scholarly journals Gravastars in f(š•‹,š’Æ) gravity

2020 ā—½ Ā 
Vol 35 (04) ā—½ Ā 
pp. 2050017 ā—½ Ā 
Author(s): Ā 
Shounak Ghosh ā—½ Ā 
A. D. Kanfon ā—½ Ā 
Amit Das ā—½ Ā 
M. J. S. Houndjo ā—½ Ā 
Ines G. Salako ā—½ Ā 
... Ā 
Keyword(s): Ā 
Energy Density ā—½ Ā 
Momentum Tensor ā—½ Ā 
Stellar Model ā—½ Ā 
Repulsive Force ā—½ Ā 
Schwarzschild Solution ā—½ Ā 
Interior Region ā—½ Ā 
Exterior Region ā—½ Ā 
Tensor Formula ā—½ Ā 
Matter Energy ā—½ Ā 
Shell Region

We propose a stellar model under the [Formula: see text] gravity following Mazurā€“Mottolaā€™s conjecture[Formula: see text] known as gravastar which is generally believed as a viable alternative to black hole. The gravastar consists of three regions, viz., (I)Ā interior region, (II)Ā intermediate shell region, and (III)Ā exterior region. The pressure within the interior core region is assumed to be equal to the constant negative matter-energy density which provides a constant repulsive force over the thin shell region. The shell is assumed to be made up of fluid of ultrarelativistic plasma and following the Zelā€™dovichā€™s conjecture of stiff fluid3 it is also assumed that the pressure which is directly proportional to the matter-energy density according to Zelā€™dovichā€™s conjecture, does cancel the repulsive force exerted by the interior region. The exterior region is completely vacuum and it can be described by the Schwarzschild solution. Under all these specifications, we find out a set of exact and singularity-free solutions of the gravastar presenting several physically valid features within the framework of alternative gravity, namely [Formula: see text] gravity,4 where the part of the gravitational Lagrangian in the corresponding action is taken as an arbitrary function of torsion scalar [Formula: see text] and the trace of the energyā€“momentum tensor [Formula: see text].

BULK VISCOUS BRANSā€“DICKE COSMOLOGICAL MODELS AND LATE-TIME ACCELERATION OF THE UNIVERSE

2003 ā—½ Ā 
Vol 12 (05) ā—½ Ā 
pp. 925-939 ā—½ Ā 
Author(s): Ā 
M. K. MAK ā—½ Ā 
T. HARKO
Keyword(s): Ā 
Energy Density ā—½ Ā 
Momentum Tensor ā—½ Ā 
Physical Parameters ā—½ Ā 
Late Time ā—½ Ā 
Field Equations ā—½ Ā 
Bulk Viscous ā—½ Ā 
Order Of Magnitude ā—½ Ā 
Viscous Pressure ā—½ Ā 
The Universe ā—½ Ā 
Matter Energy

We consider the dynamics of a causal bulk viscous cosmological fluid filled flat homogeneous Universe in the framework of the Bransā€“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 Bransā€“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.

scholarly journals Dynamical system analysis of FLRW models with Modified Chaplygin gas

Scientific Reports ā—½ Ā 
2021 ā—½ Ā 
Vol 11 (1) ā—½ Ā 
Author(s): Ā 
Ali Osman Yılmaz ā—½ Ā 
Ertan GĆ¼dekli
Keyword(s): Ā 
Dynamical System ā—½ Ā 
Equation Of State ā—½ Ā 
Energy Density ā—½ Ā 
System Analysis ā—½ Ā 
Chaplygin Gas ā—½ Ā 
Modified Chaplygin Gas ā—½ Ā 
State Spaces ā—½ Ā 
The Universe ā—½ Ā 
Matter Energy ā—½ Ā 
Far Future

AbstractWe investigate Friedmannā€“Lamaitreā€“Robertsonā€“Walker (FLRW) models with modified Chaplygin gas and cosmological constant, using dynamical system methods. We assume $$p=(\gamma -1)\mu -\dfrac{A}{\mu ^\alpha }$$ p = ( Ī³ - 1 ) Ī¼ - A Ī¼ Ī± as equation of state where $$\mu$$ Ī¼ is the matter-energy density, p is the pressure, $$\alpha$$ Ī± is a parameter which can take on values $$0<\alpha \le 1$$ 0 < Ī± ā‰¤ 1 as well as A and $$\gamma$$ Ī³ are positive constants. We draw the state spaces and analyze the nature of the singularity at the beginning, as well as the fate of the universe in the far future. In particular, we address the question whether there is a solution which is stable for all the cases.

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

2021 ā—½ Ā 
Author(s): Ā 
Deep Bhattacharjee
Keyword(s): Ā 
Energy Density ā—½ Ā 
Energy Momentum Tensor ā—½ Ā 
Momentum Tensor ā—½ Ā 
Space Time ā—½ Ā 
Exotic Matter ā—½ Ā 
Positive Energy ā—½ Ā 
Spacelike Hypersurfaces ā—½ Ā 
Stress Energy ā—½ Ā 
Maxwell Fields ā—½ Ā 
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.

scholarly journals A geometrical model for the evolution of spherical planetary nebulae based on thin-shell formalism

2021 ā—½ Ā 
pp. 2150191
Author(s): Ā 
Ibrahim Gullu ā—½ Ā 
S. Habib Mazharimousavi ā—½ Ā 
S. Danial Forghani
Keyword(s): Ā 
Energy Density ā—½ Ā 
Planetary Nebula ā—½ Ā 
Thin Shell ā—½ Ā 
Energy Momentum Tensor ā—½ Ā 
Geometric Model ā—½ Ā 
Geometrical Model ā—½ Ā 
Momentum Tensor ā—½ Ā 
Evolution Pattern ā—½ Ā 
Simple Power Function ā—½ Ā 
Curvature Tensors

A spherical planetary nebula is described as a geometric model. The nebula itself is considered as a thin-shell, which is visualized as a boundary of two spacetimes. The inner and outer curvature tensors of the thin-shell are found in order to get an expression of the energy-momentum tensor on the thin-shell. The energy density and pressure expressions are derived using the energy-momentum tensor. The time evolution of the radius of the thin-shell is obtained in terms of the energy density. The model is tested by using a simple power function for decreasing energy density and the evolution pattern of the planetary nebula is attained.

scholarly journals On homogeneous and isotropic universe

2015 ā—½ Ā 
Vol 30 (34) ā—½ Ā 
pp. 1550186 ā—½ Ā 
Author(s): Ā 
M. O. Katanaev
Keyword(s): Ā 
Vector Fields ā—½ Ā 
Energy Momentum Tensor ā—½ Ā 
Killing Vector ā—½ Ā 
Momentum Tensor ā—½ Ā 
Killing Vector Fields ā—½ Ā 
Full System ā—½ Ā 
Independent Functions ā—½ Ā 
Definition Of ā—½ Ā 
Usual Scale ā—½ Ā 
Matter Energy

We give a simple example of spacetime metric, illustrating that homogeneity and isotropy of space slices at all moments of time is not obligatory lifted to a full system of six Killing vector fields in spacetime, thus it cannot be interpreted as a symmetry of a four-dimensional metric. The metric depends on two arbitrary and independent functions of time. One of these functions is the usual scale factor. The second function cannot be removed by coordinate transformations. We prove that it must be equal to zero, if the metric satisfies Einsteinā€™s equations and the matter energyā€“momentum tensor is homogeneous and isotropic. A new, equivalent, definition of homogeneous and isotropic spacetime is given.

scholarly journals SPECTRA OF RELIC GRAVITONS AND BRANSā€“DICKE THEORY

2005 ā—½ Ā 
Vol 20 (02) ā—½ Ā 
pp. 127-134 ā—½ Ā 
Author(s): Ā 
B. K. SAHOO
Keyword(s): Ā 
Scalar Field ā—½ Ā 
Energy Density ā—½ Ā 
Time Dependence ā—½ Ā 
Gravitational Waves ā—½ Ā 
Hubble Parameter ā—½ Ā 
Frequency Dependent ā—½ Ā 
Cosmological Expansion ā—½ Ā 
Relic Gravitational Waves ā—½ Ā 
Matter Energy ā—½ Ā 
Inflationary Models

The spectra of relic gravitational waves produced as a result of cosmological expansion of the inflationary models are derived in Bransā€“Dicke (BD) theory of gravity. The time dependence of the very early Hubble parameter and matter energy density are derived from frequency-dependent spectrum of relic gravitational waves. Also it is found that Bransā€“Dicke scalar field contributes to the energy density of relic gravitons.

Kinetics and Mechanism of Cationic Micelle/Flexible Nanoparticle Catalysis: A Review

2018 ā—½ Ā 
Vol 43 (1) ā—½ Ā 
pp. 1-20 ā—½ Ā 
Author(s): Ā 
Mohammad Niyaz Khan ā—½ Ā 
Ibrahim Isah Fagge
Keyword(s): Ā 
Rate Increase ā—½ Ā 
Molecular Level ā—½ Ā 
Binding Constants ā—½ Ā 
Exchange Constant ā—½ Ā 
Interior Region ā—½ Ā 
Bimolecular Reactions ā—½ Ā 
Exterior Region ā—½ Ā 
Different Shapes ā—½ Ā 
Molecular Origin ā—½ Ā 
Kinetics Of

The aqueous surfactant (Surf) solution at [Surf] > cmc (critical micelle concentration) contains flexible micelles/nanoparticles. These particles form a pseudophase of different shapes and sizes where the medium polarity decreases as the distance increases from the exterior region of the interface of the Surf/H2O particle towards its furthest interior region. Flexible nanoparticles (FNs) catalyse a variety of chemical and biochemical reactions. FN catalysis involves both positive catalysis ( i.e. rate increase) and negative catalysis ( i.e. rate decrease). This article describes the mechanistic details of these catalyses at the molecular level, which reveals the molecular origin of these catalyses. Effects of inert counterionic salts (MX) on the rates of bimolecular reactions (with one of the reactants as reactive counterion) in the presence of ionic FNs/micelles may result in either positive or negative catalysis. The kinetics of cationic FN (Surf/MX/H2O)-catalysed bimolecular reactions (with nonionic and anionic reactants) provide kinetic parameters which can be used to determine an ion exchange constant or the ratio of the binding constants of counterions.

scholarly journals Generalized Phenomenological Models of Dark Energy

10.1155/2020/5249839 ā—½ Ā 
2020 ā—½ Ā 
Vol 2020 ā—½ Ā 
pp. 1-8
Author(s): Ā 
Prasenjit Paul ā—½ Ā 
Rikpratik Sengupta
Keyword(s): Ā 
General Relativity ā—½ Ā 
Dark Energy ā—½ Ā 
Energy Density ā—½ Ā 
Cosmological Constant ā—½ Ā 
Phenomenological Models ā—½ Ā 
Einstein Field Equations ā—½ Ā 
Field Equations ā—½ Ā 
Free Parameters ā—½ Ā 
The Universe ā—½ Ā 
Matter Energy

It was first observed at the end of the last century that the universe is presently accelerating. Ever since, there have been several attempts to explain this observation theoretically. There are two possible approaches. The more conventional one is to modify the matter part of the Einstein field equations, and the second one is to modify the geometry part. We shall consider two phenomenological models based on the former, more conventional approach within the context of general relativity. The phenomenological models in this paper consider a Ī› term firstly a function of aĀØ/a and secondly a function of Ļ, where a and Ļ are the scale factor and matter energy density, respectively. Constraining the free parameters of the models with the latest observational data gives satisfactory values of parameters as considered by us initially. Without any field theoretic interpretation, we explain the recent observations with a dynamical cosmological constant.

scholarly journals Applications of light speed expansion and gravitational self-energy density in black hole cosmology

2015 ā—½ Ā 
Vol 3 (2) ā—½ Ā 
pp. 123
Author(s): Ā 
Satya Seshavatharam UV ā—½ Ā 
Terry Tatum E ā—½ Ā 
Lakshminarayana S
Keyword(s): Ā 
Black Hole ā—½ Ā 
Energy Density ā—½ Ā 
Thermal Energy ā—½ Ā 
Planck Scale ā—½ Ā 
Cosmic Inflation ā—½ Ā 
Self Energy ā—½ Ā 
Light Speed ā—½ Ā 
Cosmological Observations ā—½ Ā 
The Universe ā—½ Ā 
Matter Energy

<p>From the beginning of Planck scale to the scale of the current Hubble radius: 1) Considering the relation, subjects of black holes and cosmology, both can be integrated into evolving black hole cosmology and cosmic horizon problem can be relinquished. 2) Considering ā€˜continuous light speed expansionā€™ of the cosmic black hole horizon, attributed results of cosmic inflation can be re-addressed completely. If ā€˜natureā€™ of the universe is to expand with light speed, then there is no need to think about the existence of currently believed ā€˜Lambda termā€™. In addition, ā€˜light speed expanding cosmic spaceā€™ can be called as ā€˜flat spaceā€™. 3) Considering the ratio of gravitational self-energy density and thermal energy density to be Ā (where Ā is the Planck scale temperature, and is cosmic temperature at any time). Quantum gravity can be implemented in low energy scale current cosmological observations. Considering the above concepts, currently believed dark matter energy density and visible matter energy density both can be accurately fitted with the ratio of current gravitational self-energy density and current thermal energy density. To proceed further, the authors would like to highlight the following three points: 1) Deep-space red shift non-linearity can be expected to be connected with cosmological gravitational and relativistic effects and cannot be considered as a major criterion of cosmic evolution. 2) Until one finds solid applications of super luminal speeds and super luminal expansions in other areas of physics like astrophysics and nuclear astrophysics, currently believed ā€˜cosmic inflationā€™ cannot be considered as a real physical model and alternative proposals of inflation can be given a chance in exploring the evolving history of the universe. 3) Implementing Planck scale in current paradigm of cosmological observations and standard cosmology is very challenging and is inevitable.</p>

scholarly journals Surge motion on a floating cylinder in water of finite depth

2003 ā—½ Ā 
Vol 2003 (57) ā—½ Ā 
pp. 3643-3656 ā—½ Ā 
Author(s): Ā 
Dambaru D. Bhatta
Keyword(s): Ā 
Velocity Potential ā—½ Ā 
Separation Of Variables ā—½ Ā 
Finite Depth ā—½ Ā 
Added Mass ā—½ Ā 
Computational Results ā—½ Ā 
Complex Matrix ā—½ Ā 
Interior Region ā—½ Ā 
Exterior Region ā—½ Ā 
Damping Coefficients ā—½ Ā 
Calm Water

We derived added mass and damping coefficients of a vertical floating circular cylinder due to surge motion in calm water of finite depth. This is done by deriving the velocity potential for the cylinder by considering two regions, namely, interior region and exterior region. The velocity potentials for these two regions are obtained by the method of separation of variables. The continuity of the solutions has been maintained at the imaginary interface of these regions by matching the functions and gradients of each solution. The complex matrix equation is numerically solved to determine the unknown coefficients. Some computational results are presented for different depth-to-radius and draft-to-radius ratios.

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