$\sigma_8$ tension and Horndeski Gravity

Abstract: A few observations have suggested that the present-day amplitude of matter density fluctuations is lower than predicted by the Planck-calibrated $\Lambda$CDM model. This tension, known as $\sigma_8$ tension, has motivated explorations of models beyond $\Lambda$CDM model. I will discuss possible solutions within the context of Horndeski gravity, which is the most general theory of gravity with second derivatives in the equations of motion.