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

Abstract

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.