  Dark energy cosmologies with an equation of state parameter $w$ less than -1
are often found to violate the null energy condition and show unstable
behaviour. A solution to this problem may require the existence of a consistent
effective theory that violates the null energy condition only momentarily and
does not develop any instabilities or other pathological features for a late
time cosmology. A model which incorporates a dynamical scalar field $\phi$
coupled to the quadratic Riemann invariant of the Gauss-Bonnet form is a viable
proposal. Such an effective theory is shown to admit nonsingular cosmological
evolutions for a wide range of scalar-Gauss-Bonnet coupling. We discuss the
conditions for which our model yields observationally supported spectra of
scalar and tensor fluctuations, under cosmological perturbations. The model can
provide a reasonable explanation for the transition from matter dominance to
dark energy regime and the late time cosmic acceleration, offering an
interesting testing ground for investigations of the cosmological modified
gravity.
