A STUDY ON COSMOLOGICAL MODELS IN GENERAL RELATIVITY AND MODIFIED GRAVITY THEORIES

Abstract

Cosmology is a branch of science which deals with the study of the origin of the u niverse, its evolution and its eventual fate. The modern cosmology is based on the Big Bang theory, where the universe is considered as emerged out of the Big Bang, which occurred about 13.7 billion years ago. The cosmology assumes the homo geneous (no change during linear motion) and isotropic (no change during angular motion) universe, which is justified on the scales of larger than 100 Mpc. These prop erties lead us to make an assumption about the model of the universe, called the Cosmological Principle. This principle is the basis of the Big Bang cosmology. For the evolution of the universe, various models have been proposed by scientists from time to time. Since the development of general relativity, cosmology has changed our perception of the laws of the universe remarkably. The First Three Minutes written by Steven Weinberg and A Brief History of Time by Stephen Hawking are the famous books which create the interest in this subject. The rapid development in observational cosmology witnesses that the universe is expanding with an accelerated rate. Several theories have been proposed to ex plain the accelerated phenomena for past two decades. It has been observed that a large part of the universe has One a mysterious component with negative pressure, so-called dark energy (DE). The most natural and successful candidate of DE is the cosmological constant which was introduced by Albert Einstein to obtain a static uni verse. Some other DE candidates like scalar fields, Chaplygin gas, holographic dark energy, Ricci dark energy, etc. have been proposed to explain the accelerated expan sion of the universe. Recently, it has been studied that the bulk viscosity and matter creation are another alternative candidates to explain the present acceleration of the universe. The motive of this thesis’s work is to explore the effects of bulk viscosity and matter creation in explaining the dark energy phenomena within the framework of a spatial ly homogeneous and isotropic flat Friedmann-Lemaître-Robertson-Walker metric in xi general relativity and its modified theories. We extract the useful information about the bulk viscosity and matter creation by using observational data to fit the model ac cording to the accepted model. Chapter 1 is introductory in nature. Chapters 2 – 6 are based on the research work published in the form of research papers in reputed refereed journals. The last chapter contains the conclusion and future scope of the thesis work. Each chapter begins with a brief outline of the work carried out in that chapter.