An updated analysis of two classes of f(R) theories of gravity
  The observed accelerated cosmic expansion can be a signature of
fourth\,-\,order gravity theories, where the acceleration of the Universe is a
consequence of departures from Einstein General Relativity, rather than the
sign of the existence of a fluid with negative pressure. In the
fourth\,-\,order gravity theories, the gravity Lagrangian is described by an
analytic function $f(R)$ of the scalar curvature $R$ subject to the demanding
conditions that no detectable deviations from standard GR is observed on the
Solar System scale. Here we consider two classes of $f(R)$ theories able to
pass Solar System tests and investigate their viability on cosmological scales.
To this end, we fit the theories to a large dataset including the combined
Hubble diagram of Type Ia Supernovae and Gamma Ray Bursts, the Hubble parameter
$H(z)$ data from passively evolving red galaxies, Baryon Acoustic Oscillations
extracted from the seventh data release of the Sloan Digital Sky Survey (SDSS)
and the distance priors from the Wilkinson Microwave Anisotropy Probe seven
years (WMAP7) data. We find that both classes of $f(R)$ fit very well this
large dataset with the present\,-\,day values of the matter density, Hubble
constant and deceleration parameter in agreement with previous estimates;
however, the strong degeneracy among the $f(R)$ parameters prevents us from
strongly constraining their values. We also derive the growth factor $g =
d\ln{\delta}/d\ln{a}$, with $\delta = \delta \rho_M/\rho_M$ the matter density
perturbation, and show that it can still be well approximated by $g(z) \propto
\Omega_M(z)^{\gamma}$. We finally constrain $\gamma$ (on some representative
scales) and investigate its redshift dependence to see whether future data can
discriminate between these classes of $f(R)$ theories and standard dark energy
models.
