Nonminimal Coupling, Quartic Potential and Perfect Fluid Cosmologies

Perfect-fluid matter, satisfying the equation of state p=(γ−1)ρ, is considered in cosmologies where the geometry is nonminimally coupled with a scalar field ɸ and the potential of ɸ is λɸ 4 +Λ. Exact solutions are found when γ is a constant describing the ordinary forms of matter (γ=1, dust, γ=4/3, radiation, γ=2, stiff matter and γ=0, scalar field matter) and a discussion is done in order to recover Einstein gravity and the Newton constant observed today. The various solutions can be classified according to the different values of γ, λ and Λ.