Résumé
Phys.Rev.Lett.98:131302,2007 Allf(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 allf(R)theories that behave as a power ofRat large or smallR(which include most of those proposed so far in the literature) the scale factor during the matter phase grows ast^(1/2)instead of the standard lawt^(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.