Assessing observational constraints on dark energy

TL;DR

This paper shows that common observational constraints on dark energy's equation of state can be misleading, as simple models consistent with fundamental physics can produce similar apparent preferences.

Contribution

It demonstrates that the apparent observational preference for certain dark energy parameters is due to degeneracies and assumptions in the parameterization, not actual physical evidence.

Findings

Simple quintessence models satisfying NEC predict similar observational preferences.

Best-fit dark energy equation of state can differ from parameterized values.

Degeneracy in the $w_0$-$w_a$ parameterization explains likelihood contour shapes.

Abstract

Observational constraints on time-varying dark energy ({\it e.g.}, quintessence) are commonly presented on a $w_0$-$w_a$ plot that assumes the equation of state of dark energy strictly satisfies $w(z)= w_0+ w_a z/(1+z)$ as a function of the redshift $z$. Recent observations favor a sector of the $w_0$-$w_a$ plane in which $w_0 > -1$ and $w_0+w_a< -1$, 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 $w_0$ obtained assuming the parameterization above. In addition, the analysis reveals an approximate degeneracy of the $w_0$-$w_a$ parameterization that explains the eccentricity and orientation of the likelihood contours presented in recent observational studies.