Is the dark energy density constant in time?
In plain words
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 statement
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.
What would settle it
Final DESI BAO, Euclid clustering and Rubin LSST supernovae combined with CMB, giving a $5\ \sigma$ result stable across supernova samples.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
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).
Problems built on this one
Special cases
- Does the dark energy equation of state cross $w = -1$? · open
- Are evolving dark energy hints driven by supernova systematics? · open
Results
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See also
- Related Bound on scalar potential slopes in asymptotic field space
- Related Is $\alpha$ drifting today at the $10^{-19}$ per year level?
- Related Can averaging over cosmic inhomogeneities mimic dark energy?
- Related Why is the vacuum energy density so small yet nonzero?
- Related Why are the dark energy and matter densities comparable today?
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