Effect of curvature-squared terms on cosmology

We consider the effect on Friedmann cosmology of adding all possible algebraic terms quadratic in the curvature tensor to the usual Einstein-Hilbert action for gravitation. Assuming that the vacuum energy density (cosmological constant) is not extraordinarily large in any phase of the evolution of the universe, we find that the cosmological solutions of this more general theory are indistinguishable from standard Friedmann models all the way from the initial singularity to the present.