  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.
