Strong energy condition and the repulsive character of f(R) gravity
  The Raychaudhuri equation enables to examine the whole spacetime structure
without specific solutions of Einstein's equations, playing a central role for
the understanding of the gravitational interaction in Cosmology. In General
Relativity, without considering a cosmological constant, a non-positive
contribution in the Raychaudhuri equation is usually interpreted as the
manifestation of the attractive character of gravity. In this case, particular
energy conditions -- indeed the strong energy condition -- must be assumed in
order to guarantee the attractive character. In the context of f(R) gravity,
however, even assuming the standard energy conditions one may have a positive
contribution to the Raychaudhuri equation. Besides providing a simple way to
explain the observed cosmic acceleration, this fact opens the possibility of a
repulsive character of this kind of gravity. In order to discuss physical
bounds on f(R) models, we address the attractive/non-attractive character of
f(R) gravity considering the Raychaudhuri equation and assuming the strong
energy condition along with recent estimates of the cosmographic parameters.
