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 $\textit{Planck}$ 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 $z_{\rm OFF}$ at which the interaction becomes effectively inactive, finding a firm degeneracy between these two parameters which holds true regardless of when the scaling regime begins.