Are f(R) dark energy models cosmologically viable ?
  All $f(R)$ modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a strong coupling. This 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 most of 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. 
Luca Amendola (INAF/OAR Italy), David Polarski (LPTA France) and   Shinji Tsujikawa (Gunma Nat. Coll. Techn. Japan)