Does the dark energy equation of state cross $w = -1$?
In plain words
The current best fits say dark energy was once stronger than empty-space energy and is now weaker. A simple rolling field cannot do that, so either the fits mislead or the physics is more exotic.
Precise statement
DESI $w0-wa$ best fits imply $w < -1$ at redshift $z > \sim 0.4$ and $w > -1$ today, which a single minimally coupled canonical scalar field cannot produce. Determine whether the data require crossing $w = -1$ when fitted with physically consistent models (thawing quintessence, coupled dark energy and dark matter, scalar-tensor gravity) or whether the crossing is an artifact of the linear parametrization and priors.
What would settle it
Model comparison using reconstructions of w(z) in redshift bins and physical models, showing whether non-crossing models fit as well as crossing ones.
Related problems
- Special case of Is the dark energy density constant in time?