Abstract:
The main purpose of this thesis is to explain the dark matter and dark energy contribution to standard Friedmann Equation which relates the expansion rate of the universe, H; to the various fractions of the present energy densities of the universe by using Brans-Dicke theory of gravity in late-time regime. Firstly, vacuum solutions of the field equations will be analyzed and it will be shown that there is some anomaly in explaining the dark matter issue if Brans-Dicke field Á is solely assumed to be complex. Then, for the next step, a hyperbolic phase of Brans-Dicke field Á is worked out in the field equations and hence the anomaly seen under the assumption of complex Brans-Dicke field Á has been disappeared automat- ically. Therefore, we showed that Brans-Dicke scalar tensor theory well accounts for dark matter and dark energy provided that the Brans-Dicke field Á is modified suitably.
