Energy conditions in modified Gauss-Bonnet gravity
  In considering alternative higher-order gravity theories, one is liable to be
motivated in pursuing models consistent and inspired by several candidates of a
fundamental theory of quantum gravity. Indeed, motivations from string/M-theory
predict that scalar field couplings with the Gauss-Bonnet invariant, G, are
important in the appearance of non-singular early time cosmologies. In this
work, we discuss the viability of an interesting alternative gravitational
theory, namely, modified Gauss-Bonnet gravity or f(G) gravity. We consider
specific realistic forms of f(G) analyzed in the literature that account for
the late-time cosmic acceleration and that have been found to cure the
finite-time future singularities present in the dark energy models. We present
the general inequalities imposed by the energy conditions and use the recent
estimated values of the Hubble, deceleration, jerk and snap parameters to
examine the viability of the above-mentioned forms of f(G) imposed by the weak
energy condition.
