arXiv subjects

Saibal Ray

Publications and source records attributed to Saibal Ray.

At least 163 records · Page 9Linked to original sources

arXiv2010

Variable Equation of State for Generalized Dark Energy Model

We present a model for the present accelerating Universe and focus on the different important physical variables involved in the model under the phenomenological assumption $Λ\propto H^2$ with a prescription for equation of state parameter in the form $ω(t)=ω_0+\frac{ω_1τ}{t^2}$, where $ω_0$ and $ω_1$ are two constants and $τ$ is a parameter having dimension of time $t^2$. General expressions for the density parameter $Ω$ and deceleration parameter $q$ are obtained which under specific bound reproduce some of the previous results. We explore physical features of these parameters which (i) provide the scenario of complete evolution of the cosmos with $ω(t)$ and (ii) agree mostly with the observational status of the present phase of the accelerating Universe.

physics.gen-ph↗

arXiv2010

Dark Energy Models With Variable Equation Of State Parameter

Two variable $Λ$ models, viz. $Λ\sim (\dot a/a)^2$ and $Λ\sim ρ$ have been studied under the assumption that the equation of state parameter $ω$ is a function of time. The selected $Λ$ models are found to be equivalent both in four and five dimensions. The possibility of signature flip of the deceleration parameter is also shown.

gr-qc↗

arXiv2010

Higher Dimensional Dark Energy Investigation with Variable $Λ$ and $G$

Time variable $Λ$ and $G$ are studied here under a phenomenological model of $Λ$ through an ($n+2$) dimensional analysis. The relation of Zeldovich (1968) $|Λ| = 8πG^2m_p^6/h^4$ between $Λ$ and $G$ is employed here, where $m_p$ is the proton mass and $h$ is Planck's constant. In the present investigation some key issues of modern cosmology, viz. the age problem, the amount of variation of $G$ and the nature of expansion of the Universe have been addressed.

gr-qc↗

arXiv2009

Features of galactic halo in a brane world model and observational constraints

Several aspects of the 4d imprint of the 5d bulk Weyl radiation are investigated within a recently proposed model solution. It is shown that the solution has a number of physically interesting properties. The constraints on the model imposed by combined measurements of rotation curve and lensing are discussed. A brief comparison with a well known scalar field model is also given.

gr-qc↗

arXiv2009

Scenarios of cosmic string with variable cosmological constant

Exact solutions of the Einstein field equations with cosmic string and space varying cosmological constant, viz., $Λ= Λ(r)$, in the energy-momentum tensors are presented. Three cases have been studied: where variable cosmological constant (1) has power law dependence, (2) is proportional to the string fluid density, and (3) is purely a constant. Some cases of interesting physical consequences have been found out such that (i) variable cosmological constant can be represented by a power law of the type $Λ=3r^{-2}$, (ii) variable cosmological constant and cosmic string density are interdependent to each other according to the relation $Λ=-8πρ_s$, and (iii) cosmic string density can be scaled by a power law of the type $ρ_s=r^{-2}$. It is also shown that several known solutions can be recovered from the general form of the solutions obtained here.

astro-ph↗

arXiv2008

Electromagnetic mass model admitting conformal motion

We study charged fluid spheres under the 4-dimensional Einstein-Maxwell space-time. The solutions thus obtained admitting conformal motion. We also investigate whether the solutions set provide electromagnetic mass models such that the physical parameters including the gravitational mass arise from the electromagnetic field alone. In this connection three cases are studied here in detail with the propositions: (1) $p = - ρ$, (2) $σe^{λ/2} = σ_0$ and (3) $8 πp - E^2 = p_0$ where $ρ$, $p$, $σ$ are respectively the usual matter density, fluid pressure and charge density of the spherical distribution. Based on these assumptions several features are explored which seems physically very interesting.

gr-qc↗

arXiv2008

Scenario of inflationary cosmology from the phenomenological $Λ$ models

Choosing the three phenomenological models of the dynamical cosmological term $Λ$, viz., $Λ\sim (\dot a/a)^2$, $Λ\sim {\ddot a/a}$ and $Λ\sim ρ$ where $a$ is the cosmic scale factor, it has been shown by the method of numerical analysis that the three models are equivalent for the flat Universe $k=0$. The evolution plots for dynamical cosmological term $Λ$ vs. time $t$ and also the cosmic scale factor $a$ vs. $t$ are drawn here for $k=0, +1$. A qualitative analysis has been made from the plots which supports the idea of inflation and hence expanding Universe.

gr-qc↗

arXiv2008

Galactic rotation curves and brane world models

In the present investigation flat rotational curves of the galaxies are considered under the framework of brane-world models where the 4d effective Einstein equation has extra terms which arise from the embedding of the 3-brane in the $5d$ bulk. It has been shown here that these long range bulk gravitational degrees of freedom can act as a mechanism to yield the observed galactic rotation curves without the need for dark matter. The present model has the advantage that the observed rotation curves result solely from well-established non-local effects of gravitation, such as dark radiation and dark pressure under a direct use of the condition of flat rotation curves and does not invoke any exotic matter field.

astro-ph↗

arXiv2008

$Λ$-CDM Universe: A Phenomenological Approach With Many Possibilities

A time-dependent phenomenological model of $Λ$, viz. $\dot Λ\sim H^3$ is selected to investigate the $Λ$-CDM cosmology. Time-dependent form of the equation of state parameter $ω$ is derived and it has been possible to obtain the sought for flip of sign of the deceleration parameter q. Present age of the Universe, calculated for some specific values of the parameters agrees very well with the observational data.

gr-qc↗

arXiv2008

The dark energy equation of state

We perform a study of cosmic evolution with an equation of state parameter $ω(t)=ω_0+ω_1(t\dot H/H)$ by selecting a phenomenological $Λ$ model of the form, $\dotΛ\sim H^3$. This simple proposition explains both linearly expanding and inflationary Universes with a single set of equations. We notice that the inflation leads to a scaling in the equation of state parameter, $ω(t)$, and hence in equation of state. In this approach, one of its two parameters have been pin pointed and the other have been delineated. It has been possible to show a connection between dark energy and Higgs-Boson.

gr-qc↗

arXiv2008

Generalized Model for $Λ$-Dark Energy

Einstein field equations under spherically symmetric space-times are considered here in connection to dark energy investigation. A set of solutions are obtained for a kinematical $Λ$ model, viz., $Λ\sim (\dot a/a)^2$ without assuming any {\it a priori} value for the curvature constant and the equation of state parameter $ω$. Some interesting results, such as the nature of cosmic density $Ω$ and deceleration parameter $q$, have been obtained with the consideration of two-fluid structure instead of usual uni-fluid cosmological model.

gr-qc↗

arXiv2007

Relativistic Gravitational mass in Tolman-VI type solution

Some known solutions for static charged fluid spheres of Tolman-VI type in general relativity are reexamined. The gravitational mass that appears in the Pant and Sah \cite{pan79} and Tikekar \cite{tik84} solutions is shown to be of electromagnetic origin in the sense that the gravitational mass, along with all other physical quantities, depends on the electromagnetic field alone. The energy condition, singularity and stability problems of the models are discussed thoroughly.

astro-ph↗

arXiv2007

Lorentz's electromagnetic mass: a clue for unification?

We review in the present article the conjecture of electromagnetic mass by Lorentz. The philosophical perspectives and historical accounts of this idea are described, especially, in the light of Einstein's special relativistic formula {E = mc^2}. It is shown that the Lorentz's electromagnetic mass model has taken various shapes through its journey and the goal is not yet reached.

physics.hist-ph↗

arXiv2007

Do Solar system tests permit higher dimensional general relativity?

We perform a survey whether higher dimensional Schwarzschild space-time is compatible with some of the solar system phenomena. As a test we examine five well known solar system effects, viz., (1) Perihelion shift, (2) Bending of light, (3) Gravitational redshift, (4) Gravitational time delay and (5) Motion of test particle in the framework of general relativity with higher dimensions. It is shown that the results related to all these physical phenomena are mostly incompatible with the higher dimensional version of general relativity except that of Motion of test particle. We compare all these results with the available data in the literature.

gr-qc↗

arXiv2007

Periodicity of ~155 days in solar electron fluence

In this paper we have investigated the occurrence rate of high energetic(E>10 MeV) solar electron flares measured by IMP-8 spacecraft of NASA for solar cycle 21 (June, 1976 to August, 1986) first time by three different methods to detect periodicities accurately. Power-spectrum analysis confirms a periodicity ~155 days which is in consistent with the result of Chowdhury and Ray (2006), that "Rieger periodicity" was operated throughout the cycle 21 and it is independent on the energy of the electron fluxes.

astro-ph↗

arXiv2007

Accelerating Universe with a dynamic cosmological term

Keeping in mind the current picture of an accelerating and flat Universe, some specific dynamical models of the cosmological term $Λ$ have been selected for investigating the nature of dark energy. Connecting the free parameters of the models with the cosmic matter and vacuum energy density parameters, it is shown that the models are equivalent. Using the selected models, the present values of some of the physical parameters have been estimated, and a glimpse at the past decelerating universe has also been presented. It is observed that most of these cosmological parameters nicely agree with the values suggested by the Type Ia Supernovae and other experimental data.

astro-ph↗

arXiv2007

Physical properties of Tolman-Bayin solutions: some cases of static charged fluid spheres in general relativity

In this article, Einstein-Maxwell space-time has been considered in connection to some of the astrophysical solutions as previously obtained by Tolman (1939) and Bayin (1978). The effect of inclusion of charge into these solutions has been investigated thoroughly and also the nature of fluid pressure and mass density throughout the sphere have been discussed. Mass-radius and mass-charge relations have been derived for various cases of the charged matter distribution. Two cases are obtained where perfect fluid with positive pressures give rise to electromagnetic mass models such that gravitational mass is of purely electromagnetic origin.

astro-ph↗

arXiv2007

Large Number Hypothesis: A Review

Large dimensionless numbers, arising out of ratios of various physical constants, intrigued many scientists, especially Dirac. Relying on the coincidence of large numbers, Dirac arrived at the revolutionary hypothesis that the gravitational constant $G$ should vary inversely as the cosmic time $t$. This hypothesis of Dirac, known as Large Number Hypothesis (LNH), sparked off many speculations, arguments and new ideas in terms of applications. Works done by several authors with LNH as their basic platform are reviewed in this work. Relationship between some of those works are pointed out here. Possibility of time-variations of physical constants other than $G$ are also discussed.

gr-qc↗