Assessing observational constraints on dark energy

Observational constraints on time-varying dark energy ({ e.g.}, quintessence) are commonly presented on a w₀-wₐ plot that assumes the equation of state of dark energy strictly satisfies w(z)= w₀+ wₐ z/(1+z) as a function of the redshift z. Recent observations favor a sector of the w₀-wₐ plane in which w₀ > -1 and w₀+wₐ< -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-wₐ parameterization that explains the eccentricity and orientation of the likelihood contours presented in recent observational studies.