Late-time cosmology in a (phantom) scalar-tensor theory: Dark energy and the cosmic speed-up

We consider late-time cosmology in a (phantom) scalar-tensor theory with an exponential potential, as a dark-energy model with equation of state parameter close to $-1$ (a bit above or below this value). Scalar (and also other kinds of) matter can be easily taken into account. An exact spatially flat FRW cosmology is constructed for such theory, which admits (eternal or transient) acceleration phases for the current universe, in correspondence with observational results. Some remarks on the possible origin of the phantom, starting from a more fundamental theory, are also made. It is shown that quantum gravity effects may prevent (or, at least, delay or soften) the cosmic doomsday catastrophe associated with the phantom, i.e., the otherwise unavoidable finite-time future singularity (Big Rip). A dark-energy model (higher-derivative scalar-tensor theory) is introduced, and it is shown to admit an effective phantom and/or quintessence description with a transient acceleration phase. In this case, gravity favors that an initially insignificant portion of dark energy becomes dominant over the standard matter and radiation components in the evolution process.