Negative cosmological constant in the dark sector?

Abstract

We consider the possibility that the dark sector of our Universe contains a negative cosmological constant dubbed $\ensuremath{\lambda}$. 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$\mathrm{\ensuremath{\Lambda}}$ cold dark matter ($\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$) model. For a large class of our models, the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with ${w}_{X}>\ensuremath{-}1$ 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 $\ensuremath{\lambda}$. 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 $\ensuremath{\lambda}$, the best models are obtained with a phantom behavior on redshifts $z\ensuremath{\gtrsim}1$ with a higher evidence for nonzero $\ensuremath{\lambda}$. An observed value for $h$ substantially higher than 0.70 would be a decisive test of their viability.