COSMO In the literature: contested

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

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).