Evolution of Dark Energy Reconstructed from the Latest Observations

We reconstruct the evolution of the dark energy (DE) density using a nonparametric Bayesian approach from a combination of the latest observational data. We caution against parameterizing DE in terms of its equation of state as it can be singular in modified gravity models, and using it introduces a bias preventing negative effective DE densities. We find a 3.7σ preference for an evolving effective DE density with interesting features. For example, it oscillates around the Λ cold dark matter prediction at z ≲ 0.7, and could be negative at z ≳ 2.3; DE can be pressure-less at multiple redshifts, and a short period of cosmic deceleration is allowed at 0.1 ≲ z ≲ 0.2. We perform the reconstruction for several choices of the prior, as well as a evidence-weighted reconstruction. We find that some of the dynamical features, such as the oscillatory behavior of the DE density, are supported by the Bayesian evidence, which is a first detection of a dynamical DE with a positive Bayesian evidence. The evidence-weighted reconstruction prefers a dynamical DE at a (2.5 ± 0.06)σ significance level.