scholarly journals Intrinsically symmetric cosmological model in the presence of dissipative fluids

2021 ◽  
pp. 2150033
Author(s):  
Eduardo Bittencourt ◽  
Leandro Gomes ◽  
Grasiele Santos
Keyword(s):  
Exact Solution ◽  
Energy Density ◽  
Exotic Matter ◽  
Luminosity Distance ◽  
Field Equations ◽  
Spatial Gradients ◽  
Inhomogeneous Models ◽  
The Standard Model ◽  
Dissipative Terms ◽  
Dissipative Fluids

Based upon the intrinsic symmetries approach to inhomogeneous cosmologies, we propose an exact solution to Einstein’s field equations where the spatial sections are flat and the source is a nonperfect fluid such that the dissipative terms can be written in terms of spatial gradients of the energy density under a suitable choice of the coordinate system. It is shown through the calculation of the luminosity distance as a function of the redshift that the presence of such inhomogeneities may lead to an effective deceleration parameter compatible with either the standard [Formula: see text]CDM model or LTB models depending on the choice of boundary conditions with no exotic matter. This fact is another evidence that different inhomogeneous models should be carefully investigated in order to verify which model may be compatible with observations and still be as close as possible to the standard model regarding the underlying assumptions, without resorting necessarily to exotic matter components.

scholarly journals THE GENERAL EXACT SOLUTION FOR RELATIVISTIC SPHERICAL SHELLS

2009 ◽  
Vol 18 (12) ◽  
pp. 1801-1808 ◽  
Author(s):  
J. KIJOWSKI ◽  
G. MAGLI ◽  
D. MALAFARINA
Keyword(s):  
Exact Solution ◽  
Equation Of State ◽  
Closed Form ◽  
Energy Condition ◽  
Matter Field ◽  
The Other ◽  
Exotic Matter ◽  
Spherically Symmetric ◽  
Field Equations ◽  
Spherical Shells

The general exact solution to the Einstein matter field equations describing spherically symmetric shells satisfying an equation of state in closed form is discussed under general assumptions of physical reasonableness. The solutions split into two classes: one describes the behavior of "ordinary" matter satisfying the weak energy condition, while the other describes "exotic" matter for which this condition can be violated. In both cases all those solutions which are already known are recovered as particular cases.

Features of an analytical solution in Eddington inspired Born–Infeld (EiBI) gravity

2021 ◽  
pp. 2150001
Author(s):  
Mithun Ghosh
Keyword(s):  
Exact Solution ◽  
Energy Density ◽  
Analytical Solution ◽  
Compact Star ◽  
Paper Analysis ◽  
Energy Conditions ◽  
Spherically Symmetric ◽  
Field Equations ◽  
Equation Of States ◽  
Future Implication

Recently, Harko et al. [Phys. Rev. D 88, 044032 (2013)] have derived an exact solution of the spherically symmetric field equations in EiBI gravity, describing a compact star with decreasing pressure but increasing energy density. We have explored some features of this solution by restricting the range of the parameter [Formula: see text] which satisfies four energy conditions for 10 equation of states (EOSs). Viability and deviations of these features in the light of updated observed properties of neutron stars (NS) have been explored completely. A comparison of those features with recent observations may uncover the future implication of this paper. Analysis of our results indicates that this solution may be used as an alternative EOS.

scholarly journals NONUNIFORM BRANEWORLD STARS: AN EXACT SOLUTION

2009 ◽  
Vol 18 (05) ◽  
pp. 837-852 ◽  
Author(s):  
J. OVALLE
Keyword(s):  
Exact Solution ◽  
Effective Density ◽  
Interior Solution ◽  
Effective Pressure ◽  
Gravity Effect ◽  
Field Equations ◽  
Matching Conditions ◽  
Physical Behavior ◽  
Exterior Solution ◽  
Weyl Scalar

In this paper the first exact interior solution to Einstein's field equations for a static and nonuniform braneworld star with local and nonlocal bulk terms is presented. It is shown that the bulk Weyl scalar [Formula: see text] is always negative inside the stellar distribution, and in consequence it reduces both the effective density and the effective pressure. It is found that the anisotropy generated by bulk gravity effect has an acceptable physical behavior inside the distribution. Using a Reissner–Nördstrom-like exterior solution, the effects of bulk gravity on pressure and density are found through matching conditions.

FRW cosmological models with cosmological constant in f (R,T) theory of gravity

2021 ◽  
Author(s):  
Anirudh Pradhan ◽  
Priyanka Garg ◽  
Archana Dixit
Keyword(s):  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Apparent Magnitude ◽  
Physical Parameters ◽  
Distance Modulus ◽  
Luminosity Distance ◽  
Field Equations ◽  
Eos Parameter ◽  
Accelerating Expansion ◽  
Frw Cosmological Model

In the present paper, we have generalized the behaviors of {\color{blue}transit-decelerating to accelerating} FRW cosmological model in f (R, T) gravity theory, where R, T are Ricci scalar and trace of energy-momentum tensor respectively. The solution of the corresponding field equations is obtained by assuming a linear function of the Hubble parameter H, i.e., q = c<sub>1</sub> + c<sub>2</sub>H which gives a time-dependent DP (deceleration parameter) q(t)=-1+\frac{c_2}{\sqrt{2c_2 t +c_3}}, where c<sub>3</sub> and c<sub>2</sub> are arbitrary integrating constants [Tiwari et al., Eur. Phys. J. Plus: 131, 447 (2016); 132, 126 (2017)]. There are two scenarios in which we explain the particular form of scale factor thus obtained  (i) By using the recent constraints from OHD and JLA data which shows a cosmic deceleration to acceleration and (ii) By using new constraints from supernovae type la union data which shows accelerating expansion universe (q<0) throughout the evolution. We have observed that the EoS parameter, energy density parameters, and important cosmological planes yield the results compatible with the modern observational data. For the derived models, we have calculated various physical parameters as Luminosity distance, Distance modulus, and Apparent magnitude versus redshift for both supporting current observations.

scholarly journals TRAVERSABLE WORMHOLES IN (2+1) AND (3+1) DIMENSIONS WITH A COSMOLOGICAL CONSTANT

1995 ◽  
Vol 04 (02) ◽  
pp. 231-245 ◽  
Author(s):  
M.S.R. DELGATY ◽  
R.B. MANN
Keyword(s):  
Energy Density ◽  
Cosmological Constant ◽  
Positive Energy ◽  
Mass Energy ◽  
Field Equations ◽  
Dimensional Solution ◽  
Partial Stability ◽  
Traversable Wormholes ◽  
Spacetime Curvature ◽  
Radial Tension

Macroscopic traversable wormhole solutions to Einstein’s field equations in (2+1) and (3+1) dimensions with a cosmological constant are investigated. Ensuring traversability severely constrains the material used to generate the wormhole’s spacetime curvature. Although the presence of a cosmological constant modifies to some extent the type of matter permitted [for example it is possible to have a positive energy density for the material threading the throat of the wormhole in (2+1) dimensions], the material must still be “exotic,” that is matter with a larger radial tension than total mass-energy density multiplied by c2. Two specific solutions are applied to the general cases and a partial stability analysis of a (2+1) dimensional solution is explored.

scholarly journals Positive Energy Driven CTCs In ADM 3+1 Space – Time of Unprotected Chronology

2021 ◽  
Author(s):  
Deep Bhattacharjee
Keyword(s):  
Energy Density ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Space Time ◽  
Exotic Matter ◽  
Positive Energy ◽  
Spacelike Hypersurfaces ◽  
Stress Energy ◽  
Maxwell Fields ◽  
Stress Energy Momentum Tensor

Chronology unprotected mechanisms are considered with a very low gravitational polarization to make the wormhole traversal with positive energy density everywhere. No need of exotic matter has been considered with the assumption of the Einstein-Dirac-Maxwell Fields, encountering above the non-zero stress-energy-momentum tensor through spacelike hypersurfaces by a hyperbolic coordinate shift.

Wormhole solutions in embedding class 1 space–time

2021 ◽  
Vol 36 (02) ◽  
pp. 2150015
Author(s):  
Nayan Sarkar ◽  
Susmita Sarkar ◽  
Farook Rahaman ◽  
Safiqul Islam
Keyword(s):  
Dimensional Space ◽  
Vacuum Solution ◽  
Energy Condition ◽  
Radial Distance ◽  
Space Time ◽  
Exotic Matter ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Asymptotically Flat ◽  
Class 1

The present work looks for new spherically symmetric wormhole solutions of the Einstein field equations based on the well-known embedding class 1, i.e. Karmarkar condition. The embedding theorems have an interesting property that connects an [Formula: see text]-dimensional space–time to the higher-dimensional Euclidean flat space–time. The Einstein field equations yield the wormhole solution by violating the null energy condition (NEC). Here, wormholes solutions are obtained corresponding to three different redshift functions: rational, logarithm, and inverse trigonometric functions, in embedding class 1 space–time. The obtained shape function in each case satisfies the flare-out condition after the throat radius, i.e. good enough to represents wormhole structure. In cases of WH1 and WH2, the solutions violate the NEC as well as strong energy condition (SEC), i.e. here the exotic matter content exists within the wormholes and strongly sustains wormhole structures. In the case of WH3, the solution violates NEC but satisfies SEC, so for violating the NEC wormhole preserve due to the presence of exotic matter. Moreover, WH1 and WH2 are asymptotically flat while WH3 is not asymptotically flat. So, indeed, WH3 cutoff after some radial distance [Formula: see text], the Schwarzschild radius, and match to the external vacuum solution.

Color-flavor locked compact stars: An exact solution approach

2021 ◽  
Author(s):  
Ksh. Newton Singh ◽  
Shyam Das ◽  
Piyali Bhar ◽  
Monsur Rahaman ◽  
Farook Rahaman
Keyword(s):  
Exact Solution ◽  
Compact Star ◽  
Density Perturbation ◽  
Compact Stars ◽  
Stable Configuration ◽  
Physical Parameters ◽  
Superconducting Gap ◽  
Field Equations ◽  
Critical Limit ◽  
Solution Approach

We present an exact solution that could describe compact star composed of color-flavor locked (CFL) phase. Einstein’s field equations were solved through CFL equation of state (EoS) along with a specific form of [Formula: see text] metric potential. Further, to explore a generalized solution we have also included pressure anisotropy. The solution is then analyzed by varying the color superconducting gap [Formula: see text] and its effects on the physical parameters. The stability of the solution through various criteria is also analyzed. To show the physical validity of the obtained solution we have generated the [Formula: see text] curve and fitted three well-known compact stars. This work shows that the anisotropy of the pressure at the interior increases with the color superconducting gap leading to decrease in adiabatic index closer to the critical limit. Further, the fluctuating range of mass due to the density perturbation is larger for lower color superconducting gap leading to more stable configuration.

Signature of Topology of the Universe

2018 ◽  
Vol 2 (01) ◽  
Author(s):  
Vipin Kumar Sharma
Keyword(s):  
General Relativity ◽  
Energy Density ◽  
Local Geometry ◽  
Microwave Background ◽  
Main Motivation ◽  
And Topology ◽  
The Standard Model ◽  
Topology Of The Universe ◽  
The Universe ◽  
Insight Into

The main motivation to write this article is to relate the cosmology and topology in order to gain some insight into the topological signatures of the Standard model of Universe. The theory of General Relativity as given by Einstein only describes the local geometry of space but not global, hence leaves the possibility to explore the topology of the space (simply- or multi-connected). By expressing the cosmological model in trms of energy density parameters, we attempt to understand the geometry of spacetime. This is followed by a discussion on the possibility to detect the signatures of topology of space imprinted on the Cosmic Microwave Background (CMB).

Exact wormholes solutions without exotic matter in f(R,T) gravity

2019 ◽  
Vol 16 (03) ◽  
pp. 1950046 ◽  
Author(s):  
M. Zubair ◽  
Rabia Saleem ◽  
Yasir Ahmad ◽  
G. Abbas
Keyword(s):  
Momentum Tensor ◽  
Exotic Matter ◽  
Gravity Models ◽  
Energy Conditions ◽  
Anisotropic Fluid ◽  
Energy Tensor ◽  
Field Equations ◽  
Stress Energy ◽  
Symmetric Geometry ◽  
The Stability

This paper is aimed to evaluate the existence of wormholes in viable [Formula: see text] gravity models (where [Formula: see text] is the scalar curvature and [Formula: see text] is the trace of stress–energy tensor of matter). The exact solutions for energy–momentum tensor components depending on different shapes and redshift functions are calculated without some additional constraints. To investigate this, we consider static spherically symmetric geometry with matter contents as anisotropic fluid and formulate the Einstein field equations for three different [Formula: see text] models. For each model, we derive expression for weak and null energy conditions and graphically analyzed its violation near the throat. It is really interesting that wormhole solutions do not require the presence of exotic matter — like that in general relativity. Finally, the stability of the solutions for each model is presented using equilibrium condition.

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