  We find bounds on the Wilson coefficients of effective field theories (EFTs)
living in a Universe undergoing expansion by requiring that its modes do not
propagate further than a minimally coupled photon by a resolvable amount. To do
so, we compute the spatial shift suffered by the EFT modes at a fixed time
slice within the WKB approximation and the regime of validity of the EFT. We
analyze the bounds arising on shift-symmetric scalars and curved space
generalizations of Galileons.
