Gravity at the horizon: from the cosmic dawn to ultra-large cosmological scales

Recent advances in cosmology provide both the motivation and the data to probe gravity on the largest scales available to observation. I will revise the landscape of gravitational theories, focusing on modern scalar-tensor theories and their cosmological implications. Then I will present the ongoing effort to test gravity in novel regimes such as the early universe and ultra-large scales, as well as using mildly non-linear effects. I will also introduce the hi_class code (www.hiclass-code.net), which is central to this program.