DARK ENERGY PHENOMENA IN COSMOLOGY

Abstract

In the thesis, we have carried out a dark energy phenomena of the Universe which has been predicted by many cosmological observations. In chapter 2, we have studied HDE model in well motivated and established BD theory. We have assumed a logarithmic form of BD scalar field, proposed by Kumar and Singh [181]. We have extended our study to HDE model with future event horizon as an IR cutoff and have shown that this model explains the evolution and solve the coincidence problem more effectively with the logarithmic form of BD scalar field in comparison of power-law form. In chapter 3, the concept of bulk viscosity with new HDE has been analysed to explain the recent accelerated expansion of the Universe. It is thought that the negative pressurecausedbythebulkviscositycanplaytheroleofdarkenergycomponentand drive the accelerated expansion of the Universe. We have observed that the accelerated expansion may be possible for non-viscous case but the phase transition is not possible. It has been tried to demonstrate that the bulk viscosity can play the role as a possible candidate of dark energy. Using statefinder parameters and Om diagnostic, it has been found that our model shows the similar behavior as quintessence model and Chaplygin gas model for different values of the viscosity coefficient. Chapter4istheextensionoftheworkcarriedoutinchapter3, in f(R,T) gravitytheory. We have studied new HDE model with constant bulk viscosity in f(R,T) gravity theory. We have observed that the accelerated expansion may be possible for nonviscouscasein f(R,T) gravitybutdoesnotshowphasetransition. Theintroductionof bulk viscosity not only makes the phase transition possible but also presents a wide range of possible evolutions of the Universe depending on parameters of the model. We have also distinguished this model from other existing dark energy models using two geometrical diagnostics: statefinder parameter and Om diagnostic. The ther 163 164 modynamics and the local entropy have been discussed for this model. The model preserves the validity of the second law of thermodynamics as bulk viscous coefficient remains positive through the evolution of the Universe. Big-Rip and Type III singularities are obtained depending on values of model parameter. In chapter 5, we have generalised the previous work for more general form of the bulk viscous coefficient with new HDE in the framework of f(R,T) gravity theory. We have classified all possible scenarios (deceleration, acceleration and their transition) with different parameter regions chosen properly for positive and negative ranges of model parameter and viscosity coefficients to analyze the evolution of the Universe. WehavealsoinvestigatedthestatefinderpairandOmdiagnosticforthisviscousmodel to discriminate from other existing DE models. The model evolution behaviors are shown by plotting the statefinder and Om−z trajectories. The evolution of effective EoS parameter is also shown graphically. The entropy and generalized second law of thermodynamics are found to be valid for this model under some constraints on bulk viscous coefficients. Thus,wehaveobservedthatGTRaswellas f(R,T) gravityhavepotentialtoexplain therecentacceleratedexpansionoftheUniverseinthepresenceofbulkviscositywith new HDE. The concept of bulk viscosity presents a mechanism to observe accelerated expansion as well as phase transition of the Universe. The spatially homogeneous and anisotropic Bianchi-I and Bianchi-V models with the scalar field have been studied in the chapter 6 and 7. In chapter 6, we have investigated the effect of non-interacting scalar field in the framework of Bianchi-I. We have assumed the average scale factor as an exponential function of scalar field. We have considered the solution for two cases: flat potential and exponential potential. We have obtained that the zero-rest-mass model expands with decelerated rate and behaves like a stiff matter. In case of exponential potential function, the model expands with decelerated, accelerated or shows the transition depending on the model parameters. The isotropization is observed at late-time evolution of the Universe in exponential potential model. Chapter7istheextensionofthepreviouschapterforhomogeneousandanisotropic Bianchi-V model filled with perfect fluid and scalar field. The two sources have been assumed to be non-interacting. The average scale factor and scalar potential have been considered as an exponential functions of the scalar field. The observational data has been used here to find out the values of model parameters. The model 165 behaves like ΛCDM or SCDM depending on the values of model parameters. The anisotropic models explains that in late time of evolution the anisotropic behaviour is damped out and the Universe become isotropic one which is the good harmony with the current observational data.