Gravity, Horizons and Open EFTs

Abstract: Wilsonian effective theories exploit hierarchies of scale to simplify the description of low-energy behaviour and play as central a role for gravity as for the rest of physics. They are useful both when hierarchies of scale are explicit in a gravitating system and more generally for understanding precisely what controls the size of quantum corrections in gravitational systems. But effective descriptions are also relevant for open systems (e.g. fluid mechanics as a long-distance description of statistical systems) for which the `integrating out' of unobserved low-energy degrees of freedom complicate a straightforward application of Wilsonian methods. Observations performed only on one side of an apparent horizon provide examples where open system descriptions also arise in gravitational physics. This chapter describes some early adaptations of Open Effective Theories (i.e. techniques for exploiting hierarchies of scale in open systems) in gravitational settings. Besides allowing the description of new types of phenomena (such as decoherence) these techniques also have an additional benefit: they sometimes can be used to resum perturbative expansions at late times and thereby to obtain controlled predictions in a regime where perturbative predictions otherwise generically fail.