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.

References

We now find ourselves with three open challenges: what are the microphysical underpinnings of inflation, dark matter, and dark energy?

— The Spectre of Underdetermination in Modern Cosmology  (2501.06095 - Ferreira et al., 10 Jan 2025) in Section 4: The spectre of underdetermination

A successful description of the observed late-time accelerated expansion of the Universe is provided by the ΛCDM model, although the nature of the dark sector and the underlying gravitational dynamics remain open questions.

— Cosmological Reconstruction in $f(Q, T)$ Gravity: $Λ$CDM Background and Matter-Sector Dependence  (2608.21003 - Pal et al., 21 Aug 2026) in Section 5, Conclusion

Another related problem is the possibility of describing cosmological dark energy within the proposed framework. Presumably, it can be realized by introducing a more complicated dependence on the curvature scalar $R$, analogously to the known description of dark energy by modified gravity through an $F(R)$ generalization of General Relativity. This is left for future studies.

— Vacuum energy problem in anti-de Sitter space  (2608.25860 - Arbuzova et al., 26 Aug 2026) in Section Conclusion

Indeed, it remains to be explored what the true micro-physical nature of these results are.

— The optimal redshift for dark energy II: application to cosmological data and the evidence for the phantom crossing of the CPL equation of state  (2608.25278 - Rippentrop et al., 26 Aug 2026) in Section Conclusion