Do consistent F(R) models mimic general relativity plus Λ?

Modified gravity models are subject to a number of consistency requirements which restrict the form that the function F(R) can take. We study a particular class of F(R) functions which satisfy various constraints that have been found in the literature. These models have a late time accelerating epoch, and an acceptable matter era. We calculate the Friedmann equation for our models, and show that in order to satisfy the constraints we impose, they must mimic general relativity plus A throughout the cosmic history, with exponentially suppressed corrections. We also find that the free parameters in our model must be fine tuned to obtain an acceptable late time accelerating phase. We discuss the generality of this conclusion. (C) 2007 Published by Elsevier BV.