A STUDY ON ACCELERATING COSMOLOGICAL MODELS OF THE UNIVERSE

Abstract

In the thesis work, we have carried out a study on the accelerating cosmological models of the Universe which has been predicted by many cosmological observations. We have worked on modified theories of gravity to explain it. In chapter 2, the concept of bulk viscosity has been analyzed in f(R,T) gravity to explain the recent accelerated expansion of the Universe. We have observed that the accelerated expansion may be possible for perfect fluid in our model of f(R,T) gravity but the phase transitionisnotpossible. Theintroductionofbulkviscositymakesthephasetransition possible and presents a wide range of possible evolutions of the Universe depending on parameters of the model. Further, chapter 3 deals with the bulk viscosity and matter creation together in f(R,T) gravity. In the literature, both the phenomena have been treated as same cosmologicalphenomenaandsomepaperstreatbothasdifferentphenomena. Inour study, we have found that although both the phenomena generate effective negative pressure which may explain the accelerated expansion but both have considerably different effects which make both the concepts separate and they exist independently. It has been shown in chapter 4 that HDE model with Hubble horizon as an IR cut– off may explain the accelerated expansion in f(R,T) gravity without taking interaction between DM and HDE which is not possible in GTR. Further, we have shown that if we consider the viscous HDE then the phase transition may also be achieved. Using statefinder parameters, it has ben found that our model shows the similar behavior as ΛCDM model and quintessence model for different values of the parameters. Thus,

we have observed that f(R,T) gravity has potential to explain the recent accelerated expansion of the Universe. The concept of bulk viscosity presents a mechanism to observe accelerated expansion as well as phase transition of the Universe. In chapter 5, we have studied HDE model in well motivated and established BD theory. We have pointed out that the power–law form of BD scalar field gives a constant as well as time–dependent DP for HDE models in BD theory. It means that the same model describes two types of behavior of the Universe, evolution without phase transition and evolution with phase transition. Therefore, we have proposed a logarithmic form of BD scalar field which is free from this problem. We have shown that HDE model with Hubble horizon as IR cut–off in BD theory with this form of BD scalar field explains early time inflation, medieval time deceleration and late time acceleration of the Universe. This form also plays an important role to avoid cosmic coincidence problem. We have extended our study to NADE model in chapter 6 and have shown that this model explains the evolution and coincidence problem more effectively with logarithmic form of BD scalar field in comparison of power–law form. Thus, first advantage of this form is that it is free from the constant deceleration parameter problem of power– law, and second is that it explains the evolution of the Universe more effectively and may explain the cosmic coincidence problem.