Quintessence Problem and Brans-Dicke Theory

For the last few decades, it is generally believed that after a (small) period of inflationary era (when the universe was accelerated) the universe has been evolving in a Big bang scenario (deceleration of the universe) and most of the present day observations are in accord with this cosmological model (known as Standard Cosmological Model). But difficulty has started when recent data from high redshift supernovae [1,2] (in fact from luminosity redshift relation of type I a supernovas upto about z = 1) suggest an accelerated universe at the present epoch (known as quintessence problem). So there must be some matter field (known as Quintessence matter or Q-matter) which is either neglected or unknown, responsible for this accelerated universe. From theoretical point of view, a lot of works [3-8] has been done to solve this quintessence problem and possible candidates for Q-matter are Cosmological Constant (or more generally a variable cosmological term), a scalar field [9,10] with a potential giving rise to a negative pressure at the present epoch, a dissipative fluid with an effective negative stress [11] and more exotic matter like a frustrated network of non-abelian cosmic strings or domain walls [12,13]. In these works, it is generally assumed that Q-matter behaves as a perfect fluid with barotropic equation of state and most of them have considered spatially flat model of the universe (only the work of Chimento etal [11] has been done for open model of the universe). Recently, Banerjee etal [14] have shown that it is possible to have an accelerated universe with BD-theory in Friedmann model without any matter.