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Open AccessArticle

Chandrasekhar Mass Limit of White Dwarfs in Modified Gravity

by

Artyom V. Astashenok

,

Sergey D. Odintsov

and

Vasilis K. Oikonomou

Symmetry 2023, 15(6), 1141; https://doi.org/10.3390/sym15061141 - 24 May 2023

Viewed by 209

Abstract

We investigate the Chandrasekhar mass limit of white dwarfs in various models of

f (R)

gravity. Two equations of state for stellar matter are used: the simple relativistic polytropic equation with polytropic index

n = 3

and the realistic Chandrasekhar equation [...] Read more.

We investigate the Chandrasekhar mass limit of white dwarfs in various models of

f (R)

gravity. Two equations of state for stellar matter are used: the simple relativistic polytropic equation with polytropic index

n = 3

and the realistic Chandrasekhar equation of state. For calculations, it is convenient to use the equivalent scalar–tensor theory in the Einstein frame and then to return to the Jordan frame picture. For white dwarfs, we can neglect terms containing relativistic effects from General Relativity and we consider the reduced system of equations. Its solution for any model of

f (R) = R + β R^{m}

(

m \geq 2

,

β > 0

) gravity leads to the conclusion that the stellar mass decreases in comparison with standard General Relativity. For realistic equations of state, we find that there is a value of the central density for which the mass of a white dwarf peaks. Therefore, in frames of modified gravity, there is a lower limit on the radius of stable white dwarfs, and this minimal radius is greater than in General Relativity. We also investigate the behavior of the Chandrasekhar mass limit in

f (R)

gravity. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

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Open AccessArticle

Viability of Baryon to Entropy Ratio in Modified Hořava–Lifshitz Gravity

by

Abdul Jawad

,

Abdul Malik Sultan

and

Shamaila Rani

Symmetry 2023, 15(4), 824; https://doi.org/10.3390/sym15040824 - 29 Mar 2023

Viewed by 585

Abstract

In this paper, we study the matter–antimatter imbalance in the universe through baryogenesis (also known as baryosynthesis), which is a physical process that took off just a little while after the big bang explosion, producing a supremacy of matter over antimatter. In this [...] Read more.

In this paper, we study the matter–antimatter imbalance in the universe through baryogenesis (also known as baryosynthesis), which is a physical process that took off just a little while after the big bang explosion, producing a supremacy of matter over antimatter. In this work, we commit the reproduction of the baryon to entropy ratio (

\frac{η_{_{B}}}{S} = \frac{η_{_{β}} - η_{\bar{_{β}}}}{S}

), where

η_{_{β}} (η_{\bar{_{β}}})

is a baryon(anti-baryon) number and S is the entropy of the universe in the presence of modified Hořava-Lifshitz

F (R)

gravity, which is also called

F (\tilde{R})

-gravity. We inspect different baryogenesis interactions proportional to

\tilde{R}

(where

\tilde{R}

is the argument of general function F used for the development of modified Hořava-Lifshitz gravity). For this study, we examine two models by choosing different values of

F (\tilde{R})

. In the first model, the functional value of

F (\tilde{R}) = \tilde{R} + α {\tilde{R}}^{2}

(where

α

is a real constant). The second model is more generalized and extended as compare to first one. Mathematically, this model is given by

F (\tilde{R}) = \tilde{R} + α {\tilde{R}}^{2} + β {\tilde{R}}^{m}

, where

α

,

β

are real constants and

m > 2

is a real model parameter. Our results for both models and different values of m point out that matter-antimatter asymmetry does not vanish under the effect of the modified Hořava-Lifshitz theory of gravity, which shows a consistent and compatible fact of gravitational baryogenesis with recent observational data. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

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Open AccessArticle

The Effects of Reduced Gravity and Radiative Heat Transfer on the Magnetohydrodynamic Flow Past a Non-Rotating Stationary Sphere Surrounded by a Porous Medium

by

,

,

,

,

and

Symmetry 2023, 15(4), 806; https://doi.org/10.3390/sym15040806 - 26 Mar 2023

Cited by 2 | Viewed by 442

Abstract

In the present study, the effects of reduced gravity and solar radiation on the magnetohydrodynamics (MHD) fluid flow and heat transfer past a solid and stationary sphere embedded in a porous medium are investigated. A model describing the considered configuration is put in [...] Read more.

In the present study, the effects of reduced gravity and solar radiation on the magnetohydrodynamics (MHD) fluid flow and heat transfer past a solid and stationary sphere embedded in a porous medium are investigated. A model describing the considered configuration is put in dimensionless form using appropriate dimensionless variables and then transformed to primitive form for a smooth algorithm on a computing tool. A primitive form of the model is solved by employing the finite difference method. Solutions for variables of interest, such as velocity distribution and temperature field, along with their gradients, are depicted in graphs and tables. The main goal of the paper is to study the physical impact of reduced gravity on heat transfer and fluid flow around a sphere surface inserted in a porous medium in the presence of an applied magnetic field and solar radiation. The effects of the governing parameters, which are the reduced gravity parameter, magnetic field parameter, radiation parameter, porous medium parameter, and the Prandtl number, are discussed and physically interpreted. The displayed solutions indicate that velocity rises with the reduced gravity and solar radiation parameters but decreases with augmenting the Prandtl number, magnetic field parameter, and porous medium parameter. It is deduced from the presented results that the temperature becomes lower by increasing the values of the reduced gravity parameter and the Prandtl number, but, on the other hand, it becomes higher by increasing the values of the magnetic field, the porous medium, and the radiation parameters at all the considered positions of the surface of the sphere. A comparison between the present and already published results is performed to check the validity of the proposed numerical model. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

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Open AccessArticle

Study of Propeller Vortex Characteristics under Loading Conditions

by

,

,

,

,

and

Symmetry 2023, 15(2), 445; https://doi.org/10.3390/sym15020445 - 07 Feb 2023

Viewed by 648

Abstract

Marine load is an important factor affecting propeller propulsion efficiency, and the study of the wake evolution mechanism under different conditions is an essential part of the propeller equipment design, which needs to meet the requirements of complex engineering. Based on the large [...] Read more.

Marine load is an important factor affecting propeller propulsion efficiency, and the study of the wake evolution mechanism under different conditions is an essential part of the propeller equipment design, which needs to meet the requirements of complex engineering. Based on the large eddy simulation (LES) method, the wake instability characteristics are researched with the hydrodynamic load and wake dynamics theory, and the vortices composition and evolution mechanism under various load conditions are analyzed. Meanwhile, the propeller wake using the unsteady Reynolds-averaged Navier–Stokes (URANS) and LES methods is numerically simulated and compared. In addition, a comparison between a simulation and an experiment is carried out. The vortices evolution is described by dimensionless values of the velocity, pressure field, and vorticity field. The breaking and reassembling of different vortices are discussed. The results show that the pitch of the helicoidal tip vortices is larger under light loading conditions with high advance coefficients, and the wake is more stable, in contrast, which is smaller and the vortices break down earlier. By comparison, the topology of the vortices system is more complex under the low advance coefficient. Considering the interference effect between adjacent tip vortices, the energy dissipation is accelerated, resulting in the increased instability of vortices. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

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Open AccessArticle

Type I Shapovalov Wave Spacetimes in the Brans–Dicke Scalar-Tensor Theory of Gravity

by

,

,

and

Symmetry 2022, 14(12), 2636; https://doi.org/10.3390/sym14122636 - 13 Dec 2022

Viewed by 444

Abstract

Exact solutions for Shapovalov wave spacetimes of type I in Brans–Dicke’s scalar-tensor theory of gravity are constructed. Shapovalov wave spacetimes describe gravitational wave models that allow for the the separation of wave variables in privileged coordinate systems. In contrast to general relativity, the [...] Read more.

Exact solutions for Shapovalov wave spacetimes of type I in Brans–Dicke’s scalar-tensor theory of gravity are constructed. Shapovalov wave spacetimes describe gravitational wave models that allow for the the separation of wave variables in privileged coordinate systems. In contrast to general relativity, the vacuum field equations of the Brans–Dicke scalar-tensor theory of gravity lead to exact solutions for type I Shapovalov spaces, allowing for the the construction of observational tests to detect such wave disturbances. Furthermore, the equations for the trajectories of the test particles are obtained for the models considered. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

Open AccessArticle

Maxwell Equations in Homogeneous Spaces with Solvable Groups of Motions

by

V. V. Obukhov

Symmetry 2022, 14(12), 2595; https://doi.org/10.3390/sym14122595 - 08 Dec 2022

Cited by 3 | Viewed by 431

Abstract

The classification of exact solutions of Maxwell vacuum equations for the case where the electromagnetic fields and metrics of homogeneous spaces are invariant with respect to the motion group

G_{3} (V I I)

was completed. All non-equivalent exact solutions of [...] Read more.

The classification of exact solutions of Maxwell vacuum equations for the case where the electromagnetic fields and metrics of homogeneous spaces are invariant with respect to the motion group

G_{3} (V I I)

was completed. All non-equivalent exact solutions of Maxwell vacuum equations for electromagnetic fields and spaces with such symmetry were obtained. The vectors of the canonical frame of a homogeneous space of type VII according to the Bianchi classification and the electromagnetic field potentials were found. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

Open AccessArticle

A Proposal for a Novel Formulation Based on the Hyperbolic Cattaneo’s Equation to Describe the Mechano-Transduction Process Occurring in Bone Remodeling

by

Daria Scerrato

,

Ivan Giorgio

,

Alberto Maria Bersani

and

Daniele Andreucci

Symmetry 2022, 14(11), 2436; https://doi.org/10.3390/sym14112436 - 17 Nov 2022

Viewed by 562

Abstract

In this paper, we propose a model for the mechanical stimulus involved in the process of bone remodeling together with its evolution over time. Accumulated evidence suggests that bone remodeling could be interpreted as a feedback control process in which the mechanical state [...] Read more.

In this paper, we propose a model for the mechanical stimulus involved in the process of bone remodeling together with its evolution over time. Accumulated evidence suggests that bone remodeling could be interpreted as a feedback control process in which the mechanical state of the bone tissue is monitored, then appropriate signals are derived from the daily mechanical usage of the bone, these signals are transmitted into the surrounding region, and then they are detected by other agents whose purpose is to adapt the bone mass to the mechanical requirements of the environment. Therefore, we employ the diffusion equation for mass transport which is improved with Cattaneo’s correction to model the stimulus. This last improvement considers the effects of relaxation and non-locality, which we believe play essential roles in signaling messengers transport phenomena and are essential to match the evidence that suggests time-dependent excitations provide a more significant response at specific frequencies. To illustrate this particular behavior, numerical simulations have been performed in a 2D framework. The results fit the central aspect addressed, related to the dependency of the time of the adaptive process of bone, suggesting that our model is promising and deserves further investigation, both theoretical and experimental. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

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Open AccessArticle

Attractive and Repulsive Fluctuation-Induced Pressure in Peptide Films Deposited on Semiconductor Substrates

by

Galina L. Klimchitskaya

,

Vladimir M. Mostepanenko

and

Oleg Yu. Tsybin

Symmetry 2022, 14(10), 2196; https://doi.org/10.3390/sym14102196 - 19 Oct 2022

Cited by 2 | Viewed by 653

Abstract

We consider the fluctuation-induced (Casimir) pressure in peptide films deposited on GaAs, Ge, and ZnS substrates which are either in a dielectric or metallic state. The calculations of the Casimir pressure are performed in the framework of the fundamental Lifshitz theory employing the [...] Read more.

We consider the fluctuation-induced (Casimir) pressure in peptide films deposited on GaAs, Ge, and ZnS substrates which are either in a dielectric or metallic state. The calculations of the Casimir pressure are performed in the framework of the fundamental Lifshitz theory employing the frequency-dependent dielectric permittivities of all involved materials. The electric conductivity of semiconductor substrates is taken into account within the experimentally and thermodynamically consistent approach. According to our results, the Casimir pressure in peptide films deposited on dielectric-type semiconductor substrates vanishes for some definite film thickness and is repulsive for thinner and attractive for thicker films. The dependence of this effect on the fraction of water in the film and on the static dielectric permittivity of the semiconductor substrate is determined. For the metallic-type semiconductor substrates, the Casimir pressure in peptide coatings is shown to be always repulsive. The possible applications of these results to the problem of stability of thin coatings in microdevices are discussed. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

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Open AccessArticle

Symmetries of Quantum Fisher Information as Parameter Estimator for Pauli Channels under Indefinite Causal Order

by

Francisco Delgado

Symmetry 2022, 14(9), 1813; https://doi.org/10.3390/sym14091813 - 01 Sep 2022

Cited by 1 | Viewed by 908

Abstract

Quantum Fisher Information is considered in Quantum Information literature as the main resource to determine a bound in the parametric characterization problem of a quantum channel by means of probe states. The parameters characterizing a quantum channel can be estimated until a limited [...] Read more.

Quantum Fisher Information is considered in Quantum Information literature as the main resource to determine a bound in the parametric characterization problem of a quantum channel by means of probe states. The parameters characterizing a quantum channel can be estimated until a limited precision settled by the Cramér–Rao bound established in estimation theory and statistics. The involved Quantum Fisher Information of the emerging quantum state provides such a bound. Quantum states with dimension

d = 2

, the qubits, still comprise the main resources considered in Quantum Information and Quantum Processing theories. For them, Pauli channels are an important family of parametric quantum channels providing the most faithful deformation effects of imperfect quantum communication channels. Recently, Pauli channels have been characterized when they are arranged in an Indefinite Causal Order. Thus, their fidelity has been compared with single or sequential arrangements of identical channels to analyse their induced transparency under a joint behaviour. The most recent characterization has exhibited important features for quantum communication related with their parametric nature. In this work, a parallel analysis has been conducted to extended such a characterization, this time in terms of their emerging Quantum Fisher Information to pursue the advantages of each kind of arrangement for the parameter estimation problem. The objective is to reach the arrangement stating the best estimation bound for each type of Pauli channel. A complete map for such an effectivity is provided for each Pauli channel under the most affordable setups considering sequential and Indefinite Causal Order arrangements, as well as discussing their advantages and disadvantages. Full article

(This article belongs to the Special Issue Physics and Symmetry Section: Feature Papers 2022)

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