Nayem S k
University of Kalyani
F(R) theory of gravity unifies early inflation with late time cosmic acceleration, admits Newtonian limit and passes the solar test singlehandedly. However, in order to select a particular form of F(R) out of indefinitely many, Noether symmetry was invoked as a selection rule. We have shown that Noether symmetry gives R^(3/2) along with a conserved current d/dt a√R in R-W metric when coupled to pressure-less dust or in vacuum and it does not yield any symmetry when the configuration space is enlarged by including scalar field or in anisotropic model or in tachyonic field. Inclusion of gauge term does not improve the situation. Exact solution in R-W metric does not satisfy all cosmological data and a linear term may be necessary in addition with F(R). However a non-Noether conserved current exists in general for gravity when a non-minimally coupled scalar field or tachyonic field is added to F(R). We show that it is useful to obtain improved exact solution.