Anisotropic Matter Research Articles

Quintessence compact stars with Vaidya–Tikekar type grr for anisotropic fluid

The present study provides a new solution to the Einstein field equations for anisotropic matter configuration in static and spherically symmetric space–time. By taking benefit from the conformal Killing vector (CKV) technique and quintessence field specified by a parameter ωq as –1 < ωq < –1/3, we generate an exact solution to the field equations. For this investigation, we have used a specific form of metric potential taken fromVaidya–Tikekar (J. Astrophys. Astron. 3, 325 (1982)) geometry. To canvass the physical plausibility of the presented solution, we explored some analytical expressions such as energy conditions, the TOV equation, stability analysis, and equation of state parameters. We present graphical analysis of the necessary analytical expressions that revealed that our solution satisfies the necessary physical conditions.

Stability of realistic stellar objects in f(R,T) gravity with anisotropic matter configuration

In this paper, we have obtained a new class of solutions of modified field equations (FEqs) in [Formula: see text] gravity with barotropic equation of state (EoS) i.e. [Formula: see text]. For this purpose, we assume spherically symmetric metric with anisotropic matter distribution and formulated the FEqs. We apply Durgapal transformation and choose suitable gravitational potential to solve the dynamical equations numerically. By varying parametric values [Formula: see text] and [Formula: see text] (appearing in gravitational potential), we generate special cases of solutions. We plot physical quantities like energy density, anisotropy parameter, radial and tangential pressures in all particular cases. It is concluded that all the plots hold physical acceptability criteria and show consistency which admits modified [Formula: see text] gravity for compact star models.

Compact star in Tolman\u2013Kuchowicz spacetime in the background of Einstein\u2013Gauss\u2013Bonnet gravity

The present work is devoted to the study of anisotropic compact matter distributions within the framework of five-dimensional Einstein–Gauss–Bonnet gravity. To solve the field equations, we have considered that the inner geometry is described by Tolman–Kuchowicz spacetime. The Gauss–Bonnet Lagrangian mathcal {L}_{GB} is coupled to the Einstein–Hilbert action through a coupling constant, namely alpha . When this coupling tends to zero general relativity results are recovered. We analyze the effect of this parameter on the principal salient features of the model, such as energy density, radial and tangential pressure and anisotropy factor. These effects are contrasted with the corresponding general relativity results. Besides, we have checked the incidence on an important mechanism: equilibrium by means of a generalized Tolman–Oppenheimer–Volkoff equation and stability through relativistic adiabatic index and Abreu’s criterion. Additionally, the behavior of the subliminal sound speeds of the pressure waves in the principal directions of the configuration and the conduct of the energy-momentum tensor throughout the star are analyzed employing the causality condition and energy conditions, respectively. All these subjects are illuminated by means of physical, mathematical and graphical surveys. The M–I and the M–R graphs imply that the stiffness of the equation of state increases with alpha ; however, it is less stiff than GR.

A completely deformed anisotropic class one solution for charged compact star: a gravitational decoupling approach

In this article, we have investigated a new completely deformed embedding class one solution for the compact star in the framework of charged anisotropic matter distribution. For determining of this new solution, we deformed both gravitational potentials as nu ~mapsto ~xi +alpha , h(r) and e^{-lambda } mapsto ~e^{-{mu }} + alpha ,f(r) by using Ovalle (Phys Lett B 788:213, 2019) approach. The gravitational deformation divides the original coupled system into two individual systems which are called the Einstein’s system and Maxwell-system (known as quasi-Einstein system), respectively. The Einstein’s system is solved by using embedding class one condition in the context of anisotropic matter distribution while the solution of Maxwell-system is determined by solving of corresponding conservation equation via assuming a well-defined ansatz for deformation function h(r). In this way, we obtain the expression for the electric field and another deformation function f(r). Moreover, we also discussed the physical validity of the solution for the coupled system by performing several physical tests. This investigation shows that the gravitational decoupling approach is a powerful methodology to generate a well-behaved solution for the compact object.

Anisotropic relativistic fluid spheres: an embedding class I approach

In this work, we present a new class of analytic and well-behaved solution to Einstein’s field equations describing anisotropic matter distribution. It’s achieved in the embedding class one spacetime framework using Karmarkar’s condition. We perform our analysis by proposing a new metric potential g_{rr} which yields us a physically viable performance of all physical variables. The obtained model is representing the physical features of the solution in detail, analytically as well as graphically for strange star candidate SAX J1808.4-3658 (Mass=0.9 ~M_{odot }, radius=7.951 km), with different values of parameter n ranging from 0.5 to 3.4. Our suggested solution is free from physical and geometric singularities, satisfies causality condition, Abreu’s criterion and relativistic adiabatic index varGamma , and exhibits well-behaved nature, as well as, all energy conditions and equilibrium condition are well-defined, which implies that our model is physically acceptable. The physical sensitivity of the moment of inertia (I) obtained from the solutions is confirmed by the Bejger−Haensel concept, which could provide a precise tool to the matching rigidity of the state equation due to different values of n viz., n=0.5, 1.08, 1.66, 2.24, 2.82 and 3.4.

Relativistic anisotropic model of strange star SAX J1808.4-3658 admitting quadratic equation of state

In this paper, we study the behavior of static spherically symmetric relativistic model of the strange star SAX J1808.4-3658 by exploring a new exact solution for anisotropic matter distribution. We analyze the comprehensive structure of the space–time within the stellar configuration by using the Einstein field equations amalgamated with quadratic equation of state (EoS). Further, we compare solutions of quadratic EoS model with modified Bose–Einstein condensation EoS and linear EoS models which can be generated by a suitable choice of parameters in quadratic EoS model. Subsequently, we compare the properties of strange star SAX J1808.4-3658 for all the three EoS models with the help of graphical representations.

Dynamical instability of anisotropic homogeneous cylinder

In this paper, we consider a non-static cylindrically symmetric self-gravitating system with anisotropic matter configuration and investigate its stability regions by using a homogenous model. We establish perturbed form of dynamical equations by using Eulerian and Lagrangian approaches. The conservation of baryon number is applied to obtain adiabatic index as well as perturbed pressure. A variational principle is used to find characteristic frequency which helps to compute the instability criteria. It is found that dynamical instability can be prevented till the radius of a cylinder exceeds the limit R[Formula: see text]18. We conclude that the system becomes unstable against radial oscillations as the radial pressure increases relative to tangential pressure.

A new class of compact stellar model compatible with observational data

In this work, a physically reasonable metric potential g_{rr} and a specific choice of the anisotropy has been utilized to obtain closed-form solutions of the Einstein field equation for a spherically symmetric anisotropic matter distribution. This class of solution has been used to develop viable models for observed pulsars. Smooth matching of interior spacetime metric with the exterior Schwarzschild metric and utilizing the condition that radial pressure is zero across the boundary leads us to determine the model parameters. A particular pulsar 4U1820-30 having current estimated mass and radius (mass=1.58 M_odot and radius=9.1 km) has been allowed for testing the physical acceptability of the developed model. The gross physical nature of the observed pulsar has been analyzed graphically. The stability of the model is also discussed given causality conditions, adiabatic index and generalized Tolman–Oppenheimer–Volkov (TOV) equation under the forces acting on the system. To show that this model is compatible with observational data, few more pulsars have been considered, and all the requirements of a realistic star are highlighted. Additionally, the mass-radius (M–R) relationship of compact stellar objects analyzed for this model. The maximum mass for the presented model is approx 4M_odot which is compared with the realization of Rhoades and Ruffini (Phys Rev Lett 32:324, 1974).

Overturning and apparent anisotropic pressure in Eddington-inspired Born-Infeld theory on compact stars

The advantage of mapping modified gravity theories with standard matter into general relativity (GR) with modified matter is that we can quickly implement the well-established analytical and numerical methods developed within GR to study modified gravity theories and compare the modified gravity theories with current observational data. In this paper, we explicitly show that overturning/cracking instability may occur in an isotropic Eddington-inspired Born-Infeld (EiBI) stellar structure. Analyses are done by reducing the Tolman-Oppenheimer-Volkoff-like equations of EiBI into the ones of GR with an additional anisotropiclike nonlinear term. We obtain the term by expanding the stellar structure equations with respect to $\ensuremath{\gamma}\ensuremath{\kappa}\ensuremath{\epsilon}$ and $\ensuremath{\gamma}\ensuremath{\kappa}P$ and by using numerical calculations. We find that, for a specific value of $\ensuremath{\kappa}$, the overturning/cracking may occur. In the nonrelativistic regime, cracking tends to occur when $\ensuremath{\kappa}g0$. In the relativistic regime, for neutron stars (NS) with equation of state (EOS) based on a relativistic mean field model (RMF), the overturning occurs at $\ensuremath{\kappa}\ensuremath{\gtrsim}0$, and the overturning point is located near the core of the star. We also show that the stellar structure equations within EiBI theory with isotropic matter can be recast into GR defined in a physical metric $g$ with a modified (apparent) anisotropic matter. In this representation, the signature of a potentially overturning instability within the apparent anisotropic structure is present and related to the asymptotic behavior of the apparent speed of sound near the center of the star and an imaginary value of the apparent tangential speed of sound near the edge of the star.

Existence of realistic stellar objects in Rastall gravity with linear equation of state

In this paper, we have discussed the anisotropic matter configuration to explore the existence of realistic stellar objects in non-conservative theory named as Rastall theory of gravity. We have assumed a static spherically symmetric metric with linear equation of state (EoS) to formulate the dynamical equations. The Durgapal and Banerji transformation is employed to investigate the gravitational behavior of compact objects. In this regard, a particular gravitational potential is selected to solve the system of dynamical equations numerically. We compared change in behavior of physical quantities like energy density, anisotropy parameter, and radial and tangential pressures by plotting three particular cases. With the help of physical analysis, it can be seen that the solutions of compact spheres hold physical acceptability criteria and depict stability.

Spherically symmetric wormholes with anisotropic matter

We study the geometry of a wormhole spacetime filled with anisotropic matter in the context of general relativity. In the course of the study, new static and spherically-symmetric solutions, analytic and numerical ones, are found. We specify the existence condition for a wormhole throat. We analyze properties of the solutions after categorizing them based on the spacetime regularity and the signature of energy density. The necessary conditions which allow a wormhole spacetime to be nonsingular are described.

Study of anisotropic strange stars in f(R,T) gravity: An embedding approach under the simplest linear functional of the matter-geometry coupling

The present work is focused on the investigation of the existence of compact structures describing anisotropic matter distributions within the framework of modified gravity theories, specifically f(R,$\mathcal{T}$) gravity theory. Additionally, we have taken f(R,$\mathcal{T}$) as a linear function of the Ricci scalar $R$ and the trace of the energy-momentum tensor $\mathcal{T}$ as $f(R,\mathcal{T})$=$R+2\chi\mathcal{T}$,where $\chi$ is a dimensionless coupling parameter, and the Lagrangian matter $\mathcal{L}_m=-\frac{1}{3}\left(2p_{t}+p_{r}\right)$, to describe the complete set of field equations for the anisotropic matter distribution. We follow the embedding class one procedure using Eisland condition to obtain a full space-time description inside the stellar configuration. Once the space-time geometry is specified we determine the complete solution of the modified Einstein equations by using the MIT bag model equation of state $p_{r}=\frac{1}{3}\left(\rho-4B\right)$ that describes the strange quark matter (SQM) distribution inside the stellar system, where $B$ denotes a bag constant. The physical validity of our anisotropic solution is confirmed by executing several physical tests. It is worth mentioning that with the help of the observed mass values for the various strange star candidates we have predicted the exact radii by taking different values for $\chi$ and $B$. These predicted radii show monotonic decreasing nature as the parameter $\chi$ is moved from $-0.8$ to $0.8$ progressively. In this case, our anisotropic stellar system becomes more massive and transforms into more dense compact stars.

Dynamic Spirals of Nanoparticles in Light-Responsive Polygonal Fields.

Nanoparticles tend to aggregate once integrated into soft matter and consequently, self-assembling nanoparticles into large-scale, regular, well-defined, and ultimately chiral patterns remains an ongoing challenge toward the design and realization of organized superstructures of nanoparticles. The patterns of nanoparticles that are reported in liquid crystals so far are all static, and this lack of responsiveness extends to assemblies of nanoparticles formed in topological singularities and other localized structures of anisotropic matter. Here, it is shown that gold nanoparticles form spiral superstructures in polygonal fields of cholesteric liquid crystals. Moreover, when the cholesteric liquid crystals incorporate molecular photoswitches in their composition, the pitch of the nanoparticulate spirals follows the light-induced reorganization of the cholesteric liquid crystals. These experimental findings indicate that chiral liquid crystals can be used as chiral and dynamic templates for soft photonic nanomaterials. Controlling the geometry of these spirals of nanoparticles will ultimately allow modulating the plasmonic signature of hybrid and chiral systems.

Entropy of self-gravitating anisotropic matter

We examine the entropy of self-gravitating anisotropic matter confined to a box in the context of general relativity. The configuration of self-gravitating matter is spherically symmetric, but has anisotropic pressure of which angular part is different from the radial part. We deduce the entropy from the relation between the thermodynamical laws and the continuity equation. The variational equation for this entropy is shown to reproduce the gravitational field equation for the anisotropic matter. This result re-assures us the correspondence between gravity and thermodynamics. We apply this method to calculate the entropies of a few objects such as compact star and wormholes.

Anisotropic generalization of Vaidya-Tikekar superdense stars

We study superdense relativistic stars with anisotropic matter distributions with spheroidal spatial hypersurfaces. We propose a methodology to make an anisotropic generalization of the Vaidya-Tikekar superdense star model. The anisotropic Einstein field equations can be solved in terms of hypergeometric functions for our choice of gravitational potential and anisotropy. Particular parameter choices allow us to generate models of anisotropic stars in terms of elementary functions. Also, isotropic stars can be generated in the limit of vanishing anisotropy. In particular, we obtain the well known superdense models of Tikekar which are isotropic and have specific spheroidal geometries. The impact of anisotropy on the gross physical behaviour of a compact star is studied.

Effect of pressure anisotropy on Buchdahl-type relativistic compact stars

We consider exact models for dense relativistic stars with anisotropic pressures and containing Buchdahl-type spacetime geometry. The Buchdahl condition can be transformed to an Euler–Cauchy equation for the gravitational potentials. We solve this condition to find a new exact solution to the Einstein field equations with anisotropic matter distribution. We show that the exact solution produces a realistic model of a compact relativistic star satisfying all physical requirements. The regularity, equilibrium, casuality, stability, energy conditions and compactness limits for a well behaved compact sphere are satisfied.

2+1-dimensional gravitational decoupled anisotropic solutions

This paper investigates exact models for spherically symmetric anisotropic matter distribution in 2+1-dimensions via gravitational decoupling approach. For this purpose, we choose known spherical solutions with perfect fluid in the absence as well as the presence of cosmological constant and extend them to anisotropic models by imposing a constraint on matter components. The physical viability and stability of our developed solutions are investigated through graphical analysis of density, radial/tangential pressure, energy conditions, and causality criterion. It is found that both solutions are stable and satisfy all the physical requirements for the feasible choice of the model parameters.

Stellar systems and structure scalars

The work is devoted to analyzing the effects of dark source polynomial curvature corrections in the mathematical modeling of radiating stars. In this scenario, we have used a particular f(R, T) model and consider the spherically symmetric geometry of relativistic interior. We assumed that our geometry is coupled with anisotropic shearing matter distribution undergoing radiating epoch with free streaming and diffusion approximation. We have calculated spherically symmetric total matter content with the help of Misner–Sharp formalism. A particular relation among anisotropic pressure, shearing viscosity, radiating parameters, energy density, and tidal forces is obtained. We then expressed this equation with the help of f(R, T) structure scalar, the scalar obtained by orthogonal decomposition of the Riemann tensor. The role of the logarithmic Ricci and trace of stress–energy tensor terms are also observed through Weyl scalar, shear, expansion scalar differential equations.

On the role of f (G, T) terms in structure scalars

This work is devoted to explore the effects of $f(G,T)$ terms on the study of structure scalars and their influences in the formulations of the Raychaudhuri, shear and Weyl scalar equations. For this purpose, we have assumed non-static spherically symmetric geometry coupled with shearing viscous locally anisotropic dissipative matter content. We have developed relations among the Misner-Sharp mass, Weyl scalar, matter and structure variables. We have also formulated set of $f(G,T)$ structure scalars after orthogonally breaking down of the Riemann curvature tensor. The influences of these scalar functions in the modeling of relativistic radiating spheres are also studied. The factor involved in the emergence of inhomogeneities is also explored for the constant and varying modified curvature corrections. We inferred that $f(G,T)$ structure scalars could lead provide an effective tool to study Penrose-Hawking singularity theorems and Newman-Penrose formalism.

New classes of generalized anisotropic polytropes pertaining radiation density

In this article, we consider the generalized polytropic equation of state with anisotropic matter distribution in isotropic coordinates. The static spherically symmetric configuration is considered for the development of mathematical models of compact objects incorporating the radiation factor. We have examined 12 different stars with the developed models. Analysis of the models shows that they are well-behaved and physically viable.