Observational preference for the stable small-phantom regime

Determine whether the theoretically stable region of the Einstein-scalar-Gauss–Bonnet parameter space, particularly the small-phantom regime with sufficiently small Gauss–Bonnet coupling, is favored by current low-redshift cosmological observations.

Background

The paper identifies parameter regions in which the effective dark-energy equation of state crosses the phantom divide while the scalar and tensor sound speeds remain positive and the tensor kinetic coefficient remains positive, thereby avoiding gradient and ghost instabilities. However, larger Gauss–Bonnet couplings can produce more negative equation-of-state values and induce gradient instabilities. The authors therefore distinguish a theoretically stable small-phantom regime from parameter regions that are observationally or perturbatively problematic.

The paper does not perform a direct statistical confrontation of the Einstein-scalar-Gauss–Bonnet model with cosmological data. It leaves unresolved whether observations favor the small-phantom regime over other regions of the model’s parameter space.

References

However, whether the model genuinely prefers the small-phantom regime, i.e., small values of $u$, will ultimately be determined by confronting it directly with cosmological observational datasets.

— Oscillatory dark energy with phantom crossings in Einstein-Gauss-Bonnet gravity  (2609.37437 - Hussain et al., 29 Sep 2026) in Section 4, subsection “Cosmological parameter evolution for representative values”