Addressing the Hubble and S8 tensions with a kinetically mixed dark sector

We present a kinetically mixed dark sector (KMIX) model to address the Hubble and S8 tensions. Inspired from string theory, our model includes two fields: an axion, which plays a role similar to the scalar field in early dark energy models, and a dilaton. This theory differs from other axio-dilaton models aimed at the Hubble tension in that there is necessarily kinetic mixing between the two fields which allows for efficient energy transfer from the axion into the dilaton which has w approximate to 1. As a direct consequence of these dynamics, we find the model does not need to resort to a fine-tuned potential to solve the Hubble tension and naturally accommodates a standard axion potential. Furthermore, the axion will necessarily makeup a small (fuzzy) fraction of SZcdm once it begins to oscillate at the bottom of its potential and will suppress the growth of perturbations on scales sensitive to S8. Interestingly, the scale of the potential for the dilaton has to be small, <= O(10 meV)4, suggesting the possibility for a connection to dark energy. Implementing the dynamics for the background and perturbations in a modified Boltzmann code we calculate the CMB and matter power spectra for our theory. Exploring the parameter space of our model, we find regions which can accommodate a similar to 10% increase in H0 from the Planck inferred value and S8 values that are consistent with large-scale structure constraints.