Microphysical nature of dark energy

Determine the microphysical underpinnings of dark energy by identifying the underlying field(s) or modified-gravity mechanism, along with their equations of motion and action, that drive late-time cosmic acceleration and are consistent with expansion and structure-growth measurements.

Background

For dark energy, the paper emphasizes that cosmological observations primarily constrain the expansion history and the growth of structure, yielding only a small set of parameters (e.g., an equation-of-state parameterization and limited growth modifications). Many scalar-field and modified-gravity models produce empirically similar behavior over the narrow redshift range probed, leaving them effectively indistinguishable.

Consequently, even with improved measurements, a large family of microphysical models can match the same bulk properties, reinforcing the diagnosis of persistent underdetermination for the dark energy microphysics.