Ujjal Debnath

Role of modified Chaplygin gas in accelerated universe

In this paper, we have considered a model of modified Chaplygin gas and its role in the accelerating phase of the universe. We have assumed that the equation of state of this modified model is valid from the radiation era to the ΛCDM model. We have used recently developed statefinder parameters in characterizing different phases of the universe diagrammatically.

EMERGENT UNIVERSE AND THE PHANTOM TACHYON MODEL

In this work, I have considered that the universe is filled with normal matter and a phantom field (or tachyonic field). If the universe is filled with a scalar field, Ellis et al have shown that an emergent scenario is possible only for k = +1, i.e. for a closed universe. Here I have shown that the emergent scenario is possible for a closed universe if the universe contains the normal tachyonic field. But for a phantom field (or tachyonic field), the negative kinetic term can generate the emergent scenario for all values of k(=0, ± 1). From recently developed statefinder parameters, the behaviour of different stages of the evolution of the emergent universe has been studied. The static Einstein universe and the stability analysis have been briefly discussed for both phantom and tachyon models.

EMERGENT UNIVERSE IN THE CHAMELEON, f(R) AND f(T) GRAVITY THEORIES

In this work, we consider an emergent Universe in generalized gravity theories like the chameleon, f(R) and f(T) gravities. We reconstruct the potential of the chameleon field under the emergent scenario of the Universe and observe its increasing nature with the evolution of the Universe. We reveal that in the emergent Universe scenario, the equation-of-state parameter behaves like quintessence in the case of f(R) gravity and like phantom in the case of f(T) gravity.

Variable modified Chaplygin gas and accelerating universe

Abstract In this letter, I have proposed a model of variable modified Chaplygin gas and shown its role in accelerating phase of the universe. I have shown that the equation of state of this model is valid from the radiation era to quiessence model. The graphical representations of statefinder parameters characterize different phase of evolution of the universe. All results presented in the letter concerns the case k=0.

NAKED SINGULARITIES IN HIGHER DIMENSIONAL GRAVITATIONAL COLLAPSE

Spherically symmetric inhomogeneous dust collapse has been studied in higher dimensional space–time and the factors responsible for the appearance of a naked singularity are analyzed in the region close to the centre for the marginally bound case. It is clearly demonstrated that in the former case naked singularities do not appear in the space–time having more than five dimension, which appears to a strong result. The non-marginally bound collapse is also examined in five dimensions and the role of shear in developing naked singularities in this space–time is discussed in details. The five-dimensional space–time is chosen in the later case because we have exact solution in closed form only in five dimension and not in any other case.

Interacting modified Chaplygin gas in loop quantum cosmology

We investigate the background dynamics when dark energy is coupled to dark matter in the universe described by loop quantum cosmology. We consider dark energy of the form modified Chaplygin gas. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. It henceforth resolves the famous cosmic coincidence problem in modern cosmology. The statefinder parameters are also calculated to classify this dark energy model.

Correspondence between DBI-essence and modified Chaplygin gas and the generalized second law of thermodynamics

In this work, we have considered the DBI-essence dark energy model in FRW Universe. We have found the exact solutions of potential, warped brane tension and DBI scalar field. We also calculate the statefinder parameters for our model that make it distinguishable among numerous dark energy models. Moreover, we establish correspondence between DBI-essence and modified Chaplygin gas (MCG) and hence reconstruct the potential and warped brane tension. By this reconstruction, we observe that DBI scalar field and potential increase and warped brane tension decreases during evolution of the Universe. Finally, we investigate the validity of the generalized second law (GSL) of thermodynamics in the presence of DBI-essence and modified Chaplygin gas. It is observed that the GSL breaks down for DBI-essence model but GSL always satisfied for MCG model.

FRW Cosmology with Variable G and Λ

We have considered a cosmological model of the FRW universe with variable G and Λ. The solutions have been obtained for flat model with particular form of cosmological constant. The cosmological parameters have also been obtained for dust, radiation and stiff matter. The statefinder parameters are analyzed and have shown that these depends only on w and ε. Further the lookback time, proper distance, luminosity distance and angular diameter distance have also been calculated for our model.

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 in new variable modified Chaplygin gas (NVMCG) and generalized cosmic Chaplygin gas (GCCG) models onto 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 the Kerr–Newman black hole. We also find the expression for the change of mass 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 dark energy, the mass of the black hole increases and angular momentum increases in the case of a Kerr–Newman black hole.

Dynamics of interacting phantom and quintessence dark energies

We present models, in which phantom energy interacts with two different types of dark energies including variable modified Chaplygin gas (VMCG) and new modified Chaplygin gas (NMCG). We then construct potentials for these cases. It has been shown that the potential of the phantom field decreases from a higher value with the evolution of the Universe.