Cosmic Self-Acceleration from Modified Gravity before/after GW170817

Modifications of gravity have long been considered as an alternative explanation for the late-time accelerated expansion of our Universe. The recent gravitational wave measurement GW170817 with its electromagnetic counterparts brought the challenge to the concept of cosmic self-acceleration from modifying gravity that had been anticipated for such an event. I will first discuss why a rigorous discrimination between acceleration from modified gravity and from a cosmological constant or dark energy was not possible with observations of the large-scale structure alone and how that measurement has particularly impacted the landscape of scalar-tensor gravity theories. I will conclude with an outlook on how surviving self-accelerated models will ultimately only be exhaustively probed with a large number of Standard Sirens. Finally, I will sketch new concepts that have been brought forward of how an evolving speed of gravity may nevertheless be the driver of cosmic acceleration yet remain compatible with the tight GW170817 constraint.