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Title: Is the dark energy density constant in time?
Plain: If dark energy is the energy of empty space it never changes, but if it comes from a slowly rolling field its density should drift. New maps of millions of galaxies hint at a drift.
Precise: In the parametrization $w(a) = w0 + wa (1 - a)$, with w the pressure-to-density ratio and a the scale factor, determine whether $(w0, wa) = (-1, 0)$ is excluded at $5\sigma$ by baryon acoustic oscillations, CMB and type Ia supernovae. DESI DR2 BAO plus CMB plus DES-Dovekie supernovae give $w0 = -0.803 \pm 0.054, wa = -0.72 \pm 0.21$. Answer yes or no at $5\sigma$ with datasets whose systematics are independently validated.
Settled by: Final DESI BAO, Euclid clustering and Rubin LSST supernovae combined with CMB, giving a $5\ \sigma$ result stable across supernova samples.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: contested
Status note: DESI DR2 BAO plus CMB gives $3.1\sigma$ (2025); adding supernovae gives $2.8\sigma$ (Pantheon+), $3.8\sigma$ (Union3) and $3.2\sigma$ (DES-Dovekie, 2025).
Posed since: 1998