Cubic Galileon Gravity in the CMB

Among the models addressing the Hubble tension, those introducing a dynamical dark component around recombination have been the most promising thus far. Their study has highlighted that, in fact, cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) observations can allow for such components before and near recombination. The new dynamical degree of freedom can be early dark energy (EDE) or early modified gravity depending on its coupling to gravity. We study a new model, GEDE, featuring the cubic Galileon operator Xφ and test it against the most recent Planck PR4 CMB and Cepheid calibrated Pantheon+ type Ia Supernovae data. Thanks to the kinetic braiding effects, GEDE gives a better fit to the data, with a higher H₀, and is preferred over the canonical EDE with a Bayes factor ln B=0.9, despite introducing one more parameter. This calls for further explorations of modified gravity near and before last scattering. To facilitate these, we introduce a substantial extension of the cosmological code EFTCAMB that allows to fully evolve the background and linear dynamics of any covariant theory, oscillatory or not, belonging to the Horndeski class.