Energy conditions in modified gravity
  We consider generalized energy conditions in modified theories of gravity by
taking into account the further degrees of freedom related to scalar fields and
curvature invariants. The latter are usually recast as generalized {\it
geometrical fluids} that have different meanings with respect to the standard
matter fluids generally adopted as sources of the field equations. More
specifically, in modified gravity the curvature terms are grouped in a tensor
$H^{ab}$ and a coupling $g(\Psi^i)$ that can be reorganized in effective
Einstein field equations, as corrections to the energy-momentum tensor of
matter. The formal validity of such inequalities does not assure some basic
requirements such as the attractive nature of gravity, so that the energy
conditions have to be considered in a wider sense.
