Nonlocal gravity and gravitational-wave observations

We discuss a modified gravity model which fits cosmological observations at a level statistically indistinguishable from ΛCDM and at the same time predicts very large deviations from General Relativity (GR) in the propagation of gravitational waves (GWs) across cosmological distances. The model is a variant of the RT nonlocal model proposed and developed by our group, with initial conditions set during inflation, and predicts a GW luminosity distance that, at the redshifts accessible to LISA or at a third-generation GW detectors such as the Einstein Telescope (ET), can differ from that in GR by as much as 60%. An effect of this size could be detected with just a single standard siren with counterpart by LISA or ET. At the redshifts accessible to the LIGO/Virgo/Kagra network the effect is smaller but still potentially detectable.