Negative cosmological constant in the dark sector?

We consider the possibility that the dark sector of our Universe contains a negative cosmological constant dubbed. 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 () model. For a large class of our models, the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with wₗ>-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 H₀ is taken into account. While we find no decisive evidence for a nonzero, the best models are obtained with a phantom behavior on redshifts z1 with a higher evidence for nonzero. An observed value for h substantially higher than 0. 70 would be a decisive test of their viability.