space and time coordinates, typically treated as classical numbers, are replaced by complementary quantum dual fields. Within this framework, consistent with the Bekenstein criterion and the Hawking–Hertog multiverse conception, singularities merge into a helix-like cosmic scale factor that encodes the topological transition between the contraction and expansion phases of the universe analytically continued into the complex plane. This scale factor captures the essence of an intricate topological quantum-leap transition between two phases of the branching universe: a contraction phase preceding the now-surpassed conventional concept of a primordial singularity and a subsequent expansion phase, whose transition region is characterized by a Riemannian topological foliated structure. The present linearized formulation, based on a slight gravitational field perturbation, also reveals a high sensitivity of relic gravitational wave amplitudes to the primordial matter and energy content during the universe’s phase transition. It further predicts stochastic homogeneous distributions of gravitational wave intensities arising from the interplay of short- and long-spacetime effects within the non-commutative algebraic framework. These results align with the anticipated future observations of relic gravitational waves, expected to pervade the universe as a stochastic, homogeneous background. Full article

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

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19 pages, 417 KiB

Open AccessArticle

Statistical Strong Lensing as a Test of Conformal Gravity

by Li-Xue Yue and Da-Ming Chen

Universe 2025, 11(6), 178; https://doi.org/10.3390/universe11060178 - 31 May 2025

Viewed by 751

Abstract

As an alternative gravitational theory to General Relativity (GR), Conformal Gravity (CG) can be verified through astronomical observations. Currently, Mannheim and Kazanas have provided vacuum solutions for cosmological and local gravitational systems, and these solutions may resolve the dark matter and dark energy [...] Read more.

As an alternative gravitational theory to General Relativity (GR), Conformal Gravity (CG) can be verified through astronomical observations. Currently, Mannheim and Kazanas have provided vacuum solutions for cosmological and local gravitational systems, and these solutions may resolve the dark matter and dark energy issues encountered in GR, making them particularly valuable. For static, spherically symmetric systems, CG predicts an additional linear potential generated by luminous matter in addition to the conventional Newtonian potential. This extra potential is expected to account for the observations of galaxies and galaxy clusters without the need of dark matter. It is characterized by the parameter

γ^{*}

, which corresponds to the linear potential generated by the unit of the solar mass, and it is thus a universal constant. The value of

γ^{*}

was determined by fitting the rotation curve data of spiral galaxies. These predictions of CG should also be verified by the observations of strong gravitational lensing. To date, in the existing literature, the observations of strong lensing employed to test CG have been limited to a few galaxy clusters. It has been found that the value of

γ^{*}

estimated from strong lensing is several orders of magnitude greater than that obtained from fitting rotation curves. In this study, building upon the previous research, we tested CG via strong lensing statistics. We used a well-defined sample that consisted of both galaxies and galaxy clusters. This allowed us to test CG through statistical strong lensing in a way similar to the conventional approach in GR. As anticipated, our results were consistent with previous studies, namely that the fitted

γ^{*}

is much larger than that from rotation curves. Intriguingly, we further discovered that, in order to fit the strong lensing data of another sample, the value of

γ^{*}

cannot be a constant, as is required in CG. Instead, we derived a formula for

γ^{*}

as a function of the stellar mass

M_{*}

of the galaxies or galaxy clusters. It was found that

γ^{*}

decreases as

M_{*}

increases. Full article

(This article belongs to the Section Gravitation)

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18 pages, 2382 KiB

Open AccessArticle

Bethe–Heitler Cascades and Hard Gamma-Ray Spectra in Flaring TeV Blazars: 1ES 0414009 and 1ES 1959650

by Samuel Victor Bernardo da Silva, Luiz Augusto Stuani Pereira and Rita de Cássia Dos Anjos

Universe 2025, 11(6), 177; https://doi.org/10.3390/universe11060177 - 31 May 2025

Viewed by 1376

Abstract

In this work, we present updated models of the spectral energy distributions (SEDs) for two high-frequency-peaked BL Lac objects (HBLs), that is, 1ES 0414+009 and 1ES 1959+650. The hard gamma-ray spectra observed during their flaring states suggest the presence of an additional emission [...] Read more.

In this work, we present updated models of the spectral energy distributions (SEDs) for two high-frequency-peaked BL Lac objects (HBLs), that is, 1ES 0414+009 and 1ES 1959+650. The hard gamma-ray spectra observed during their flaring states suggest the presence of an additional emission component beyond the standard synchrotron self-Compton (SSC) scenario. We explore the possibility that this hard gamma-ray emission arises from inverse Compton (IC) scattering by Bethe–Heitler pairs produced along the line of sight, pointing to a more complex high-energy emission mechanism in these sources. Full article

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

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27 pages, 3100 KiB

Open AccessArticle

Harmonizing Sunspot Datasets Consistency: Focusing on SOHO/MDI and SDO/HMI Data

by Barbara Góra-Gálik, Emese Forgács-Dajka and Istvan Ballai

Universe 2025, 11(6), 176; https://doi.org/10.3390/universe11060176 - 31 May 2025

Viewed by 1647

Abstract

To ensure the long-term consistency of sunspot group data, it is essential to harmonize measurements from SOHO/MDI and SDO/HMI, two major solar observatories with overlapping coverage. In our analysis, we use two complementary sets of data: SOHO/MDI–Debrecen Sunspot Data (SDD) and SDO/HMI–Debrecen Sunspot [...] Read more.

To ensure the long-term consistency of sunspot group data, it is essential to harmonize measurements from SOHO/MDI and SDO/HMI, two major solar observatories with overlapping coverage. In our analysis, we use two complementary sets of data: SOHO/MDI–Debrecen Sunspot Data (SDD) and SDO/HMI–Debrecen Sunspot Data (HMIDD). Our objective is to identify systematic differences between their recorded parameters and to assess whether their data can be combined into a coherent time series. While the overlap between the datasets spans only about one year, this period allows for a direct statistical comparison without the need for additional image processing. Though the two instruments do not measure identical area values, our results reveal a strong linear relationship between them, which is in line with earlier studies. On the other hand, a systematic discrepancy in their magnetic field strength measurements was observed. Contrary to previous findings, SDO/HMI magnetic field values tend to be higher than those from SOHO/MDI. These differences may arise from the use of different calibration procedures and measurement techniques, or from the physical characteristics of the sunspot groups themselves. These results highlight the challenges involved in unifying data from multiple solar instruments that have been captured over extended time periods. While broad consistencies are observable, the differences between sunspot groups and measurement parameters demonstrate the importance of using careful, instrument-aware calibration approaches when combining such datasets. Full article

(This article belongs to the Special Issue Solar and Stellar Activity: Exploring the Cosmic Nexus)

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24 pages, 541 KiB

Open AccessArticle

New Black Hole Solution in f(R) Theory and Its Related Physics

by G. G. L. Nashed and Ali Eid

Universe 2025, 11(6), 175; https://doi.org/10.3390/universe11060175 - 30 May 2025

Cited by 1 | Viewed by 1197

Abstract

Recent observations suggest that General Relativity (GR) faces challenges in fully explaining phenomena in regimes of strong gravitational fields. A promising alternative is the

f (R)

theory of gravity, where R denotes the Ricci scalar. This modified theory aims to address [...] Read more.

Recent observations suggest that General Relativity (GR) faces challenges in fully explaining phenomena in regimes of strong gravitational fields. A promising alternative is the

f (R)

theory of gravity, where R denotes the Ricci scalar. This modified theory aims to address the limitations observed in standard GR. In this study, we derive a black hole (BH) solution without introducing nonlinear electromagnetic fields or imposing specific constraints on R or the functional form of

f (R)

gravity. The BH solution obtained here is different from the classical Schwarzschild solution in GR and, under certain conditions, reduces to the Schwarzschild (A)dS solution. This BH is characterized by the gravitational mass of the system and an additional parameter, which distinguishes it from GR BHs, particularly in the asymptotic regime. We show that the curvature invariants of this solution remain well defined at both small and large values of r. Furthermore, we analyze their thermodynamic properties, demonstrating consistency with established principles such as Hawking radiation, entropy, and quasi-local energy. This analysis supports their viability as alternative models to classical GR BHs. 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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19 pages, 617 KiB

Open AccessArticle

Flare Set-Prediction Transformer: A Transformer-Based Set-Prediction Model for Detailed Solar Flare Forecasting

by Liang Qiao and Gang Qin

Universe 2025, 11(6), 174; https://doi.org/10.3390/universe11060174 - 29 May 2025

Viewed by 356

Abstract

Solar flare prediction models typically use classification, predicting only the probability of categorized events within a time window. This misses critical information, such as how many flares occur, their precise timings, and their intensities. To address this, we propose a paradigm shift to [...] Read more.

Solar flare prediction models typically use classification, predicting only the probability of categorized events within a time window. This misses critical information, such as how many flares occur, their precise timings, and their intensities. To address this, we propose a paradigm shift to set prediction, directly forecasting a variable-sized set of flare events with detailed characteristics. We demonstrate this approach with FSPT (Flare Set-Prediction Transformer), a transformer-based model adapted from object detection principles. FSPT predicts sets containing individual flare start, peak, and end time offsets, as well as peak X-ray intensity. This work presents the set-prediction framework and the FSPT model, showing its potential for more informative flare forecasting. 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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15 pages, 276 KiB

Open AccessArticle

Quantum Fields and the Cosmological Constant

by Renata Ferrero, Vincenzo Naso and Roberto Percacci

Universe 2025, 11(6), 173; https://doi.org/10.3390/universe11060173 - 28 May 2025

Viewed by 487

Abstract

It has been shown that if one solves self-consistently the semiclassical Einstein equations in the presence of a quantum scalar field, with a cutoff on the number of modes, spacetime become flatter when the cutoff increases. Here, we extend the result to include [...] Read more.

It has been shown that if one solves self-consistently the semiclassical Einstein equations in the presence of a quantum scalar field, with a cutoff on the number of modes, spacetime become flatter when the cutoff increases. Here, we extend the result to include the effect of fields with spin 0, 1/2, 1 and 2. With minor adjustments, the main result persists. Remarkably, one can have positive curvature even if the cosmological constant in the bare action is negative. Full article

(This article belongs to the Section Cosmology)

20 pages, 542 KiB

Open AccessArticle

Neutron Star Inner Crust at Finite Temperatures: A Comparison Between Compressible Liquid Drop and Extended Thomas–Fermi Approaches

by Guilherme Grams, Nikolai N. Shchechilin, Théau Diverrès, Anthea F. Fantina, Nicolas Chamel and Francesca Gulminelli

Universe 2025, 11(6), 172; https://doi.org/10.3390/universe11060172 - 27 May 2025

Viewed by 680

Abstract

We investigate the effects of temperature on the properties of the inner crust of a non-accreting neutron star. To this aim, we employ two different treatments: the compressible liquid drop model (CLDM) and the temperature-dependent extended Thomas–Fermi (TETF) method. Our systematic comparison shows [...] Read more.

We investigate the effects of temperature on the properties of the inner crust of a non-accreting neutron star. To this aim, we employ two different treatments: the compressible liquid drop model (CLDM) and the temperature-dependent extended Thomas–Fermi (TETF) method. Our systematic comparison shows an agreement between the two methods on their predictions for the crust thermodynamic properties. We find that the CLDM description can also reproduce reasonably well the TETF composition especially if the surface energy is optimized on the ETF calculation. However, the neglect of neutron skin in CLDM leads to an overestimation of the proton radii. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024 – Compact Objects)

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17 pages, 858 KiB

Open AccessArticle

Optical Photometric Monitoring of the Blazar OT 355 and Local Standard Stars’ Calibration

by R. Bachev, Tushar Tripathi, Alok C. Gupta, A. Kurtenkov, Y. Nikolov, A. Strigachev, S. Boeva, G. Latev, B. Spassov, M. Minev, E. Ovcharov, W.-X. Yang, Yi Liu and J.-H. Fan

Universe 2025, 11(6), 171; https://doi.org/10.3390/universe11060171 - 27 May 2025

Viewed by 563

Abstract

OT 355 (4FGL J1734.3 + 3858) is a relatively rarely studied but highly variable, moderate-redshift (z = 0.975) flat-spectrum radio quasar (blazar). With this work, we aim to study its optical variability on different timescales, which can help us to better understand the [...] Read more.

OT 355 (4FGL J1734.3 + 3858) is a relatively rarely studied but highly variable, moderate-redshift (z = 0.975) flat-spectrum radio quasar (blazar). With this work, we aim to study its optical variability on different timescales, which can help us to better understand the physical processes in relativistic jets operating in blazar-type active galactic nuclei. OT 355 was observed in four colors (BVRI) during 41 nights between 2017 and 2023 using three 1 and 2 m class telescopes. The object was also monitored on intra-night timescales, for about 100 h in total. In addition, secondary standard stars in the field of OT 355 were calibrated in order to facilitate future photometric studies. We detected significant intra-night and night-to-night variations of up to 0.5 mag. Variability characteristics, color changes, and a possible “rms-flux” relation were studied and discussed. Using simple arguments, we show that a negative “rms-flux” relation should be expected if many independent processes/regions drive the short-term variability via Doppler factor changes, which is not observed in this and other cases. This finding raises arguments for the idea that more complex multiplicative processes are responsible for blazar variability. Studying blazar variability, especially on the shortest possible timescales, can help to estimate the strength and geometry of their magnetic fields, the linear sizes of the emitting regions, and other aspects, which may be of importance for constraining and modeling blazars’ emitting mechanisms. Full article

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

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13 pages, 1902 KiB

Open AccessArticle

A Novel Mid-Infrared Narrowband Filter for Solar Telescopes

by Junfeng Hou

Universe 2025, 11(6), 170; https://doi.org/10.3390/universe11060170 - 27 May 2025

Viewed by 678

Abstract

The mid-infrared band is the last major observational window for the ground-based large solar telescopes in the 21st century. Achieving ultra-narrowband filter imaging is a fundamental challenge that all solar telescopes encounter as they progress towards the mid-infrared spectrum. The guided-mode resonance filtering [...] Read more.

The mid-infrared band is the last major observational window for the ground-based large solar telescopes in the 21st century. Achieving ultra-narrowband filter imaging is a fundamental challenge that all solar telescopes encounter as they progress towards the mid-infrared spectrum. The guided-mode resonance filtering (GMRF) technology provides a promising solution to this critical issue. This paper describes in detail the fundamental principles and calculation procedure of guided-mode resonance filtering. Building upon this foundation, a preliminary design and simulation of a mid-infrared guided-mode resonance filter are carried out. The results show that when the thickness of the sub-wavelength grating is an even multiple of the half-wavelength, it is feasible to attain ultra-narrowband filtering with a bandwidth below 0.03 nm by increasing the grating thickness and decreasing the grating fill factor. Nevertheless, the high sensitivity of the resonant wavelength to the angle of incidence still stands as a formidable obstacle that demands further investigation and resolution. 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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15 pages, 2361 KiB

Open AccessArticle

Estimation of Day-Time Seeing Changes at Huairou Solar Observing Station Based on Neural Networks from 1989 to 2010

by Xing Hu, Shangbin Yang, Tengfei Song, Xingming Bao, Wenjun Sun, Yuanyong Deng, Yu Liu and Mingyu Zhao

Universe 2025, 11(6), 169; https://doi.org/10.3390/universe11060169 - 27 May 2025

Viewed by 339

Abstract

Seeing is a key factor affecting the image quality of astronomical observations and can be quantitatively described by the Fried parameter

r_{0}

. The larger the

r_{0}

value (in unit of cm), the better the seeing conditions. Currently, daytime seeing measurements [...] Read more.

Seeing is a key factor affecting the image quality of astronomical observations and can be quantitatively described by the Fried parameter

r_{0}

. The larger the

r_{0}

value (in unit of cm), the better the seeing conditions. Currently, daytime seeing measurements are primarily conducted using the Solar Differential Image Motion Monitor (SDIMM) or the spectral ratio method. In this work, we propose a neural network model for estimating daytime

r_{0}

. The experimental results of the training set and the test set show that this model can currently estimate

r_{0}

with an accuracy exceeding

99 %

. Using this model, we estimate the

r_{0}

of the Huairou Solar Observing Station (HSOS) in 22 consecutive years from 1989 to 2010. The median

r_{0}

of HSOS in 22 consecutive years was around 2.5 cm, and the best seeing condition was in April and September of one year. This result confirmed the long-term stability of seeing conditions. In addition, we conducted an error analysis comparing the seeing measured by SDIMM and the results obtained by the spectral ratio method both under domeless and domed conditions. The results indicate a significant correlation between the SDIMM results and the spectral ratio method results, with first-order fitting coefficients of 2.2 and 2.9, respectively. 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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23 pages, 1090 KiB

Open AccessArticle

A Novel Search Technique for Low-Frequency Periodic Gravitational Waves

by Harshit Raj, Sanjeev Dhurandhar and Massimo Tinto

Universe 2025, 11(6), 168; https://doi.org/10.3390/universe11060168 - 24 May 2025

Viewed by 348

Abstract

We quantify the advantages of a recently proposed data processing technique to search for continuous gravitational wave (GW) signals from isolated rotating asymmetric neutron stars in data measured by ground-based GW interferometers. This technique relies on the symmetry of the motion around the [...] Read more.

We quantify the advantages of a recently proposed data processing technique to search for continuous gravitational wave (GW) signals from isolated rotating asymmetric neutron stars in data measured by ground-based GW interferometers. This technique relies on the symmetry of the motion around the Sun of an Earth-bound gravitational wave interferometer. By multiplying the measured data time series with a half-year time-shifted copy of it, we obtain two advantages: (i) the main Doppler phase modulation of a monochromatic gravitational wave signal is exactly removed, and (ii) the signal in the product data are located at twice the GW signal frequency. The first significantly reduces the size of the signal’s parameter space over which a search is to be performed. The second is advantageous at low frequencies; we find that, with currently available computer processing speeds, this technique is capable of achieving sensitivity that is comparable to or even better than coherent and other possibly non-coherent methods. Further, since our proposed method is implemented over a year-long data segment, it requires processing time comparable to the data acquisition time of currently available computers. Full article

(This article belongs to the Collection Gravitational Waves as a New Probe for Astronomy and Fundamental Physics)

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