## Abstract

Observational constraints on time-varying dark energy (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.

Original language | English (US) |
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Article number | 138826 |

Journal | Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics |

Volume | 855 |

DOIs | |

State | Published - Aug 2024 |

## All Science Journal Classification (ASJC) codes

- Nuclear and High Energy Physics

## Keywords

- Baryon acoustic oscillations
- Cyclic cosmology
- Dark energy
- Quintessence
- Swampland