Assessing observational constraints on dark energy

Observational constraints on time-varying dark energy (e.g., quintessence) are commonly presented on a w₀–w_a plot that assumes the equation of state of dark energy strictly satisfies w(z) = w₀ + w_az/(1 + z) as a function of the redshift z. Recent observations favor a sector of the w₀–w_a plane in which w₀ > –1 and w₀ + w_a < –1, suggesting that the equation of state underwent a transition from violating the null energy condition (NEC) at large z to obeying it at small z. In this paper, we demonstrate that this impression is misleading by showing that simple quintessence models satisfying the NEC for all z predict an observational preference for the same sector. We also find that quintessence models that best fit observational data can predict a value for the dark energy equation of state at present that is significantly different from the best-fit value of w₀ obtained assuming the parameterization above. In addition, the analysis reveals an approximate degeneracy of the w₀–w_a parameterization that explains the eccentricity and orientation of the likelihood contours presented in recent observational studies.. Authors: Shlivko, David [Princeton University, NJ (United States)] (ORCID:0000000295241282); Steinhardt, Paul J. [Princeton University, NJ (United States); Harvard University, Cambridge MA (United States)] (ORCID:0000000334881603). DOE Contract: FG02-91ER40671; SC0007968. Subjects: 79 ASTRONOMY AND ASTROPHYSICS; Baryon acoustic oscillations; Cyclic cosmology; Dark energy; Quintessence; Swampland