Cosmological bound from the neutron star merger GW170817 in scalar-tensor and F(R) gravity theories

We consider the evolution of cosmological gravitational waves in scalar-tensor theory and F(R) gravity theory as typical models of the modified gravity. Although the propagation speed is not changed from the speed of light, the propagation phase changes when we compare the propagation in these modified gravity theories with the propagation in the ΛCDM model. The phase change might be detected in future observations.