arXiv subjects

Ujjal Debnath

Publications and source records attributed to Ujjal Debnath.

At least 109 records · Page 6Linked to original sources

arXiv2012

Observational Constraints of Homogeneous Higher Dimensional Cosmology with Modified Chaplygin Gas

In this work, we have considered the flat FRW model of the universe in $(n+2)$-dimensions filled with the dark matter (perfect fluid with negligible pressure) and the modified Chaplygin gas (MCG) type dark energy. We present the Hubble parameter in terms of the observable parameters $Ω_{m0}$, $Ω_{x0}$ and $H_{0}$ with the redshift $z$ and the other parameters like $A$, $B$, $C$, $n$ and $α$. From Stern data set (12 points), we have obtained the bounds of the arbitrary parameters by minimizing the $χ^{2}$ test. The best-fit values of the parameters are obtained by 66%, 90% and 99% confidence levels. Now to find the bounds of the parameters and to draw the statistical confidence contour, we first fixed three parameters $C, n, α$ and then fixed the three parameters $A, n, α$. In the first case we find the bounds of $(A, B)$ and draw the contour between them for 4D$(n=2)$, 5D$(n=3)$ and 6D$(n=4)$. In the second case we fixed three different values of A as 1, 1/3, -1/3 to find the bounds of $(B, C)$ and draw the contour between them. Here the parameter $n$ determines the higher dimensions and we perform comparative study between three cases : 4D $(n=2)$, 5D $(n=3)$ and 6D $(n=4)$ respectively. Next due to joint analysis with BAO observation, we have also obtained the bounds of the parameters ($A,B$) by fixing some other parameters $α$ and $A$ for 4D, 5D and 6D.

physics.gen-ph↗

arXiv2012

Thermodynamics of an Evolving Lorentzian Wormhole with Entropy Corrections

In this work, we study the generalized second law of thermodynamics (GSL) at the apparent horizon of an evolving Lorentzian wormhole. We obtain the expressions of thermal variables at the apparent horizon. Choosing the two well-known entropy functions i.e. power-law and logarithmic, we obtain the expressions of GSL. We have analyzed the GSL using a power-law form of scale factor $a(t)=a_0t^n$ and the special form of shape function $b(r)=\frac{b_{0}}{r}$. It is shown that GSL is valid in the evolving wormhole spacetime for both choices of entropies if the power-law exponent is small, but for large values of $n$, the GSL is satisfied at initial stage and after certain stage of the evolution of the wormhole, it violates.

gr-qc↗

arXiv2012

Periodic Cosmological Evolutions of Equation of State for Dark Energy

We demonstrate two periodic or quasi-periodic generalizations of the Chaplygin gas (CG) type models to explain the origins of dark energy as well as dark matter by using the Weierstrass $\wp(t)$, $σ(t)$ and $ζ(t)$ functions with two periods being infinite. If the universe can evolve periodically, a non-singular universe can be realized. Furthermore, we examine the cosmological evolution and nature of the equation of state (EoS) of dark energy in the Friedmann-Lemaître-Robertson-Walker cosmology. It is explicitly illustrated that there exist three type models in which the universe always stays in the non-phantom (quintessence) phase, whereas it always evolves in the phantom phase, or the crossing of the phantom divide can be realized. The scalar fields and the corresponding potentials are also analyzed for different types of models.

gr-qc↗

arXiv2012

Thermodynamics in Higher Dimensional Vaidya Space-Time

In this work, we have considered the Vaidya spacetime in null radiating fluid with perfect fluid in higher dimension and have found the solution for barotropic fluid. We have shown that the Einstein's field equations can be obtained from Unified first law i.e., field equations and unified first law are equivalent. The first law of thermodynamics has also been constructed by Unified first law. From this, the variation of entropy function has been derived on the horizon. The variation of entropy function inside the horizon has been derived using Gibb's law of thermodynamics. So the total variation of entropy function has been constructed at apparent and event horizons both. If we do not assume the first law, then the entropy on the both horizons can be considered by area law and the variation of total entropy has been found at both the horizons. Also the validity of generalized second law (GSL) of thermodynamics has been examined at both apparent and event horizons by using the first law and the area law separately. When we use first law of thermodynamics and Bekenstein-Hawking area law of thermodynamics, the GSL for apparent horizon in any dimensions are satisfied, but the GSL for event horizon can not be satisfied in any dimensions.

gr-qc↗

arXiv2012

Study of Generalized Second Law of Thermodynamics in Loop Quantum Cosmology with the Effect of Non-Linear Electrodynamics

In this work, we have discussed the Maxwell's electrodynamics in non-linear forms in FRW universe. The energy density and pressure for non-linear electrodynamics have been written in the electro-magnetic universe. The Einstein's field equations for flat FRW model in loop quantum cosmology have been considered if the universe is filled with the matter and electro-magnetic field. We separately assumed the magnetic universe and electric universe. The interaction between matter and magnetic field have been considered in one section and for some particular form of interaction term, we have found the solutions of magnetic field and the energy density of matter. We have also considered the interaction between the matter and electric field and another form of interaction term has been chosen to solve the field equations. The validity of generalized second law of thermodynamics has been investigated on apparent and event horizons using Gibb's law and the first law of thermodynamics for magnetic and electric universe separately.

physics.gen-ph↗

arXiv2012

Dynamical Study of DBI-essence in Loop Quantum Cosmology and Braneworld

We have studied homogeneous isotropic FRW model having dynamical dark energy DBI-essence with scalar field. The existence of cosmological scaling solutions restricts the Lagrangian of the scalar field $ϕ$. Choosing $p=X g(X e^{λϕ})$, where $X=-g^{μν} \partial_μϕ\partial_νϕ/2$ with $g$ is any function of $X e^{λϕ}$ and defining some suitable transformations, we have constructed the dynamical system in different gravity: (i) Loop Quantum Cosmology (LQC), (ii) DGP BraneWorld and (iii) RS-II Brane World. We have investigated the stability of this dynamical system around the critical point for three gravity models and investigated the scalar field dominated attractor solution in support of accelerated universe. The role of physical parameters have also been shown graphically during accelerating phase of the universe.

gr-qc↗

arXiv2012

Observational Constraints of Modified Chaplygin Gas in Loop Quantum Cosmology

We have considered the FRW universe in loop quantum cosmology (LQC) model filled with the dark matter (perfect fluid with negligible pressure) and the modified Chaplygin gas (MCG) type dark energy. We present the Hubble parameter in terms of the observable parameters $Ω_{m0}$, $Ω_{x0}$ and $H_{0}$ with the redshift $z$ and the other parameters like $A$, $B$, $C$ and $α$. From Stern data set (12 points), we have obtained the bounds of the arbitrary parameters by minimizing the $χ^{2}$ test. The best-fit values of the parameters are obtained by 66%, 90% and 99% confidence levels. Next due to joint analysis with BAO and CMB observations, we have also obtained the bounds of the parameters ($B,C$) by fixing some other parameters $α$ and $A$. From the best fit of distance modulus $μ(z)$ for our theoretical MCG model in LQC, we concluded that our model is in agreement with the union2 sample data.

physics.gen-ph↗

arXiv2012

Thermodynamics of Evolving Lorentzian Wormhole at Apparent and Event Horizons

We have investigated the non-static Lorentzian Wormhole model in presence of anisotropic pressure. We have presented some exact solutions of Einstein equations for anisotropic pressure case. Introducing two EoS parameters we have shown that these solutions give very rich dynamics of the universe yielding to the different expansion history of it in the $r$ - direction and in the $T$ - direction. The corresponding explicit forms of the shape function $b(r)$ is presented.We have shown that the Einstein's field equations and unified first law are equivalent for the dynamical wormhole model. The first law of thermodynamics has been derived by using the Unified first law. The physical quantities including surface gravity and the temperature are derived for the wormhole. Here we have obtained all the results without any choice of the shape function. The validity of generalized second law (GSL) of thermodynamics has been examined at apparent and event horizons for the evolving Lorentzian wormhole.

physics.gen-ph↗

arXiv2012

Variable $G$ Correction for Dark Energy Model in Higher Dimensional Cosmology

In this work, we have considered $N (=4+d)$-dimensional Einstein field equations in which 4-dimensional space-time which is described by a FRW metric and that of the extra $d$-dimensions by an Euclidean metric. We have calculated the corrections to statefinder parameters due to variable gravitational constant $G$ in higher dimensional Cosmology. We have considered two special cases whether dark energy and dark matter interact or not. In a universe where gravitational constant is dynamic, the variable $G$-correction to statefinder parameters is inevitable. The statefinder parameters are also obtained for generalized Chaplygin gas in the effect of the variation of $G$ correction.

physics.gen-ph↗

arXiv2012

Presence of Dark Energy and Dark Matter : Does Cosmic Acceleration signifies a Weak Gravitational collapse?

In this work the collapsing process of a spherically symmetric star, made of dust cloud, in the background of dark energy is studied for two different gravity theories separately, i.e., DGP Brane gravity and Loop Quantum gravity. Two types of dark energy fluids, namely, Modified Chaplygin gas and Generalised Cosmic Chaplygin gas are considered for each model. Graphs are drawn to characterize the nature and the probable outcome of gravitational collapse. A comparative study is done between the collapsing process in the two different gravity theories. It is found that in case of dark matter, there is a great possibility of collapse and consequent formation of Black hole. In case of dark energy possibility of collapse is far lesser compared to the other cases, due to the large negative pressure of dark energy component. There is an increase in mass of the cloud in case of dark matter collapse due to matter accumulation. The mass decreases considerably in case of dark energy due to dark energy accretion on the cloud. In case of collapse with a combination of dark energy and dark matter, it is found that in the absence of interaction there is a far better possibility of formation of black hole in DGP brane model compared to Loop quantum cosmology model.

physics.gen-ph↗

arXiv2012

Dynamics of Modified Chaplygin Gas in Brane World Scenario: Phase Plane Analysis

In this work we investigate the background dynamics when dark energy is coupled to dark matter with a suitable interaction in the universe described by brane cosmology. Here DGP and the RSII brane models have been considered separately. Dark energy in the form of modified Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is considered in order to at least alleviate (if not solve) the cosmic coincidence problem. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. A significant attempt towards the solution of the cosmic coincidence problem is taken. The statefinder parameters are also calculated to classify the dark energy models. Graphs and phase diagrams are drawn to study the variations of these parameters. It is also seen that the background dynamics of modified Chaplygin gas is completely consistent with the notion of an accelerated expansion in the late universe. Finally, it has been shown that the universe in both the models follows the power law form of expansion around the critical point, which is consistent with the known results.

gr-qc↗

arXiv2012

Nature of singularity formed by the gravitational collapse in Husain space-time with electromagnetic field and scalar field

In this work, we have investigated the outcome of gravitational collapse in Husain space-time in the presence of electro-magnetic and a scalar field with potential. In order to study the nature of the singularity, global behavior of radial null geodesics have been taken into account. The nature of singularities formed has been thoroughly studied for all possible variations of the parameters. These choices of parameters has been presented in tabular form in various dimensions. It is seen that irrespective of whatever values of the parameters chosen, the collapse always results in a naked singularity in all dimensions. There is less possibility of formation of a black hole. Hence this work is a significant counterexample of the cosmic censorship hypothesis.

gr-qc↗

arXiv2012

Generalized Second Law of Thermodynamics for FRW Cosmology with Power-Law Entropy Correction

In this work, we have considered the power law correction of entropy on the horizon. If the flat FRW Universe is filled with the n components fluid with interactions, the GSL of thermodynamics for apparent and event horizons have been investigated for equilibrium and non-equilibrium cases. If we consider a small perturbation around the de Sitter space-time, the general conditions of the validity of GSL have been found. Also if a phantom dominated Universe has a polelike type scale factor, the validity of GSL has also been analyzed. Further we have obtained constraints on the power-law parameter α in the phantom and quintessence dominated regimes. Finally we obtain conditions under which GSL breaks down in a cosmological background.

gr-qc↗

arXiv2012

Statefinder Parameters for Different Dark Energy Models with Variable G Correction in Kaluza-Klein Cosmology

In this work, we have calculated the deceleration parameter, statefinder parameters and EoS parameters for different dark energy models with variable $G$ correction in homogeneous, isotropic and non-flat universe for Kaluza-Klein Cosmology. The statefinder parameters have been obtained in terms of some observable parameters like dimensionless density parameter, EoS parameter and Hubble parameter for holographic dark energy, new agegraphic dark energy and generalized Chaplygin gas models.

physics.gen-ph↗

arXiv2012

Accretion of New Variable Modified Chaplygin Gas and Generalized Cosmic Chaplygin Gas onto Schwarzschild and Kerr-Newman Black holes

In this work, we have studied accretion of the dark energies NVMCG and GCCG onto the Schwarzschild and Kerr-Newman Black holes. We find the expression of the critical four velocity component which gradually decreases for the fluid flow towards the Schwarzschild as well as Kerr-Newman Black hole. We also find the expression for change of masses of the black hole in both cases. For the Kerr-Newman black hole which is rotating and charged we calculate the specific angular momentum and total angular momentum. We showed that in both cases due to accretion of the dark energy mass of the black hole increases and angular momentum increases in case of Kerr-Newman black hole.

physics.gen-ph↗

arXiv2012

Variable Modified Chaplygin Gas in Anisotropic Universe with Kaluza-Klein Metric

In this work, we have consider Kaluza-Klein Cosmology for anisotropic universe where the universe is filled with variable modified chaplygin gas (VMCG). Here we find normal scalar field $ϕ$ and the self interacting potential $V(ϕ)$ to describe the VMCG Cosmology. Also we graphically analyzed the geometrical parameters named {\it statefinder parameters} in anisotropic Kaluza-Klein model. Next, we consider a Kaluza-Klein model of interacting VMCG with dark matter in the Einstein gravity framework. Here we construct the three dimensional autonomous dynamical system of equations for this interacting model with the assumption that the dark energy and the dark matter are interact between them and for that we also choose the interaction term. We convert that interaction terms to its dimensionless form and perform stability analysis and solve them numerically. We obtain a stable scaling solution of the equations in Kaluza-Klein model and graphically represent solutions.

physics.gen-ph↗

arXiv2011

Large Dynamical System Analysis for Anisotropic Universe in Brans-Dicke Theory

In this work, we have studied the Brans-Dicke (BD) cosmology in anisotropic models. We present three dimensional dynamical system describing the evolution of anisotropic models containing perfect fluid and BD scalar field with self-interacting potential. The relevant equations have been transformed into the dynamical system. The critical points and the corresponding eigen values have been found in radiation, dust, dark energy, $Λ$CDM and phantom phases of the universe. The natures and the stability around the critical points have also been investigated.

physics.gen-ph↗

arXiv2011

Validity of Generalized Second Law of Thermodynamics in the Logamediate and Intermediate scenarios of the Universe

In this work, we have investigated the validity of the generalized second law of thermodynamics in logamediate and intermediate scenarios of the universe bounded by the Hubble, apparent, particle and event horizons using and without using first law of thermodynamics. We have observed that the GSL is valid for Hubble, apparent, particle and event horizons of the universe in the logamediate scenario of the universe using first law and without using first law. Similarly the GSL is valid for all horizons in the intermediate scenario of the universe using first law. Also in the intermediate scenario of the universe, the GSL is valid for Hubble, apparent and particle horizons but it breaks down whenever we consider the universe enveloped by the event horizon.

physics.gen-ph↗