Testing gravity with gravitational wave friction and gravitational slip

Understand

Gravitational waves (GWs) emitted by binary sources are interesting signals for testing gravity on cosmological scales since they allow measurements of the luminosity distance.

• When followed by electromagnetic counterparts, in particular, they enable a reconstruction of the GW-distance-redshift relation.
• In the context of several modified gravity (MG) theories, even when requiring that the speed of propagation is equal to that of light, this GW distance differs from the standard electromagnetic luminosity distance due to the presence of a modified friction in the GW propagation.
• The very same source of this friction, which is the running of an effective Planck mass, also affects the scalar sector generating gravitational slip, i.e.