# Are f(R) dark energy models cosmologically viable ?

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## Abstract

We show that all f(R) modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a large coupling. This strong coupling induces a cosmological evolution radically different from standard cosmology. We find that in all f(R) theories that behave as a power of R at large or small R (which include all those proposed so far in the literature) the scale factor during the matter phase grows as t^{1/2} instead of the standard law t^{2/3}. This behaviour is grossly inconsistent with cosmological observations (e.g. WMAP), thereby ruling out these models even if they pass the supernovae test and can escape the local gravity constraints.