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What we have not learned about the dark energy equation of state

Savvas Nesseris1,*, Yashar Akrami1,2,3,†, and Glenn D. Starkman2,‡

  • *Contact author: savvas.nesseris@csic.es
  • †Contact author: yashar.akrami@csic.es
  • ‡Contact author: glenn.starkman@case.edu
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Phys. Rev. D 114, L041305 – Published 21 August, 2026

DOI: https://doi.org/10.1103/q2gb-n61g

Abstract

We show that using a Taylor expansion for the dark energy equation-of-state parameter and limiting it to the zeroth and first-order terms, i.e., the so-called Chevallier-Polarski-Linder (CPL) parametrization in regimes where it has been shown to fail as a physics-based two-parameter model, instead of allowing for higher-order terms and then marginalizing over them, adds extra information not present in the data and leads to markedly different and potentially misleading conclusions. Fixing the higher-order terms to zero, one concludes that vacuum energy that is currently nondynamical (e.g., the cosmological constant) is excluded at several σ significance as the explanation of cosmic acceleration, even in Dark Energy Spectroscopic Survey (DESI) DR1 data. Meanwhile, instead marginalizing over the higher-order terms shows that we know neither the current dark energy equation of state nor its current rate of change well enough to make such a claim. The CPL parametrization also implies that dark energy exhibits phantom behavior at high redshifts, while we show that by allowing the higher-order terms—which is required in order to capture the behavior of the dark energy equation of state in regimes beyond the validity of the CPL parametrization—the evidence for this phantomlike dark energy significantly weakens. This is not an argument for the higher-order phenomenological parametrizations, but rather a caution regarding such parametrizations in general. This issue has become more prominent now with the recent release of high-quality Stage IV galaxy survey data. The results of analyses using simple parametrizations should be interpreted with great care.

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Outline

Information

Phys. Rev. D 114, L041305– Published 21 August, 2026
  • Received 25 July 2025
  • Revised 28 March 2026
  • Accepted 20 July 2026
Article part of CHORUS
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