Phantom crossing or dark interaction?

Recent results from DESI BAO measurements, together with Planck CMB and Pantheon+ data, suggest that there may be a `phantom’ phase ($w_{\rm de}<-1$) in the expansion of the Universe. This inference follows when the $w_0, w_a$ parametrization for the dark energy equation of state $w_{\rm de}$ is used to fit the data. Since phantom dark energy in general relativity is unphysical, we investigate the possibility that the phantom behaviour is not intrinsic, but effective – due to a non-gravitational interaction between dark matter and non-phantom dark energy. To this end, we assume a physically motivated thawing quintessence-like form of the intrinsic dark energy equation of state $w_{\rm de}$. Then we use a $w_0, w_a$ model for the \emph{effective} equation of state of dark energy. We find that the data favours a phantom crossing for the effective dark energy, but only at low significance. The intrinsic equation of state of dark energy is non-phantom, without imposing any non-phantom priors. A nonzero interaction is favoured at more than $3σ$ at $z\sim0.3$. The energy flows from dark matter to dark energy at early times and reverses at later times.

💡 Research Summary

The paper investigates whether the apparent phantom‑dark‑energy phase (effective equation‑of‑state w < −1) inferred from recent DESI BAO, Planck CMB, and Pantheon+ data is truly a physical property of dark energy or an artefact caused by a non‑gravitational interaction between dark matter (DM) and a non‑phantom dark energy (DE) component.

Theoretical framework
In a spatially flat FLRW universe the total energy density obeys the Friedmann equation. When a coupling Q(z) is introduced between DM and DE, the continuity equations become
\