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We consider a disformal coupling between Standard Model matter and a cubicGalileon scalar sector, assumed to be a relict of some other physics thatsolves the cosmological constant problem rather than a solution in its ownright. This allows the energy density carried by the Galileon scalar to besufficiently small that it evades stringent constraints from the integratedSachs-Wolfe effect, which otherwise rules out the cubic Galileon theory.Although the model with disformal coupling does not exhibit screening, we showthere is a `screening-like' phenomenon in which the energy density carried bythe Galileon scalar is suppressed during matter domination when the quadraticand cubic Galileon operators are both relevant and the quadratic sector has astable kinetic term. We obtain the explicit 3+1 form of Maxwell's equations inthe presence of the disformal coupling, and the wave equations that governelectromagnetic waves. The disformal coupling is known to generate a small massthat modifies their velocity of propagation. We use the WKB approximation tostudy electromagnetic waves in this theory and show that, despite remarkablerecent constraints from the LIGO/Virgo observatories that restrict thedifference in propagation velocity between electromagnetic and gravitationalradiation to roughly 1 part in $10^{15}$, the disformal coupling is too weak tobe constrained by events such as GW170817 or by the dispersion ofelectromagnetic radiation at different wavelengths.

Constraints on a cubic Galileon disformally coupled to Standard Model matter