Cosmological tests of chameleon dark energy with dark sector coupling

Recent DESI measurements of baryon acoustic oscillations favor a value of $Ω_{\mathrm m}$ different from that preferred by Planck cosmic microwave background observations. Combined with supernova data, these observations suggest an evolving dark energy equation of state that crosses $w=-1$. As a potential solution to multiple cosmological problems, chameleon dark energy has attracted increasing attention in recent years. In this work, we constrain a chameleon model in which late-time cosmic acceleration is driven by a canonical scalar field nonminimally coupled to dark matter. At early times, the scalar field evolves under the combined influence of its potential and the nonminimal coupling, and its effective mass greatly exceeds the Hubble parameter. The chameleon model with energy transfer from dark energy to dark matter yields a best-fit improvement of $Δχ^2=-7.25$ relative to $Λ\mathrm{CDM}$, comparable to that for energy transfer in the opposite direction. Joint sampling over both coupling directions favors a nonzero interaction at $3.3σ$. Although no phantom field is introduced, the modified dark matter evolution produces an effective dark energy equation of state that crosses $w=-1$ at $z\simeq0.5$.