Universe

15 pages, 308 KiB

Open AccessFeature PaperArticle

Quadratic Theory of Gravity with a Scalar Field and Type I Shapovalov Wave Spacetimes

by Konstantin Osetrin, Ilya Kirnos and Altair Filippov

Universe 2022, 8(12), 664; https://doi.org/10.3390/universe8120664 - 19 Dec 2022

Cited by 1 | Viewed by 1499

For the quadratic theory of gravity with a scalar field, exact solutions are found for gravitational-wave models in Shapovalov I-type spacetimes, which do not arise in models of the general theory of relativity. The theory of gravity under consideration can effectively describe the [...] Read more.

For the quadratic theory of gravity with a scalar field, exact solutions are found for gravitational-wave models in Shapovalov I-type spacetimes, which do not arise in models of the general theory of relativity. The theory of gravity under consideration can effectively describe the early stages of the universe. Type I Shapovalov spaces are the most general forms of gravitational-wave Shapovalov spacetimes, whose metrics in privileged coordinate systems depend on three variables, including the wave variable. For Einstein vacuum spacetimes, these wave models degenerate into simpler types. The exact models of gravitational waves in the quadratic theory of gravity can be used to test the realism of such theories of gravity. Full article

(This article belongs to the Special Issue Advances in Cosmology and Subatomic Particle Physics)

13 pages, 305 KiB

Open AccessArticle

Scattering in Geometric Approach to Quantum Theory

by Albert Schwarz

Universe 2022, 8(12), 663; https://doi.org/10.3390/universe8120663 - 16 Dec 2022

Cited by 7 | Viewed by 3860

Abstract

We define inclusive scattering matrix in the framework of a geometric approach to quantum field theory. We review the definitions of scattering theory in the algebraic approach and relate them to the definitions in the geometric approach. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Field Theory)

3 pages, 179 KiB

Open AccessEditorial

Editorial to the Special Issue “Feature Papers—Compact Objects”

by Sergei B. Popov and Ziri Younsi

Universe 2022, 8(12), 662; https://doi.org/10.3390/universe8120662 - 16 Dec 2022

Viewed by 1160

Abstract

Astrophysical studies of compact objects unite very different fields of research: from observations of known sources to sophisticated theoretical models involving new physics [...] Full article

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

12 pages, 2648 KiB

Open AccessCommunication

A Journey into the Proton Structure: Progresses and Challenges

by Francesco Giovanni Celiberto

Universe 2022, 8(12), 661; https://doi.org/10.3390/universe8120661 - 15 Dec 2022

Cited by 10 | Viewed by 1861

Abstract

Unraveling the inner dynamics of gluons and quarks inside nucleons is a primary target of studies at new-generation colliding machines. Finding an answer to fundamental problems of Quantum ChromoDynamics, such as the origin of nucleon mass and spin, strongly depends on our ability [...] Read more.

Unraveling the inner dynamics of gluons and quarks inside nucleons is a primary target of studies at new-generation colliding machines. Finding an answer to fundamental problems of Quantum ChromoDynamics, such as the origin of nucleon mass and spin, strongly depends on our ability of reconstructing the 3D motion of partons inside the parent hadrons. We present progresses and challenges in the extraction of TMD parton densities, with particular attention to the ones describing polarization states of gluons, which still represent a largely unexplored field. Then, we highlight connections with corresponding parton densities in the high-energy limit, the so-called unintegrated gluon distributions or UGDs and, more in general, to recent developments in high-energy physics. Full article

(This article belongs to the Special Issue Advance in Quark-Gluon-Plasma (QGP) Physics)

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15 pages, 311 KiB

Open AccessArticle

Scattering in Algebraic Approach to Quantum Theory—Associative Algebras

by Albert Schwarz

Universe 2022, 8(12), 660; https://doi.org/10.3390/universe8120660 - 15 Dec 2022

Cited by 7 | Viewed by 1224

Abstract

The definitions of scattering matrix and inclusive scattering matrix in the framework of formulation of quantum field theory in terms of associative algebras with involution are presented. The scattering matrix is expressed in terms of Green functions on shell (LSZ formula), and the [...] Read more.

The definitions of scattering matrix and inclusive scattering matrix in the framework of formulation of quantum field theory in terms of associative algebras with involution are presented. The scattering matrix is expressed in terms of Green functions on shell (LSZ formula), and the inclusive scattering matrix is expressed in terms of generalized Green functions on shell. The expression for inclusive scattering matrix can be used also for quasi-particles (for elementary excitations of any translation-invariant stationary state, for example, for elementary excitations of equilibrium state). An interesting novelty is the consideration of associative algebras over real numbers. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2023—Field Theory)

9 pages, 272 KiB

Open AccessArticle

Lorentz Gauge and Coulomb Gauge for Tetrad Field of Gravity

by Tao Zhou

Universe 2022, 8(12), 659; https://doi.org/10.3390/universe8120659 - 15 Dec 2022

Cited by 1 | Viewed by 1552

Abstract

In general relativity, an inertial frame can only be established in a small region of spacetime, and local inertial frames are mathematically represented by a tetrad field in gravity. The tetrad field is not unique due to the freedom to perform Lorentz transformations [...] Read more.

In general relativity, an inertial frame can only be established in a small region of spacetime, and local inertial frames are mathematically represented by a tetrad field in gravity. The tetrad field is not unique due to the freedom to perform Lorentz transformations in local inertial frames, and there exists freedom to choose the local inertial frame at each spacetime point. The local Lorentz transformations are known as non-Abelian gauge transformations for the tetrad field, and to fix the gauge freedom corresponding to the Lorentz gauge

\partial^{μ} A_{μ} = 0

and the Coulomb gauge

\partial^{i} A_{i} = 0

in electrodynamics, the Lorentz gauge and Coulomb gauge for tetrad fields are proposed in the present work. Moreover, properties of the Lorentz gauge and the Coulomb gauge for tetrad fields are discussed to show their similarities to those in electromagnetic fields. Full article

(This article belongs to the Section Field Theory)

10 pages, 263 KiB

Open AccessArticle

Generation of Primordial Magnetic Fields from QED and Higgs-like Domain Walls in Einstein–Cartan Gravity

by L. C. Garcia de Andrade

Universe 2022, 8(12), 658; https://doi.org/10.3390/universe8120658 - 14 Dec 2022

Cited by 1 | Viewed by 1331

Abstract

Spacetime torsion is known to be highly suppressed at the end of inflation, which is called preheating. This result was recently shown in (EPJ C (2022)) in the frame of Einstein–Cartan–Brans–Dicke inflation. In this paper, it is shown that a torsionful magnetogenesis in [...] Read more.

Spacetime torsion is known to be highly suppressed at the end of inflation, which is called preheating. This result was recently shown in (EPJ C (2022)) in the frame of Einstein–Cartan–Brans–Dicke inflation. In this paper, it is shown that a torsionful magnetogenesis in QED effective Lagrangean drives a torsion damping in order to be subsequently amplified by the dynamo effect after the generation of these magnetic fields seeds. This damping on amplification would depend upon the so-called torsion chirality. Here, a cosmic factor

g k K

is present where K is the contortion vector and k is the wave vector which is connected to the inverse of magnetic coherence length. In a second example, we find Higgs inlationary fields in Einstein–Cartan gravity thick domain walls (DWs). Recently, a modified Einstein–Cartan gravity was given by Shaposhnikov et al. [PRL (2020)] to obtain Higgs-like inflatons as a portal to dark energy. In the case of thick DW, we assume that there is a torsion squared influence, since we are in the early universe where torsion is not so weak as in the late universe as shown by Paul and SenGupta [EPJ C (2019)] in a 5D brane-world. A static DW solution is obtained when the inflationary potential vanishes and Higgs potential is a helical function. Recently, in the absence of inflation, domain wall dynamos were obtained in Einstein–Cartan gravity (EC) where the spins of the nucleons were orthogonal to the wall. Full article

(This article belongs to the Section Cosmology)

10 pages, 1040 KiB

Open AccessArticle

Chiral Magnetic Josephson Junction as a Base for Low-Noise Superconducting Qubits

by Maxim N. Chernodub, Julien Garaud and Dmitri E. Kharzeev

Universe 2022, 8(12), 657; https://doi.org/10.3390/universe8120657 - 14 Dec 2022

Cited by 3 | Viewed by 1891

Abstract

The lack of space inversion symmetry endows non-centrosymmetric superconducting materials with various interesting parity-breaking phenomena, including the anomalous Josephson effect. Our paper considers a Josephson junction of two non-centrosymmetric superconductors connected by a uniaxial ferromagnet. We show that this “Chiral Magnetic Josephson junction” [...] Read more.

The lack of space inversion symmetry endows non-centrosymmetric superconducting materials with various interesting parity-breaking phenomena, including the anomalous Josephson effect. Our paper considers a Josephson junction of two non-centrosymmetric superconductors connected by a uniaxial ferromagnet. We show that this “Chiral Magnetic Josephson junction” (CMJ junction) exhibits a direct analog of the Chiral Magnetic Effect, which has already been observed in Weyl and Dirac semimetals. We suggest that the CMJ can serve as an element of a qubit with a Hamiltonian tunable by the ferromagnet’s magnetization. The CMJ junction avoids using an offset magnetic flux in inductively shunted qubits, thus enabling a simpler and more robust architecture. Furthermore, when the uniaxial ferromagnet’s easy axis is directed across the junction, the resulting “chiral magnetic qubit” provides robust protection from the noise caused by magnetization fluctuations. Full article

(This article belongs to the Section Mathematical Physics)

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13 pages, 3632 KiB

Open AccessArticle

Self-Supervised Learning for Solar Radio Spectrum Classification

by Siqi Li, Guowu Yuan, Jian Chen, Chengming Tan and Hao Zhou

Universe 2022, 8(12), 656; https://doi.org/10.3390/universe8120656 - 14 Dec 2022

Cited by 3 | Viewed by 1586

Abstract

Solar radio observation is an important way to study the Sun. Solar radio bursts contain important information about solar activity. Therefore, real-time automatic detection and classification of solar radio bursts are of great value for subsequent solar physics research and space weather warnings. [...] Read more.

Solar radio observation is an important way to study the Sun. Solar radio bursts contain important information about solar activity. Therefore, real-time automatic detection and classification of solar radio bursts are of great value for subsequent solar physics research and space weather warnings. Traditional image classification methods based on deep learning often require considerable training data. To address insufficient solar radio spectrum images, transfer learning is generally used. However, the large difference between natural images and solar spectrum images has a large impact on the transfer learning effect. In this paper, we propose a self-supervised learning method for solar radio spectrum classification. Our method uses self-supervised training with a self-masking approach in natural language processing. Self-supervised learning is more conducive to learning the essential information about images compared with supervised methods, and it is more suitable for transfer learning. First, the method pre-trains using a large amount of other existing data. Then, the trained model is fine-tuned on the solar radio spectrum dataset. Experiments show that the method achieves a classification accuracy similar to that of convolutional neural networks and Transformer networks with supervised training. Full article

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

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18 pages, 3499 KiB

Open AccessArticle

Simultaneous Analysis of Midrapidity

p_{T}

Spectra of Identified Particle Species in Pb + Pb Collisions at

\sqrt{s_{n n}}

= 2.76 TeV Using Tsallis Distribution with Transverse Flow

by Khusniddin K. Olimov, Igor A. Lebedev, Anastasiya I. Fedosimova, Fu-Hu Liu, Shakhnoza Z. Kanokova, Maratbek Z. Shodmonov and Boburbek J. Tukhtaev

Universe 2022, 8(12), 655; https://doi.org/10.3390/universe8120655 - 13 Dec 2022

Cited by 5 | Viewed by 1785

Abstract

The midrapidity transverse momentum distributions of the charged pions, kaons, protons, and antiprotons in ten groups of centrality of Pb + Pb collisions at

\sqrt{s_{n n}}

= 2.76 TeV, measured by the ALICE Collaboration, have been analyzed successfully using both thermodynamically consistent [...] Read more.

The midrapidity transverse momentum distributions of the charged pions, kaons, protons, and antiprotons in ten groups of centrality of Pb + Pb collisions at

\sqrt{s_{n n}}

= 2.76 TeV, measured by the ALICE Collaboration, have been analyzed successfully using both thermodynamically consistent and non-consistent Tsallis distribution functions with transverse flow. The collision centrality dependencies of the extracted parameters of two kinds of Tsallis functions with transverse flow have been investigated. The significantly different behavior (growth rates) of ⟨β_T⟩ in regions

⟨ N_{p a r t} ⟩

< 71 and

⟨ N_{p a r t} ⟩

> 71 with the temperature T₀ becoming constant in region

⟨ N_{p a r t} ⟩

> 71 has been observed. This could indicate that

⟨ N_{p a r t} ⟩

= 71 ± 5 (corresponding to

⟨ d N_{c h} / d η ⟩

= 205 ± 15) is a threshold border value of collision centrality for crossover phase transition from the dense hadronic state to the QGP state (or a mixed state of QGP and hadrons) in Pb + Pb collisions at

\sqrt{s_{n n}}

= 2.76 TeV. This conjecture is supported further by the observed, significantly different correlations between T₀ and

⟨ β_{T} ⟩

parameters in the corresponding

⟨ β_{T} ⟩

< 0.44 and

⟨ β_{T} ⟩

> 0.44 ranges. The strong positive linear correlation between non-extensivity parameter q for pions and kaons, between q for pions and (anti)protons, and between q for kaons and (anti)protons has been obtained. The parameter q for all studied particle species has proven to be strongly anticorrelated with the average transverse flow velocity, ⟨β_T⟩. Quite a large positive linear correlation has been obtained between the q of the studied particle species and temperature parameter T₀. Analysis of q versus

⟨ N_{p a r t} ⟩

dependencies for the studied particle species suggests that the highly thermalized and equilibrated QGP is produced in central Pb + Pb collisions at

\sqrt{s_{n n}}

= 2.76 TeV with

⟨ N_{p a r t} ⟩

> 160. Full article

(This article belongs to the Special Issue Collectivity in High-Energy Proton-Proton and Heavy-Ion Collisions)

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17 pages, 572 KiB

Open AccessArticle

The Contribution of Charged Bosons with Right-Handed Neutrinos to the Muon g − 2 Anomaly in the Twin Higgs Models

by Guo-Li Liu and Ping Zhou

Universe 2022, 8(12), 654; https://doi.org/10.3390/universe8120654 - 11 Dec 2022

Cited by 4 | Viewed by 1365

Abstract

We examine the charged boson and right-handed neutrino contribution to the muon

g - 2

anomaly in twin Higgs models with joint constraints of Higgs global fit data, precision electroweak data, leptonic flavor-changing decay

μ \to e γ

, and the mass requirement [...] Read more.

We examine the charged boson and right-handed neutrino contribution to the muon

g - 2

anomaly in twin Higgs models with joint constraints of Higgs global fit data, precision electroweak data, leptonic flavor-changing decay

μ \to e γ

, and the mass requirement of heavy-gauge bosons. It comes with the conclusion that some parameters, such as the coupling of charged Higgs to the lepton

y_{μ}

, the top Yukawa

y_{t}

, and heavy-gauge boson coupling to the lepton

V_{μ}

are constrained roughly in the range of

0.12 ≲ y_{μ} ≲ 0.4

,

0.4 ≲ y_{t} ≲ 0.9

, and

0.47 ≲ V_{μ} ≲ 1

, respectively. Full article

(This article belongs to the Special Issue Origin of the Flavor Structure in the Standard Model and Beyond)

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12 pages, 897 KiB

Open AccessArticle

Radio Emission from Supernova Remnants: Model Comparison with Observations

by Denis A. Leahy, Felicity Merrick and Miroslav Filipović

Universe 2022, 8(12), 653; https://doi.org/10.3390/universe8120653 - 8 Dec 2022

Cited by 8 | Viewed by 1568

Abstract

Supernova remnants (SNRs) are an integral part in studying the properties of the Galaxy and its interstellar medium. For the current work, we compare the observed radio luminosities of SNRs to predictions based on a recent analytic model applied to 54 SNRs with [...] Read more.

Supernova remnants (SNRs) are an integral part in studying the properties of the Galaxy and its interstellar medium. For the current work, we compare the observed radio luminosities of SNRs to predictions based on a recent analytic model applied to 54 SNRs with X-ray observations. We use the X-ray data to determine the properties of shock velocities, ages and circumstellar densities for the SNRs, whereas shock radii are determined from catalogs. With this set of SNR properties, we can calculate the model radio emission and compare it to the observed radio emission for a sample of SNRs. This is the first time that this test has been carried out—previously the SNR properties were assumed instead of derived from X-ray data. With the assumption that the radio emission process depends on SNR properties in the form of power-law functions, we explore ways to improve the radio emission model. The main results of this study are (i) the model has significant deficiencies and cannot reproduce observed radio emission; and (ii) the model can be improved significantly by changing its dependence on SNR parameters, although the improved model is still not accurate. Significant work remains to improve the components of radio emission models, including changes to the SNR evolution model, the radio emitting volume, and the efficiencies for conversion of shock energy into relativistic electrons and for magnetic field amplification. Full article

(This article belongs to the Special Issue Supernovae Observations and Researches)

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14 pages, 383 KiB

Open AccessArticle

PossibleExistence of Dark-Matter-Admixed Pulsar in the Disk Region of the Milky Way Galaxy

by Nilofar Rahman, Masum Murshid, Sajahan Molla and Mehedi Kalam

Universe 2022, 8(12), 652; https://doi.org/10.3390/universe8120652 - 7 Dec 2022

Cited by 1 | Viewed by 1295

Abstract

In our previous study, (Eur Phys J Plus 135:362, 2020 & Eur Phys J Plus 135:637, 2020), we have discussed the possible existence of the dark-matter-admixed pulsars, located in dwarf as well as in massive spiral galaxies (based on Singular Isothermal Sphere dark-matter [...] Read more.

In our previous study, (Eur Phys J Plus 135:362, 2020 & Eur Phys J Plus 135:637, 2020), we have discussed the possible existence of the dark-matter-admixed pulsars, located in dwarf as well as in massive spiral galaxies (based on Singular Isothermal Sphere dark-matter density profile) and in the Milky Way galaxy (based on Universal Rotational Curve dark-matter density profile). In this article, we use the Navarro–Frenk–White (NFW) dark-matter density profile to get analogous results for the pulsars in the disk region of the Milky Way galaxy. These findings may be treated as valuable complements to the previous findings. We conclude from our findings that there is a unique possibility of the presence of dark-matter-admixed pulsars in all the regions of the galaxies. Full article

(This article belongs to the Section Compact Objects)

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27 pages, 565 KiB

Open AccessArticle

Multiverse Predictions for Habitability: Element Abundances

by McCullen Sandora, Vladimir Airapetian, Luke Barnes, Geraint F. Lewis and Ileana Pérez-Rodríguez

Universe 2022, 8(12), 651; https://doi.org/10.3390/universe8120651 - 7 Dec 2022

Cited by 5 | Viewed by 2974

Abstract

We investigate the dependence of elemental abundances on physical constants, and the implications this has for the distribution of complex life for various proposed habitability criteria. We consider three main sources of abundance variation: differing supernova rates, alpha burning in massive stars, and [...] Read more.

We investigate the dependence of elemental abundances on physical constants, and the implications this has for the distribution of complex life for various proposed habitability criteria. We consider three main sources of abundance variation: differing supernova rates, alpha burning in massive stars, and isotopic stability, and how each affects the metal-to-rock ratio and the abundances of carbon, oxygen, nitrogen, phosphorus, sulfur, silicon, magnesium, and iron. Our analysis leads to several predictions for which habitability criteria are correct by determining which ones make our observations of the physical constants, as well as a few other observed features of our universe, most likely. Our results indicate that carbon-rich or carbon-poor planets are uninhabitable, slightly magnesium-rich planets are habitable, and life does not depend on nitrogen abundance too sensitively. We also find suggestive but inconclusive evidence that metal-rich planets and phosphorus-poor planets are habitable. These predictions can then be checked by probing regions of our universe that closely resemble normal environments in other universes. If any of these predictions are found to be wrong, the multiverse scenario would predict that the majority of observers are born in universes differing substantially from ours, and so can be ruled out, to varying degrees of statistical significance. Full article

(This article belongs to the Section Planetary Sciences)

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17 pages, 776 KiB

Open AccessArticle

Bulk Viscous Flat FLRW Model with Observational Constraints in f(T, B) Gravity

by Archana Dixit and Anirudh Pradhan

Universe 2022, 8(12), 650; https://doi.org/10.3390/universe8120650 - 7 Dec 2022

Cited by 5 | Viewed by 1569

Abstract

This paper investigates the impact of bulk viscosity within the framework of

f (T, B)

gravity. We consider a time-dependent viscosity model with a particular Hubble parameter expression. Here, we looked into the viability of well-motivated [...] Read more.

This paper investigates the impact of bulk viscosity within the framework of

f (T, B)

gravity. We consider a time-dependent viscosity model with a particular Hubble parameter expression. Here, we looked into the viability of well-motivated

f (T, B)

gravity model, which takes the form

f = α log (B) + β T

, and has free parameters

α

and

β

. The 46 observational Hubble data (OHD) in the range

0 \leq z \leq 2.36

were used to constrain the model parameters to achieve the solution. We have used the Markov Chain Monte Carlo (MCMC) method to estimate model parameters and observe that the model appears to be in good agreement with the observations. In addition, we evaluate the effective viscous equation of state parameter for the

f (T, B)

model. We have examined the characteristics of different energy conditions for the stability analysis. The model is valid based on the positive behavior of null energy conditions (NEC), weak energy conditions (WEC), and dominant energy conditions (DEC); however, strong energy conditions (SEC) are in violation, suggesting that the universe is expanding faster. Our model was found in the quintom region. We also discussed how the tachyon scalar field corresponds to

f (T, B)

gravity. Full article

(This article belongs to the Section Gravitation)

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35 pages, 2394 KiB

Open AccessArticle

Probing the Low-Mass End of the Black Hole Mass Function via a Study of Faint Local Spiral Galaxies

by Michael S. Fusco, Benjamin L. Davis, Julia Kennefick, Daniel Kennefick and Marc S. Seigar

Universe 2022, 8(12), 649; https://doi.org/10.3390/universe8120649 - 6 Dec 2022

Cited by 5 | Viewed by 2260

Abstract

We present an analysis of the pitch angle distribution function (PADF) for nearby galaxies and its resulting black hole mass function (BHMF) via the well-known relationship between pitch angle and black hole mass. Our sample consists of a subset of 74 spiral galaxies [...] Read more.

We present an analysis of the pitch angle distribution function (PADF) for nearby galaxies and its resulting black hole mass function (BHMF) via the well-known relationship between pitch angle and black hole mass. Our sample consists of a subset of 74 spiral galaxies from the Carnegie-Irvine Galaxy Survey with absolute B-band magnitude

M_{B} > - 19.12

mag and luminosity distance

D_{L} \leq 25.4

Mpc, which is an extension of a complementary set of 140 more luminous (

M_{B} \leq - 19.12

mag) late-type galaxies. We find the PADFs of the two samples are, somewhat surprisingly, not strongly dissimilar; a result that may hold important implications for spiral formation theories. Our data show a distinct bimodal population manifest in the pitch angles of the Sa–Sc types and separately the Scd–Sm types, with Sa–Sc types having tighter spiral arms on average. Importantly, we uncover a distinct bifurcation of the BHMF, such that the Sa–Sc galaxies typically host so-called “supermassive” black holes (

M_{•} ≳ 10^{6} M_{⊙}

), whereas Scd–Sm galaxies accordingly harbor black holes that are “less-than-supermassive” (

M_{•} ≲ 10^{6} M_{⊙}

). It is amongst this latter population of galaxies where we expect fruitful bounties of elusive intermediate-mass black holes (IMBHs), through which a better understanding will help form more precise benchmarks for future generations of gravitational wave detectors. Full article

(This article belongs to the Section Galaxies and Clusters)

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11 pages, 277 KiB

Open AccessArticle

Quantum Scalar-Field Propagator in a Stochastic Gravitational-Plane Wave

by Zbigniew Haba

Universe 2022, 8(12), 648; https://doi.org/10.3390/universe8120648 - 5 Dec 2022

Cited by 2 | Viewed by 2137

Abstract

A stochastic metric can appear in classical as well as in quantum gravity. We show that if the linearized stochastic Gaussian gravitational-plane wave has the frequency spectrum

ω^{4 γ - 1}

(

0 \leq γ < 1

), then the equal-time propagator [...] Read more.

A stochastic metric can appear in classical as well as in quantum gravity. We show that if the linearized stochastic Gaussian gravitational-plane wave has the frequency spectrum

ω^{4 γ - 1}

(

0 \leq γ < 1

), then the equal-time propagator of the scalar field behaves as

p^{- \frac{1}{1 - γ}}

for large momenta. We discuss models of quantum-field theory where such anomalous behavior can appear. Full article

(This article belongs to the Special Issue Quantum Physics including Gravity: Highlights and Novelties)

13 pages, 400 KiB

Open AccessArticle

Cooling Process of White Dwarf Stars in Palatini f(R) Gravity

by Surajit Kalita, Lupamudra Sarmah and Aneta Wojnar

Universe 2022, 8(12), 647; https://doi.org/10.3390/universe8120647 - 5 Dec 2022

Cited by 7 | Viewed by 1546

Abstract

A simple cooling model of white dwarf stars is re-analyzed in Palatini

f (R)

gravity. Modified gravity affects the white dwarf structures and consequently their ages. We find that the resulting super-Chandrasekhar white dwarfs need more time to cool down than [...] Read more.

A simple cooling model of white dwarf stars is re-analyzed in Palatini

f (R)

gravity. Modified gravity affects the white dwarf structures and consequently their ages. We find that the resulting super-Chandrasekhar white dwarfs need more time to cool down than sub-Chandrasekhar ones, or when compared to the Newtonian models. Full article

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

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8 pages, 1854 KiB

Open AccessFeature PaperArticle

Coronal Field Geometry and Solar Wind Speed

by Ivan Berezin and Andrey Tlatov

Universe 2022, 8(12), 646; https://doi.org/10.3390/universe8120646 - 5 Dec 2022

Cited by 6 | Viewed by 1671

Abstract

The Wang–Sheeley–Arge (WSA) solar wind (SW) model is based on the idea that weakly expanding coronal magnetic field tubes are associated with sources of fast SWs and vice versa. A parameter called the “flux tube expansion” (FTE) is used to determine the degree [...] Read more.

The Wang–Sheeley–Arge (WSA) solar wind (SW) model is based on the idea that weakly expanding coronal magnetic field tubes are associated with sources of fast SWs and vice versa. A parameter called the “flux tube expansion” (FTE) is used to determine the degree of expansion of magnetic tubes. The FTE is calculated based on the coronal magnetic field model, usually in the potential approximation. The second input parameter for the WSA model is the great circle distance from the base of the open magnetic field line in the photosphere to the boundary of the corresponding coronal hole (DCHB). These two coronal magnetic field parameters are related by an empirical relationship with the solar wind velocity near the Sun. The WSA model has shortcomings and does not fully explain the solar wind formation mechanisms. In the present work, we model various coronal magnetic field parameters in the potential-field source-surface (PFSS) approximation from a long series of magnetographic observations: the Solar Telescope-magnetograph for Operative Prognoses (STOP) (Kislovodsk Mountain Astronomical Station), the Helioseismic and magnetic imager (SDO/HMI), and data from the Wilcox Solar Observatory (WSO). Our main goal is to identify correlations between the coronal magnetic field parameters and the observed SW velocity in order to use them for modeling SW. We found that the SW velocity correlates relatively well with some geometric properties of the magnetic tubes, including the force line length, the latitude of the force line footpoints, and the DCHB. We propose a formula for calculating the SW velocity based on these parameters. The presented relationship does not use FTE and showed a better correlation with observations compared to the WSA model. Full article

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

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6 pages, 241 KiB

Open AccessCommunication

Bekenstein Bound and Non-Commutative Canonical Variables

by Fabio Scardigli

Universe 2022, 8(12), 645; https://doi.org/10.3390/universe8120645 - 5 Dec 2022

Viewed by 1359

Abstract

A universal upper limit on the entropy contained in a localized quantum system of a given size and total energy is expressed by the so-called Bekenstein bound. In a previous paper [Buoninfante, L. et al. 2022], on the basis of general thermodynamic arguments, [...] Read more.

A universal upper limit on the entropy contained in a localized quantum system of a given size and total energy is expressed by the so-called Bekenstein bound. In a previous paper [Buoninfante, L. et al. 2022], on the basis of general thermodynamic arguments, and in regimes where the equipartition theorem still holds, the Bekenstein bound has been proved practically equivalent to the Heisenberg uncertainty relation. The smooth transition between the Bekenstein bound and the holographic bound suggests a new pair of canonical non-commutative variables, which could be thought to hold in strong gravity regimes. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

10 pages, 1750 KiB

Open AccessArticle

Linear Polarization Signatures of Particle Acceleration in High-Synchrotron-Peak Blazars

by Alan P. Marscher and Svetlana G. Jorstad

Universe 2022, 8(12), 644; https://doi.org/10.3390/universe8120644 - 4 Dec 2022

Cited by 12 | Viewed by 1785

Abstract

Blazars whose synchrotron spectral energy distribution (SED) peaks at X-ray energies need to accelerate electrons to energies in the >100 GeV range in relativistic plasma jets at distances of parsecs from the central engine. Compton scattering by the same electrons can explain high [...] Read more.

Blazars whose synchrotron spectral energy distribution (SED) peaks at X-ray energies need to accelerate electrons to energies in the >100 GeV range in relativistic plasma jets at distances of parsecs from the central engine. Compton scattering by the same electrons can explain high luminosities at very high photon energies (>100 GeV) from the same objects. Turbulence combined with a standing conical shock can accomplish this. Such a scenario can also qualitatively explain the level and variability of linear polarization observed at optical frequencies in these objects. Multi-wavelength polarization measurements, including those at X-ray energies by the Imaging X-ray Polarimetry Explorer (IXPE), find that the degree of polarization is several times higher at X-ray than at optical wavelengths, in general agreement with the turbulence-plus-shock picture. Some detailed properties of the observed polarization can be naturally explained by this scenario, while others pose challenges that may require modifications to the model. Full article

(This article belongs to the Special Issue Black Holes and Relativistic Jets)

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22 pages, 872 KiB

Open AccessArticle

Ultraviolet and X-ray Light-Curves of Novae Observed by the Neil Gehrels Swift Observatory

by Kim L. Page, N. Paul M. Kuin and Julian P. Osborne

Universe 2022, 8(12), 643; https://doi.org/10.3390/universe8120643 - 4 Dec 2022

Cited by 4 | Viewed by 1668

Abstract

With rapid response capabilities, and a daily planning of its observing schedule, the Neil Gehrels Swift Observatory is ideal for monitoring transient and variable sources. Here we present a sample of the 12 novae with the most detailed ultraviolet (UV) follow-up by Swift [...] Read more.

With rapid response capabilities, and a daily planning of its observing schedule, the Neil Gehrels Swift Observatory is ideal for monitoring transient and variable sources. Here we present a sample of the 12 novae with the most detailed ultraviolet (UV) follow-up by Swift—the first uniform analysis of such UV light-curves. The fading of these specific light-curves can be modelled as power-law decays (plotting magnitude against log time), showing that the same physical processes dominate the UV emission for extended time intervals in individual objects. After the end of the nuclear burning interval, the X-ray emission drops significantly, fading by a factor of around 10–100. The UV changes, however, are of a lower amplitude, declining by 1–2 mag over the same time period. The UV light-curves typically show a break from flatter to steeper around the time at which the X-ray light-curve starts a steady decline from maximum, ∼0.7–1.3 T

_{SSSend}

. Considering populations of both classical and recurrent novae, and those with main sequence or giant companions, we do not find any strong differences in the UV light-curves or their evolution, although the long-period recurrent novae are more luminous than the majority of the classical novae. Full article

(This article belongs to the Special Issue 18 Years of Science with the Neil Gehrels Swift Observatory's Ultra-Violet/Optical Telescope)

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16 pages, 569 KiB

Open AccessArticle

Scalar Field Models of Barrow Holographic Dark Energy in f(R,T) Gravity

by Umesh Kumar Sharma, Mukesh Kumar and Gunjan Varshney

Universe 2022, 8(12), 642; https://doi.org/10.3390/universe8120642 - 2 Dec 2022

Cited by 10 | Viewed by 1681

Abstract

This research study investigates Barrow holographic dark energy with an energy density of

ρ_{Λ} = C H^{2 - Δ}

by considering the Hubble horizon as the IR cut-off in the

f (R, T)

gravity framework. We employ Barrow [...] Read more.

This research study investigates Barrow holographic dark energy with an energy density of

ρ_{Λ} = C H^{2 - Δ}

by considering the Hubble horizon as the IR cut-off in the

f (R, T)

gravity framework. We employ Barrow holographic dark energy to obtain the equation of the state for the Barrow holographic energy density in a flat FLRW Universe. Concretely, we study the correspondence between quintessence, k-essence, and dilation scalar field models with the Barrow holographic dark energy in a flat

f (R, T)

Universe. Furthermore, we reconstruct the dynamics and potential for all these models for different values of the Barrow parameter:

Δ

. Via this study, we can show that for Barrow holographic quintessence, k-essence, and dilation scalar field models, if the corresponding model parameters satisfy some limitations, the accelerated expansion can be achieved. Full article

(This article belongs to the Special Issue Dark Matter and Dark Energy: Particle Physics, Cosmology, and Experimental Searches)

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41 pages, 1887 KiB

Open AccessReview

Pulsar Glitches: A Review

by Shiqi Zhou, Erbil Gügercinoğlu, Jianping Yuan, Mingyu Ge and Cong Yu

Universe 2022, 8(12), 641; https://doi.org/10.3390/universe8120641 - 1 Dec 2022

Cited by 30 | Viewed by 4299

Abstract

∼

6 %

of all known pulsars have been observed to exhibit sudden spin-up events, known as glitches. For more than fifty years, these phenomena have played an important role in helping to understand pulsar (astro)physics. Based on the review of pulsar glitches [...] Read more.

∼

6 %

of all known pulsars have been observed to exhibit sudden spin-up events, known as glitches. For more than fifty years, these phenomena have played an important role in helping to understand pulsar (astro)physics. Based on the review of pulsar glitches search method, the progress made in observations in recent years is summarized, including the achievements obtained by Chinese telescopes. Glitching pulsars demonstrate great diversity of behaviours, which can be broadly classified into four categories: normal glitches, slow glitches, glitches with delayed spin-ups, and anti-glitches. The main models of glitches that have been proposed are reviewed and their implications for neutron star structure are critically examined regarding our current understanding. Furthermore, the correlations between glitches and emission changes, which suggest that magnetospheric state-change is linked to the pulsar-intrinsic processes, are also described and discussed in some detail. Full article

(This article belongs to the Special Issue Frontiers in Pulsars Astrophysics)

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6 pages, 315 KiB

Open AccessCommunication

A Causality Preserving Evolution of a Pair of Strings

by Serguei Krasnikov

Universe 2022, 8(12), 640; https://doi.org/10.3390/universe8120640 - 1 Dec 2022

Cited by 2 | Viewed by 1117

Abstract

As follows from Gott’s discovery, a pair of straight string-like singularities moving in opposite directions, when they have suitable speed and impact parameter, produce closed timelike curves. I argue in this paper that there always is a not-so-frightening alternative: the Universe may prefer [...] Read more.

As follows from Gott’s discovery, a pair of straight string-like singularities moving in opposite directions, when they have suitable speed and impact parameter, produce closed timelike curves. I argue in this paper that there always is a not-so-frightening alternative: the Universe may prefer to produce a certain (surprisingly simple and absolutely mild) singularity instead. Full article

(This article belongs to the Special Issue The Physics of Time Travel)

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9 pages, 262 KiB

Open AccessArticle

Double Hawking Temperature: From Black Hole to de Sitter

by Grigory E. Volovik

Universe 2022, 8(12), 639; https://doi.org/10.3390/universe8120639 - 1 Dec 2022

Cited by 7 | Viewed by 1331

Abstract

The double Hawking temperature

T = 2 T_{H}

appears in some approaches to the Hawking radiation when the radiation is considered in terms of the quantum tunneling. We consider the origin of such unusual temperature for the black hole horizon and also [...] Read more.

The double Hawking temperature

T = 2 T_{H}

appears in some approaches to the Hawking radiation when the radiation is considered in terms of the quantum tunneling. We consider the origin of such unusual temperature for the black hole horizon and also for the cosmological horizon in de Sitter spacetime. In the case of the black hole horizon, there are two contributions to the tunneling process of radiation, each being governed by the temperature

T = 2 T_{H}

. These processes are coherently combined to produce the radiation with the Hawking temperature

T_{H}

. This can be traditionally interpreted as the pair creation of two entangled particles, of which one goes towards the center of the black hole, while the other one escapes from the black hole. In the case of the cosmological horizon, the temperature

T = 2 T_{H}

is physical. While the creation of the entangled pair is described by the Hawking temperature, the de Sitter spacetime allows for another process, in which only a single (non-entangled) particle inside the cosmological horizon is created. This process is characterized by the local temperature

T = 2 T_{H}

. The local single-particle process also takes place outside the black hole horizon, but it is exponentially suppressed. Full article

(This article belongs to the Section Cosmology)

15 pages, 514 KiB

Open AccessEditor’s ChoiceReview

Four-top quark physics at the LHC

by Freya Blekman, Fréderic Déliot, Valentina Dutta and Emanuele Usai

Universe 2022, 8(12), 638; https://doi.org/10.3390/universe8120638 - 30 Nov 2022

Cited by 9 | Viewed by 1704

Abstract

The production of four top quarks presents a rare process in the Standard Model that provides unique opportunities and sensitivity to Standard Model observables including potential enhancement of many popular new physics extensions. This article summarises the latest experimental measurements of the four-top [...] Read more.

The production of four top quarks presents a rare process in the Standard Model that provides unique opportunities and sensitivity to Standard Model observables including potential enhancement of many popular new physics extensions. This article summarises the latest experimental measurements of the four-top quark production cross section at the LHC. An overview is provided detailing interpretations of the experimental results regarding the top quark Yukawa coupling in addition to the limits on physics beyond the Standard Model. Further, prospects for future measurements and opportunities offered by this challenging final state are given herein. Full article

(This article belongs to the Special Issue Top Quark at the New Physics Frontier)

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9 pages, 1808 KiB

Open AccessArticle

Radiation Dosimetry Estimations in the Venusian Atmosphere during Different Periods of Solar Activity

by Anastasia Tezari, Argyris N. Stassinakis, Pavlos Paschalis, Helen Mavromichalaki, Christina Plainaki, Anastasios Kanellakopoulos, Norma Crosby, Mark Dierckxsens and Pantelis Karaiskos

Universe 2022, 8(12), 637; https://doi.org/10.3390/universe8120637 - 30 Nov 2022

Cited by 2 | Viewed by 1735

Abstract

The new space era has expanded the exploration of other planets of our solar system. In this work, radiation quantities are estimated in the Venusian atmosphere using the software tool DYASTIMA/DYASTIMA-R, such as the energy deposit and the ambient dose equivalent rate. Monte [...] Read more.

The new space era has expanded the exploration of other planets of our solar system. In this work, radiation quantities are estimated in the Venusian atmosphere using the software tool DYASTIMA/DYASTIMA-R, such as the energy deposit and the ambient dose equivalent rate. Monte Carlo simulations of the secondary particle cascades for different atmospheric layers were performed during solar minimum and solar maximum conditions, as well as during the extreme solar particle event that took place in October 1989, with a focus on the so-called Venusian zone of habitability. Full article

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

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15 pages, 1166 KiB

Open AccessArticle

Bouncing Cosmology in Modified Gravity with Higher-Order Gauss–Bonnet Curvature Term

by Santosh V. Lohakare, Francisco Tello-Ortiz, S. K. Tripathy and B. Mishra

Universe 2022, 8(12), 636; https://doi.org/10.3390/universe8120636 - 29 Nov 2022

Cited by 10 | Viewed by 1601

Abstract

In this paper, we studied the bouncing behavior of the cosmological models formulated in the background of the Hubble function in the

F (R, G)

theory of gravity, where R and

G

, respectively, denote the Ricci scalar and Gauss–Bonnet [...] Read more.

In this paper, we studied the bouncing behavior of the cosmological models formulated in the background of the Hubble function in the

F (R, G)

theory of gravity, where R and

G

, respectively, denote the Ricci scalar and Gauss–Bonnet invariant. The actions of the bouncing cosmology are studied with a consideration of the different viable models that can resolve the difficulty of singularity in standard Big Bang cosmology. Both models show bouncing behavior and satisfy the bouncing cosmological properties. Models based on dynamical, deceleration, and energy conditions indicate the accelerating behavior at the late evolution time. The phantom at the bounce epoch is analogous to quintessence behavior. Finally, we formulate the perturbed evolution equations and investigate the stability of the two bouncing solutions. Full article

(This article belongs to the Special Issue Dark Matter and Dark Energy: Particle Physics, Cosmology, and Experimental Searches)

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13 pages, 343 KiB

Open AccessArticle

Weak Field Limit for Embedding Gravity

by Stanislav Kuptsov, Mikhail Ioffe, Sergey Manida and Sergey Paston

Universe 2022, 8(12), 635; https://doi.org/10.3390/universe8120635 - 29 Nov 2022

Cited by 2 | Viewed by 1209

Abstract

We study a perturbation theory for embedding gravity equations in a background for which corrections to the embedding function are linear with respect to corrections to the flat metric. The remaining arbitrariness after solving the linearized field equations is fixed by an assumption [...] Read more.

We study a perturbation theory for embedding gravity equations in a background for which corrections to the embedding function are linear with respect to corrections to the flat metric. The remaining arbitrariness after solving the linearized field equations is fixed by an assumption that the solution is static in the second order. A nonlinear differential equation is obtained, which allows for finding the gravitational potential for a spherically symmetric case if a background embedding is given. An explicit form of a spherically symmetric background parameterized by one function of radius is proposed. It is shown that this function can be chosen in such a way that the gravitational potential is in a good agreement with the observed distribution of dark matter in a galactic halo. Full article

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

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Universe, Volume 8, Issue 12 (December 2022) – 55 articles

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