Gravitational Rainbows: LIGO and Dark Energy at its Cutoff
  The recent direct detection of a neutron star merger with optical counterpart
has been used to severely constrain models of dark energy that typically
predict a modification of the speed of gravitational waves. We point out that
the energy scales observed at LIGO, and the particular frequency of the neutron
star event, lie very close to the strong coupling scale or cutoff associated
with many dark energy models. While it is true that at very low energies one
expects gravitational waves to travel at a speed different than light in these
models, the same is no longer necessarily true as one reaches energy scales
close to the cutoff. We show explicitly how this occurs in a simple model with
a known partial UV completion. Within the context of Horndeski, we show how the
operators that naturally lie at the cutoff scale can affect the speed of
propagation of gravitational waves and bring it back to unity at those scales.
We discuss how further missions including LISA and PTAs could play an essential
role in testing such models.
