f(T) gravity after GW170817 and GRB170817A

The combined observation of GW170817 and its electromagnetic counterpart GRB170817A reveals that gravitational waves propagate at the speed of light in high precision. We apply the standard analysis of cosmological perturbations, as well as the effective field theory approach, to investigate the experimental consequences for the theory of f(T) gravity. Our analysis verifies for the first time that the speed of gravitational waves within f(T) gravity is equal to the light speed, and hence, the constraints from GW170817 and GRB170817A are trivially satisfied. Nevertheless, by examining the dispersion relation and the frequency of cosmological gravitational waves, we observe a deviation from the results of general relativity, quantified by a new parameter. Although its value is relatively small in viable f(T) models, its possible future measurement in advancing gravitational-wave astronomy would be the smoking gun of testing this type of modified gravity.