Universe

24 pages, 365 KB

Open AccessArticle

Thermodynamic Blocking in Self-Gravitating Systems

by Barnabás Deme and Jean-Baptiste Fouvry

Universe 2025, 11(12), 419; https://doi.org/10.3390/universe11120419 - 18 Dec 2025

Viewed by 274

Building upon a thermodynamic formalism, we show that one-dimensional self-gravitating systems in hydrostatic equilibrium with a uniform density are in maximal entropy states when submitted to adiabatic perturbations, i.e., perturbations that maintain the hydrostatic equilibrium. We also show that the same result holds [...] Read more.

Building upon a thermodynamic formalism, we show that one-dimensional self-gravitating systems in hydrostatic equilibrium with a uniform density are in maximal entropy states when submitted to adiabatic perturbations, i.e., perturbations that maintain the hydrostatic equilibrium. We also show that the same result holds for three-dimensional spherical systems when submitted to spherically symmetric adiabatic perturbations. We coin this phenomenon “thermodynamic blocking”, given its similarity to the more general “kinetic blocking”. This result underlines the importance of the thermodynamic formalism, which proves useful when kinetic equations break down. Full article

(This article belongs to the Section Gravitation)

2 pages, 127 KB

Open AccessEditorial

The 10th Anniversary of Universe

by Lorenzo Iorio

Universe 2025, 11(12), 418; https://doi.org/10.3390/universe11120418 - 18 Dec 2025

Viewed by 142

Abstract

The adventure of the journal Universe began about ten years ago, when I received an invitation from Dr [...] Full article

23 pages, 4022 KB

Open AccessReview

On the Counter-Rotating Tori and Counter-Rotating Parts of the Kerr Black Hole Shadows

by Daniela Pugliese and Zdenek Stuchlík

Universe 2025, 11(12), 417; https://doi.org/10.3390/universe11120417 - 17 Dec 2025

Viewed by 295

Abstract

We review some aspects of accretion disks physics, spacetime photon shell and photon orbits, related to retrograde (counter-rotating) motion in Kerr black hole (BH) spacetimes. In this brief review, we examine the counter-rotating components of the Kerr BH shadow boundary, under [...] Read more.

We review some aspects of accretion disks physics, spacetime photon shell and photon orbits, related to retrograde (counter-rotating) motion in Kerr black hole (BH) spacetimes. In this brief review, we examine the counter-rotating components of the Kerr BH shadow boundary, under the influence of counter-rotating accretion tori, accreting flows and proto-jets (open critical funnels of matter, associated with the tori) orbiting around the central BH. We also analyze the redshifted emission arising from counter-rotating structures. Regions of the shadows and photon shell are constrained in their dependence of the BH spin and observational angle. The effects of the counter-rotating structures on these are proven to be typical of the fast-spinning BHs, and accordingly can be observed only in the restricted classes of the Kerr BH spacetimes. This review is intended as a concise guide to the main properties of counter-rotating fluxes and counter-rotating disks in relation to the photon shell and the BH shadow boundary. Our findings may serve as the basis for different theoretical frameworks describing counter-rotating accretion flows with observable imprints manifesting at the BH shadow boundary. The results can eventually enable the distinction of counter-rotating fluxes through their observable imprints, contributing to constraints on both the BH spin and the structure of counter-rotating accretion disks. In particular, photon trajectories and their impact parameters can manifest in the morphology of the BH shadow. Such features, when accessible through high-resolution imaging and spectral or polarization measurements, could provide a direct avenue for testing different theoretical models on accretion disk dynamics and their BH attractors. Full article

(This article belongs to the Collection Open Questions in Black Hole Physics)

► Show Figures

Figure 1

19 pages, 1014 KB

Open AccessArticle

Toward Precision Physics Tests with Future COHERENT Detectors

by Mattia Atzori Corona, Matteo Cadeddu, Nicola Cargioli, Francesca Dordei, Carlo Giunti and Riccardo Pavarani

Universe 2025, 11(12), 416; https://doi.org/10.3390/universe11120416 - 16 Dec 2025

Viewed by 171

Abstract

We present a comprehensive sensitivity study of future CE

ν

NS detectors, focusing on a cryogenic cesium iodide detector and a tonne-scale liquid argon one, currently being developed by the COHERENT Collaboration. These setups will enable precision measurements of the weak mixing angle [...] Read more.

We present a comprehensive sensitivity study of future CE

ν

NS detectors, focusing on a cryogenic cesium iodide detector and a tonne-scale liquid argon one, currently being developed by the COHERENT Collaboration. These setups will enable precision measurements of the weak mixing angle at low energies and allow accurate extraction of the neutron nuclear distribution radius. We also demonstrate that next-generation detectors will place constraints on the neutrino charge radius comparable to or better than current global fits. In addition, we explore the sensitivity to non-standard neutrino electromagnetic properties, such as magnetic moments and millicharges, as well as new mediators. These findings reinforce the role of CE

ν

NS experiments in the upcoming precision era, with future detectors playing a key role in advancing our understanding of neutrino interactions and electroweak physics at low energies. Full article

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

► Show Figures

Figure 1

25 pages, 718 KB

Open AccessReview

Measuring Supermassive Black Hole Masses with H₂O Megamasers: Observations, Methods, and Implications for Black Hole Demographics

by Cheng-Yu Kuo

Universe 2025, 11(12), 415; https://doi.org/10.3390/universe11120415 - 12 Dec 2025

Viewed by 227

Abstract

Measuring supermassive black hole (SMBH) masses is fundamental to understanding active galactic nuclei (AGN) and their coevolution with host galaxies. Among existing techniques, H₂O megamaser observations with Very Long Baseline Interferometry (VLBI) provide the most direct and geometric determinations of SMBH [...] Read more.

Measuring supermassive black hole (SMBH) masses is fundamental to understanding active galactic nuclei (AGN) and their coevolution with host galaxies. Among existing techniques, H₂O megamaser observations with Very Long Baseline Interferometry (VLBI) provide the most direct and geometric determinations of SMBH masses by tracing molecular gas in sub-parsec Keplerian disks. Over the past two decades, the Megamaser Cosmology Project (MCP) has surveyed thousands of nearby AGNs and obtained high-sensitivity VLBI maps of dozens of maser disks that lead to accurate SMBH masses with uncertainties typically below 10%. In this paper, we present a comprehensive review that summarizes the essential elements required to obtain accurate black hole masses with the H₂O megamaser technique—including the physical conditions for maser excitation, observational requirements, disk modeling, and sources of SMBH mass uncertainty—and we discuss the implications of maser-based measurements for exploring SMBH demographics. In particular, we will show that maser-derived black hole masses, largely free from the systematic biases of stellar or gas-dynamical methods, provide critical anchors at the low-mass end of the SMBH population (M_BH∼10⁷M_⊙), and reveal possible deviations from the canonical M_BH–σ_∗ relation. With forthcoming spectroscopic surveys and advances in millimeter/submillimeter VLBI, the maser technique promises to extend precise dynamical mass measurements to both larger local samples and high-redshift galaxies. Full article

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

► Show Figures

Figure 1

78 pages, 730 KB

Open AccessReview

Pfaffian Systems, Cartan Connections, and the Null Surface Formulation of General Relativity

by Emanuel Gallo and Carlos Kozameh

Universe 2025, 11(12), 414; https://doi.org/10.3390/universe11120414 - 11 Dec 2025

Viewed by 257

Abstract

This review examines the role of differential forms, Pfaffian systems, and hypersurfaces in general relativity. These mathematical constructions provide the essential tools for general relativity, in which the curvature of spacetime—described by the Einstein field equations—is most elegantly formulated using the Cartan calculus [...] Read more.

This review examines the role of differential forms, Pfaffian systems, and hypersurfaces in general relativity. These mathematical constructions provide the essential tools for general relativity, in which the curvature of spacetime—described by the Einstein field equations—is most elegantly formulated using the Cartan calculus of differential forms. Another important subject in this discussion is the notion of conformal geometry, where the relevant invariants of a metric are characterized by Élie Cartan’s normal conformal connection. The previous analysis is then used to develop the null surface formulation (NSF) of general relativity, a radical framework that postulates the structure of light cones rather than the metric itself as the fundamental gravitational variable. Defined by a central Pfaffian system, this formulation allows the entire spacetime geometry to be reconstructed from a single scalar function, Z, whose level surfaces are null. Full article

(This article belongs to the Section Gravitation)

20 pages, 392 KB

Open AccessArticle

D_N-Orthogonal Freedom in the Canonical Seesaw: Flavor Invariants and Physical Non-Equivalence of F-Classes

by Jianlong Lu

Universe 2025, 11(12), 413; https://doi.org/10.3390/universe11120413 - 11 Dec 2025

Viewed by 181

Abstract

We study basis-independent structures in the Type-I seesaw mechanism for light Majorana neutrinos, assuming the canonical scenario with three heavy right-handed (sterile) neutrinos. Let

m_{ν}

denote the

3 \times 3

mass matrix of light neutrinos, obtained at tree level from heavy Majorana [...] Read more.

We study basis-independent structures in the Type-I seesaw mechanism for light Majorana neutrinos, assuming the canonical scenario with three heavy right-handed (sterile) neutrinos. Let

m_{ν}

denote the

3 \times 3

mass matrix of light neutrinos, obtained at tree level from heavy Majorana singlets with a diagonal mass matrix

D_{N} = diag (M_{1}, M_{2}, M_{3})

and a Dirac matrix

m_{D}

. We show that all right actions F on the seesaw matrix that leave

m_{ν}

unchanged form the group

G = D_{N}^{1 / 2} O (3, C) D_{N}^{- 1 / 2}

. While oscillation data determine the PMNS matrix

U_{PMNS}

and the mass-squared splittings, they do not fix the F-class within G. We classify basis-invariant quantities into those that are class-blind (e.g.,

det η

) and class-sensitive (e.g.,

Tr η

,

Tr η^{2}

, an alignment measure, and CP-odd traces relevant to leptogenesis), where

η

denotes the non-unitarity matrix of the light sector. We provide explicit formulas and both high-scale and GeV-scale benchmark examples that illustrate these invariant fingerprints and their scaling with

D_{N}

. This converts the degeneracy at fixed

m_{ν}

into measurable, basis-invariant fingerprints. Full article

(This article belongs to the Special Issue Neutrino Insights: Peering into the Subatomic Universe)

50 pages, 4804 KB

Open AccessReview

A Brief Review of Unsupervised Machine Learning Algorithms in Astronomy: Dimensionality Reduction and Clustering

by Chih-Ting Kuo, Duo Xu and Rachel Friesen

Universe 2025, 11(12), 412; https://doi.org/10.3390/universe11120412 - 11 Dec 2025

Viewed by 588

Abstract

This review investigates the application of unsupervised machine learning algorithms to astronomical data. Unsupervised machine learning enables researchers to analyze large, high-dimensional, and unlabeled datasets and is sometimes considered more helpful for exploratory analysis because it is not limited by present knowledge and [...] Read more.

This review investigates the application of unsupervised machine learning algorithms to astronomical data. Unsupervised machine learning enables researchers to analyze large, high-dimensional, and unlabeled datasets and is sometimes considered more helpful for exploratory analysis because it is not limited by present knowledge and can therefore be used to extract new knowledge. Unsupervised machine learning algorithms that have been repeatedly applied to analyze astronomical data are classified according to their usage, including dimension reduction and clustering. This review also discusses anomaly detection and symbolic regression. For each algorithm, this review discusses the algorithm’s functioning in mathematical and statistical terms, the algorithm’s characteristics (e.g., advantages and shortcomings and possible types of inputs), and the different types of astronomical data analyzed with the algorithm. Example figures are generated. The algorithms are tested on synthetic datasets. This review aims to provide an up-to-date overview of both the high-level concepts and detailed applications of various unsupervised learning methods in astronomy, highlighting their advantages and disadvantages to help researchers new to unsupervised learning. Full article

(This article belongs to the Special Issue Applications of Artificial Intelligence in Modern Astronomy)

► Show Figures

Figure 1

19 pages, 1107 KB

Open AccessArticle

DAMPS: DArk Matter Profile Around Massive Schwarzschild Black Holes

by Hai-Chao Yuan, Zi-Chang Zhang and Yong Tang

Universe 2025, 11(12), 411; https://doi.org/10.3390/universe11120411 - 11 Dec 2025

Viewed by 279

Abstract

Dark matter constitutes the predominant component of the universe, yet its fundamental nature remains elusive, motivating diverse physical and astrophysical investigations. Recently, gravitational waves have emerged as a new probe for detecting the distribution of dark matter around massive black holes by measuring [...] Read more.

Dark matter constitutes the predominant component of the universe, yet its fundamental nature remains elusive, motivating diverse physical and astrophysical investigations. Recently, gravitational waves have emerged as a new probe for detecting the distribution of dark matter around massive black holes by measuring the dynamical friction exerted on compact objects within their orbits. The dark matter density profile plays a critical role in such analyses. In this study, we compute the relativistic density and velocity distributions of dark matter surrounding Schwarzschild black holes and develop a corresponding Python package DAMPS (v1.0.0). We provide a detailed derivation of the theoretical framework, present numerical results for two types of initial dark matter profiles—Hernquist and single power-law—and demonstrate an application to gravitational-waveform calculations for extreme mass-ratio inspirals. We anticipate that this software tool will benefit the broader community and advance the understanding of black hole–dark matter systems important to future space-based gravitational-wave detectors. Full article

(This article belongs to the Special Issue Exploring Low-Frequency Gravitational Wave Sources: Waveforms, Detection and Sciences)

► Show Figures

Figure 1

10 pages, 1152 KB

Open AccessArticle

Logamediate Warm Axion Inflation in Light of Planck Data

by Zahra Shamloui, Vahid Kamali and Saeid Ebrahimi

Universe 2025, 11(12), 410; https://doi.org/10.3390/universe11120410 - 10 Dec 2025

Viewed by 150

Abstract

Axion warm inflation is studied within the framework of Logamediate inflation. Using a novel approach, we constrain the parameter space of the model and find a reasonable region of free parameters compatible with the temperature, polarization, and lensing CMB data. We focus on [...] Read more.

Axion warm inflation is studied within the framework of Logamediate inflation. Using a novel approach, we constrain the parameter space of the model and find a reasonable region of free parameters compatible with the temperature, polarization, and lensing CMB data. We focus on an inflaton evolution characterized by a high dissipative regime, where particle production impacts the inflaton dynamics more than the expansion rate. Specifically, we consider the cubic form of the dissipation coefficient,

Υ = Υ_{0} T^{3}

, as proposed in minimal warm inflation. We show that this parameter remains large during the slow-roll epoch for a broad range of the free parameter

Υ_{0}

, as indicated by our data analysis. Full article

(This article belongs to the Special Issue Bouncing Cosmology and the Early Universe: From Theory to Observations)

► Show Figures

Figure 1

19 pages, 1305 KB

Open AccessArticle

A Study of Compact Stellar Objects in f(R, T) Theory of Gravity

by Anupama Roy Chowdhury, Shyam Das and Farook Rahaman

Universe 2025, 11(12), 409; https://doi.org/10.3390/universe11120409 - 10 Dec 2025

Viewed by 175

Abstract

In this paper, we investigate the stability and feasibility of an anisotropic stellar model under

f (R, T)

gravity that embraces the Karmarkar condition. In order to develop the

f (R, T)

gravity model, the functional form [...] Read more.

In this paper, we investigate the stability and feasibility of an anisotropic stellar model under

f (R, T)

gravity that embraces the Karmarkar condition. In order to develop the

f (R, T)

gravity model, the functional form of

f (R, T)

is taken into consideration as the linear function of the trace of the energy-momentum tensor T and the Ricci scalar R, respectively. This study proposes a well-known form of the radial metric function and finds another metric function by employing the Karmakar condition, which provides the exact solution to the field equation. The expression of the model parameters is derived by matching the obtained interior solutions with the Schwarzschild exterior metric over the bounding surface of a celestial object, along with the requirement that the radial pressure vanish at the boundary. The current estimated data of the star, pulsar 4U1608-52, is used to graphically explore the model. The physical attributes of the celestial object are thoroughly examined within the framework of the present model. Adjusting the model parameter, a detailed analysis of the stability criterion is presented that involves the adiabatic index, the Herrera cracking technique, and the causality condition. Furthermore, the Tolman–Oppenheimer–Volkhoff equation is used to analyze the stellar model’s equilibrium state. In order to maintain the stability condition of the anisotropic stellar structure, a suitable range for the model parameter is determined by the graphical analysis of the present model in this study. In addition, the numerical values of the physical parameters related to the compact stars Her X-1, LMC X-4, Cen X-3 and KS1731-207 are used to examine the model solution within the desired range of the model parameter. Full article

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

► Show Figures

Figure 1

25 pages, 692 KB

Open AccessArticle

Noncommutative Bianchi I and III Cosmology Models: Radiation Era Dynamics and γ Estimation

by Gil Oliveira-Neto and Yuri Soncco Apaza

Universe 2025, 11(12), 408; https://doi.org/10.3390/universe11120408 - 10 Dec 2025

Viewed by 299

Abstract

In this work, we analyze the dynamical evolution of locally rotationally symmetric anisotropic cosmological models of Bianchi type I (flat curvature) and Bianchi type III (open curvature) within a noncommutative phase space framework characterized by a deformation parameter

γ

. Using a Hamiltonian [...] Read more.

In this work, we analyze the dynamical evolution of locally rotationally symmetric anisotropic cosmological models of Bianchi type I (flat curvature) and Bianchi type III (open curvature) within a noncommutative phase space framework characterized by a deformation parameter

γ

. Using a Hamiltonian formulation based on Schutz’s formalism for a perfect radiation fluid, we introduce noncommutative Poisson brackets that allow for geometric corrections to commutative dynamics. The resulting equations are solved numerically, which allows for the study of the impact of

γ

and the energy density C on the expansion of the universe and the evolution of anisotropy. The results show that

γ < 0

improves expansion and favors isotropization, while

γ > 0

tends to slow expansion and preserve residual anisotropy, especially in the open curvature model. It is estimated that the influence of noncommutativity was significant during the early stages of the universe, decreasing toward the present time, suggesting that this approach could serve as an effective alternative to the cosmological constant in describing the evolution of the early universe. Full article

(This article belongs to the Section Cosmology)

► Show Figures

Figure 1

13 pages, 11300 KB

Open AccessArticle

Conceptual Analog to Wave Interference with Discrete Particles

by Ioannis Contopoulos and Evangelos Chaniadakis

Universe 2025, 11(12), 407; https://doi.org/10.3390/universe11120407 - 9 Dec 2025

Viewed by 208

Abstract

We present a numerical implementation of the proposed Source–Detector Resonance (SDR) as a conceptual analog of a Double-Slit Interference Experiment with discrete particles. Two periodic streams of particles are emitted from two point sources at random integer multiples of a fundamental period P [...] Read more.

We present a numerical implementation of the proposed Source–Detector Resonance (SDR) as a conceptual analog of a Double-Slit Interference Experiment with discrete particles. Two periodic streams of particles are emitted from two point sources at random integer multiples of a fundamental period P and corresponding frequency

ω = 2 π / P

and fly out towards a detection screen. The screen consists of a deep set of identical oscillators with eigenfrequency

ω_{0} = 2 π / P_{0}

. In the SDR scenario,

ω \approx ω_{0}

. When the particles reach the screen, they implement a periodic forcing of its oscillators at the stream’s fundamental frequency

ω_{0}

. As a result, an oscillating pattern develops along the screen. The amplitude of oscillation of each oscillator saturates at a value that is determined by the balance between the periodic particle forcing and the damping of each oscillator. This is clearly proportional to the number of particles that reach a certain oscillator per unit time times the fraction of particles that reach it at its resonant frequency. The latter fraction is equal to the ratio of the Power Spectral Density (PSD) of the time series of the particles that reach the oscillator at its resonance frequency PSD(

ω_{0}

) over the PSD at zero frequency PSD(0). If we further assume that each oscillator absorbs a particle and announces a detection with a probability that is proportional to the square of the ratio PSD(

ω_{0}

)/PSD(0); the pattern of particle detections that develops over a thick layer of oscillators is shown to be the same as that of a Double-Slit Interference Experiment. Our result shows that when macroscopic resonant detectors interact with and detect periodic streams of discrete particles, they may create the illusion of an interference measurement, as if each discrete particle manifests a phase of its own. Full article

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

► Show Figures

Figure 1

15 pages, 280 KB

Open AccessReview

Quantum Field Theory in the Weyl–Wigner Representation

by Emilio Santos

Universe 2025, 11(12), 406; https://doi.org/10.3390/universe11120406 - 9 Dec 2025

Viewed by 247

Abstract

The Wigner representation for quantum mechanics of particles is generalized to Bose fields. The standard Hilbert space quantization becomes, via the Weyl transform, a quantization method that consists of adding a Gaussian zeropoint field distribution to the vacuum. I comment on the possible [...] Read more.

The Wigner representation for quantum mechanics of particles is generalized to Bose fields. The standard Hilbert space quantization becomes, via the Weyl transform, a quantization method that consists of adding a Gaussian zeropoint field distribution to the vacuum. I comment on the possible advantages of the method in order to study quantum fields in curved spaces. I study a unified formulation of non-relativistic quantum electrodynamics in the Weyl–Wigner formalism, in terms of (classical-like) c-numbers. Full article

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

7 pages, 216 KB

Open AccessOpinion

A Modest Proposal for Naming a Hypothetical Distant Planet in the Solar System

by Lorenzo Iorio

Universe 2025, 11(12), 405; https://doi.org/10.3390/universe11120405 - 8 Dec 2025

Viewed by 479

Abstract

The need to choose appropriate and meaningful names for the objects of scientific inquiry, in the spirit of Michael Faraday and, on a different level, of the ancient Chinese doctrine of rectification of names (正名, Zhèngmíng), is illustrated here in the case [...] Read more.

The need to choose appropriate and meaningful names for the objects of scientific inquiry, in the spirit of Michael Faraday and, on a different level, of the ancient Chinese doctrine of rectification of names (正名, Zhèngmíng), is illustrated here in the case of the so-called Planet Nine. Since before the discovery of Neptune, the fascinating hypothesis of the possible existence of a new, distant planet in the solar system, yet to be discovered, has regularly surfaced in the pages of astronomy journals in various guises. Its most recent incarnations have been tentatively given names such as Planet X, Planet Y, and, most famously, Planet Nine. Such labels are unsatisfactory because they reveal no significant physical or orbital properties of the object which they are attributed to. I propose here the name Telisto, from the ancient Greek word τήλɩστoς for ‘farthest, most remote’ which captures a feature common to all versions of this scenario that seems destined to remain at the forefront of astronomical research for a long time to come: its supposedly great heliocentric distance, estimated at several hundred astronomical units. By exploring the history of astronomy, I also respond to some criticisms that might be leveled at this proposal. Among other things, I also draw a comparison with the naming of the so-called axions, which are hypothetical elementary particles proposed almost fifty years ago and which continue to be an active object of research. Full article

(This article belongs to the Section Planetary Sciences)

11 pages, 12575 KB

Open AccessArticle

Exploring the Role of Vector Potential and Plasma-β in Jet Formation from Magnetized Accretion Flows

by Ishika Palit, Miles Angelo Paloma Sodejana and Hsiang-Yi Karen Yang

Universe 2025, 11(12), 404; https://doi.org/10.3390/universe11120404 - 8 Dec 2025

Viewed by 296

Abstract

In this work, we investigate how the choice of initial vector potential and plasma parameters influences the development of accretion columns and jet formation in magnetized accretion flows. Using general relativistic magnetohydrodynamic simulations, we explore two different configurations of the vector potential [...] Read more.

In this work, we investigate how the choice of initial vector potential and plasma parameters influences the development of accretion columns and jet formation in magnetized accretion flows. Using general relativistic magnetohydrodynamic simulations, we explore two different configurations of the vector potential

A_{ϕ}

and three plasma beta values (

β = 50, 100, 500

). We analyze how variations in the poloidal magnetic field strength and plasma magnetization affect magnetic flux accumulation near the black hole and the subsequent growth of the accretion column. Our results highlight the dependence of jet launching efficiency and accretion dynamics on the initial magnetic field topology and plasma beta, offering insight into the conditions that favor magnetically arrested disk or standard and normal evolution states. Full article

(This article belongs to the Special Issue New Progress of Black Hole Accretion Disk)

► Show Figures

Figure 1

23 pages, 2117 KB

Open AccessArticle

Inferring Cosmological Parameters with Evidential Physics-Informed Neural Networks

by Hai Siong Tan

Universe 2025, 11(12), 403; https://doi.org/10.3390/universe11120403 - 5 Dec 2025

Viewed by 345

Abstract

We examine the use of a novel variant of Physics-Informed Neural Networks to predict cosmological parameters from recent supernovae and baryon acoustic oscillations (BAO) datasets. Our machine learning framework generates uncertainty estimates for target variables and the inferred unknown parameters of the underlying [...] Read more.

We examine the use of a novel variant of Physics-Informed Neural Networks to predict cosmological parameters from recent supernovae and baryon acoustic oscillations (BAO) datasets. Our machine learning framework generates uncertainty estimates for target variables and the inferred unknown parameters of the underlying PDE descriptions. Built upon a hybrid of the principles of Evidential Deep Learning, Physics-Informed Neural Networks, Bayesian Neural Networks, and Gaussian Processes, our model enables learning the posterior distribution of the unknown PDE parameters through standard gradient-descent-based training. We apply our model to an up-to-date BAO dataset (Bousis et al. 2024) calibrated with the CMB-inferred sound horizon, and the Pantheon+ Sne Ia distances (Scolnic et al. 2018), examining the relative effectiveness and mutual consistency among the standard

Λ

CDM, wCDM and

Λ_{s}

CDM models. Unlike previous results arising from the standard approach of minimizing an appropriate

χ^{2}

function, the posterior distributions for parameters in various models trained purely on Pantheon+ data were found to be largely contained within the

2 σ

contours of their counterparts trained on BAO data. Our study illustrates how a data-driven machine learning approach can be suitably adapted for cosmological parameter inference. Full article

(This article belongs to the Section Cosmology)

► Show Figures

Figure 1

13 pages, 293 KB

Open AccessArticle

Hydrodynamics of Two-Dimensional CFTs

by Kevin Nguyen

Universe 2025, 11(12), 402; https://doi.org/10.3390/universe11120402 - 5 Dec 2025

Viewed by 220

Abstract

We demonstrate that the geometric action on a coadjoint orbit of the Virasoro group appropriately describes non-dissipative two-dimensional conformal fluids. While this action has already appeared in the context of AdS₃ gravity, the hydrodynamical interpretation given here is new. We use this [...] Read more.

We demonstrate that the geometric action on a coadjoint orbit of the Virasoro group appropriately describes non-dissipative two-dimensional conformal fluids. While this action has already appeared in the context of AdS₃ gravity, the hydrodynamical interpretation given here is new. We use this to argue that the geometric action manifestly controls both sides of the fluid/gravity correspondence, and that the gravitational ‘hologram’ gives an effective hydrodynamical description of the dual CFT. As a byproduct, our work sheds light on the nature of the AdS₃ reparametrization theory used to effectively compute Virasoro vacuum blocks at large central charge, since the reparametrization mode is now understood as a fluctuation of the fluid velocity. Full article

(This article belongs to the Section Field Theory)

► Show Figures

Figure 1

15 pages, 15216 KB

Open AccessArticle

Investigation of Prominence Oscillations with High-Resolution Observations from the New Vacuum Solar Telescope

by Mingbao Gao, Zongjun Ning, Yuzhi Yang, Yijie Han, Guotianci Xu, Xiaoyu Tang, Kangyi Li and Yu Huang

Universe 2025, 11(12), 401; https://doi.org/10.3390/universe11120401 - 5 Dec 2025

Viewed by 236

Abstract

In this paper, we investigate the thread oscillations in a quiescent prominence observed by New Vacuum Solar Telescope at the H

α

line center on 17 April 2024. Each individual thread is traced by the local maximum intensity on the time–distance maps. Although [...] Read more.

In this paper, we investigate the thread oscillations in a quiescent prominence observed by New Vacuum Solar Telescope at the H

α

line center on 17 April 2024. Each individual thread is traced by the local maximum intensity on the time–distance maps. Although there are numerous threads in this prominence, 24 oscillating threads are identified at eight slits parallel to the solar surface. A sinusoidal function is used to fit them, and about 1.5 cycles of oscillations and a mean period of 27.7 min are detected. We find that all these 24 threads display the oscillation with almost a constant amplitude with an average value of about 0.92 Mm, with no damping or expansion during their lifetimes. Furthermore, we find the oscillations at different positions on a same thread almost have a similar period of 88.7 min in phase, which indicates that the thread oscillations could be triggered by a standing-type wave. Using the typical parameters in the prominence, the magnetic field strength (B) is estimated in the range of 4 G ≤ B ≤ 21 G, which is consistent with the previous results. Our findings would provide the clues for the thread oscillation mechanism in the prominence. Full article

(This article belongs to the Special Issue Oscillations and Instabilities of Solar Filaments)

► Show Figures

Figure 1

9 pages, 240 KB

Open AccessArticle

Second-Order Pseudo-Hermitian Spin-1/2 Bosons

by Armando de la C. Rangel-Pantoja, I. Díaz-Saldaña and Carlos A. Vaquera-Araujo

Universe 2025, 11(12), 400; https://doi.org/10.3390/universe11120400 - 5 Dec 2025

Viewed by 281

Abstract

The canonical quantization of a field theory for spin-1/2 massive bosons that satisfy the Klein–Gordon equation is presented. The breakdown of the usual spin–statistics connection is due to the redefinition of the dual field, rendering the theory pseudo-Hermitian. The normal-ordered Hamiltonian is bounded [...] Read more.

The canonical quantization of a field theory for spin-1/2 massive bosons that satisfy the Klein–Gordon equation is presented. The breakdown of the usual spin–statistics connection is due to the redefinition of the dual field, rendering the theory pseudo-Hermitian. The normal-ordered Hamiltonian is bounded from below with real eigenvalues, and the theory is consistent with microcausality and invariant under parity, charge conjugation and time reversal. Full article

(This article belongs to the Section Field Theory)

15 pages, 414 KB

Open AccessArticle

Evolution of the Magnetic Activity of the Single Giant OP Andromedae Between 1993 and 2025

by Stefan Georgiev, Renada Konstantinova-Antova, Ana Borisova, Rumen Bogdanovski, Dimitar Kolev, Michel Aurière, Pascal Petit, Dimitar Churalski, Alexander Kurtenkov, Maya Galabova, Nikolay Tomov, Haralambi Markov, Borislav Spassov, Radoslav Zamanov, Milen Minev and Miroslav Moyseev

Universe 2025, 11(12), 399; https://doi.org/10.3390/universe11120399 - 5 Dec 2025

Viewed by 204

Abstract

We investigate the long-term magnetic variability of OP And, a magnetically active single K giant, between 1993 and 2025. To track magnetic activity, we analyze the variability of the H

α

line and two lines of the calcium infrared triplet. The variability of [...] Read more.

We investigate the long-term magnetic variability of OP And, a magnetically active single K giant, between 1993 and 2025. To track magnetic activity, we analyze the variability of the H

α

line and two lines of the calcium infrared triplet. The variability of the H

α

line reveals that the activity level of OP And is higher in the period 1993–2000, while during the period 2010–2016 it is lower, possibly close to an eventual minimum. Recent data (2020–2025) indicate an increase of the activity level again. The flare frequency rate and the calcium infrared triplet data (when available) follow the same behavior. In addition, the structure of the H

α

line also changes with the activity level: when the activity is higher, we observe a blue-shifted component of this line, interpreted as an expanding hot area above the photosphere, but during the lower activity interval it is almost absent. Our results are in a good agreement with the idea that the magnetic field influences the mass outflow in this giant. Additionally, we examine how flare frequency correlates with overall activity. While a complete activity cycle remains undetermined, the recent upward trend suggests that the eventual activity cycle of OP And seems to be slightly longer than 30 years. More years of observations are necessary to reach the next maximum and to determine the exact duration of the cycle. Full article

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

► Show Figures

Figure 1

14 pages, 436 KB

Open AccessArticle

Jet X-Ray Properties of EXO 1846-031 During Its 2019 Outburst

by Riya Bhowmick, Sujoy Kumar Nath, Dipak Debnath and Hsiang-Kuang Chang

Universe 2025, 11(12), 398; https://doi.org/10.3390/universe11120398 - 4 Dec 2025

Viewed by 205

Abstract

The Galactic X-ray transient EXO 1846-031 was first discovered during an outburst in 1985 by the EXOSAT mission. The source remained in a quiescent state for nearly 34 years after the first outburst. The source started its second outburst on 23 July 2019. [...] Read more.

The Galactic X-ray transient EXO 1846-031 was first discovered during an outburst in 1985 by the EXOSAT mission. The source remained in a quiescent state for nearly 34 years after the first outburst. The source started its second outburst on 23 July 2019. We studied the accretion flow properties using the Two Component Advective Flow (TCAF) paradigm of this 2019 outburst. During the outburst, the source went through all the four spectral states, though, due to data constraints, it was not possible to define the date of the state transitions during the declining intermediate states. During this outburst, the black hole candidate (BHC) exhibited significant jet activity. In the TCAF solution, the model normalization is expected to remain constant for a given source. Therefore, any need for a significantly different normalization to achieve a better spectral fit suggests the presence of additional X-ray contributions from components not accounted for in the current TCAF model fit’s file. By comparing with the expected normalization, we estimate the X-ray contribution originating from jets and outflows. We further analyze the origin of the jet. Our analysis shows that, on some days, up to

\sim 92 %

of the total X-ray flux originates from the base of the jet itself. Full article

(This article belongs to the Special Issue New Progress of Black Hole Accretion Disk)

► Show Figures

Figure 1

24 pages, 1646 KB

Open AccessArticle

Statistical Properties of Prompt Emission and X-Ray Afterglow Plateau Emission of Gamma-Ray Bursts with Jet Features

by Da-Ling Ma, Si-Yuan Zhu, Wan-Peng Sun and Fu-Wen Zhang

Universe 2025, 11(12), 397; https://doi.org/10.3390/universe11120397 - 3 Dec 2025

Viewed by 187

Abstract

Gamma-ray bursts (GRBs) are widely recognized to exhibit jet-like emission structures, though previous studies often assumed isotropic emission due to observational constraints. This assumption limited our understanding of the intrinsic properties of GRBs. Here, we analyze 40 GRBs with observed X-ray plateaus and [...] Read more.

Gamma-ray bursts (GRBs) are widely recognized to exhibit jet-like emission structures, though previous studies often assumed isotropic emission due to observational constraints. This assumption limited our understanding of the intrinsic properties of GRBs. Here, we analyze 40 GRBs with observed X-ray plateaus and jet features, all with measured redshifts. By applying jet corrections to prompt and plateau-phase quantities, we probe their intrinsic behavior. We find that the jet-corrected prompt emission energy (

E_{j e t}

) depends less strongly on the jet-corrected X-ray luminosity at the end of the plateau (

L_{X, j e t}

). An anti-correlation is also observed between the jet opening angle (

θ_{j e t}

) and the rest frame peak energy (

E_{p, z}

):

E_{p, z} \propto θ_{j e t}^{- 0.44 \pm 0.13}

for ISM and

E_{p, z} \propto θ_{j e t}^{- 0.78 \pm 0.13}

for wind environments, indicating that more collimated jets yield higher peak energies. After jet correction, the

L_{X}

-

T_{a, z}

correlation and the three-parameter

L_{X}

-

T_{a, z}

-

E_{γ, i s o}

,

L_{X}

-

T_{a, z}

-

L_{p}

and

L_{X}

-

T_{a, z}

-

E_{p, z}

relations are generally weakened. Among these, the first three remain relatively stable, suggesting they reflect intrinsic GRB physics, whereas the

L_{X}

-

T_{a, z}

-

E_{p, z}

relation weakens significantly, implying it may be an artifact of the isotropic assumption. We also identify a new three-parameter correlation:

θ_{j e t} (I S M) \propto E_{j e t} {(I S M)}^{0.36 \pm 0.06} E_{p, z}^{- 0.62 \pm 0.09}

,

θ_{j e t} (W i n d) \propto E_{j e t}

{(W i n d)}^{0.29 \pm 0.09}

E_{p, z}^{- 0.61 \pm 0.09}

. Full article

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

► Show Figures

Figure 1

35 pages, 1898 KB

Open AccessReview

Three Decades of FCNC Studies in 3-3-1 Model with Right-Handed Neutrinos: From Z′-Dominance to the Alignment Limit

by Patricio Escalona, João Paulo Pinheiro, Vinícius Oliveira, Adriano Doff and Carlos Antonio De Sousa Pires

Universe 2025, 11(12), 396; https://doi.org/10.3390/universe11120396 - 3 Dec 2025

Viewed by 191

Abstract

Flavor-changing neutral current (FCNC) processes play a prominent role in the search for physics beyond the Standard Model (SM) due to their sensitivity to new physics at the TeV scale. Meson–antimeson transitions and rare meson decays provide stringent constraints on new physics through [...] Read more.

Flavor-changing neutral current (FCNC) processes play a prominent role in the search for physics beyond the Standard Model (SM) due to their sensitivity to new physics at the TeV scale. Meson–antimeson transitions and rare meson decays provide stringent constraints on new physics through precision measurements of observables such as mass differences, CP asymmetries, and branching ratios. Extensions of the SM based on the

SU {(3)}_{C} \times SU {(3)}_{L} \times U {(1)}_{N}

gauge group offer a compelling framework for flavor physics, as FCNC processes emerge inexorably at tree level due to the non-universal transformations of the quark families. Among its various realizations, the version incorporating right-handed neutrinos (331RHNs) is the most phenomenologically viable. This review synthesizes three decades of theoretical developments in FCNC phenomenology within the 331RHN model, from early

Z^{'}

-dominated studies to the recent recognition of the decisive role played by the SM-like Higgs boson and the identification of the alignment limit. We demonstrate that viable parameter space spans orders of magnitude—from

m_{Z^{'}} \sim

a few hundred GeV to ∼100 TeV—depending critically on quark mixing parameterizations and scalar alignment configurations, with significant implications for experimental searches at current and future colliders. Full article

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

► Show Figures

Figure 1

14 pages, 2454 KB

Open AccessArticle

Characteristics of the Solar Differential Rotation and Activity During Solar Cycle No. 24

by Ivana Poljančić Beljan, Luka Šibenik, Klaudija Lončarić, Tomislav Jurkić, Rajka Jurdana-Šepić, Werner Pötzi, Damir Hržina, Roman Brajša, Astrid M. Veronig and Arnold Hanslmeier

Universe 2025, 11(12), 395; https://doi.org/10.3390/universe11120395 - 3 Dec 2025

Viewed by 298

Abstract

An analysis of the solar differential rotation (DR) during solar cycle No. 24 (SC24) (2009–2019), based on the Kanzelhöhe Observatory for Solar and Environmental Research (KSO) data set, is presented. The white-light images were processed and positions of sunspot groups were extracted using [...] Read more.

An analysis of the solar differential rotation (DR) during solar cycle No. 24 (SC24) (2009–2019), based on the Kanzelhöhe Observatory for Solar and Environmental Research (KSO) data set, is presented. The white-light images were processed and positions of sunspot groups were extracted using the morphological image processing technique. The sample was constrained to ±58° in central meridian distance (CMD). Two methods were applied to derive the sidereal angular rotation rate (ω) and, in turn, the solar rotation parameters A and B: (a) calculating synodic rotation velocities from daily CMD differences and elapsed time (daily shift method); (b) applying a robust linear least-squares fit to the time series CMD(t) for each sunspot group. To assess the relationship between rotation parameters and solar activity, we analyzed the yearly mean total sunspot number from the Sunspot Index and Long-term Solar Observations (SILSO). This study marks the first complete analysis of SC24 using the KSO sunspot groups’ data. Our goal is to extend the previous analysis of DR from the KSO data to the present, especially because the Solar Optical Observing Network/United States Air Force/National Oceanic and Atmospheric Administration data set (SOON/USAF/NOAA) and Debrecen Photoheliographic Data (DPD) catalogues do not provide data after 2018. Full article

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

► Show Figures

Figure 1

12 pages, 442 KB

Open AccessArticle

Black-Hole Evaporation for Cosmological Observers

by Thiago de L. Campos, C. Molina and J. A. S. Lima

Universe 2025, 11(12), 394; https://doi.org/10.3390/universe11120394 - 30 Nov 2025

Viewed by 538

Abstract

This work investigates the evaporation of black holes immersed in a de Sitter environment, using the Vaidya-de Sitter spacetime. The role of cosmological observers is highlighted in the development and Hayward thermodynamics for non-stationary geometries is employed in the description of the compact [...] Read more.

This work investigates the evaporation of black holes immersed in a de Sitter environment, using the Vaidya-de Sitter spacetime. The role of cosmological observers is highlighted in the development and Hayward thermodynamics for non-stationary geometries is employed in the description of the compact objects. The results of the proposed dynamical model are compared with the usual description based on stationary geometries, with specific results for primordial black holes (PBHs). The timescale of evaporation is shown to depend significantly on the choice of cosmological observer and can differ substantially from predictions based on stationary models at late times. Deviations are also shown with respect to the standard assertion that there is a fixed initial mass just below

10^{15} g \sim 10^{- 18} M_{⊙}

for the PBHs which are completing their evaporation process at the present epoch. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity, 2nd Edition)

► Show Figures

Figure 1

30 pages, 1535 KB

Open AccessArticle

(Pre)-Inflationary Dynamics with Starobinsky Potential in Noncommutative Effective LQC

by Luis Rey Díaz-Barrón, Abraham Espinoza-García, Sinuhé Pérez-Payán and José Socorro

Universe 2025, 11(12), 393; https://doi.org/10.3390/universe11120393 - 29 Nov 2025

Viewed by 307

Abstract

In this work, we investigate the (pre)-inflationary dynamics of a flat, homogeneous, and isotropic universe governed by the Starobinsky potential within the framework of noncommutative effective loop quantum cosmology. The field equations are solved numerically for various initial conditions and different values of [...] Read more.

In this work, we investigate the (pre)-inflationary dynamics of a flat, homogeneous, and isotropic universe governed by the Starobinsky potential within the framework of noncommutative effective loop quantum cosmology. The field equations are solved numerically for various initial conditions and different values of the noncommutative parameter. We analyze the background dynamics for three representative regimes—the extreme kinetic-energy domination, kinetic-energy domination, and potential-energy domination. A complementary analysis is performed from the viewpoint of dynamical systems, highlighting the qualitative features of the scalar field evolution. Finally, a discussion comparing our results with previous studies employing the chaotic (quadratic) potential in the same formalism is presented. Full article

(This article belongs to the Section Cosmology)

► Show Figures

Figure 1

24 pages, 4114 KB

Open AccessArticle

Building a Radio AGN Sample from Cosmic Morning—The Radio High-Redshift Quasar Catalog (RHzQCat): I. Catalog from SDSS Quasars and Radio Surveys at z ≥ 3

by Yingkang Zhang, Ruqiu Lin, Krisztina Perger, Sándor Frey, Tao An, Xiang Ji, Qiqi Wu and Shilong Liao

Universe 2025, 11(12), 392; https://doi.org/10.3390/universe11120392 - 28 Nov 2025

Viewed by 441

Abstract

Radio-loud high-redshift quasars (RHRQs) provide crucial insights into the evolution of relativistic jets and their connection to the growth of supermassive black holes. Beyond the extensively studied population at

z \geq 5

, the cosmic morning epoch (

3 ≲ z ≲ 5

[...] Read more.

Radio-loud high-redshift quasars (RHRQs) provide crucial insights into the evolution of relativistic jets and their connection to the growth of supermassive black holes. Beyond the extensively studied population at

z \geq 5

, the cosmic morning epoch (

3 ≲ z ≲ 5

) marks the peak of active galactic nucleus (AGN) activity and black hole accretion, yet remains relatively unexplored. In this work, we compiled the radio high-redshift quasar catalog (RHzQCat) by cross-matching the SDSS DR16Q catalog with four major radio surveys—FIRST, NVSS, RACS, and GLEAM. Our tier-based cross-matching framework and visual validation ensured reliable source identification across surveys with diverse beam sizes. The catalog included 1629 reliable and 315 candidate RHRQs, with radio luminosities uniformly spanning

10^{25.5}

–

10^{29.3}

W Hz⁻¹. About 95% of the confirmed sources exhibited compact morphologies, consistent with Doppler-boosted or young AGN populations at high redshifts. Our catalog increases the number of known RHRQs at

z \geq 3

by an order of magnitude, representing the largest and most homogeneous catalog of radio quasars at cosmic morning, filling the observational gap between the early (

z > 6

) and local Universe. It provides a robust reference for future statistical studies of jet evolution, AGN feedback, and cosmic magnetism with next-generation facilities such as the Square Kilometer Array (SKA). Full article

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

► Show Figures

Figure 1

21 pages, 590 KB

Open AccessArticle

Nonrelativistic Quantum Dynamics in a Twisted Screw Spacetime

by Faizuddin Ahmed and Edilberto O. Silva

Universe 2025, 11(12), 391; https://doi.org/10.3390/universe11120391 - 27 Nov 2025

Viewed by 380

Abstract

We investigate the nonrelativistic quantum dynamics of a spinless particle in a screw-type spacetime endowed with two independent twist controls that interpolate between a pure screw dislocation and a homogeneous twist. From the induced spatial metric, we build the covariant Schrödinger operator, separate [...] Read more.

We investigate the nonrelativistic quantum dynamics of a spinless particle in a screw-type spacetime endowed with two independent twist controls that interpolate between a pure screw dislocation and a homogeneous twist. From the induced spatial metric, we build the covariant Schrödinger operator, separate variables to obtain a single radial eigenproblem, and include a uniform axial magnetic field and an Aharonov–Bohm (AB) flux by minimal coupling. Analytically, we identify a clean separation between a global, AB-like reindexing set by the screw parameter and a local, curvature-driven mixing generated by the distributed twist. We derive the continuity equation and closed expressions for the azimuthal and axial probability currents, establish practical parameter scalings, and recover limiting benchmarks (AB, Landau, and flat space). Numerically, a finite-difference Sturm–Liouville solver (with core excision near the axis and Langer transform) resolves spectra, wave functions, and currents. The results reveal AB periodicity and reindexing with the screw parameter, Landau fan trends, twist-induced level tilts and avoided crossings, and a geometry-induced near-axis backflow of the axial current with negligible weight in cross-section integrals. The framework maps the geometry and fields directly onto measurable spectral shifts, interferometric phases, and persistent-current signals. Full article

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

► Show Figures

Figure 1

15 pages, 767 KB

Open AccessArticle

On the Properties of the Power-Law Cosmological Solutions in Lovelock Gravity

by Sergey Pavluchenko

Universe 2025, 11(12), 390; https://doi.org/10.3390/universe11120390 - 26 Nov 2025

Viewed by 225

Abstract

In this paper we study the properties of Kasner cosmological solutions in Lovelock gravity. Recent progress in the investigation of flat cosmological models in Lovelock gravity unveiled the fact that in quadratic (Gauss–Bonnet) and cubic Lovelock gravities, the higher-order power-law solutions could play [...] Read more.

In this paper we study the properties of Kasner cosmological solutions in Lovelock gravity. Recent progress in the investigation of flat cosmological models in Lovelock gravity unveiled the fact that in quadratic (Gauss–Bonnet) and cubic Lovelock gravities, the higher-order power-law solutions could play the role of both future and past asymptotes, and under some conditions, there could exist a smooth transition between them. So it is natural to question if this feature is unique to Gauss–Bonnet and cubic Lovelock gravities, or if it is a general feature of Lovelock gravity. Our analysis suggests that starting from quartic and in all higher-order Lovelock gravities, the high-order Kasner solution cannot play the role of a past asymptote, not only preventing the abovementioned transition from happening, but also potentially hindering the possibility of reaching viable compactification. Full article

(This article belongs to the Section Gravitation)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Universe, Volume 11, Issue 12 (December 2025) – 39 articles

Further Information

Guidelines

MDPI Initiatives

Follow MDPI