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

We study the cosmological constant (Λ) in the standard Λ cold dark matter model by introducing the graduated dark energy (gDE) characterized by a minimal dynamical deviation from the null inertial mass density of the Λ in the form ρinert ρλ<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 datasets of Planck+baryon acoustic oscillations+supernova+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, Δχmin2=6.4, for the new maxima, while it remains at insignificant levels, Δχmin2 0.02, 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 H0 measurements, high-precision Ly-α data, and model-independent Omh2 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 anti-de Sitter vacua to de Sitter vacua, at a redshift z≈2.32, and triggered the late-time acceleration, and suggests looking for such mechanisms in string theory constructions.