EXACT MODELS FOR ANISOTROPIC RELATIVISTIC STARS

2002 ◽  
Vol 11 (02) ◽  
pp. 207-221 ◽  
Author(s):  
M. K. MAK ◽  
PETER N. DOBSON ◽  
T. HARKO
Keyword(s):  
Energy Density ◽  
Density Ratio ◽  
Anisotropy Parameter ◽  
Integral Form ◽  
Radial Pressure ◽  
Physical Parameters ◽  
Field Equations ◽  
Gravitational Field Equations ◽  
Anisotropic Star ◽  
Decreasing Functions

We present a class of exact solutions of the Einstein gravitational field equations describing spherically symmetric and static anisotropic stellar type configurations. The solution is represented in a closed integral form. The energy density and both radial and tangential pressure are finite and positive inside the anisotropic star. The energy density, radial pressure, pressure-density ratio and the adiabatic speed of sound are monotonically decreasing functions. Several stellar models with the anisotropy coefficient proportional to r2 are discussed, the values of the basic physical parameters of the star (radius, mass and red shift) and bound on anisotropy parameter is obtained.

Charged anisotropic static cylindrically symmetric models

2013 ◽  
Vol 91 (2) ◽  
pp. 113-119 ◽  
Author(s):  
M. Sharif ◽  
H. Ismat Fatima
Keyword(s):  
Electromagnetic Field ◽  
Equation Of State ◽  
Energy Density ◽  
Symmetric Space ◽  
Radial Pressure ◽  
Matter Distribution ◽  
Field Equations ◽  
Stellar Objects ◽  
Cylindrically Symmetric ◽  
Barotropic Equation

In this paper, we investigate exact solutions of the field equations for charged, anisotropic, static, cylindrically symmetric space–time. We use a barotropic equation of state linearly relating the radial pressure and energy density. The analysis of the matter variables indicates a physically reasonable matter distribution. In the most general case, the central densities correspond to realistic stellar objects in the presence of anisotropy and charge. Finally, we conclude that matter sources are less affected by the electromagnetic field.

Anisotropic charged compact star of embedding class I

2016 ◽  
Vol 26 (06) ◽  
pp. 1750053 ◽  
Author(s):  
Piyali Bhar ◽  
Megan Govender
Keyword(s):  
Compact Star ◽  
Radial Pressure ◽  
Stellar Model ◽  
Class I ◽  
Necessary And Sufficient Condition ◽  
Field Equations ◽  
Central Singularity ◽  
Anisotropic Star ◽  
Necessary And Sufficient ◽  
Chaplygin Equation

In this paper, we present a model of a compact relativistic anisotropic star in the presence of an electric field. In order to obtain an exact solution of the Einstein–Maxwell field equations, we assume that the stellar material inside the star obeys a Chaplygin equation of state which is a nonlinear relationship between the radial pressure and the matter density. Using Tolman’s metric potential for [Formula: see text], we obtain the other metric co-efficient by employing the Karmarkar condition which is a necessary and sufficient condition for the interior spacetime of our model to be of embedding class I. Our stellar model is free from central singularity and obeys all the conditions for a realistic stellar object.

BULK VISCOUS BRANS–DICKE COSMOLOGICAL MODELS AND LATE-TIME ACCELERATION OF THE UNIVERSE

2003 ◽  
Vol 12 (05) ◽  
pp. 925-939 ◽  
Author(s):  
M. K. MAK ◽  
T. HARKO
Keyword(s):  
Energy Density ◽  
Momentum Tensor ◽  
Physical Parameters ◽  
Late Time ◽  
Field Equations ◽  
Bulk Viscous ◽  
Order Of Magnitude ◽  
Viscous Pressure ◽  
The Universe ◽  
Matter Energy

We consider the dynamics of a causal bulk viscous cosmological fluid filled flat homogeneous Universe in the framework of the Brans–Dicke theory. Three classes of exact solutions of the field equations are obtained and the behavior of the physical parameters is considered in detail. In this model the energy density associated to the Brans–Dicke scalar field is of the same order of magnitude as the matter energy density. The inclusion of the bulk viscous pressure term in the matter energy-momentum tensor leads to a non-decelerating evolution of the Universe.

A New Model for Charged Anisotropic Matter with Modified Chaplygin Equation of State

2020 ◽  
Author(s):  
Manuel Malaver ◽  
Hamed Daei Kasmaei
Keyword(s):  
Equation Of State ◽  
Energy Density ◽  
Compact Star ◽  
Radial Pressure ◽  
Extended Version ◽  
Matter Distribution ◽  
Field Equations ◽  
New Model ◽  
Anisotropic Matter ◽  
Chaplygin Equation

In this paper, we found a new model for compact star with charged anisotropic matter distribution considering an extended version of the Chaplygin equation of state. We specify a particular form of the metric potential Z(x) that allows us to solve the Einstein-Maxwell field equations. The obtained model satisfies all physical properties expected in a realistic star such that the expressions for the radial pressure, energy density, metric coefficients, measure of anisotropy and the mass are fully well defined and are regular in the interior of star. The solution obtained in this work can have multiple applications in astrophysics and cosmology.

scholarly journals Anisotropic strange star with Tolman V potential

2018 ◽  
Vol 27 (08) ◽  
pp. 1850089 ◽  
Author(s):  
Dibyendu Shee ◽  
Debabrata Deb ◽  
Shounak Ghosh ◽  
Saibal Ray ◽  
B. K. Guha
Keyword(s):  
Stellar System ◽  
Energy Condition ◽  
Anisotropy Parameter ◽  
Stellar Model ◽  
Compact Stars ◽  
Anisotropic Fluid ◽  
Physical Parameters ◽  
Model Matching ◽  
Field Equations ◽  
Bag Constant

In this paper, we present a strange stellar model using Tolman [Formula: see text]-type metric potential employing simplest form of the MIT bag equation of state (EOS) for the quark matter. We consider that the stellar system is spherically symmetric, compact and made of an anisotropic fluid. Choosing different values of [Formula: see text] we obtain exact solutions of the Einstein field equations and finally conclude that for a specific value of the parameter [Formula: see text], we find physically acceptable features of the stellar object. Further, we conduct different physical tests, viz., the energy condition, generalized Tolman–Oppeheimer–Volkoff (TOV) equation, Herrera’s cracking concept, etc., to confirm the physical validity of the presented model. Matching conditions provide expressions for different constants whereas maximization of the anisotropy parameter provides bag constant. By using the observed data of several compact stars, we derive exact values of some of the physical parameters and exhibit their features in tabular form. It is to note that our predicted value of the bag constant satisfies the report of CERN-SPS and RHIC.

scholarly journals Neutral physical compact spherically symmetric stars with non-exotic matters in Einstein’s cluster model using Weitzenböck geometry

2020 ◽  
Vol 80 (12) ◽  
Author(s):  
G. G. L. Nashed ◽  
Amare Abebe ◽  
Kazuharu Bamba
Keyword(s):  
Energy Momentum Tensor ◽  
Compact Star ◽  
Radial Pressure ◽  
Static Stability ◽  
Momentum Tensor ◽  
Field Equations ◽  
Tangential Pressure ◽  
Tetrad Field ◽  
Gravitational Field Equations ◽  
Anisotropic Energy

AbstractWe revisit the neutral (uncharged) solutions that describe Einstein’s clusters with matters in the frame of Weitzenböck geometry. To this end, we use a tetrad field with non-diagonal spherical symmetry which gives vanishing of the off-diagonal components of the gravitational field equations. The cluster solutions are calculated by using an anisotropic energy–momentum tensor. We solve the field equations using two novel assumptions. First, we use an equation of state that relates density with tangential pressure, and then we assume a specific form of one of the metric potentials in addition to the assumption of the vanishing of radial pressure to make the system of differential equations in a closed-form. The resulting solutions are coincide with the literature $$ however \, \,in\, \,this\, \,study\, \,we\, \,constrain\,\, the\,\, constants \, \,of\, \, integration\, \, from\, \, \,the\, \, matching\,\, of\, \,boundary $$ h o w e v e r i n t h i s s t u d y w e c o n s t r a i n t h e c o n s t a n t s o f i n t e g r a t i o n f r o m t h e m a t c h i n g o f b o u n d a r y $$ condition\, \, in a\,\, way \,\,different\,\, from\,\, that\,\, presented \,\,in \,\,the\,\, literature. $$ c o n d i t i o n i n a w a y d i f f e r e n t f r o m t h a t p r e s e n t e d i n t h e l i t e r a t u r e . Among many things presented in this study, we investigate the static stability specification and show that our model is consistent with a real compact start except that the tangential pressure has a vanishing value at the center of the star which is not accepted from the physical viewpoint of a real compact star. We conclude that the model that has vanishing radial pressure in the frame of Einstein’s theory is not a physical model. Therefore, we extend this study and derive a new compact star without assuming the vanishing of the redial pressure but instead we assume new form of the metric potentials. We repeat our procedure done in the case of vanishing radial pressure and show in details that the new compact star is more realistic from different physical viewpoints of real compact stellar.

scholarly journals Cosmological Particle Production and Causal Thermodynamics

1999 ◽  
Vol 52 (4) ◽  
pp. 659 ◽  
Author(s):  
M. K. Mak ◽  
T. Harko
Keyword(s):  
Particle Production ◽  
Particle Creation ◽  
Phenomenological Theory ◽  
Physical Parameters ◽  
Field Equations ◽  
Gravitational Field Equations ◽  
Inflationary Phase ◽  
Bulk Viscous ◽  
Minkowskian Geometry ◽  
The Universe

The full linear causal Israel–Stewart–Hiscock theory of bulk viscous processes in relativistic cosmological fluids is reformulated as an effective phenomenological theory for describing particle production processes in the early universe. Explicit expressions for the particle balance law and particle production rates are obtained that relate the particle creation rate to the bulk viscous (creation) pressure. The general formalism is applied to the case of a full causal cosmological fluid with bulk viscosity coecient proportional to the Hubble function. In this case the general solution of the gravitational field equations can be expressed in an exact parametric form. For an appropriate choice of the physical parameters, the dynamics of the universe can be modelled as starting from a vacuum quasi-Minkowskian geometry, followed by an inflationary period but ending in a non-inflationary phase. The influence of the matter creation processes on the evolution of the universe and the behaviour of the energy density, temperature and entropy are investigated.

scholarly journals Dynamical Analysis and Cosmological Evolution in Weyl Integrable Gravity

Universe ◽  
2021 ◽  
Vol 7 (12) ◽  
pp. 468
Author(s):  
Andronikos Paliathanasis
Keyword(s):  
Geometric Approach ◽  
Chaplygin Gas ◽  
Ideal Gas ◽  
Cosmological Evolution ◽  
Dynamical Analysis ◽  
Stationary Points ◽  
Physical Parameters ◽  
Field Equations ◽  
Gravitational Field Equations ◽  
Variational Symmetries

We investigate the cosmological evolution for the physical parameters in Weyl integrable gravity in a Friedmann–Lemaître–Robertson–Walker universe with zero spatial curvature. For the matter component, we assume that it is an ideal gas, and of the Chaplygin gas, from the Weyl integrable gravity a scalar field is introduced by a geometric approach which provides an interaction with the matter component.We calculate the stationary points for the field equations and we study their stability properties. Furthermore, we solve the inverse problem for the case of an ideal gas and prove that the gravitational field equations can follow from the variation of a Lagrangian function. Finally, variational symmetries are applied for the construction of analytic and exact solutions.

scholarly journals Model of Charged Anisotropic Strange Stars in Minimally Coupled f R Gravity

2021 ◽  
Vol 2021 ◽  
pp. 1-25
Author(s):  
H. Nazar ◽  
G. Abbas
Keyword(s):  
Energy Density ◽  
Graphical Representation ◽  
Radial Pressure ◽  
Mass Function ◽  
Compact Stars ◽  
Energy Conditions ◽  
Anisotropic Fluid ◽  
Field Equations ◽  
Strange Matter ◽  
New Family

In the present article, we have investigated a new family of nonsingular solutions of static relativistic compact sphere which incorporates the characteristics of anisotropic fluid and electromagnetic field in the context of minimally coupled f R theory of gravity. The strange matter MIT bag model equation of state (EoS) has been considered along with the usual forms of the Karori–Barua KB metric potentials. For this purpose, we derived the Einstein–Maxwell field equations in the assistance of strange matter EoS and KB type ansatz by employing the two viable and cosmologically well-consistent models of f R = R + γ R 2 and f R = R + γ R R + α R 2 . Thereafter, we have checked the physical acceptability of the proposed results such as pressure, energy density, energy conditions, TOV equation, stability conditions, mass function, compactness, and surface redshift by using graphical representation. Moreover, we have investigated that the energy density and radial pressure are nonsingular at the core or free from central singularity and always regular at every interior point of the compact sphere. The numerical values of such parameters along with the surface density, charge to radius ratio, and bag constant are computed for three well-known compact stars such as CS1 SAXJ 1808 . 4 − 3658 ( x ˜ = 7.07   km , CS2 VelaX − 1 x ˜ = 9.56   km , and CS3 4U1820 − 30 x ˜ = 10   km and are presented in Tables 1–6. Conclusively, we have noticed that our presented charged compact stellar object in the background of two well-known f R models obeys all the necessary conditions for the stable equilibrium position and which is also perfectly fit to compose the strange quark star object.

f(T) gravity and static wormhole solutions

2014 ◽  
Vol 29 (27) ◽  
pp. 1450137 ◽  
Author(s):  
Muhammad Sharif ◽  
Shamaila Rani
Keyword(s):  
Equation Of State ◽  
Energy Density ◽  
Radial Pressure ◽  
Energy Conditions ◽  
Spherically Symmetric ◽  
Shape Functions ◽  
Field Equations ◽  
Torsion Scalar ◽  
Transverse Pressure

In this paper, we study static spherically symmetric wormhole solutions in the framework of f(T) gravity, where T represents torsion scalar. We consider non-diagonal tetrad and anisotropic distribution of the fluid. We construct expressions for matter components such as energy density, radial pressure and transverse pressure from the field equations. Taking into account a particular equation of state (EoS) in terms of traceless fluid, we discuss the behavior of energy conditions for wormhole solutions with well-known f(T) and shape functions. We conclude that physically acceptable static wormhole solutions are obtained for both these functions.

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