Viscous String Cosmological Models in Alternative Theory of Gravity

Recently observational data indicates that the discrepancy arises at late time, when the age of the universe is some billions years. In this context the Friedmann-RobertsonWalker model is a unique correspondence between the expansion rate and the distance scale. Although general theory of relativity (GTR) is the most suitable theory for describing universe as a whole, but still there are some physical phenomena which are to be addressed by GTR. For this purpose there is a need either to modify the theory or to introduce alternate theory which is suitable to address the universe in all aspects. During the investigations of research findings it have been noticed that many alternate/modified theories of gravity were proposed by cosmologists as and when required. Among all the available alternative theories, the scalartensor Brans-Dicke theory (BD) [1] of gravity is the most promising existing theory which has very effectively solves the problems of early time inflation and late time accelerating behaviour of the universe [2]. According to BD theory, the gravitational constant G is not a constant but it varies with space and time. The BransDicke gravity also relates the gravitation constant G with scalar field φ along with relation 1 G φ − ≅ ). The action principle for the Brans-Dicke gravity is given as