Cosmological Constraints on Horndeski Gravity in Light of GW170817
  The discovery of the electromagnetic counterpart to GW170817 severely
constrains the tensor mode propagation speed, eliminating a large model space
of Horndeski theory. We use the cosmic microwave background data from Planck
and the joint analysis of the BICEP2/Keck Array and Planck, galaxy clustering
data from the SDSS LRG survey, BOSS baryon acoustic oscillation data, and
redshift space distortion measurements to place constraints on the remaining
Horndeski parameters. We evolve the Horndeski parameters as power laws with
both the amplitude and power law index free. We find a 95% CL upper bound on
the present-day coefficient of the Hubble friction term in the cosmological
propagation of gravitational waves is 2.38, whereas General Relativity gives 2
at all times. While an enhanced friction suppresses the amplitude of the
reionization bump of the primordial B-mode power spectrum at $\ell < 10$, our
result limits the suppression to be less than 0.8%. This constraint is
primarily due to the scalar integrated Sachs-Wolfe effect in temperature
fluctuations at low multipoles.
