scholarly journals Charged stars in 4D Einstein–Gauss–Bonnet gravity

2021 ◽  
Vol 81 (9) ◽  
Author(s):  
Ayan Banerjee ◽  
Sudan Hansraj ◽  
Lushen Moodly
Keyword(s):  
Graphical Analysis ◽  
Compact Objects ◽  
Gravity Theory ◽  
Compact Stars ◽  
Model Parameters ◽  
The Novel ◽  
Coupling Constant ◽  
Curvature Effects ◽  
New Exact Solutions ◽  
Local Gravity

AbstractAn alternative gravity theory that has attracted considerable attention recently is the novel four-dimensional Einstein–Gauss–Bonnet (4EGB) gravity. This idea was proposed to bypass the Lovelock’s theorem and to permit nontrivial higher curvature effects on the four-dimensional local gravity. In this approach, the Gauss–Bonnet (GB) coupling constant $$\alpha $$ α is rescaled by a factor of $$\alpha /(D -4)$$ α / ( D - 4 ) in D dimensions and taking the limit $$D \rightarrow 4$$ D → 4 . In this article, we analyze the effects of charge on static compact stars in the regularized 4D EGB gravity theory. Two classes of new exact solutions are found for a particular choice of the gravitational potential and assuming a relationship between the electric field intensity and the spatial potential. A graphical analysis indicates that the matter and electromagnetic variables are well behaved for specific values of the parameter space. Finally, based on physical grounds appropriate bounds on the model parameters we show that compact objects with the value of adiabatic index $$\gamma $$ γ is consistent with expectations.

scholarly journals An analytical anisotropic compact stellar model of embedding class I

2021 ◽  
Vol 36 (05) ◽  
pp. 2150028
Author(s):  
Lipi Baskey ◽  
Shyam Das ◽  
Farook Rahaman
Keyword(s):  
Radial Pressure ◽  
Stellar Model ◽  
Compact Objects ◽  
Compact Stars ◽  
Model Parameters ◽  
Field Equations ◽  
Principal Stresses ◽  
Schwarzschild Solution ◽  
Spherical Fluid ◽  
Physical Profile

A class of solutions of Einstein field equations satisfying Karmarkar embedding condition is presented which could describe static, spherical fluid configurations, and could serve as models for compact stars. The fluid under consideration has unequal principal stresses i.e. fluid is locally anisotropic. A certain physically motivated geometry of metric potential has been chosen and codependency of the metric potentials outlines the formation of the model. The exterior spacetime is assumed as described by the exterior Schwarzschild solution. The smooth matching of the interior to the exterior Schwarzschild spacetime metric across the boundary and the condition that radial pressure is zero across the boundary lead us to determine the model parameters. Physical requirements and stability analysis of the model demanded for a physically realistic star are satisfied. The developed model has been investigated graphically by exploring data from some of the known compact objects. The mass-radius (M-R) relationship that shows the maximum mass admissible for observed pulsars for a given surface density has also been investigated. Moreover, the physical profile of the moment of inertia (I) thus obtained from the solutions is confirmed by the Bejger–Haensel concept.

scholarly journals Anisotropic stellar model of neutron stars in f(T) gravity with off-diagonal tetrad

2022 ◽  
Vol 82 (1) ◽  
Author(s):  
Jay Solanki ◽  
Jackson Levi Said
Keyword(s):  
Neutron Stars ◽  
Graphical Analysis ◽  
Quadratic Equation ◽  
Stellar Model ◽  
Compact Stars ◽  
Quadratic Model ◽  
Model Parameters ◽  
Stellar Objects ◽  
Wide Range ◽  
Quadratic Equation Of State

AbstractIn this paper, we develop a new class of analytical solutions describing anisotropic stellar structures of observed neutron stars using modified f(T) gravity. We use the off-diagonal tetrad that is best suitable for studying spherically symmetric objects in f(T) gravity. We develop exact solutions in the quadratic model of f(T) gravity by introducing physically reliable metric potentials that can describe a wide range of astrophysical systems. We then apply the model to investigate the stellar structures of four observed compact stars, 4U 1538-52, J0437-4715, J0030+0451, and 4U 1820-30. We calculate the values of model parameters for the stellar objects under examination in this paper. Comprehensive graphical analysis shows that the model describing anisotropic stellar structures is physically acceptable, causal, and stable. The model inherently exhibits the quadratic equation of state that can be utilized to investigate the material composition and stellar structures of the observed compact stars.

Analysis of physically realizable stellar models in embedded class one spacetime manifold

2019 ◽  
Vol 35 (04) ◽  
pp. 2050001 ◽  
Author(s):  
Ritu Tamta ◽  
Pratibha Fuloria
Keyword(s):  
Transverse Velocity ◽  
Compact Star ◽  
Graphical Analysis ◽  
Compact Stars ◽  
Field Equations ◽  
Star Models ◽  
New Exact Solutions ◽  
Spacetime Manifold ◽  
Stellar Models ◽  
The Stability

In this paper, we searched two new exact solutions of Einstein’s field equations for modeling of compact cold stars using embedded class one spacetime continuum. We find out the expressions for pressure, density, anisotropy, redshift, metric potentials and other physical variables in terms of algebraic and trigonometric expressions and observe that all variables show well-behaved trends inside the compact stellar configurations. The causality condition is well maintained by our stellar models, i.e. the radial velocity and transverse velocity are less than l. The stability of our models is assessed via different stability criteria. The Buchdahl condition holds good for our solution. Herrera’s cracking method is applied to check the stability of stellar models. We generate anisotropic compact star models, whose masses and radii are in close agreement with the observed values for compact stars 4U 1538-52, LMCX-4, PSRJ1614-2230. A comparative analysis of the proposed models is carried out based on two different solutions reported in the paper. The appropriate graphical analysis is provided to authenticate the viability of the models.

scholarly journals Compact Stars Admitting Noether Symmetries in Energy-Momentum Squared Gravity

2021 ◽  
Vol 2021 ◽  
pp. 1-14
Author(s):  
M. Sharif ◽  
M. Zeeshan Gul
Keyword(s):  
Initial Conditions ◽  
Early Time ◽  
Coupling Model ◽  
Big Bang ◽  
Compact Objects ◽  
Compact Stars ◽  
Model Parameters ◽  
Conserved Quantities ◽  
Noether Symmetries ◽  
Stellar Objects

This paper investigates the geometry of compact stellar objects through the Noether symmetry approach in the energy-momentum squared gravity. This newly developed theory overcomes the problems of big bang singularity and provides the viable cosmological consequences in the early time universe. Moreover, its implications occur in high curvature regime where the deviations of energy-momentum squared gravity from general relativity is confirmed. We consider the minimal coupling model of this modified theory and formulate symmetry generators as well as corresponding conserved quantities. We use conservation relation and apply some suitable initial conditions to evaluate the metric potentials. Finally, we explore some interesting features of the compact objects for appropriate values of the model parameters through numeric analysis. It is found that compact stellar objects in this particular framework depend on the model parameters as well as conserved quantities. We conclude that Noether symmetries generate solutions that are consistent with the astrophysical observational data and hence confirm the viability of this procedure.

scholarly journals Analysis and Prediction of COVID-19 Using SIR, SEIQR, and Machine Learning Models: Australia, Italy, and UK Cases

Information ◽  
2021 ◽  
Vol 12 (3) ◽  
pp. 109 ◽  
Author(s):  
Iman Rahimi ◽  
Amir H. Gandomi ◽  
Panagiotis G. Asteris ◽  
Fang Chen
Keyword(s):  
Machine Learning ◽  
Logistic Function ◽  
Prediction Performance ◽  
Machine Learning Algorithms ◽  
Model Parameters ◽  
The Novel ◽  
Chinese City ◽  
Limited Memory ◽  
Increasing Trend ◽  
Novel Coronavirus

The novel coronavirus disease, also known as COVID-19, is a disease outbreak that was first identified in Wuhan, a Central Chinese city. In this report, a short analysis focusing on Australia, Italy, and UK is conducted. The analysis includes confirmed and recovered cases and deaths, the growth rate in Australia compared with that in Italy and UK, and the trend of the disease in different Australian regions. Mathematical approaches based on susceptible, infected, and recovered (SIR) cases and susceptible, exposed, infected, quarantined, and recovered (SEIQR) cases models are proposed to predict epidemiology in the above-mentioned countries. Since the performance of the classic forms of SIR and SEIQR depends on parameter settings, some optimization algorithms, namely Broyden–Fletcher–Goldfarb–Shanno (BFGS), conjugate gradients (CG), limited memory bound constrained BFGS (L-BFGS-B), and Nelder–Mead, are proposed to optimize the parameters and the predictive capabilities of the SIR and SEIQR models. The results of the optimized SIR and SEIQR models were compared with those of two well-known machine learning algorithms, i.e., the Prophet algorithm and logistic function. The results demonstrate the different behaviors of these algorithms in different countries as well as the better performance of the improved SIR and SEIQR models. Moreover, the Prophet algorithm was found to provide better prediction performance than the logistic function, as well as better prediction performance for Italy and UK cases than for Australian cases. Therefore, it seems that the Prophet algorithm is suitable for data with an increasing trend in the context of a pandemic. Optimization of SIR and SEIQR model parameters yielded a significant improvement in the prediction accuracy of the models. Despite the availability of several algorithms for trend predictions in this pandemic, there is no single algorithm that would be optimal for all cases.

Time-scales to reach chemical equilibrium in ices at snowline distance around compact objects: the influence of accretion mass in the central object

2021 ◽  
Vol 503 (2) ◽  
pp. 2973-2978
Author(s):  
G A Carvalho ◽  
S Pilling
Keyword(s):  
Time Scales ◽  
Chemical Equilibrium ◽  
Accretion Rate ◽  
Radiation Reaction ◽  
Compact Objects ◽  
Compact Stars ◽  
X Ray ◽  
Accretion Rates ◽  
Central Compact Objects ◽  
Astrophysical Ices

ABSTRACT In this work, we analyse soft X-ray emission due to mass accretion on to compact stars and its effects on the time-scale to reach chemical equilibrium of eventual surrounding astrophysical ices exposed to that radiation. Reaction time-scales due to soft X-ray in water-rich and pure ices of methanol, acetone, acetonitrile, formic acid, and acetic acid were determined. For accretion rates in the range $\dot{m}=10^{-12}\!-\!10^{-8}\,{\rm M}_\odot$ yr−1 and distances in the range 1–3 LY from the central compact objects, the time-scales lie in the range 10–108 yr, with shorter time-scales corresponding to higher accretion rates. Obtained time-scales for ices at snow-line distances can be small when compared to the lifetime (or age) of the compact stars, showing that chemical equilibrium could have been achieved. Time-scales for ices to reach chemical equilibrium depend on X-ray flux and, hence, on accretion rate, which indicates that systems with low accretion rates may not have reached chemical equilibrium.

scholarly journals A Study on the Generalized Approximation Modeling Method Based on Fitting Sensitivity for Prediction of Engine Performance

2017 ◽  
Vol 2017 ◽  
pp. 1-12
Author(s):  
Lin Lin ◽  
Fang Wang ◽  
Shisheng Zhong
Keyword(s):  
Engine Performance ◽  
Grey Model ◽  
Model Parameters ◽  
Bayesian Regularization ◽  
Safety Engineering ◽  
The Novel ◽  
Mean Square ◽  
Approximation Model ◽  
Mean Square Errors ◽  
Fitting Accuracy

Prediction technology for aeroengine performance is significantly important in operational maintenance and safety engineering. In the prediction of engine performance, to address overfitting and underfitting problems with the approximation modeling technique, we derived a generalized approximation model that could be used to adjust fitting precision. Approximation precision was combined with fitting sensitivity to allow the model to obtain excellent fitting accuracy and generalization performance. Taking the Grey model (GM) as an example, we discussed the modeling approach of the novel GM based on fitting sensitivity, analyzed the setting methods and optimization range of model parameters, and solved the model by using a genetic algorithm. By investigating the effect of every model parameter on the prediction precision in experiments, we summarized the change regularities of the root-mean-square errors (RMSEs) varying with the model parameters in novel GM. Also, by analyzing the novel ANN and ANN with Bayesian regularization, it is concluded that the generalized approximation model based on fitting sensitivity can achieve a reasonable fitting degree and generalization ability.

scholarly journals Compact stars in f(ℛ,𝒢,𝒯 ) gravity

2021 ◽  
Vol 36 (24) ◽  
pp. 2150165
Author(s):  
M. Ilyas
Keyword(s):  
Test Particle ◽  
Gravitational Theory ◽  
Momentum Tensor ◽  
Conservation Equation ◽  
Compact Objects ◽  
Compact Stars ◽  
Energy Conditions ◽  
Field Equations ◽  
Physical Features ◽  
The Impact

This work is to introduce a new kind of modified gravitational theory, named as [Formula: see text] (also [Formula: see text]) gravity, where [Formula: see text] is the Ricci scalar, [Formula: see text] is Gauss–Bonnet invariant and [Formula: see text] is the trace of the energy–momentum tensor. With the help of different models in this gravity, we investigate some physical features of different relativistic compact stars. For this purpose, we develop the effectively modified field equations, conservation equation, and the equation of motion for test particle. Then, we check the impact of additional force (massive test particle followed by a nongeodesic line of geometry) on compact objects. Furthermore, we took three notable stars named as [Formula: see text], [Formula: see text] and [Formula: see text]. The physical behavior of the energy density, anisotropic pressures, different energy conditions, stability, anisotropy, and the equilibrium scenario of these strange compact stars are analyzed through various plots. Finally, we conclude that the energy conditions hold, and the core of these stars is so dense.

scholarly journals Anisotropic cosmological reconstruction in f(R, T) gravity

2018 ◽  
Vol 33 (29) ◽  
pp. 1850170 ◽  
Author(s):  
B. Mishra ◽  
Sankarsan Tarai ◽  
S. K. Tripathy
Keyword(s):  
Simple Approach ◽  
Cosmological Models ◽  
Gravity Theory ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Coupling Constant ◽  
Anisotropic Cosmological Models

Anisotropic cosmological models are constructed in f(R, T) gravity theory to investigate the dynamics of universe concerning the late time cosmic acceleration. Using a more general and simple approach, the effect of the coupling constant and anisotropy on the cosmic dynamics have been investigated. In this study, it is found that cosmic anisotropy substantially affects cosmic dynamics.

Study of electron-phonon coupling and its effect on electrical resistivity in HF systems

2019 ◽  
Vol 33 (04) ◽  
pp. 1950012
Author(s):  
P. C. Baral
Keyword(s):  
Electrical Resistivity ◽  
Mean Field Theory ◽  
Tight Binding ◽  
Harmonic Approximation ◽  
Mean Field ◽  
Graphical Analysis ◽  
Kondo Lattice ◽  
Model Parameters ◽  
Binding Model ◽  
Model Hamiltonian

In this work, we report on theoretical study of the effect of electron-phonon (EP) interaction in THz frequency and temperature dependence of the electrical resistivity in heavy fermion (HF) systems. For this purpose, a model Hamiltonian is considered which consists of the Heisenberg type exchange interaction between localized moments and a tight binding model called the Kondo lattice model (KLM). The effect of EP coupling on electrical resistivity is presented by considering phonon interaction to bare f-electrons, band electrons and to the hybridization between band and f-electrons as a perturbed term. The phonon Hamiltonian in harmonic approximation is also included. The model Hamiltonian is solved by employing the mean-field theory (MFT) along with the Hubbard model of approximation. The temperature- and frequency-dependent electrical resistivity exhibits change in slopes at T[Formula: see text] as well as at T[Formula: see text]. The theoretical findings from the graphical analysis by varying the model parameters g[Formula: see text], g[Formula: see text] and g[Formula: see text] are compared to some of the experimental results in HF systems.

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