arXiv subjects

Saibal Ray

Publications and source records attributed to Saibal Ray.

At least 91 records · Page 5Linked to original sources

arXiv2018

Anisotropic strange stars with Tolman V potential

In this paper we present a strange stellar model using Tolman $V$ 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 $n$ we obtain exact solutions of the Einstein field equations and finally conclude that for a specific value of the parameter $n=1/2$ we find physically acceptable features of the stellar object. Further we conduct different physical tests, viz., the energy condition, generalized TOV equation, Herrera's cracking concept, etc., to confirm 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 a tabular form. It is to note that our predicted value of the bag constant satisfies the report of CERN-SPS and RHIC.

gr-qc↗

arXiv2018

Gravastars with higher dimensional spacetimes

We present a new model of gravastar in the higher dimensional Einsteinian spacetime including Einstein's cosmological constant $Λ$. Following Mazur and Mottola (2001,~2004) we obtain a set of solutions for gravastar. This gravastar is described by three different regions, namely, (I) Interior region, (II) Intermediate thin spherical shell and (III) Exterior region. The pressure within the interior region is equal to the negative matter density which provides a repulsive force over the shell. This thin shell is formed by ultra relativistic plasma, where the pressure is directly proportional to the matter-energy density which does counter balance the repulsive force from the interior whereas the exterior region is completely vacuum assumed to be de Sitter spacetime which can be described by the generalized Schwarzschild solution. With this specification we find out a set of exact and non-singular solutions of the gravastar which seems physically very interesting and reasonable.

gr-qc↗

arXiv2018

Strange stars in $f(R,\mathcal{T})$ gravity

In this article we try to present spherically symmetric isotropic strange star model under the framework of $f(R,\mathcal{T})$ theory of gravity. To this end, we consider that the Lagrangian density is an arbitrary linear function of the Ricci scalar $R$ and the trace of the energy momentum tensor~$\mathcal{T}$ given as $f\left(R,\mathcal{T}\right)=R+2χT$. We also assume that the quark matter distribution is governed by the simplest form of the MIT bag model equation of state (EOS) as $p=\frac{1}{3}\left(ρ-4B\right)$, where $B$ is the bag constant. We have obtained an exact solution of the modified form of the the Tolman-Oppenheimer-Volkoff (TOV) equation in the framework of $f(R,\mathcal{T})$ gravity theory and studied the dependence of different physical properties, viz., total mass, radius, energy density and pressure on the chosen values of $χ$. Further, to examine physical acceptability of the proposed stellar model in detail, we conducted different tests, viz. energy conditions, modified TOV equation, mass-radius relation, causality condition etc. We have precisely explained the effects arising due to the coupling of the matter and geometry on the compact stellar system. For a chosen value of the Bag constant we have predicted numerical values of different physical parameters in tabular format for the different strange stars. It is found that as the factor $χ$ increases the strange stars shrink gradually and become less massive to turn into a more compact stellar system. The maximum mass point is well within the observational limits and hence our proposed model is suitable to explain the ultra dense compact stars. For $χ=0$ we retrieve as usual the standard results of general relativity (GR).

gr-qc↗

arXiv2018

Anisotropic strange stars under simplest minimal matter-geometry coupling in the $f(R,\mathcal{T})$ gravity

We study strange stars in the framework of $f\left(R,\mathcal{T}\right)$ theory of gravity where the strange quark matter distribution inside the stellar system is governed by the phenomenological MIT Bag model equation of state (EOS). Further, for a specific value of $B$ and observed values of mass of the strange star candidates we obtain the exact solution of the modified Tolman-Oppenheimer-Volkoff (TOV) equation in the framework of $f\left(R,\mathcal{T}\right)$ gravity and have studied in detail the dependence of the different physical parameters due to the chosen different values of $χ$. To check the physical acceptability and stability of the stellar system based on the obtained solutions we have performed different physical tests, viz., the energy conditions, Herrera cracking concept, adiabatic index etc. In this work, we also have explained the effects, those are arising due to the interaction between the matter and the curvature terms in $f\left(R,\mathcal{T}\right)$ gravity, on the anisotropic compact stellar system. It is interesting to note that as the values of $χ$ increase the strange stars become more massive and their radius increase gradually so that eventually they gradually turn into less dense compact objects. The present study reveals that the modified $f\left(R,\mathcal{T}\right)$ gravity is a suitable theory to explain massive stellar systems like recent magnetars, massive pulsars and super-Chandrasekhar stars, which can not be explained in the framework of GR. However, for $χ=0$ the standard results of Einsteinian gravity are retrieved.

physics.gen-ph↗

arXiv2018

Compact stars: a generalized model

This paper discusses a generalized model for compact stars, assumed to be anisotropic in nature due to the spherical symmetry and high density. After embedding the four-dimensional spacetime in a five-dimensional flat spacetime, which may be treated as an alternative to Karmarkar's condition of embedding class 1 spacetime, the Einstein field equations were solved by employing a class of physically acceptable metric functions proposed by Lake \cite{Lake2003}. The physical properties determined include the anisotropic factor showing that the anisotropy is zero at the center and maximal at the surface. Other boundary conditions yielded the values of various parameters needed for rendering the numerous plots and also led to the EOS parameters. It was further determined that the usual energy conditions are satisfied and that the compact structures are stable, based on several criteria, starting with the TOV equation. The calculation of the effective gravitational mass shows that the models satisfy the Buchdahl condition. Finally, the values of the numerous constants and physical parameters were determined specifically for the strange star LMCX-4. It is shown that the present generalized model can justify most off the compact stars including white dwarfs and ultra dense compact stars for a suitable tuning of the parametric values of $n$.

physics.gen-ph↗

arXiv2018

A generalized family of anisotropic compact object in general relativity

We present model for anisotropic compact star under the general theory of relativity of Einstein. In the study a 4-dimensional spacetime has been considered which is embedded into the 5-dimensional flat metric so that the spherically symmetric metric has class 1 when the condition $e^λ=\left(\,1+C\,e^ν \,{ν'}^2\,\right)$ is satisfied ($λ$ and $ν$ being the metric potentials along with a constant $C$). A set of solutions for the field equations are found depending on the index $n$ involved in the physical parameters. The interior solutions have been matched smoothly at the boundary of the spherical distribution to the exterior Schwarzschild solution which necessarily provides values of the unknown constants. We have chosen the values of $n$ as $n=2$ and $n$=10 to 20000 for which interesting and physically viable results can be found out. The numerical values of the parameters and arbitrary constants for different compact stars are assumed in the graphical plots and tables as follows: (i) LMC X-4 : $a=0.0075$, $b=0.000821$ for $n=2$ and $a=0.0075$, $nb=0.00164$ for $n\ge 10$, (ii) SMC X-1: $a=0.00681$, $b=0.00078$ for $n=2$, and $a=0.00681$, $nb=0.00159$ for $n \ge 10$. The investigations on the physical features of the model include several astrophysical issues, like (i) regularity behavior of stars at the centre, (ii) well behaved condition for velocity of sound, (iii) energy conditions, (iv) stabilty of the system via the following three techniques - adiabatic index, Herrera cracking concept and TOV equation, (v) total mass, effective mass and compactification factor and (vi) surface redshift. Specific numerical values of the compact star candidates LMC X-4 and SMC X-1 are calculated for central and surface densities as well as central pressure to compare the model value with actual observational data.

physics.gen-ph↗

arXiv2018

Anisotropic strange stars in the Einstein-Maxwell spacetime

We present here a detailed analysis on the effects of charge on the anisotropic strange star candidates by considering a spherically symmetric interior spacetime metric. To obtain exact solution of the Einstein-Maxwell field equations we have considered the anisotropic strange quark matter (SQM) distribution governed by the simplified MIT bag equation of state (EOS), $p=\frac{1}{3}\left( ρ-4\,B \right)$, where $B$ is the bag constant and the distribution of the electrical charge is given as $q(r)=Q\left({r}/{R}\right)^3=α{r^3}$, where $α$ is a constant. To this end, to calculate different constants we have described the exterior spacetime by the Reissner-Nordstr{ö}m metric. By using the values of the observed mass for the different strange star candidates we have maximized anisotropic stress at the surface to predict the exact values of the radius for the different values of $α$ and a specific value of the bag constant. Further, we perform different tests to study the physical validity and the stability of the proposed stellar model. We found accumulation of the electric charge distribution is maximum at the surface having electric charge of the order ${{10}^{20}}~C$ and electric field of the order ${10}^{21-22}~V/cm$. To study the different physical parameters and the effects of charge on the anisotropic stellar system we have presented our analysis graphically and in the tabular format by considering $LMC~X-4$ as the representative of the strange star candidates.

gr-qc↗

arXiv2017

Exact Radiation Model For Perfect Fluid Under Maximum Entropy Principle

We find the Euler-Lagrangian equation by maximising the total entropy. Hence we obtain an expression for mass of the spherically symmetric system by solving the Euler-Lagrangian equation where the Homotopy Perturbation Method has been employed. With the help of this expression and the Einstein field equations we obtain an interior solution set. Thereafter, we explain different aspects of the solution describing the system in connection to the mass, density, pressures, energy, stability, mass-radius ratio, compactness factor and surface redshift. This analysis shows that all the physical properties, in connection to brown dwarf stars, are valid with the observed features.

physics.gen-ph↗

arXiv2017

Relativistic model for anisotropic strange stars

In this article, we attempt to find a singularity free solution of Einstein's field equations for compact stellar objects, precisely strange (quark) stars, considering Schwarzschild metric as the exterior spacetime. To this end, we consider that the stellar object is spherically symmetric, static and anisotropic in nature and follows the density profile given by Mak and Harko (2002), which satisfies all the physical conditions. To investigate different properties of the ultra-dense strange stars we have employed the MIT bag model for the quark matter. Our investigation displays an interesting feature that the anisotropy of compact stars increases with the radial coordinate and attains its maximum value at the surface which seems an inherent property for the singularity free anisotropic compact stellar objects. In this connection we also perform several tests for physical features of the proposed model and show that these are reasonably acceptable within certain range. Further, we find that the model is consistent with the energy conditions and the compact stellar structure is stable with the validity of the TOV equation and Herrera cracking concept. For the masses bellow the maximum mass point in mass vs radius curve the typical behavior achieved within the framework of general relativity. We have calculated the maximum mass and radius of the strange stars for the three finite values of bag constant $B_g$.

gr-qc↗

arXiv2017

All spherically symmetric charged anisotropic solutions for compact star

In the present paper we develop an algorithm for all spherically symmetric anisotropic charged fluid distribution. Considering a new source function $ν(r)$ we find out a set of solutions which is physically well behaved and represent compact stellar models. A detailed study specifically shows that the models actually correspond to strange stars in terms of their mass and radius. In this connection we investigate about several physical properties like energy conditions, stability, mass-radius ratio, electric charge content, anisotropic nature and surface redshift through graphical plots and mathematical calculations. All the features from these studies are in excellent agreement with the already available evidences in theory as well as observations.

gr-qc↗

arXiv2017

Gravastars in $f(R,\mathcal{T})$ gravity

We propose a unique stellar model under the $f(R,\mathcal{T})$ gravity by using the conjecture of Mazur-Mottola [P. Mazur and E. Mottola, Report number: LA-UR-01-5067., P. Mazur and E. Mottola, Proc. Natl. Acad. Sci. USA 101, 9545 (2004)] which is known as gravastar and a viable alternative to the black hole as available in literature. This gravastar is described by the three different regions, viz., (I) Interior core region, (II) Intermediate thin shell, and (III) Exterior spherical region. The pressure within the interior region is equal to the constant negative matter density which provides a repulsive force over the thin spherical shell. This thin shell is assumed to be formed by a fluid of ultra relativistic plasma and the pressure, which is directly proportional to the matter-energy density according to Zel'dovich's conjecture of stiff fluid [Y.B. Zel'dovich, Mon. Not. R. Astron. Soc. 160, 1 (1972)], does counter balance the repulsive force exerted by the interior core region. The exterior spherical region is completely vacuum and assumed to be de Sitter spacetime which can be described by the Schwarzschild solution. Under this specification we find out a set of exact and singularity-free solution of the collapsing star which presents several other physically valid features within the framework of alternative gravity.

gr-qc↗

arXiv2017

Compact star in pseudo-spheroidal spacetime

We investigate perfect fluid stars in $(2+1)$ dimension in pseudo spheroidal spacetime with the help of Vaidya-Tikekar metric where the physical $3$-space ($t=$ constant) is described by pseudo-spheroidal geometry. Here the spheroidicity parameter $a$, plays an important role for determining the properties of a compact star. In the present work a class of interior solutions corresponding to the Ba$\tilde{n}$ados-Teitelboim-Zanelli $(BTZ)$ (Bañados et al., Phys. Rev. Lett. 69:1849, 1992) exterior metric has been provided which describes a static circularly symmetric star with negative cosmological constant in equilibrium. It is shown that asymptotically anti-de Sitter $(2+1)$ dimensional spacetime described by BTZ admits a compact star solution with reasonable physical features.

physics.gen-ph↗

arXiv2017

On the features of Matese-Whitman mass function

In the present paper we exhaustively examine the physical status of the socalled Matese-Whitman mass function [J.J. Matese and P.G. Whitman, Phys. Rev. D, 22, 1270 (1980)]. As a first step, we construct the relevant Einstein field equations with an anisotropic matter distribution under the approach of Conformal killing Vector. In the intermideate step we find a set of exact solutions by using the Matese-Whitman mass function. Eventually we conduct several physical tests to explore features of the applied mass function in connection to the specific compact stars. It can be observed that all the features of the model based on the Matese-Whitman mass function are of physical interests.

gr-qc↗

arXiv2017

Charged gravastars in higher dimensions

We explore possibility to find out a new model of gravastars in the extended $D$-dimensional Einstein-Maxwell spacetime. The class of solutions as obtained by Mazur and Mottola of a neutral gravastar \cite{Mazur2001,Mazur2004} have been observed as an alternative to $D$-dimensional versions of the Schwarzschild-Tangherlini black hole. To tackle the spherical system in a convenient way we have configured that the gravastar consists of three distinct regions with different equations of state as follows: [I] Interior region $0 \leq r < r_1$,$ρ= -p$, [II] Thin shell region $r_1 \leq r < r_2$,$ρ= p$, and [III] Exterior region $r_2 < r$,$ρ= p =0$. The outer region of this gravastar model therefore corresponds to a higher dimensional Reissner-Nordstr{ö}m black hole. In connection to this junction conditions are provided and therefore we have formulated mass and the related Equation of State of the gravastar. It has been shown that the model satisfies all the requirements of the physical features. However, overall observational survey of the results also provide probable indication of non-applicability of higher dimensional approach for construction of a gravastar with or without charge from an ordinary $4$-dimensional seed as far as physical ground is concerned.

physics.gen-ph↗

arXiv2016

Compact stars in $f(R,T)$ gravity

In the present paper we generate a set of solutions describing the interior of a compact star under $f(R,T)$ theory of gravity which admits conformal motion. We consider the equation of state (EOS) $p=ωρ$ with $0<ω<1$ for the fluid distribution consisting normal matter, $ω$ being the EOS parameter. We therefore explore several aspects of the model analytically along with graphical representations to check the physical validity as well as acceptability of it within specified observational constraint in connection to a dozen of the compact star candidates. It is shown from the presented model that these objects are nothing but radiating compact stars.

gr-qc↗

arXiv2016

FLRW viscous cosmological models

In this paper we solve Friedmann equations by considering a universal media as a non-perfect fluid with bulk viscosity and is described by a general "gamma law" equation of state of the form $p= (γ-1) ρ+ Λ(t)$, where the adiabatic parameter $γ$ varies with scale factor $R$ of the metric and $Λ$ is the time dependent cosmological constant. A unified description of the early evolution of the universe is presented by assuming the bulk viscosity and cosmological parameter in a linear combination of two terms of the form: $Λ(t)=Λ_{0} + Λ_{1}\frac{\dot{R}}{R}$ and $ζ= ζ_{0} + ζ_{1} \frac{\dot{R}}{R}$, where $Λ_{0},\;Λ_{1},\, ζ_{0}$ and $ ζ_{1}$ are constants, in which an inflationary phase is followed by the radiation dominated phase. For this general gamma law equation of state, an entirely integrable dynamical equation to the scale factor $R$ is obtained along with its exact solutions. In this framework we demonstrate that the model can be used to explain the dark energy dominant universe and for a special choice of the parameters we can explain the accelerating expansion of the universe also for two different phases, viz. combination of dark energy and dark matter phase as well as unified dark energy phase. A special physical check has been performed through sound speed constraint to validate the model. At last we obtain a scaling relation between the Hubble parameter with redshift.

physics.gen-ph↗

arXiv2016

$C$-field cosmological models: revisited

We investigate plane symmetric space-time filled with perfect fluid in the $C$-field cosmology of Hoyle and Narlikar. A new class of exact solutions have been obtained by considering the creation field $C$ as a function of time only. To get the deterministic solution, it has been assumed that the rate of creation of matter-energy density is proportional to the strength of the existing $C$-field energy density. Several physical aspects and geometrical properties of the models are discussed in detail, especially it is shown that some of our solutions of $C$-field cosmology are free from singularity in contrast to the Big Bang cosmology. A comparative study has been carried out between two models, one singular and the other nonsingular, by contrasting the behaviour of the physical parameters and noted that the model in a unique way represents both the features of the accelerating as well as decelerating Universe depending on the parameters and thus seems provides glimpses of the oscillating or cyclic model of the Universe without invoking any other agent or theory in allowing cyclicity.

physics.gen-ph↗

arXiv2016

Could wormholes form in dark matter galactic halos?

We estimate expression for velocity as a function of the radial coordinate $r$ by using polynomial interpolation based on the experimental data of rotational velocities at distant outer regions of galaxies. The interpolation technique has been used to estimate fifth degree polynomial followed by cubic spline interpolation. This rotational velocity is used to find the geometry of galactic halo regions within the framework of Einstein's general relativity. In this letter we have analyzed features of galactic halo regions based on two possible choices for the dark matter density profile, viz. Navarro, Frenk \& White (NFW) type~\citep{Navarro1996} and Universal Rotation Curve (URC) ~\citep{Castignani2012}. It is argued that spacetime of the galactic halo possesses some of the characteristics needed to support traversable wormholes.

gr-qc↗