is spatially flat with a cosmic dark sector constituted by cold dark matter and some sort of dark energy, associated with large negative pressure, in order to explain the observed accelerating expansion of the universe (see [@Teg04; @Spergel07; @essence; @Kowal08; @Hic09; @komatsu08; @LJC09; @BasPli10] and references therein). Despite the mounting observational evidences on the existence of the dark energy component in the universe, its nature and fundamental origin remains an intriguing enigma challenging the very foundations of theoretical physics.

Indeed, during the last decade there has been an intense theoretical debate among cosmologists regarding the nature of this exotic “dark energy”. The absence of a fundamental physical theory, concerning the mechanism inducing the cosmic acceleration, has opened a window to a plethora of alternative cosmological scenarios. Most are based either on the existence of new fields in nature (dark energy) or in some modification of Einstein’s general relativity, with the present accelerating stage appearing as a sort of geometric effect (see [@Ratra88; @curvature; @mauro; @report; @repsergei; @Oze87; @Weinberg89; @Lambdat; @Bas09c; @Wetterich:1994bg; @Caldwell98; @Brax:1999gp; @KAM; @fein02; @Caldwell; @Bento03; @chime04; @Linder2004; @LSS08; @Brookfield:2005td; @Boehmer:2007qa; @Starobinsky-2007; @Ame10] and references therein).

In order to investigate the dynamical properties of a particular dark energy model, 