Observational constraints on early coupled quintessence

We investigate an early coupled quintessence model where a light scalar mediates a fifth force stronger than gravity among dark matter particles and leads to the growth of perturbations prior to matter-radiation equality. Using cosmological data from the Planck cosmic microwave background power spectra, the Pantheon + type 1a supernovae, baryon acoustic oscillations, and big bang nucleosynthesis, we constrain the coupling strength beta and the redshift zOFF at which the interaction becomes effectively inactive, finding a firm degeneracy between these two parameters which holds regardless of when the scaling regime begins.