The unreasonable effectiveness of the Newtonian limit in cosmology and why it matters for modified gravity

The Newtonian limit in GR+LCDM cosmology is surprisingly accurate for a wide variety of cosmological purposes. Although not often used directly as such, it also provides a single coherent way to simultaneously treat the wide array of cosmological scales, beyond what can be achieved by perturbation theory. Treating many scales consistently and coherently is even more important for studying modified gravity, both when considering individual theories and when considering model independent approaches. I will briefly recap what has been learnt about the Newtonian limit in GR+LCDM. Then I will show a selection of approaches and results for modified gravity in cosmology that each use the Newtonian limit in some fashion.