Superluminality in Effective Field Theories for Cosmology

Some effective field theories popular in cosmology exhibit superluminality; they contain fluctuations that propagate faster than the speed of light around non-trivial backgrounds. There is controversy as to whether this kind of superluminality is actually a trustable physical feature of the effective theory, or merely an artifact of using an incomplete effective theory or of asking the wrong questions. Even if it is trustable, there is controversy as to whether this spells trouble for a theory, i.e. ruling out a UV completion, or whether it is harmless.  This workshop will aim to bring people on all sides to Perimeter, with the goal of making progress on these issues.