Physical Viability Research Articles

Interior solutions of compact stars in f(T,T) gravity under Karmarkar condition

Abstract This paper is devoted to develop compact stellar configuration and to evaluate exact interior solutions in the background of f ( T , T ) gravity, where T be the torsion scalar and T be the trace of energy momentum tensor. We develop a new set up for embedding class I model under well-known Karmarkar condition for LMC X-4 and Vela X-1 compact stars using specific linear function f ( T , T ) = α T ( r ) + β T ( r ) + Φ , where α , β are any arbitrary constants and Φ be the cosmological constant. Considering static spherically symmetric space–time with perfect fluid distribution, we construct the background field equations, which describe the interior of a fluid sphere. To examine the physical viability of the obtained solutions, we have discussed all the relevant physical quantities (density, pressure, equation of state parameter, square speed of sound and equilibrium condition) analytically and graphically. It is observed that our calculated interior solutions for compact stars showed consistency with all necessary and sufficient physical conditions and hence are physically acceptable and interesting.

The impact of spheroidicity on the stability of polytropic spheres

We investigate the stability of polytropic spheres in classical general relativity influenced by a departure from homogeneous spherical symmetry. We utilize the Vaidya–Tikekar ansatz to generate models of superdense stars obeying a polytropic equation of state of the form pr=κρ1+1n−β. Models with polytropic index n=1 (quadratic equation of state) and n=2 are generated for the general spheroidal parameter. We investigate the physical viability of these models and show that they describe strange star candidates such as SAX J1808.4-3658 to a good approximation. An interesting finding of this work shows that these models become unstable as the spheroidal parameter is decreased which leads us to conclude that stability is compromised with departure from homogeneous spherical symmetry.

Decelerating to accelerating scenario for Bianchi type-II string Universe in f(R,T)-gravity theory

In this paper, we have discussed string cosmological model within the framework of [Formula: see text] theory of gravity in homogeneous but anisotropic Bianchi Type-II space-time. We have considered cosmic string as a source of energy–momentum tensor. We get the solution of the corresponding field equations by assuming deceleration parameter [Formula: see text], which is time-dependent (here, [Formula: see text] and [Formula: see text] are arbitrary constants). This particular form of scale factor enables us to explain the two scenarios, (i) By using recent constraints ([Formula: see text], [Formula: see text]) from supernovae type Ia union data (Cunha, Kinematic constraints to the transition redshift from supernovae type Ia union data, Phys. Rev. D 79 (2009) 047301), we find the values of arbitrary constants [Formula: see text] and [Formula: see text] for which we have derived a cosmological model showing accelerating expansion universe ([Formula: see text]) only throughout the evolution and (ii) By using the recent constraints ([Formula: see text], and [Formula: see text]) from SNIa data in combination with BAO and CMB observations (Giostri et al. From cosmic deceleration to acceleration: new constraints from SN Ia and BAO/CMB, J. Cosmol. Astrophys. 3 (2012) 27), we find the values of arbitrary constants for which we have derived a cosmological model with phase transition from early decelerating ([Formula: see text]) to the present accelerating ([Formula: see text]) universe. Also, for the model, we have evaluated and discussed the various physical and kinematic parameters. We have also shown the variation of these cosmological parameters graphically for specific values of the constants. The stability and physical viability are also discussed for the derived models using some recently developed diagnostic tools.

New biocompatible antibacterial wound dressing candidates; agar-locust bean gum and agar-salep films

Agar has numerous applications in biomedical and biopharmaceutical fields in gel form. However the hard and tough nature of agar films and their vulnerability to microbial attacks prevent their usage in wound dressing applications. In this work, agar - locust bean gum (LBG) and agar - salep films were prepared for the first time to improve its physical, antimicrobial and cell viability properties. LBG and salep incorporated films resulted in higher antimicrobial and cell viability properties than agar films, which are very important in wound dressing applications. Agar – LBG films had higher water vapor permeabilities and were insoluble in water and in phosphate buffer solutions. Salep incorporation resulted in lower water vapor permeability and films were soluble in both media. All films were transparent, allowing good observability. With LBG and salep addition, lower tensile strength films were obtained and thicknesses of all films were appropriate for wound dressing applications. Due to their solubility, agar - salep films can be preferred especially for the cases where removal from the wound without damaging the tissue structure is a priority.

A comparative study of the linear and colour-flavour-locked equation of states for compact objects

In this work we present a general framework for obtaining exact solutions to the Einstein field equations describing strange stars obeying a colour-flavour-locked (CFL) equation of state. Starting off with a spherically symmetric metric in isotropic coordinates describing the interior of the star, we impose a CFL equation of state to reduce the problem to a single-generating function of the gravitational potentials. Our approach leads to an infinite class of solutions of the field equations. In order to test the physical viability of our solutions, we subscribe a particular model to stringent stability tests. In particular, we show that a linear equation of state described by the MIT Bag model mimics the CFL equation of state describing strange stars with interacting quark matter. This is an interesting result which connects the more robust and mathematically tractable linear equation of state to the fundamental physics describing nuclear matter in the quark regime.

Stability of gravastars with exterior regular black holes

This paper examines the stability of thin-shell gravastars in the context of regular spacetimes (Bardeen and Bardeen-de Sitter black holes). We apply cut and paste approach to construct gravastars through the matching of interior non-singular de Sitter geometry with exterior regular black hole. This model contains three regions, i.e., interior, thin-shell and exterior. The interior and exterior regions are connected at thin-shell. We investigate physical viability of the developed model by the energy conditions and explore its stability by using radial perturbation about the equilibrium shell radius. It is found that thin-shell gravastars show large stable regions for the Bardeen-de Sitter black hole as compared to the Bardeen black hole. It is concluded that stable regions exist near the formation of expected event horizon.

Anisotropic spherical solutions through extended gravitational decoupling approach

This paper is devoted to evaluating exact anisotropic spherical solutions for static self-gravitating systems through extended geometric deformation decoupling technique. For this purpose, we consider an isotropic Tolman IV solution and extend it to anisotropic domain by transforming both temporal as well as radial metric potentials. To examine the physical viability and stability of interior anisotropic solutions, we plot energy bounds, TOV equation, causality condition and adiabatic index for the stars Her X-I and PSR J 1416-2230. It is found that both obtained models show realistic behavior as they fulfill all physical constraints as well as stability criterion. We conclude that the extended gravitational decoupling approach provides more proficient results to discuss the interior configuration of stellar structures.

Anisotropic compact stars in self-interacting Brans-Dicke gravity

This paper focuses on the extension of isotropic spherically symmetric solutions to anisotropic domain by means of minimal geometric deformations in the context of self-interacting Brans-Dicke theory. These deformations decouple the system of field equations into two sets, one describing the isotropic matter field and the other governed by anisotropic source. The former array is evaluated by assuming the metric potentials of isotropic solution (Durgapal-Fuloria/Krori-Barua spacetimes) while additional constraints are applied to solve the later. The junction conditions at the hypersurface of the compact object are employed to determine the unknown constants. The effect of scalar field on physical behavior and viability of all anisotropic solutions is analyzed through regularity and energy conditions. It is observed that anisotropic Krori-Barua solution is viable only for small values of the decoupling parameter whereas the extended Durgapal-Fuloria solution is viable under all constraints.

Revisiting Vaidya–Tikekar stellar model in the linear regime

We obtain a new class of solutions by revisiting the Vaidya–Tikekar stellar model in the linear regime. Making use of the Vaidya and Tikekar (1982) describing the spacetime of static spherically symmetric relativistic star composed of an anisotropic matter distribution admitting a linear EOS, we solve the Einstein field equations and subsequently analyse physical viability of the solution. We probe the impact of the curvature parameter K of the Vaidya–Tikekar model, which characterizes a departure from homogeneous spherical distribution, on the mass–radius relationship of the star. In the context of density-dependent MIT Bag models, we show a correlation between the curvature parameter, the bag constant and total mass and radius of some of the well-known pulsars viz., 4U 1820-30, RX J1856-37, SAXJ 1808.4 and Her X-1. We explore the possibility of fine-tuning these parameters based on current observational data.

Gravitational decoupled solutions of axial string cosmology

This paper is devoted to formulating exact solutions of axially symmetric spacetime through gravitational decoupling technique. For this purpose, we first evaluate an exact solution in the framework of cosmic strings by assuming some additional constraints on the metric coefficients and extend it to obtain two concrete anisotropic cosmological models. We investigate energy conditions as well as the speed of sound constraint to ensure the physical viability of the developed solutions. It is concluded that both anisotropic models meet all the energy bounds as well as stability criterion. The expanding behavior of the universe is also confirmed through different cosmological parameters.

Quality of life in patients with pleural effusions of various etiologies

The aim of the study was to analyze the most significant factors affecting quality of life in patients with pleural effusions of various etiologies. Materials and methods. Quality of life was assessed in 135 patients with pleural effusions of various etiologies 3–5 days after thoraco-scopy with pleural biopsy. In particular, the participants completed the SF-36 (Short Form-36) Health Survey; further, the degree of dyspnea was assessed before and after surgery using the Modified Medical Research Coun-cil (mMRC) scale. Results. Participants were divided into three groups: first, 65 subjects (48.1%) with malignant pleural effusions (MPE); second, 58 subjects (43%) with inflammatory effusions; and third, 12 subjects (8.9%) with transsudative ones. The volume of effusion in the first group was 1340.3±981.1 ml (M±σ); in the second group, 770.1± 607.0 ml; and in the third group, 1524.0± 991.9 ml. The degree of dyspnea in the second group was less pronounced, and decreased post-surgery from 2.2±1.0 points (M±σ) to 1.0± 1.0; while in the first group, from 2.8±1.0 points to 1.49±0.8 points; and in the third group, from 2.91 ±0.83 points to 1.64±0.67 (p=0.013). In patients with inflammatory pleurisy, physical function-ing, general health and vitality levels were significantly higher. Patients with MPE demonstrated a significant-ly steeper decline of physical, mental and emotional health status compared to patients with pleurisy of an-other etiology. The indicators of the physical, psycho-logical and emotional components were higher in men than in women, with a more pronounced difference in case of cancer. Conclusions. Quality of life of patients with pleural effusions is decreased due to the physical component as a result of respiratory failure caused by lung compression, and it improves after draining and fluid removal. Physical health and viability indices in malignant and transsudative effusions are significantly worse than in inflammatory pleurisy.

Anisotropic generalization of isotropic models via hypergeometric equation

We study Einstein’s field equations to describe static spherically symmetric relativistic compact objects with anisotropic matter distribution, and generate two classes of exact solutions by choosing a generalized form for one of the gravitational potentials and a particular form for the measure of anisotropy. This is achieved by transforming the Einstein’s field equation to a hypergeometric equation. The generated models generalize the isotropic models of Durgapal–Bannerji, Tikekar and Vaidya–Tikekar. The physical viability of the model is examined and compared with observational results of strange star candidates.

Charged anisotropic collapsing stars with heat flux in f(R) gravity

In the present article, we have studied the effects of electromagnetic field on the conformally flat collapsing stars with anisotropic heat conducting fluid in the framework of modified f(R) theory of gravity. For this purpose, we have formulated the junction conditions between the Vaidya spacetime and interior manifold at the junction interface for the extraction of an exact collapsing solution and the continuity of scalar curvature. Thereafter the physical parameters such as pressures Pr and Pt, energy density and heat flux are elucidated in the presence of electric charge through graphical representation. Also, we have investigated the different energy conditions for the physical viability of the system. It is found that the presence of charge, divergence of a non-zero acceleration and heat flux diminishes the rate of collapse and settle down the system in a zero volume.

Dynamics of anisotropic cosmic models with different matter sources in extended scalar-tensor theory

In this work, we develop some interesting models of cosmos exhibiting anisotropic properties in the extended scalar-tensor theory. In the first place, we consider the LRS Bianchi type I (BI) geometry filled with matter contents as magnetized bulk viscous cloud of strings. We developed analytic solutions and explore the cosmological significance of some interesting physical measures like cosmic volume, directional Hubble parameter, deceleration parameter, viscosity factor, particle energy density, shear and expansion scalars, and string tension density. Moreover, modified holographic Ricci dark energy is introduced in anisotropic scenario to discuss the dynamics of anisotropic comic models. In order to construct the exact cosmic solutions, we take hybrid law of scale factor as well as some viable ansatz for scalar field and its scalar potential. The physical viability of model parameters is discussed through graphical analysis. Physical analysis of both models show that our results are in agreement with the current observations and hence are cosmologically viable and promising.

Role of curvature-matter coupling on anisotropic strange stars

In this paper, we study the physical characteristics of anisotropic spherically symmetric quark star candidates for R+2σT gravity model, where R, σ and T depict scalar curvature, coupling parameter, and the trace of the energy-momentum tensor, respectively. In order to analyze the structure formation of quark stars, we consider the Heintzmann solution and assume that strange quark matter is characterized by MIT bag model equation of state. We evaluate the unknown parameters through matching conditions and obtain the values of radii of strange quark stars using modified Tolman-Oppenheimer-Volkoff equation with observed values of masses and bag constant. The feasibility of our considered solution is analyzed by graphical analysis of matter variables, energy bounds, causality condition and adiabatic index. It is found that the strange quark stars show stable structure corresponding to Heintzmann solution and their physical viability enhances with increasing values of the model parameter σ.

Study of gravitational decoupled anisotropic solution

This paper formulates the exact static anisotropic spherically symmetric solution of the field equations through gravitational decoupling. To accomplish this work, we add a new gravitational source in the energy–momentum tensor of a perfect fluid. The corresponding field equations, hydrostatic equilibrium equation as well as matching conditions are evaluated. We obtain the anisotropic model by extending the known Durgapal and Gehlot isotropic solution and examined the physical viability as well as the stability of the developed model. It is found that the system exhibits viable behavior for all fluid variables as well as energy conditions and the stability criterion is fulfilled.

Minimally deformed anisotropic model of class one space-time by gravitational decoupling

In this article, we have presented a static anisotropic solution of stellar compact objects for self-gravitating system by using minimal geometric deformation techniques in the framework of embedding class one space-time. For solving of this coupling system, we deform this system into two separate system through the geometric deformation of radial components for the source function lambda (r) by mapping: e^{-lambda (r)}rightarrow e^{-tilde{lambda }(r)}+beta ,g(r), where g(r) is deformation function. The first system corresponds to Einstein’s system which is solved by taking a particular generalized form for source function tilde{lambda }(r) while another system is solved by choosing well-behaved deformation function g(r). To test the physical viability of this solution, we find complete thermodynamical observable as pressure, density, velocity, and equilibrium condition via. TOV equation etc. In addition to the above, we have also obtained the moment of inertia (I), Kepler frequency (v), compression modulus (K_e) and stability for this coupling system. The M–R curve has been presented for obtaining the maximum mass and corresponding radius of the compact objects.

Generalized Durgapal–Fuloria relativistic stellar models

We present a new class of closed-form solutions to the Einstein–Maxwell system of equations for a static spherically symmetric anisotropic star in the presence of an electric field by generalizing earlier approaches. The field equations are integrated by specifying the form of the electric field, anisotropic factor, and one of the gravitational potentials which are physically reasonable. We regain some of the earlier solutions of Durgapal and Fuloria (1985), Gupta and Maurya (2011) and Maurya et al. (2015) as special cases. A detailed physical analysis of the solution indicates the physical viability for modelling anisotropic charged superdense stars.

Asymptotic renormalization in flat space: symplectic potential and charges of electromagnetism

We present a systematic procedure to renormalize the symplectic potential of the electromagnetic field at null infinity in Minkowski space. We work in D ≥ 6 spacetime dimensions as a toy model of General Relativity in D ≥ 4 dimensions. Total variation counterterms as well as corner counterterms are both subtracted from the symplectic potential to make it finite. These counterterms affect respectively the action functional and the Hamiltonian symmetry generators. The counterterms are local and universal. We analyze the asymptotic equations of motion and identify the free data associated with the renormalized canonical structure along a null characteristic. This allows the construction of the asymptotic renormalized charges whose Ward identity gives the QED soft theorem, supporting the physical viability of the renormalization procedure. We touch upon how to extend our analysis to the presence of logarithmic anomalies, and upon how our procedure compares to holographic renormalization.

Strange stars with MIT bag model in the Rastall theory of gravity

The aim of this paper is to study the charged anisotropic strange stars in the Rastall framework. Basic formulation of field equations in this framework is presented in the presence of charged anisotropic source. To obtain the solutions of the Rastall field equations in spherically symmetric Karori and Barua (KB) type space-time, we have considered a linear equation of state of strange matter, using the MIT bag model. The constraints on the Rastall dimensionless parameter [Formula: see text] are also discussed to obtain the physically reasonable solution. We explore some physical features of the presented model like energy conditions, stability and hydrostatic equilibrium, which are necessary to check the physical viability of the model. We also sought for the influence of the Rastall dimensionless parameter on the behavior of the physical features of obtained solution. We plot the graphs of matter variables for different chosen values of the parameter [Formula: see text] to inspect more details of analytical investigations and predict the numerical values of these variables exhibited in the tabular form. For this analysis, we choose four different arbitrary models of strange stars with compactness [Formula: see text] 0.25, 0.30, 0.35 and 0.40. We observed that all the necessary physical conditions are satisfied and the presented model is quite reasonable to study the strange stars.