A new study shows that a class of dark‑energy theories in which a scalar field is coupled to dark matter, called the chameleon branch of non‑minimally coupled quintessence (NMCQ +), is favoured by current observations over the standard ΛCDM model. The authors examined cosmological data from the cosmic microwave background, baryon acoustic oscillations and type‑Ia supernovae and compared the fit of the coupled models to an uncoupled baseline.

In the NMCQ + framework the scalar field obeys an inverse‑power‑law potential V∝φ⁻ⁿ and interacts with dark matter through a coupling constant β. A positive β describes energy transfer from dark energy to dark matter (β>0), while a negative β represents transfer in the opposite direction (β<0). The authors performed a joint sampling over both signs of β, treating the field as initially sitting at the minimum of its effective potential. They used numerical methods to solve the highly stiff background equations, which feature an early‑time effective mass far larger than the Hubble rate.

The coupled models deliver a substantially better best‑fit: NMCQ + achieves a best‑fit Δχ² = –7.25 and NMCQ − achieves Δχ² = –7.84 compared with ΛCDM. The two branches support the data with nearly equal posterior weight; overall joint sampling favours a non‑zero coupling at the 3.3‑σ level. The best‑fit configuration results in an effective dark‑energy equation‑of‑state that crosses from below to above w = –1 near redshift z≈0.5, while the scalar field itself never enters a phantom regime.

However, the authors note that the cosmological constraints depend sensitively on the chosen initial conditions and that the large effective mass at early times causes severe differential‑equation stiffness, complicating the numerical analysis. Environmental effects may further suppress the scalar field’s influence in dense regions.