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

Abstract

We study the cosmological constant ($��$) in the standard $��$CDM model by introducing the \textit{graduated dark energy} (gDE) characterised by a minimal dynamical deviation from the null inertial mass density of the $��$ in the form $��_{\rm inert}\propto ��^��<0$ with $��<1$ being a ratio of two odd integers, for which its energy density $��$ dynamically takes negative values in the finite past. For large negative values of $��$, it creates a phenomenological model described by a smooth function that approximately describes the $��$ 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 $��$ values; the new maximum significantly excludes the $��$, and the old maximum contains the $��$. The improvement in the goodness of fit for the $��$ reaches highly significant levels, $����_{\rm min}^2=6.4$ for the new maxima, while it remains at insignificant levels, $����_{\rm min}^2\lesssim0.02$, for the old maxima. We show that, in contrast to the old maxima, which do not distinguish from the $��$, the new maxima agree with the model-independent $H_0$ measurements, high-precision Ly-$��$ 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.14 pages, 7 figures, 2 tables; matches the version to appear in PRD