Universe

7 pages, 190 KiB

Open AccessCommunication

Gravitational Wave and Quantum Graviton Interferometer Arm Detection of Gravitons

by John W. Moffat

Universe 2025, 11(1), 28; https://doi.org/10.3390/universe11010028 - 20 Jan 2025

Viewed by 388

This paper explores the quantum and classical descriptions of gravitational wave detection in interferometers like LIGO. We demonstrate that a graviton scattering and quantum optics model succeeds in explaining the observed arm displacements, while the classical gravitational wave approach and a quantum graviton [...] Read more.

This paper explores the quantum and classical descriptions of gravitational wave detection in interferometers like LIGO. We demonstrate that a graviton scattering and quantum optics model succeeds in explaining the observed arm displacements, while the classical gravitational wave approach and a quantum graviton energy method also successfully predict the correct results. We provide a detailed analysis of why the quantum graviton energy approach succeeds, highlighting the importance of collective behavior and the quantum–classical correspondence in gravitational wave physics. Our findings contribute to the ongoing discussion about the quantum nature of gravity and its observable effects in macroscopic physics. Full article

(This article belongs to the Special Issue Challenges and Synergies with Future Gravitational Wave Observatories)

10 pages, 320 KiB

Open AccessArticle

Elliptic and Quadrangular Flow of Protons in the High-Baryon-Density Region

by Shaowei Lan, Zuowen Liu, Like Liu and Shusu Shi

Universe 2025, 11(1), 27; https://doi.org/10.3390/universe11010027 - 17 Jan 2025

Viewed by 390

Abstract

The collective flow provides valuable insights into the anisotropic expansion of particles produced in heavy-ion collisions and is sensitive to the equation of the state of nuclear matter in high-baryon-density regions. In this paper, we use the hadronic transport model SMASH to investigate [...] Read more.

The collective flow provides valuable insights into the anisotropic expansion of particles produced in heavy-ion collisions and is sensitive to the equation of the state of nuclear matter in high-baryon-density regions. In this paper, we use the hadronic transport model SMASH to investigate the elliptic flow (

v_{2}

), quadrangular flow (

v_{4}

), and their ratio (

v_{4} / v_{2}^{2}

) in Au+Au collisions at high baryon density. Our results show that the inclusion of baryonic mean-field potential in the model successfully reproduces experimental data from the HADES experiment, indicating that baryonic interactions play an important role in shaping anisotropic flow. In addition to comparing the transverse momentum (

p_{T}

), rapidity, and centrality dependence of

v_{4} / v_{2}^{2}

between HADES data and model calculations, we also explore its time evolution and energy dependence across

\sqrt{s_{N N}} =

2.4 to 4.5 GeV. While the ratio

v_{4} / v_{2}^{2}

for high-

p_{T}

particles approaches 0.5, which aligns with expectations from hydrodynamic behavior, we emphasize that this result primarily reflects agreement with the HADES measurements rather than a definitive indication of ideal fluid behavior. These findings contribute to understanding the early-stage dynamics in heavy-ion collisions at high baryon density. Full article

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

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52 pages, 652 KiB

Open AccessArticle

Scalar Field Kantowski–Sachs Solutions in Teleparallel F(T) Gravity

by Alexandre Landry

Universe 2025, 11(1), 26; https://doi.org/10.3390/universe11010026 - 16 Jan 2025

Viewed by 550

Abstract

In this paper, we investigate time-dependent Kantowski–Sachs spherically symmetric teleparallel

F (T)

gravity with a scalar field source. We begin by setting the exact field equations to be solved and solve conservation laws for possible scalar field potential,

V (ϕ)

, [...] Read more.

In this paper, we investigate time-dependent Kantowski–Sachs spherically symmetric teleparallel

F (T)

gravity with a scalar field source. We begin by setting the exact field equations to be solved and solve conservation laws for possible scalar field potential,

V (ϕ)

, solutions. Then, we find new non-trivial teleparallel

F (T)

solutions by using power-law and exponential ansatz for each potential case arising from conservation laws, such as linear, quadratic, or logarithmic, to name a few. We find a general formula allowing us to compute all possible new teleparallel

F (T)

solutions applicable for any scalar field potential and ansatz. Then, we apply this formula and find a large number of exact and approximate new teleparallel

F (T)

solutions for several types of cases. Some new

F (T)

solution classes may be relevant for future cosmological applications, especially concerning dark matter, dark energy quintessence, phantom energy leading to the Big Rip event, and quintom models of physical processes. Full article

(This article belongs to the Special Issue Dark Energy and Dark Matter)

20 pages, 1799 KiB

Open AccessArticle

Impact of Charge on Strange Compact Stars in Rastall Theory

by Malick Sallah and Muhammad Sharif

Universe 2025, 11(1), 25; https://doi.org/10.3390/universe11010025 - 16 Jan 2025

Viewed by 346

Abstract

Within the framework of Rastall theory, we investigate the impact of charge on the structural development of different types of spherically symmetric anisotropic stars. To do so, we present modified field equations based upon the Finch–Skea metric potentials expressed in terms of three [...] Read more.

Within the framework of Rastall theory, we investigate the impact of charge on the structural development of different types of spherically symmetric anisotropic stars. To do so, we present modified field equations based upon the Finch–Skea metric potentials expressed in terms of three parameters

(A, B, C)

. These constants are determined using suitable matching conditions and observational data for compact objects which include Her X-1, SAX J 1808.4-3658, PSR J038-0842, LMC X-4 and SMC X-1. The equation of state offered by the

MIT

bag model for quark–gluon plasma is used to investigate the inner structure and other characteristics of these compact objects. For a fixed bag constant,

B = 60 {MeV / fm}^{3}

, and two sets of the Rastall and charge parameters,

ζ = 0.255, 0.259

and

\tilde{Q} = 0.2, 0.7

, respectively, we analyze the consistency of the matter variables in the model and other physical parameters such as energy conditions, stellar mass, compactness, and surface redshift. In addition, we assess the stability of the constructed model through two different approaches. It is found that the obtained model is physically viable and stable. Full article

(This article belongs to the Special Issue Gravity and Cosmology: Exploring the Mysteries of f(T) Gravity)

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34 pages, 423 KiB

Open AccessReview

Gravitational Algebras and Applications to Nonequilibrium Physics

by Michele Cirafici

Universe 2025, 11(1), 24; https://doi.org/10.3390/universe11010024 - 16 Jan 2025

Viewed by 327

Abstract

This note aims to offer a non-technical and self-contained introduction to gravitational algebras and their applications in the nonequilibrium physics of gravitational systems. We begin by presenting foundational concepts from operator algebra theory and exploring their relevance to perturbative quantum gravity. Additionally, we [...] Read more.

This note aims to offer a non-technical and self-contained introduction to gravitational algebras and their applications in the nonequilibrium physics of gravitational systems. We begin by presenting foundational concepts from operator algebra theory and exploring their relevance to perturbative quantum gravity. Additionally, we provide a brief overview of the theory of nonequilibrium dynamical systems in finite dimensions and discuss its generalization to gravitational algebras. Specifically, we focus on entropy production in black hole backgrounds and fluctuation theorems in de Sitter spacetime. Full article

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

17 pages, 325 KiB

Open AccessArticle

An Interior Solution for the Kerr Metric: A Novel Approach

by Yu-Ching Chou

Universe 2025, 11(1), 23; https://doi.org/10.3390/universe11010023 - 15 Jan 2025

Viewed by 956

Abstract

We present a novel approach for the construction of interior solutions for the Kerr metric, extending J. Ovalle’s foundational work through ellipsoidal coordinate transformations. By deriving a physically plausible interior solution that smoothly matches the Kerr exterior metric, we analyze the energy conditions [...] Read more.

We present a novel approach for the construction of interior solutions for the Kerr metric, extending J. Ovalle’s foundational work through ellipsoidal coordinate transformations. By deriving a physically plausible interior solution that smoothly matches the Kerr exterior metric, we analyze the energy conditions across various rotation parameters. Our findings reveal anisotropic fluid properties and energy condition behaviors in specific space-time regions, providing insights into the strong-field regime of rotating black holes. The proposed solution offers a more realistic description of rotating black hole interiors, with implications for understanding compact astrophysical objects. Full article

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

11 pages, 1319 KiB

Open AccessArticle

The Multimessenger Contribution of Ultra-High-Energy Cosmic Rays from Gamma-Ray Bursts

by Zhenjiang Li, Fangsheng Min, Yi Jin and Yiqing Guo

Universe 2025, 11(1), 22; https://doi.org/10.3390/universe11010022 - 15 Jan 2025

Viewed by 508

Abstract

It has long been debated whether gamma-ray bursts (GRBs) could serve as potential sources of ultra-high-energy cosmic rays (UHECRs). In this study, we consider GRBs as sources of UHECR injection with an injection index of

α = 2

and propagate them through the [...] Read more.

It has long been debated whether gamma-ray bursts (GRBs) could serve as potential sources of ultra-high-energy cosmic rays (UHECRs). In this study, we consider GRBs as sources of UHECR injection with an injection index of

α = 2

and propagate them through the extragalactic magnetic field within the framework of CRPropa 3. The baryon loading factor

f_{C R}

is taken into account to quantify the rate of UHECR energy injection. In the benchmark case with a jet opening angle of

θ_{j} = 1

and

f_{C R} = 1

, we find that both high- and low-luminosity populations contribute to less than 10% of the UHECR spectrum. The most constrained scenario suggests

f_{C R} \leq 15

, indicating that GRBs are less efficient in producing the all-sky UHECR intensity. The high-energy diffuse neutrinos and gamma rays resulting from interactions between UHECRs from GRBs and extragalactic background photons do not dominate the observations of Fermi-LAT or IceCube. Full article

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

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14 pages, 10756 KiB

Open AccessArticle

Gravitational Waves from Black Hole Emission

by Tousif Islam, Gaurav Khanna and Steven L. Liebling

Universe 2025, 11(1), 21; https://doi.org/10.3390/universe11010021 - 14 Jan 2025

Viewed by 431

Abstract

Using adiabatic point-particle black hole perturbation theory, we simulate plausible gravitational wave (GW) signatures in two exotic scenarios (i) where a small black hole is emitted by a larger one (‘black hole emission’) and (ii) where a small black hole is emitted by [...] Read more.

Using adiabatic point-particle black hole perturbation theory, we simulate plausible gravitational wave (GW) signatures in two exotic scenarios (i) where a small black hole is emitted by a larger one (‘black hole emission’) and (ii) where a small black hole is emitted by a larger one and subsequently absorbed back (‘black hole absorption’). While such scenarios are forbidden in general relativity (GR), alternative theories (such as certain quantum gravity scenarios obeying the weak gravity conjecture, white holes, and Hawking radiation) may allow them. By leveraging the phenomenology of black hole emission and absorption signals, we introduce straightforward modifications to existing gravitational waveform models to mimic gravitational radiation associated with these exotic events. We anticipate that these (incomplete but) initial simulations, coupled with the adjusted waveform models, will aid in the development of null tests for GR using GWs. Full article

(This article belongs to the Special Issue Recent Advances in Gravity: A Themed Issue in Honor of Prof. Jorge Pullin on His 60th Anniversary)

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31 pages, 11373 KiB

Open AccessReview

Massive Clusters and OB Associations as Output of Massive Star Formation in Gaia Era

by Ignacio Negueruela

Universe 2025, 11(1), 20; https://doi.org/10.3390/universe11010020 - 14 Jan 2025

Viewed by 537

Abstract

Over the past two decades, our understanding of star formation has undergone a major shift, driven by a wealth of data from infrared, submillimeter and radio surveys. The emerging view depicts star formation as a hierarchical process, which predominantly occurs along filamentary structures [...] Read more.

Over the past two decades, our understanding of star formation has undergone a major shift, driven by a wealth of data from infrared, submillimeter and radio surveys. The emerging view depicts star formation as a hierarchical process, which predominantly occurs along filamentary structures in the interstellar medium. These structures span a wide range of spatial scales, ultimately leading to the birth of young stars, which distribute in small groups, clusters and OB associations. Given the inherently complex and dynamic nature of star formation, a comprehensive understanding of these processes can only be achieved by examining their end products—namely, the distribution and properties of young stellar populations. In the Gaia era, the nearby OB associations are now characterised with unprecedented detail, allowing for a robust understanding of their formation histories. Nevertheless, to fully grasp the mechanisms of star formation and its typical scale, it is essential to study the much larger associations, which constitute the backbones of spiral arms. The large catalogues of young open clusters that have emerged from Gaia DR3 offer a valuable resource for investigating star formation on larger spatial scales. While the cluster parameters listed in these catalogues are still subject to many uncertainties and systematic errors, ongoing improvements in data analysis and upcoming Gaia releases promise to enhance the accuracy and reliability of these measurements. This review aims to provide a comprehensive summary of recent advancements and a critical assessment of the datasets available. Full article

(This article belongs to the Special Issue Advances in Star Formation in the Milky Way)

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18 pages, 1218 KiB

Open AccessArticle

Exploring the Enigma of Particle Dynamics and Plasma Lensing Using Einstein–Euler–Heisenberg Black Hole Geometry

by Allah Ditta, Raja Sikander Mehmood, Muhammad Fiaz, Bismillah Bibi, Sana Deen, Rimsha Jaffar and Asif Mahmood

Universe 2025, 11(1), 19; https://doi.org/10.3390/universe11010019 - 13 Jan 2025

Viewed by 365

Abstract

The unified Einstein–Euler–Heisenberg theory is utilized to investigate the particle motion and weak gravitational lensing characteristics of black holes. This black hole solution is developed using spherically symmetric possessing electric and magnetic charges. Quantum electrodynamics corrections reveal a screening effect for BH electric [...] Read more.

The unified Einstein–Euler–Heisenberg theory is utilized to investigate the particle motion and weak gravitational lensing characteristics of black holes. This black hole solution is developed using spherically symmetric possessing electric and magnetic charges. Quantum electrodynamics corrections reveal a screening effect for BH electric charges and paramagnetic impacts on magnetic charges. We analyzed the motion of massive as well as massless particles by studying their effective potential, event horizon, photon orbit and inner circular orbit. It was demonstrated that magnetic and electric fields of spherically symmetric black holes have significant impact. Then, we also delve to study the weak gravitational lensing phenomenon. A comprehensive approach was employed to investigate this phenomenon and explore the angle of deflection of light rays near magnetically and electrically charged black holes. Full article

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

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22 pages, 3351 KiB

Open AccessArticle

Distinguishing Compact Objects in Extreme-Mass-Ratio Inspirals by Gravitational Waves

by Lujia Xu, Shucheng Yang, Wenbiao Han, Xingyu Zhong, Rundong Tang and Yuanhao Zhang

Universe 2025, 11(1), 18; https://doi.org/10.3390/universe11010018 - 13 Jan 2025

Viewed by 472

Abstract

Extreme-mass-ratio inspirals (EMRIs) are promising gravitational-wave (GW) sources for space-based GW detectors. EMRI signals typically have long durations, ranging from several months to several years, necessitating highly accurate GW signal templates for detection. In most waveform models, compact objects in EMRIs are treated [...] Read more.

Extreme-mass-ratio inspirals (EMRIs) are promising gravitational-wave (GW) sources for space-based GW detectors. EMRI signals typically have long durations, ranging from several months to several years, necessitating highly accurate GW signal templates for detection. In most waveform models, compact objects in EMRIs are treated as test particles without accounting for their spin, mass quadrupole, or tidal deformation. In this study, we simulate GW signals from EMRIs by incorporating the spin and mass quadrupole moments of the compact objects. We evaluate the accuracy of parameter estimation for these simulated waveforms using the Fisher Information Matrix (FIM) and find that the spin, tidal-induced quadruple, and spin-induced quadruple can all be measured with precision ranging from

10^{- 2}

to

10^{- 1}

, particularly for a mass ratio of ∼

10^{- 4}

. Assuming the “true” GW signals originate from an extended body inspiraling into a supermassive black hole, we compute the signal-to-noise ratio (SNR) and Bayes factors between a test-particle waveform template and our model, which includes the spin and quadrupole of the compact object. Our results show that the spin of compact objects can produce detectable deviations in the waveforms across all object types, while tidal-induced quadrupoles are only significant for white dwarfs, especially in cases approaching an intermediate-mass ratio. Spin-induced quadrupoles, however, have negligible effects on the waveforms. Therefore, our findings suggest that it is possible to distinguish primordial black holes from white dwarfs, and, under certain conditions, neutron stars can also be differentiated from primordial black holes. Full article

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

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21 pages, 881 KiB

Open AccessArticle

A Study on the Evolution of Emission Altitude with Frequency Among 104 Normal Pulsars

by Chaoxin Luo, Xin Xu, Changrong Du and Qijun Zhi

Universe 2025, 11(1), 17; https://doi.org/10.3390/universe11010017 - 12 Jan 2025

Viewed by 412

Abstract

Utilizing the databases from the European Pulsar Network (EPN), the Australia Telescope National Facility (ATNF), and published literature data, a geometric method was used to investigate the multifrequency emission altitude of 104 pulsars. We found that the evolution of emission altitudes with frequency [...] Read more.

Utilizing the databases from the European Pulsar Network (EPN), the Australia Telescope National Facility (ATNF), and published literature data, a geometric method was used to investigate the multifrequency emission altitude of 104 pulsars. We found that the evolution of emission altitudes with frequency for the majority of pulsars can be fitted using a power-law function with a normalization constant. In this work, it is found that the frequency evolution of pulsar emission altitude can be divided into three groups according to their different frequency dependencies of emission altitude (emission altitude decreases with frequency (Group A,

η \leq - 0.1

), keeps relatively constant with frequency (Group B,

- 0.1 < η \leq 0.1

), and increases with frequency (Group C,

η \geq 0.1

)), where

η

is the emission altitude variation rate. We also computed the emission altitudes across multiple frequency bands for these pulsars, thereby estimating the approximate range of the pulsar emission regions. We found that most pulsar emissions occur at altitudes of tens to hundreds of kilometers above the polar cap, with differences in emission altitude between the three groups becoming more clear at lower frequencies. Full article

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

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60 pages, 821 KiB

Open AccessReview

Introduction to Thermal Field Theory: From First Principles to Applications

by Alberto Salvio

Universe 2025, 11(1), 16; https://doi.org/10.3390/universe11010016 - 11 Jan 2025

Viewed by 416

Abstract

This review article provides the basics and discusses some important applications of thermal field theory, namely, the combination of statistical mechanics and relativistic quantum field theory. In the first part, the fundamentals are covered: the density matrix, the corresponding averages, and the treatment [...] Read more.

This review article provides the basics and discusses some important applications of thermal field theory, namely, the combination of statistical mechanics and relativistic quantum field theory. In the first part, the fundamentals are covered: the density matrix, the corresponding averages, and the treatment of fields of various spin in a medium. The second part is dedicated to the computation of thermal Green’s function for scalars, vectors, and fermions with path-integral methods. These functions play a crucial role in thermal field theory as explained here. A more applicative part of the review is dedicated to the production of particles in a medium and to phase transitions in field theory, including the process of vacuum decay in a general theory featuring a first-order phase transition. To understand this review, the reader should have good knowledge of non-statistical quantum field theory. Full article

(This article belongs to the Section Field Theory)

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54 pages, 671 KiB

Open AccessArticle

Quantum-Ordering Ambiguities in Weak Chern—Simons 4D Gravity and Metastability of the Condensate-Induced Inflation

by Panagiotis Dorlis, Nick E. Mavromatos and Sotirios-Neilos Vlachos

Universe 2025, 11(1), 15; https://doi.org/10.3390/universe11010015 - 11 Jan 2025

Viewed by 406

Abstract

In this work, we elaborate further on a (3+1)-dimensional cosmological Running-Vacuum-type-Model (RVM) of inflation based on string-inspired Chern-Simons(CS) gravity, involving axions coupled to gravitational-CS(gCS) anomalous terms. Inflation in such models is caused by primordial-gravitational-waves(GW)-induced condensation of the gCS terms, which leads to a [...] Read more.

In this work, we elaborate further on a (3+1)-dimensional cosmological Running-Vacuum-type-Model (RVM) of inflation based on string-inspired Chern-Simons(CS) gravity, involving axions coupled to gravitational-CS(gCS) anomalous terms. Inflation in such models is caused by primordial-gravitational-waves(GW)-induced condensation of the gCS terms, which leads to a linear-axion potential. We demonstrate that this inflationary phase may be metastable, due to the existence of imaginary parts of the gCS condensate. These are quantum effects, proportional to commutators of GW perturbations, hence vanishing in the classical theory. Their existence is quantum-ordering-scheme dependent. We argue in favor of a physical importance of such imaginary parts, which we compute to second order in the GW (tensor) perturbations in the framework of a gauge-fixed effective Lagrangian, within a (mean field) weak-quantum-gravity-path-integral approach. We thus provide estimates of the inflation lifetime. On matching our results with the inflationary phenomenology, we fix the quantum-ordering ambiguities, and obtain an order-of-magnitude constraint on the String-Mass-Scale-to-Planck-Mass ratio, consistent with previous estimates by the authors in the framework of a dynamical-system approach to linear-axion RVM inflation. Finally, we examine the role of periodic modulations in the axion potential induced by non-perturbative effects on the slow-roll inflationary parameters, and find compatibility with the cosmological data. Full article

(This article belongs to the Special Issue Particle Physics and Cosmology: A Themed Issue in Honor of Professor Dimitri Nanopoulos)

21 pages, 3901 KiB

Open AccessArticle

A Scenario for Origin of Global 4 mHz Oscillations in Solar Corona

by Li Xue, Chengliang Jiao and Lixin Zhang

Universe 2025, 11(1), 14; https://doi.org/10.3390/universe11010014 - 9 Jan 2025

Viewed by 412

Abstract

We establish a spherically symmetric model of solar atmosphere, which consists of the whole chromosphere and low corona below the

1.25

solar radius. It is a hydrodynamic model with heating in the chromosphere through an artificial energy flux. We performed a series of [...] Read more.

We establish a spherically symmetric model of solar atmosphere, which consists of the whole chromosphere and low corona below the

1.25

solar radius. It is a hydrodynamic model with heating in the chromosphere through an artificial energy flux. We performed a series of simulations with our model and found oscillations with a peak frequency of ∼4

mHz

in the power spectrum. We confirmed that this resulted from the p-mode excited in the transition region and amplified in a resonant cavity situated in the height range ∼

4 \times 10^{3}

–

2 \times 10^{4}

km. This result is consistent with global observations of Alfvénic waves in corona and can naturally explain the observational ubiquity of

4 mHz

without the difficulty of the p-mode passing through the acoustic-damping chromosphere. We also confirmed that acoustic shock waves alone cannot heat the corona to the observed temperature, and found mass upflows in the height range ∼

7 \times 10^{3}

–

7 \times 10^{4}

km in our model, which pumped the dense and cool plasma into the corona and might be the mass supplier for solar prominences. Full article

(This article belongs to the Special Issue New Insights into High-Energy Astrophysics, Galaxies, and Cosmology—Celebrating the 10th Anniversary of the Re-establishment of the Department of Astronomy at Xiamen University (2012–2022))

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25 pages, 2288 KiB

Open AccessArticle

More Efficient and Reliable: Identifying RRab Stars with Blazhko Effect by Deep Convolutional Neural Network

by Nan Jiang, Tianrui Sun, Siyuan Pan, Lingzhi Wang, Xue Li, Bin Sheng and Xiaofeng Wang

Universe 2025, 11(1), 13; https://doi.org/10.3390/universe11010013 - 6 Jan 2025

Viewed by 466

Abstract

The physical origin of the Blazhko effect (BL), a phenomenon of a single or multiple periodic modulation(s) of the light curve, is under debate. Efficiently identifying and characterizing the BL is essential in understanding its origins and accounting for its effect on numerous [...] Read more.

The physical origin of the Blazhko effect (BL), a phenomenon of a single or multiple periodic modulation(s) of the light curve, is under debate. Efficiently identifying and characterizing the BL is essential in understanding its origins and accounting for its effect on numerous applications of RRabs in the era of large time-domain surveys. In this study, we make use of Resnet 34, a well-known convolutional neural network (CNN) architecture, to identify RRab stars with BL from phased light curves collected from OGLE. Using reliably classified RRabs from frequency analysis to train, validate, and test our model, we show that our CNN method reaches accuracies up to 94%. We then applied our CNN method to some additional RRabs located in the Magellanic Cloud (MC) and the Galactic Bulge (GB), leading to the discovery of 113 and 2496 BL candidates, respectively. The identification accuracy for the MC Sample is estimated to be 91% after cross-matching the CNN classification results with those from frequency analysis. Similarly, the light-curve parameters of these classified BL/non-BL candidates by our CNN method from the GB region resemble those observed in the literature, confirming the reliability of our CNN classifications. Our CNN method is subject to issues related to light-curve quality and sampling, but its overall reliance on light-curve quality is comparable to that of frequency analysis. Furthermore, we find that BL modulation could be primarily characterized by variations in light-curve structure. Full article

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11 pages, 289 KiB

Open AccessArticle

Precision Higgs Constraints in U(1) Extensions of the Standard Model with a Light Z′-Boson

by Zoltán Péli and Zoltán Trócsányi

Universe 2025, 11(1), 12; https://doi.org/10.3390/universe11010012 - 3 Jan 2025

Viewed by 446

Abstract

Anomaly-free

U (1)

extensions of the standard model (SM) predict a new neutral gauge boson

Z^{'}

. When

Z^{'}

obtains its mass from the spontaneous breaking of the new

U (1)

symmetry by a new complex scalar [...] Read more.

Anomaly-free

U (1)

extensions of the standard model (SM) predict a new neutral gauge boson

Z^{'}

. When

Z^{'}

obtains its mass from the spontaneous breaking of the new

U (1)

symmetry by a new complex scalar field, the model also predicts a second real scalar s, and the search for the new scalar and the search for the new gauge boson become intertwined. We present the computation of production cross sections and decay widths of such a scalar s in models with a light

Z^{'}

boson when the decay

h \to Z^{'} Z^{'}

may have a sizeable branching ratio. We show how the Higgs signal strength measurement in this channel can provide stricter exclusion bounds on the parameters of the model than those obtained from the total signal strength for Higgs boson production. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—High Energy Nuclear and Particle Physics)

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4 pages, 162 KiB

Open AccessEditorial

Pulsar Astronomy

by Wei Wang and Renxin Xu

Universe 2025, 11(1), 11; https://doi.org/10.3390/universe11010011 - 2 Jan 2025

Viewed by 388

Abstract

Pulsars are a type of fast-rotating magnetized neutron star that emits beamed multi-wavelength electromagnetic radiation [...] Full article

(This article belongs to the Special Issue Pulsar Astronomy)

13 pages, 2611 KiB

Open AccessArticle

On the Evidence of Dynamical Dark Energy

by Qing Gao, Zhiqian Peng, Shengqing Gao and Yungui Gong

Universe 2025, 11(1), 10; https://doi.org/10.3390/universe11010010 - 31 Dec 2024

Viewed by 463

Abstract

To elucidate the robustness of the baryon acoustic oscillation (BAO) data measured by the dark energy spectroscopic instrument (DESI) in capturing the dynamical behavior of dark energy, we assess the model dependence of the evidence for dynamical dark energy inferred from the DESI [...] Read more.

To elucidate the robustness of the baryon acoustic oscillation (BAO) data measured by the dark energy spectroscopic instrument (DESI) in capturing the dynamical behavior of dark energy, we assess the model dependence of the evidence for dynamical dark energy inferred from the DESI BAO data. While the DESI BAO data slightly tightens the constraints on model parameters and increases the tension between the Chevallier–Polarski–Linder (CPL) model and the

Λ

CDM model, we find that the influence of DESI BAO data on the constraint of

w_{0}

is small in the SSLCPL model. In comparison to the CPL model, the tension with the

Λ

CDM model is reduced for the SSLCPL model, suggesting that the evidence for dynamical dark energy from DESI BAO data is dependent on cosmological models. The inclusion of spatial curvature has little impact on the results in the SSLCPL model. Full article

(This article belongs to the Section Cosmology)

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18 pages, 1908 KiB

Open AccessArticle

A Model for Cosmic Magnetic Field Amplification: Effects of Pressure Anisotropy Perturbations

by Ji-Hoon Ha

Universe 2025, 11(1), 9; https://doi.org/10.3390/universe11010009 - 30 Dec 2024

Viewed by 523

Abstract

Magnetic field amplification in the early universe is a long-standing problem that has been extensively studied through theoretical and numerical approaches, focusing on turbulent dynamos and the growth of collisionless plasma instabilities. In the post-recombination era, pressure anisotropy drives plasma instabilities, and magnetic [...] Read more.

Magnetic field amplification in the early universe is a long-standing problem that has been extensively studied through theoretical and numerical approaches, focusing on turbulent dynamos and the growth of collisionless plasma instabilities. In the post-recombination era, pressure anisotropy drives plasma instabilities, and magnetic field amplification through these instabilities can be faster than that driven by turbulent dynamos. By considering the balance between isotropization through magnetic field amplification and anisotropy generation by external sources such as turbulence and anisotropic cosmic-ray scattering, it is reasonable to assume that the system evolves around an equilibrium anisotropy value. To improve the theoretical modeling of magnetic field amplification in such systems, this study specifically examines pressure anisotropy perturbations near the equilibrium anisotropy value, which may destabilize the system. By analyzing the effects of pressure anisotropy perturbations and their damping rates on the time evolution of cosmic magnetic fields, we highlight the importance of these perturbations in driving plasma instabilities and boosting cosmic magnetic field amplification. Full article

(This article belongs to the Special Issue Cosmological Models of the Universe)

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12 pages, 673 KiB

Open AccessArticle

The Effect of Outflow Launching Radial Efficiency of Accretion Disk on the Shape of Emission-Line Profiles

by Mohammad Hassan Naddaf

Universe 2025, 11(1), 8; https://doi.org/10.3390/universe11010008 - 29 Dec 2024

Viewed by 518

Abstract

This paper presents a preliminary investigation into the influence of radial behavior of disk outflow on the structure and dynamics of the broad line region (BLR) in active galactic nuclei (AGNs), with an emphasis on how the mass ejection rate contributes to shaping [...] Read more.

This paper presents a preliminary investigation into the influence of radial behavior of disk outflow on the structure and dynamics of the broad line region (BLR) in active galactic nuclei (AGNs), with an emphasis on how the mass ejection rate contributes to shaping the broad emission-line profiles. Specifically, we analyze how varying the radial efficiency of mass loss from accretion disks, driven by radiative dust-based mechanisms, contributes to the distribution of material in the BLR. By exploring different radial scenarios of disk mass loss behavior, we uncover connections between outflow radial efficiency and emission line profiles, particularly for lowly ionized lines. Our findings reveal that while the observed shape of broad emission lines is partially influenced by the radial behavior of the disk outflow, it ultimately depends more critically on the physical conditions of the clouds and the specific approach adopted regarding the emissivity for their contribution to the line formation. Full article

(This article belongs to the Section Compact Objects)

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12 pages, 4926 KiB

Open AccessArticle

Radio Detection of UHE Cosmic Rays and Neutrinos off the Moon with Two 30 m Telescopes

by Linjie Chen, Jianli Zhang, Lihong Geng and Lingmei Cheng

Universe 2025, 11(1), 7; https://doi.org/10.3390/universe11010007 - 28 Dec 2024

Viewed by 415

Abstract

Due to the Askaryan effect, radio emissions will be produced when ultra-high-energy (UHE) cosmic rays and neutrinos impact the lunar regolith. Many experiments have been proposed and performed to detect such radio emissions from the lunar regolith. However, none of the Cherenkov radio [...] Read more.

Due to the Askaryan effect, radio emissions will be produced when ultra-high-energy (UHE) cosmic rays and neutrinos impact the lunar regolith. Many experiments have been proposed and performed to detect such radio emissions from the lunar regolith. However, none of the Cherenkov radio signals has been detected in these experiments up to now. In order to improve the detectability of the UHE particles, we proposed an experiment to carry out the radio observations of the UHE cosmic rays and neutrinos with two 30 m telescopes for a far longer time than the present experiments. The expected sensitivity for the detection of such UHE particles has been calculated, both for cosmic rays and neutrinos. The results show that a few UHE particle events above 10²⁰ eV could be detected with the expected observation time of several thousand hours. Full article

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

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5 pages, 215 KiB

Open AccessCommunication

Black Holes Have More States than Those Defined by the Bekenstein–Hawking Entropy: A Simple Argument

by Carlo Rovelli

Universe 2025, 11(1), 6; https://doi.org/10.3390/universe11010006 - 28 Dec 2024

Viewed by 633

Abstract

It is often assumed that the maximum number of independent states a black hole may contain is

N_{B H} = e^{S_{B H}}

, where

S_{B H} = A / 4

is the Bekenstein–Hawking entropy and A is the horizon [...] Read more.

It is often assumed that the maximum number of independent states a black hole may contain is

N_{B H} = e^{S_{B H}}

, where

S_{B H} = A / 4

is the Bekenstein–Hawking entropy and A is the horizon area in Planck units. I present a simple and straightforward argument showing that the number of states that can be distinguished by local observers inside the hole must be greater than this number. Full article

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

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18 pages, 477 KiB

Open AccessArticle

Can Effects of a Generalized Uncertainty Principle Appear in Compact Stars?

by João Gabriel Galli Gimenez, Dimiter Hadjimichef, Peter Otto Hess, Marcelo Netz-Marzola and César A. Zen Vasconcellos

Universe 2025, 11(1), 5; https://doi.org/10.3390/universe11010005 - 26 Dec 2024

Viewed by 547

Abstract

In the present contribution, a preliminary analysis of the effects of the Generalized Uncertainty Principle (GUP) with a minimum length, in the context of compact stars, is performed. On basis of a deformed Poisson canonical algebra with a parametrized minimum length scale that [...] Read more.

In the present contribution, a preliminary analysis of the effects of the Generalized Uncertainty Principle (GUP) with a minimum length, in the context of compact stars, is performed. On basis of a deformed Poisson canonical algebra with a parametrized minimum length scale that induces deviations from conventional Quantum Mechanics, fundamental questions involving the consistence, evidences and proofs of this approach as a possible cure for unbounded energy divergence are outlined. The incorporation of GUP effects into semiclassical 2N-dimensional systems is made by means of a time-invariant distortion transformation applied to their non-deformed counterparts. Assuming the quantum hadrodynamics

σ - ω

approach as a toy-model, due to its simplicity and structured description of neutron stars, we perform a preliminary analysis of GUP effects with a minimum spacetime length on these compact objects. The corresponding results for the equation of state and the mass-radius relation for neutron stars are in tune with recent observations with a maximum mass around 2.5 M_⊙ and radius close to 12 km. Our results also indicate the smallness of the noncommutative scale. Full article

(This article belongs to the Special Issue Studies in Neutron Stars)

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10 pages, 775 KiB

Open AccessCommunication

Identifying a Point-Symmetrical Morphology in the Core-Collapse Supernova Remnant W44

by Noam Soker

Universe 2025, 11(1), 4; https://doi.org/10.3390/universe11010004 - 26 Dec 2024

Viewed by 538

Abstract

I identify a point-symmetrical morphology in the core-collapse supernova remnant (CCSNR) W44 compatible with shaping by three or more pairs of jets in the jittering jets explosion mechanism (JJEM). Motivated by recent identifications of point-symmetrical morphologies in CCSNRs and their match to the [...] Read more.

I identify a point-symmetrical morphology in the core-collapse supernova remnant (CCSNR) W44 compatible with shaping by three or more pairs of jets in the jittering jets explosion mechanism (JJEM). Motivated by recent identifications of point-symmetrical morphologies in CCSNRs and their match to the JJEM, I revisit the morphological classification of CCSNR W44. I examine a radio map of W44 and find the outer bright rim of the radio map to possess a point-symmetric structure compatible with shaping by two energetic pairs of opposite jets rather than an S-shaped morphology shaped by a precessing pair of jets. An inner pair of filaments might hint at a third powerful pair of jets. More pairs of jets were involved in the explosion process. This study adds to the growing evidence that the JJEM is the primary explosion mechanism of core-collapse supernovae. Full article

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18 pages, 309 KiB

Open AccessArticle

Symmetry Restorations in the Singlet Scalar Yukawa Model Within the Auxiliary Field Method

by Anderson A. Nogueira and Fábio L. Braghin

Universe 2025, 11(1), 3; https://doi.org/10.3390/universe11010003 - 26 Dec 2024

Viewed by 923

Abstract

The aim of this work is to investigate the connection between thermal gap-coupled equations and the concept of symmetry restoration. For that, we consider the Yukawa model, a standard model for interactions between massless fermions mediated by a real self-interacting scalar field. To [...] Read more.

The aim of this work is to investigate the connection between thermal gap-coupled equations and the concept of symmetry restoration. For that, we consider the Yukawa model, a standard model for interactions between massless fermions mediated by a real self-interacting scalar field. To explore possible symmetry restoration, we study the thermal gap-coupled equations using the auxiliary field method (Hubbard–Stratonovich), and then we derive the effective action with thermal contributions through the background external fields method. With the thermal contributions for the effective action, we investigate the phase transitions and critical phenomena in an environment featuring mixing angles arising from the quantum description of composite states. Finally, we present the Dolan–Jackiw equations to determine the critical temperatures. Full article

(This article belongs to the Section Field Theory)

17 pages, 1035 KiB

Open AccessArticle

Mapping the Λ_sCDM Scenario to f(T) Modified Gravity: Effects on Structure Growth Rate

by Mateus S. Souza, Ana M. Barcelos, Rafael C. Nunes, Özgür Akarsu and Suresh Kumar

Universe 2025, 11(1), 2; https://doi.org/10.3390/universe11010002 - 25 Dec 2024

Viewed by 519

Abstract

The concept of a rapidly sign-switching cosmological constant, interpreted as a mirror AdS-dS transition in the late universe and known as the

Λ_{s}

CDM, has significantly improved the fit to observational data, offering a promising framework for alleviating major cosmological tensions such [...] Read more.

The concept of a rapidly sign-switching cosmological constant, interpreted as a mirror AdS-dS transition in the late universe and known as the

Λ_{s}

CDM, has significantly improved the fit to observational data, offering a promising framework for alleviating major cosmological tensions such as the

H_{0}

and

S_{8}

tensions. However, when considered within general relativity, this scenario does not predict any effects on the evolution of the matter density contrast beyond modifications to the background functions. In this work, we propose a new gravitational model in which the background dynamics predicted by the

Λ_{s}

CDM framework are mapped into

f (T)

gravity, dubbed

f (T) - Λ_{s}

CDM, rendering the models indistinguishable at the background level. However, in this new scenario, the sign-switching cosmological constant dynamics modify the evolution of linear matter perturbations through an effective gravitational constant,

G_{eff}

. We investigate the evolution of the growth rate and derive new observational constraints for this scenario using RSD measurements. We also present new constraints in the standard

Λ_{s}

CDM case, incorporating the latest Type Ia supernovae data samples available in the literature, along with BAO data from DESI. Our findings indicate that the new corrections expected at the linear perturbative level, as revealed through RSD samples, can provide significant evidence in favor of this new scenario. Additionally, this model may be an excellent candidate for resolving the current

S_{8}

tension. Full article

(This article belongs to the Special Issue Gravity and Cosmology: Exploring the Mysteries of f(T) Gravity)

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17 pages, 8987 KiB

Open AccessArticle

Effects of Thermodynamics on the Concurrent Accretion and Migration of Gas Giants in Protoplanetary Disks

by Hening Wu and Ya-Ping Li

Universe 2025, 11(1), 1; https://doi.org/10.3390/universe11010001 - 25 Dec 2024

Viewed by 545

Abstract

Accretion and migration usually proceed concurrently for giant planet formation in the natal protoplanetary disks. Recent works indicate that the concurrent accretion onto a giant planet imposes significant impact on the planetary migration dynamics in the isothermal regime. In this work, we carry [...] Read more.

Accretion and migration usually proceed concurrently for giant planet formation in the natal protoplanetary disks. Recent works indicate that the concurrent accretion onto a giant planet imposes significant impact on the planetary migration dynamics in the isothermal regime. In this work, we carry out a series of 2D global hydrodynamical simulations with Athena++ to explore the effect of thermodynamics on the concurrent accretion and migration processes of the planets in a self-consistent manner. The thermodynamics effect is modeled with a thermal relaxation timescale using a

β

-cooling prescription. Our results indicate that radiative cooling has a substantial effect on the accretion and migration processes of the planet. As cooling timescales increase, we observe a slight decrease in the planetary accretion rate, and a transition from the outward migrating into inward migration. This transition occurs approximately when the cooling timescale is comparable to the local dynamical timescale (

β \sim 1

), which is closely linked to the asymmetric structures from the circumplanetary disk (CPD) region. The asymmetric structures in the CPD region which appear with an efficient cooling provide a strong positive torque driving the planet migrate outward. However, such a positive torque is strongly suppressed, when the CPD structures tend to disappear with a relatively long cooling timescale (

β ≳ 10

). Our findings may also be relevant to the dynamical evolution of accreting stellar-mass objects embedded in disks around active galactic nuclei. Full article

(This article belongs to the Special Issue New Insights into High-Energy Astrophysics, Galaxies, and Cosmology—Celebrating the 10th Anniversary of the Re-establishment of the Department of Astronomy at Xiamen University (2012–2022))

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Universe, Volume 11, Issue 1 (January 2025) – 28 articles

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