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

We study the cosmological constant ($Λ$) in the standard $Λ$CDM model by introducing the \textit{graduated dark energy} (gDE) characterised by a minimal dynamical deviation from the null inertial mass density of the $Λ$ in the form $ρ_{\rm inert}\propto ρ^λ<0$ with $λ<1$ being a ratio of two odd integers, for which its energy density $ρ$ dynamically takes negative values in the finite past. For large negative values of $λ$, it creates a phenomenological model described by a smooth function that approximately describes the $Λ$ 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 $λ$ values; the new maximum significantly excludes the $Λ$, and the old maximum contains the $Λ$. The improvement in the goodness of fit for the $Λ$ reaches highly significant levels, $Δχ_{\rm min}^2=6.4$ for the new maxima, while it remains at insignificant levels, $Δχ_{\rm min}^2\lesssim0.02$, for the old maxima. We show that, in contrast to the old maxima, which do not distinguish from the $Λ$, the new maxima agree with the model-independent $H_0$ measurements, high-precision Ly-$α$ data, and model-independent $Omh^2$ diagnostic estimates. Our results provide strong hints of a spontaneous sign switch in the cosmological constant and lead us to conjecture that the universe has transitioned from AdS vacua to dS vacua, at a redshift $z\approx 2.32$ and triggered the late-time acceleration, and suggests looking for such mechanisms in string theory constructions.