  Under the assumption that a dynamical scalar field is responsible for the
current acceleration of the Universe, we explore the possibility of probing its
physics in black hole merger processes with gravitational wave interferometers.
Remaining agnostic about the microscopic physics, we use an effective field
theory approach to describe the scalar dynamics. We investigate the case in
which some of the higher derivative operators, that are highly suppressed on
cosmological scales, instead become important on typical distances for black
holes. If a coupling to the Gauss-Bonnet operator is one of them, a non-trivial
background profile for the scalar field can be sourced in the surroundings of
the black hole, resulting in a potentially large amount of "hair". In turn,
this can induce sizeable modifications to the spacetime geometry or a mixing
between the scalar and the gravitational perturbations. Both effects will
ultimately translate into a modification of the quasi-normal mode spectrum in a
way that is also sensitive to other operators besides the one sourcing the
scalar background. The presence of deviations from the predictions of general
relativity in the observed spectrum can therefore serve as a window onto dark
energy physics.
