Graduated dark energy: Observational hints of a spontaneous sign switch in the cosmological constant

Understand

We study the cosmological constant ($\Lambda$) in the standard $\Lambda$CDM model by introducing the \textit{graduated dark energy} (gDE) characterised by a minimal dynamical deviation from the null inertial mass density of the $\Lambda$ in the form $\rho_{\rm inert}\propto \rho^{\lambda}<0$ with $\lambda<1$ being a ratio of two odd integers, for which its energy density $\rho$ dynamically takes negative values in the finite past.

• For large negative values of $\lambda$, it creates a phenomenological model described by a smooth function that approximately describes the $\Lambda$ spontaneously switching sign in the late universe to become positive today.
• We confront the model with the latest combined observational data sets of PLK+BAO+SN+$H$.
• It is striking that the data predict bimodal posterior probability distributions for the parameters of the model along with large negative $\lambda$ values; the new maximum significantly excludes the $\Lambda$, and the old maximum contains the $\Lambda$.