  To explain the cosmic speed up, brought to light by the recent SNIa and CMB
observations, we propose the following: a) In a spacetime endowed with a FRW
metric, we choose an empirical scale factor that best explains the
observations. b) We assume a modified gravity, generated by an unspecified
field lagrangian, $f(R)$. c) We use the adopted empirical scale factor to work
back retroactively to obtain $f(R)$, hence the term `Inverse $f(R)$'. d) Next
we consider the classic GR and a conventional FRW universe that, in addition to
its known baryonic content, possesses a hypothetical `Dark Energy' component.
We compare the two scenarios, and find the density, the pressure, and the
equation of the state of the Dark Energy required to make up for the
differences between the conventional and the modified GR models.
