Constraints on a cubic Galileon disformally coupled to Standard Model matter

TL;DR

This paper investigates a disformally coupled cubic Galileon scalar field with Standard Model matter, exploring its effects on cosmology and electromagnetic wave propagation, and finds it evades current observational constraints due to weak coupling.

Contribution

It introduces a detailed analysis of disformal coupling effects on Galileon fields, including a screening-like phenomenon and explicit wave equations in a cosmological context.

Findings

Disformal coupling suppresses Galileon energy density during matter domination.

Electromagnetic wave velocity is minimally affected, evading LIGO/Virgo constraints.

The model remains consistent with gravitational wave observations.

Abstract

We consider a disformal coupling between Standard Model matter and a cubic Galileon scalar sector, assumed to be a relict of some other physics that solves the cosmological constant problem rather than a solution in its own right. This allows the energy density carried by the Galileon scalar to be sufficiently small that it evades stringent constraints from the integrated Sachs-Wolfe effect, which otherwise rules out the cubic Galileon theory. Although the model with disformal coupling does not exhibit screening, we show there is a `screening-like' phenomenon in which the energy density carried by the Galileon scalar is suppressed during matter domination when the quadratic and cubic Galileon operators are both relevant and the quadratic sector has a stable kinetic term. We obtain the explicit 3+1 form of Maxwell's equations in the presence of the disformal coupling, and the wave equations that govern electromagnetic waves. The disformal coupling is known to generate a small mass that modifies their velocity of propagation. We use the WKB approximation to study electromagnetic waves in this theory and show that, despite remarkable recent constraints from the LIGO/Virgo observatories that restrict the difference in propagation velocity between electromagnetic and gravitational radiation to roughly 1 part in $10^{15}$, the disformal coupling is too weak to be constrained by events such as GW170817 or by the dispersion of electromagnetic radiation at different wavelengths.