Accelerating Expansion of the Universe
  This thesis concentrates on the accelerated expansion of the Universe
recently explored by measurements of redshift and luminosity-distance relations
of type Ia Supernovae. We have considered a model of the universe filled with
modified Chaplygin gas and barotropic fluid. The role of dynamical cosmological
constant has been explored with Modified Chaplygin Gas as the background fluid.
Various phenomenological models for \Lambda have been studied in presence of
the gravitational constant G to be constant or time dependent. A new form of
the well known Chaplygin gas model has been presented by introducing
inhomogeneity in the EOS. This model explains w=-1 crossing. An interaction of
this model with the scalar field has also been investigated through a
phenomenological coupling function. Tachyonic field has been depicted as dark
energy model to represent the present acceleration of the Universe. A mixture
of the tachyonic fluid has been considered with Generalized Chaplygin Gas to
show the role of the later as a dark energy candidate in presence of tachyonic
matter. A model of interaction has been studied with scalar field and the
inhomogeneous ideal fluid. Two forms of the ideal fluid have been analysed. A
power law expansion for the scale factor has been assumed to solve the
equations for the energy densities. Brans-Dicke theory has been used to
investigate the possibility of obtaining cosmic acceleration. For this purpose
a constant and a variable \omega (Brans-Dicke parameter) have been considered.
A self-interacting potential has been introduced to show its role in the
evolution of the Universe. This model has been studied in presence of
barotropic fluid and Generalized Chaplygin Gas.
