Negative cosmological constant in the dark sector?

We consider the possibility that the dark sector of our Universe contains a negative cosmological constant dubbed lambda. For such models to be viable, the dark sector should contain an additional component responsible for the late-time accelerated expansion rate (X). We explore the departure of the expansion history of these models from the concordance. cold dark matter (Lambda CDM) model. For a large class of our models, the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with w(X) > -1 will eventually contract and we derive an analytical expression for the scale factor a(t) in the neighborhood of its maximal value. We find also the scale factor for models ending in a Big Rip in the regime where dustlike matter density is negligible compared to.. We address further the viability of such models, in particular when a high H-0 is taken into account. While we find no decisive evidence for a nonzero lambda, the best models are obtained with a phantom behavior on redshifts z greater than or similar to 1 with a higher evidence for nonzero lambda. An observed value for h substantially higher than 0.70 would be a decisive test of their viability.