arXiv subjects

Ujjal Debnath

Publications and source records attributed to Ujjal Debnath.

At least 73 records · Page 4Linked to original sources

arXiv2019

Bouncing Cosmology for Entropy Corrected Models in Horava-Lifshitz Gravity and Fractal Universe

The modified field equations are written in logarithmic and power law versions of entropy corrected models in Einstein's gravity in the background of FRW universe. In one section, a brief review of the Ho$\check{\text r}$ava-Lifshitz gravity is discussed and the modified field equations in logarithmic and power law versions of entropy corrected models in Ho$\check{\text r}$ava-Lifshitz gravity are formulated. The stability analysis for these models are performed by describing the dynamical system. In another section, a brief review of the fractal universe is presented and the modified field equations in logarithmic and power law versions of entropy corrected models in fractal universe are formulated. The stability analysis for the dynamical system for these models in the framework of fractal universe are described. Furthermore, the bouncing scenarios of the universe in Ho$\check{\text r}$ava-Lifshitz gravity and fractal model for both logarithmic and power law entropy corrected versions in $k=0,+1,-1$ separately are analyzed. For different cases, the validity of null energy condition (NEC) at the time of bounce is examined. Finally, the behaviors of the physical quantities are depicted through diagrams.

gr-qc↗

arXiv2019

Charged Gravastars in Rastall-Rainbow Gravity

In this work, we have considered the spherically symmetric stellar system in the contexts of Rastall-Rainbow gravity theory in presence of isotropic fluid source with electromagnetic field. The Einstein-Maxwell's field equations have been written in the framework of Rastall-Rainbow gravity. The gravastar consists of three regions: interior region, thin shell region and exterior region. In the interior region, the gravastar follows the equation of sate (EoS) $p=-ρ$ and we have found the solutions of all physical quantities like energy density, pressure, electric field, charge density, gravitational mass and metric coefficients. In the exterior region, we have obtained the exterior Riessner-Nordstrom solution for vacuum model ($p=ρ=0$). Since in the shell region, the fluid source follows the EoS $p=ρ$ (ultra-stiff fluid) and the thickness of the shell of the gravastar is infinitesimal, so by the approximation $h~(\equiv A^{-1})\ll 1$, we have found the analytical solutions within the thin shell. The physical quantities like the proper length of the thin shell, entropy and energy content inside the thin shell of the charged gravastar have been computed and we have shown that they are directly proportional to the proper thickness of the shell ($ε$) due to the approximation ($ε\ll 1$). The physical parameters significantly depend on the Rastall parameter and Rainbow function. Next we have studied the matching between the surfaces of interior and exterior regions of the charged gravastar and using the matching conditions, the surface energy density and the surface pressure have been obtained. Also the equation of state parameter on the surface, mass of the thin shell, mass of the gravastar have been obtained. Finally, we have explored the stable regions of the charged gravastar in Rastall-Rainbow gravity.

physics.gen-ph↗

arXiv2019

Study of QCD generalized ghost dark energy in FRW universe

A phenomenological generalized ghost dark energy model has been studied under the framework of FRW universe. In ghost dark energy model the energy density depends linearly on Hubble parameter (H) but in this dark energy model, the energy density contains a the sub-leading term which is depends on $\mathcal{O} (H^2)$, so the energy density takes the form $ρ_D=αH+ βH^2$, where $α$ and $β$ are the constants. The solutions of the Friedman equation of our model leads to a stable universe. We have fitted our model with the present observational data including Stern data set. With the help of best fit results we find the adiabatic sound speed remains positive throughout the cosmic evolution, that claims the stability of the model. The flipping of the signature of deceleration parameter at the value of scale factor $a=0.5$ indicates that the universe is at the stage of acceleration i.e. de Sitter phase of the universe at late time. Our model shows that the acceleration of the universe begin at redshift $z_{ace}\approx 0.617$ and the model is also consistent with the current observational data.

gr-qc↗

arXiv2019

Parametrizations of Dark Energy Models in the Background of General Non-canonical Scalar Field in $D$-dimensional Fractal Universe

We explore non-canonical scalar field model in the background of non-flat $D$-dimensional fractal Universe with cosmological constant $Λ$ on the condition that the matter and scalar field are separately conserved. The potential $V$, scalar field $ϕ$, function $f$, densities, Hubble parameter and deceleration parameter can be expressed in terms of the redshift $z$ and these depend on the equation of state parameter $w_ϕ$. We also investigate four kinds of well known parametrization models and graphically we have analyzed the natures of potential, scalar field, function $f$, densities, the Hubble parameter and deceleration parameter. As a result, the best fitted values of the unknown parameters ($w_{0},w_{1}$) of the parametrizations models due to the joint data analysis (SNIa+BAO+CMB+Hubble) are found. Furthermore, the minimum values of $χ^{2}$ function are obtained. Also we have plotted the graphs for different confidence levels 66\%, 90\% and 99\% contours for ($w_{0},~w_{1}$) by fixing the other parameters.

gr-qc↗

arXiv2019

Collision of particles near charged MSW black hole in 2 + 1 dimensions

Here, we explore the dynamics of particle near the horizon of charged Mandal-Sengupta-Wadia (MSW) black hole in 2+1 dimensions. It results analysis of angular momentum and potential energy for null and time-like geodesics. We also appraise the high center-of-mass energy of coming particles from rest at infinity near the horizon of the charged MSW black hole in 2+1 dimension for the extremal case. Finally, we study the ISCO and MBCO radii for this type of black hole.

gr-qc↗

arXiv2019

Parameterizing Dark Energy Models and Study of Finite Time Future Singularities

A review on spatially flat D-dimensional Friedmann-Robertson-Walker (FRW) model of the universe has been performed. Some standard parameterizations of the equation of state parameter of the Dark Energy models are proposed and the possibilities of finite time future singularities are investigated. It is found that certain types of these singularities may appear by tuning some parameters appropriately. Moreover, for a scalar field theoretic description of the model, it was found that the model undergoes bouncing solutions in some favorable cases.

gr-qc↗

arXiv2019

Thermodynamic Black Hole with Modified Chaplygin Gas as a Heat Engine

We assume that the negative cosmological constant as a thermodynamical pressure and the asymptotically anti-de Sitter (AdS) black hole thermodynamics with modified Chaplygin gas. We have written the mass of the black hole, volume, entropy and temperature due to the thermodynamic system. We find a new solution of Einstein's field equations of AdS black hole with modified Chaplygin gas as a thermodynamic system. We also examine the weak, strong and dominant energy conditions for the source fluid of black hole. We also show that the thermodynamic black hole with Chaplygin gas can be considered as a heat engine and then we calculate work done and its efficiency by this system.

gr-qc↗

arXiv2019

Charge Gravastars in $f(T)$ Modified Gravity

In the present work, we have discussed the four dimensional spherically symmetric steller system in the framework of modified $f(T)$ gravity theory with electro-magnetic field. The field equations have been written for two cases, either $T'=0$ or $f_{TT}=0$. Next we have discussed the charged gravastar model which has three regions: interior region, shell region and exterior region. In the interior region, we have found the solutions of all physical quantities like density, pressure, electro-magnetic field and also the metric coefficients for both the cases. For $T'=0$, gravastar cannot form but it forms only for the case $f_{TT}=0$. In the exterior region, we have obtained the exterior solution for vacuum model. In the shell region, we have assumed that the interior and exterior regions join together at a place, so the intermediate region must be thin shell with the approximation $h\ll 1$. Under this approximation, we have found the analytical solutions. The proper length of the thin shell, entropy and energy content inside the thin shell have been found and they are directly proportional to the proper thickness of the shell $ε$ under the approximation ($ε\ll 1$). According to the Darmois-Israel formalism, we have studied the matching between the surfaces of interior and exterior regions of the gravastar. The energy density, pressure, equation of state parameter on the surface and mass of the thin shell have been obtained.

gr-qc↗

arXiv2018

Generalized Ghost Dark Energy in DGP Model

In 2000, Giorgi Dvali, Gregory Gabadadze and Massimo Porrati (Dvali et al. 2000) was proposed a new braneworld model named as DGP model, having two branches with $(ε=+1)$ and $(ε=-1)$. Former one $(~ε=+1)$ known as the accelerating branch, i.e. accelerating phase of the universe can be explained without adding cosmological constant or Dark energy, whereas later one represents the decelerating branch. Here we have investigated the behavior of decelerating branch $(i.e.~ε=-1)$ of DGP model with Generalized Ghost Dark Energy (GGDE). Aim of our study to find a stable solution of the universe in DGP model. To find a stable solution we have studied the behavior of different cosmological parameters such as Hubble parameter, equation of state (EoS) parameter and deceleration parameter with respect to scale factor. Then we have analysed the $ω_{D}-\acute{ω_{D}}$ to confirm no freezing region of our present study and point out thawing region. Furthermore we have checked the gradient of stability by calculating the squared sound speed. Then we extend our study to check the viability of this model under investigation through the analysis of statefinder diagnosis parameters for the present cosmological setup.

gr-qc↗

arXiv2018

Bouncing Universe in the Contexts of Generalized Cosmic Chaplygin Gas and Variable Modified Chaplygin Gas

In this work, we have considered the Friedmann-Robertson-Walker (FRW) model of the universe where bounce occurs and the universe is filled with Generalized Cosmic Chaplygin Gas (GCCG) or Variable Modified Chaplygin Gas (VMCG). We have studied the stability analysis through dynamical system for both models and found the critical points in flat, open and closed universe. In presence of scalar field, the dynamical behavior of scale factor and Hubble parameter is described in both models. Finally, we have analyzed the energy conditions for both the models in bouncing universe.

gr-qc↗

arXiv2018

Anisotropic Quintessence Strange Stars in f(T) Gravity with Modified Chaplygin Gas

In this paper, we study the existence of strange star in the background of f (T) modified gravity where T is a scalar torsion. In KB metric space, we derive the equations of motion using anisotropic property within the spherically strange star with modified chaplygin gas in the framework of modified f (T) gravity. Then we obtain many physical quantities to describe the physical status such as anisotropic behavior, energy conditions and stability. By the matching condition, we calculate the unknown parameters to evaluate the numerical values of mass, surface redshift etc. from our model to make comparison with the observational data.

gr-qc↗

arXiv2017

Dynamical System Analysis of Interacting Hessence Dark Energy in $f(T)$ Gravity

In this work, we have carried on dynamical system analysis of hessence field coupling with dark matter in $f(T)$ gravity. We have analysed the critical points due to autonomous system. The resulting autonomous system is non-linear. So, we have approached via the the theory of non-linear dynamical system.We have noticed very few papers are devoted to this kind of study.Maximum works in literature are done treating the dynamical system as done in linear dynamical analysis, which are unable to predict correct evolution.Our work is totally different from those kind of works.We have used theory of non-linear dynamical system theory,developed till date,in our analysis.This approach gives totally different stable solutions, in contrast what the linear analysis would have predicted. We have discussed the stability analysis in details due to exponential potential through computational method in tabular form and analyzed the evolution of the universe. Some plots are drawn to investigate the behaviour of the system (this plotting technique is different from usual phase plot, and devised by us.). Interestingly, the analysis shows the universe may resemble the `cosmological constant' like evolution (i.e., $Λ$CDM model is a subset of the solution set). Also, all the fixed points of our model are able to avoid Big rip singularity.

gr-qc↗

arXiv2016

Dynamics of Particles Around a Regular Black Hole with Nonlinear Electrodynamics

We investigate the dynamics of a charged particle being kicked off from its circular orbit around a regular black hole by an incoming massive particle in the presence of magnetic field. The resulting escape velocity, escape energy and the effective potential is analyzed. It is shown that the presence of even a very weak magnetic field helps the charged particles in escaping the gravitational field of the black hole. Moreover the effective force acting on the particle visibly reduces with distance. Thus particle near the black hole will experience higher effective force as compared to when it is far away.

gr-qc↗

arXiv2016

Reconstructions of $f(T)$ Gravity from Entropy Corrected Holographic and New Agegraphic Dark Energy Models in Power-law and Logarithmic Versions

Here, we peruse cosmological usage of the most promising candidates of dark energy in the framework of $f(T)$ gravity theory. We reconstruct the different $f(T)$ modifed gravity models in the spatially flat FRW universe according to entropy-corrected versions of the holographic and new agegraphic dark energy models in power-law and logarithmic corrections, which describe accelerated expansion history of the universe. We conclude that the equation of state parameter of the entropy-corrected models can transit from quintessence state to phantom regime as indicated by recent observations or can lie entirely in the phantom region. Also, using these models, we investigate the different erase of the stability with the help of the squared speed of sound.

gr-qc↗

arXiv2016

Effects of Thermal Fluctuations on the Thermodynamics of Modified Hayward Black Hole

In this work, we analyze the effects of thermal fluctuations on the thermodynamics of a modified Hayward black hole. These thermal fluctuations will produce correction terms for various thermodynamic quantities like entropy, pressure, inner energy and specific heats. We also investigate the effect of these correction terms on the first law of thermodynamics. Finally, we study the phase transition for the modified Hayward black hole. It is demonstrated that the modified Hayward black hole is stable even after the thermal fluctuations are taken into account, as long as the event horizon is larger than a certain critical value.

gr-qc↗

arXiv2015

Generalized Second Law of Thermodynamics for Non-canonical Scalar Field Model with Corrected-Entropy

In this work, we have considered a non-canonical scalar field dark energy model in the framework of flat FRW background. It has also been assumed that the dark matter sector interacts with the non-canonical dark energy sector through some interaction term. Using the solutions for this interacting non-canonical scalar field dark energy model, we have investigated the validity of generalized second law (GSL) of thermodynamics in various scenarios using first law and area law of thermodynamics. For this purpose, we have assumed two types of horizons viz apparent horizon and event horizon for the universe and using first law of thermodynamics, we have examined the validity of GSL on both apparent and event horizons. Next, we have considered two types of entropy-corrections on apparent and event horizons. Using the modified area law, we have examined the validity of GSL of thermodynamics on apparent and event horizons under some restrictions of model parameters.

gr-qc↗

arXiv2015

Particles Collision near Kerr-Sen Dilaton-Axion Black Hole

Here we consider static, axially symmetric, rotating and charged Kerr-Sen Dilaton-Axion black hole metric in generalized Boyer-Lindquist coordinates as particle accelerators. We obtain the geodesic motions of particle. We find the effective potential and discuss the circular orbit of a particle. We investigate the center of mass energy of two colliding neutral particles with different rest masses falling from rest at infinity to near the non-extremal horizons (event horizon and Cauchy horizon) and extremal horizon of the Kerr-Sen black hole. Analogous to the Compton process, we discuss the collision of a particle and a massless photon. Finally, we find the center of mass energy due to the collision of two photons in the background of Kerr-Sen black hole.

gr-qc↗

arXiv2015

Entropy Bound of Horizons for Accelerating, Rotating and Charged Plebanski-Demianski Black Hole

We first review the accelerating, rotating and charged Plebanski-Demianski (PD) black hole, which includes the Kerr-Newman rotating black hole and the Taub-NUT spacetime. The main feature of this black hole is that it has 4 horizons like event horizon, Cauchy horizon and two accelerating horizons. In the non-extremal case, the surface area, entropy, surface gravity, temperature, angular velocity, Komar energy and irreducible mass on the event horizon and Cauchy horizon are presented for PD black hole. The entropy product, temperature product, Komar energy product and irreducible mass product are found for event horizon and Cauchy horizon. Also their sums are also found for both horizons. All these relations are found to be depend on mass of the PD black hole and other parameters. So all the products are not universal for PD black hole. The entropy and area bounds for two horizons are investigated. Also we found the Christodoulou-Ruffini mass for extremal PD black hole. Finally, using first law of thermodynamics, we also found the Smarr relation for PD black hole.

gr-qc↗